Network Working Group D. Pinkas
Request for Comments: 3126 Integris
Category: Informational J. Ross
N. Pope
Security & Standards
September 2001
Electronic Signature Formats
for long term electronic signatures
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document defines the format of an electronic signature that can
remain valid over long periods. This includes evidence as to its
validity even if the signer or verifying party later attempts to deny
(i.e., repudiates the validity of the signature).
The format can be considered as an extension to RFC 2630 and RFC
2634, where, when appropriate additional signed and unsigned
attributes have been defined.
The contents of this Informational RFC is technically equivalent to
ETSI TS 101 733 V.1.2.2. The ETSI TS is under the ETSI Copyright (C).
Individual copies of this ETSI deliverable can be downloaded from
http://www.etsi.org
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RFC 3126 Electronic Signature Formats September 2001
Table of Contents
1. Introduction 4
2 Overview 5
2.1 Aim 5
2.2 Basis of Present Document 5
2.3 Major Parties 6
2.4 Electronic Signatures and Validation Data 7
2.5 Forms of Validation Data 8
2.6 Extended Forms of Validation Data 11
2.7 Archive Validation Data 13
2.8 Arbitration 15
2.9 Validation Process 15
2.10 Example Validation Sequence 16
2.11 Additional optional features 21
3. Data structure of an Electronic Signature 22
3.1 General Syntax 22
3.2 Data Content Type 22
3.3 Signed-data Content Type 22
3.4 SignedData Type 22
3.5 EncapsulatedContentInfo Type 23
3.6 SignerInfo Type 23
3.6.1 Message Digest Calculation Process 23
3.6.2 Message Signature Generation Process 24
3.6.3 Message Signature Verification Process 24
3.7 CMS Imported Mandatory Present Attributes 24
3.7.1 Content Type 24
3.7.2 Message Digest 24
3.7.3 Signing Time 24
3.8 Alternative Signing Certificate Attributes 24
3.8.1 ESS Signing Certificate Attribute Definition 25
3.8.2 Other Signing Certificate Attribute Definition 25
3.9 Additional Mandatory Attributes 26
3.9.1 Signature policy Identifier 26
3.10 CMS Imported Optional Attributes 28
3.10.1 Countersignature 29
3.11 ESS Imported Optional Attributes 29
3.11.1 Content Reference Attribute 29
3.11.2 Content Identifier Attribute 29
3.11.3 Content Hints Attribute 29
3.12 Additional Optional Attributes 30
3.12.1 Commitment Type Indication Attribute 30
3.12.2 Signer Location attribute 32
3.12.3 Signer Attributes attribute 33
3.12.4 Content Time-Stamp attribute 34
3.13 Support for Multiple Signatures 34
3.13.1 Independent Signatures 34
3.13.2 Embedded Signatures 34
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4. Validation Data 35
4.1 Electronic Signature Time-Stamp 36
4.1.1 Signature Time-Stamp Attribute Definition 36
4.2 Complete Validation Data 37
4.2.1 Complete Certificate Refs Attribute Definition 38
4.2.2 Complete Revocation Refs Attribute Definition 38
4.3 Extended Validation Data 40
4.3.1 Certificate Values Attribute Definition 40
4.3.2 Revocation Values Attribute Definition 41
4.3.3 ES-C Time-Stamp Attribute Definition 42
4.3.4 Time-Stamped Certificates and CRLs Attribute Definition 42
4.4 Archive Validation Data 43
4.4.1 Archive Time-Stamp Attribute Definition 43
5. Security Considerations 44
5.1 Protection of Private Key 44
5.2 Choice of Algorithms 44
6. Conformance Requirements 45
6.1 Signer 45
6.2 Verifier using time-stamping 46
6.3 Verifier using secure records 46
7. References 47
8. Authors' Addresses 48
Annex A (normative): ASN.1 Definitions 49
A.1 Definitions Using X.208 (1988) ASN.1 Syntax 49
A.2 Definitions Using X.680 1997 ASN.1 Syntax 57
Annex B (informative): General Description 66
B.1 The Signature Policy 66
B.2 Signed Information 67
B.3 Components of an Electronic Signature 68
B.3.1 Reference to the Signature Policy 68
B.3.2 Commitment Type Indication 69
B.3.3 Certificate Identifier from the Signer 69
B.3.4. Role Attributes 70
B.3.4.1 Claimed Role 71
B.3.4.2 Certified Role 71
B.3.5 Signer Location 72
B.3.6 Signing Time 72
B.3.7 Content Format 73
B.4 Components of Validation Data 73
B.4.1 Revocation Status Information 73
B.4.2 CRL Information 74
B.4.3 OCSP Information 74
B.4.4 Certification Path 75
B.4.5 Time-Stamping for Long Life of Signature 76
B.4.6 Time-Stamping before CA Key Compromises 77
B.4.6.1 Time-Stamping the ES with Complete validation data 77
B.4.6.2 Time-Stamping Certificates and Revocation Information 78
B.4.7 Time-Stamping for Long Life of Signature 79
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B.4.8 Reference to Additional Data 80
B.4.9 Time-Stamping for Mutual Recognition 80
B.4.10 TSA Key Compromise 81
B.5 Multiple Signatures 81
Annex C (informative): Identifiers and roles 82
C.1 Signer Name Forms 82
C.2 TSP Name Forms 82
C.3 Roles and Signer Attributes 83
Full Copyright Statement 84
This document is intended to cover electronic signatures for various
types of transactions, including business transactions (e.g.,
purchase requisition, contract, and invoice applications) where long
term validity of such signatures is important. This includes
evidence as to its validity even if the signer or verifying party
later attempts to deny (i.e., repudiates, see [ISONR]) the validity
of the signature).
Electronic signatures can be used for any transaction between an
individual and a company, between two companies, between an
individual and a governmental body, etc. This document is
independent of any environment. It can be applied to any environment
e.g., smart cards, GSM SIM cards, special programs for electronic
signatures etc.
An electronic signature produced in accordance with this document
provides evidence that can be processed to get confidence that some
commitment has been explicitly endorsed under a signature policy, at
a given time, by a signer under an identifier, e.g., a name or a
pseudonym, and optionally a role.
The European Directive on a community framework for Electronic
Signatures defines an electronic signature as: "data in electronic
form which is attached to or logically associated with other
electronic data and which serves as a method of authentication". An
electronic signature as used in the current document is a form of
advanced electronic signature as defined in the Directive.
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document (in uppercase,
as shown) are to be interpreted as described in [RFC2119].
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2 Overview
The aim of this document is to define an Electronic Signature (ES)
that remains valid over long periods. This includes evidence as to
its validity even if the signer or verifying party later attempts to
deny (repudiates) the validity of the signature.
This document specifies the use of trusted service providers (e.g.,
Time-Stamping Authorities (TSA)), and the data that needs to be
archived (e.g., cross certificates and revocation lists) to meet the
requirements of long term electronic signatures. An electronic
signature defined by this document can be used for arbitration in
case of a dispute between the signer and verifier, which may occur at
some later time, even years later. This document uses a signature
policy, referenced by the signer, as the basis for establishing the
validity of an electronic signature.
This document is based on the use of public key cryptography to
produce digital signatures, supported by public key certificates.
A Public key certificate is a public keys of a user, together with
some other information, rendered unforgeable by encipherment with the
private key of the Certification Authority (CA) which issued it
(ITU-T Recommendation X.509 [1]).
This document also specifies the uses of time-stamping services to
prove the validity of a signature long after the normal lifetime of
critical elements of an electronic signature and to support non-
repudiation. It also, as an option, defines the use of additional
time-stamps to provide very long-term protection against key
compromise or weakened algorithms.
This document builds on existing standards that are widely adopted.
This includes:
* RFC 2459 [RFC2459] Internet X.509 Public Key Infrastructure
Certificate and CRL Profile (PKIX);
* RFC 2630 [CMS] Crytographic Message Syntax (CMS);
* RFC 2634 [ESS] Enhanced Security Services (ESS);
* RFC 2439 [OCSP] One-line Certificate Status Protocol (OCSP);
* ITU-T Recommendation X.509 [1] Authentication framework;
* RFC (to be published) [TSP] PKIX Time Stamping protocol (TSP).
NOTE: See clause 8 for a full set of references.
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The following are the major parties involved in a business
transaction supported by electronic signatures as defined in this
document:
* the Signer;
* the Verifier;
* the Arbitrator;
* Trusted Service Providers (TSP).
A Signer is an entity that initially creates the electronic
signature. When the signer digitally signs over data using the
prescribed format, this represents a commitment on behalf of the
signing entity to the data being signed.
A verifier is an entity that verifies an evidence. (ISO/IEC 13888-1
[13]). Within the context of this document this is an entity that
validates an electronic signature.
An arbitrator, is an entity which arbitrates disputes between a
signer and a verifier when there is a disagreement on the validity of
a digital signature.
Trusted Service Providers (TSPs) are one or more entities that help
to build trust relationships between the signer and verifier. Use of
some specific TSP services MAY be mandated by signature policy. TSP
supporting services may provide the following information: user
certificates, cross-certificates, time-stamping tokens, CRLs, ARLs,
OCSP responses.
The following TSPs are used to support the validation or the
verification of electronic signatures:
* Certification Authorities;
* Registration Authorities;
* Repository Authorities (e.g., a Directory);
* Time-Stamping Authorities;
* One-line Certificate Status Protocol responders;
* Attribute Authorities;
* Signature Policy Issuers.
Certification Authorities provide users with public key certificates.
Registration Authorities allows the registration of entities before a
CA generates certificates.
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Repository Authorities publish CRLs issued by CAs, cross-certificates
(i.e., CA certificates) issued by CAs, signature policies issued by
Signature Policy Issuers and optionally public key certificates
(i.e., leaf certificates) issued by CAs.
Time-Stamping Authorities attest that some data was formed before a
given trusted time.
One-line Certificate Status Protocol responders (OSCP responders)
provide information about the status (i.e., revoked, not revoked,
unknown) of a particular certificate.
A Signature Policy Issuer issues signatures policies that define the
technical and procedural requirements for electronic signature
creation, validation and verification, in order to meet a particular
business need.
Attributes Authorities provide users with attributes linked to public
key certificates
Validation of an electronic signature in accordance with this
document requires:
* The electronic signature; this includes:
- the signature policy;
- the signed user data;
- the digital signature;
- other signed attributes provided by the signer;
- other unsigned attributes provided by the signer.
Validation data which is the additional data needed to validate the
electronic signature; this includes:
- certificates references;
- certificates;
- revocation status information references;
- revocation status information;
- time-stamps from Time Stamping Authorities (TSAs).
* The signature policy specifies the technical requirements on
signature creation and validation in order to meet a particular
business need. A given legal/contractual context may recognize
a particular signature policy as meeting its requirements.
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For example: a specific signature policy may be recognized by court
of law as meeting the requirements of the European Directive for
electronic commerce. A signature policy may be written using a
formal notation like ASN.1 or in an informal free text form provided
the rules of the policy are clearly identified. However, for a given
signature policy there shall be one definitive form which has a
unique binary encoded value.
Signed user data is the user's data that is signed.
The Digital Signature is the digital signature applied over the
following attributes provided by the signer:
* hash of the user data (message digest);
* signature Policy Identifier;
* other signed attributes
The other signed attributes include any additional information which
must be signed to conform to the signature policy or this document
(e.g., signing time).
According to the requirements of a specific signature policy in use,
various Validation Data shall be collected and attached to or
associated with the signature structure by the signer and/or the
verifier. The validation data includes CA certificates as well as
revocation status information in the form of certificate revocation
lists (CRLs) or certificate status information provided by an on-line
service. Additional data also includes time-stamps and other time
related data used to provide evidence of the timing of given events.
It is required, as a minimum, that either the signer or verifier
obtains a time-stamp over the signer's signature or a secure time
record of the electronic signature must be maintained. Such secure
records must not be undetectably modified and must record the time
close to when the signature was first validated.
An electronic signature may exist in many forms including:
* the Electronic Signature (ES), which includes the digital
signature and other basic information provided by the signer;
* the ES with Time-Stamp (ES-T), which adds a time-stamp to the
Electronic Signature, to take initial steps towards providing
long term validity;
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* the ES with Complete validation data (ES-C), which adds to the
ES-T references to the complete set of data supporting the
validity of the electronic signature (i.e., revocation status
information).
The signer must provide at least the ES form, but in some cases may
decide to provide the ES-T form and in the extreme case could provide
the ES-C form. If the signer does not provide ES-T, the verifier
must either create the ES-T on first receipt of an electronic
signature or shall keep a secure time record of the ES. Either of
these two approaches provide independent evidence of the existence of
the signature at the time it was first verified which should be near
the time it was created, and so protects against later repudiation of
the existence of the signature. If the signer does not provide ES-C
the verifier must create the ES-C when the complete set of revocation
and other validation data is available.
The ES satisfies the legal requirements for electronic signatures as
defined in the European Directive on electronic signatures, see Annex
C for further discussion on relationship of this document to the
Directive. It provides basic authentication and integrity protection
and can be created without accessing on-line (time-stamping)
services. However, without the addition of a time-stamp or a secure
time record the electronic signature does not protect against the
threat that the signer later denies having created the electronic
signature (i.e., does not provide non-repudiation of its existence).
The ES-T time-stamp or time record should be created close to the
time that ES was created to provide protection against repudiation.
At this time all the data needed to complete the validation may not
be available but what information is readily available may be used to
carry out some of the initial checks. For example, only part of the
revocation information may be available for verification at that
point in time. Generally, the ES-C form cannot be created at the
same time as the ES, as it is necessary to allow time for any
revocation information to be captured. Also, if a certificate is
found to be temporarily suspended, it will be necessary to wait until
the end of the suspension period.
The signer should only create the ES-C in situations where it was
prepared to wait for a sufficient length of time after creating the
ES form before dispatching the ES-C. This, however, has the
advantage that the verifier can be presented with the complete set of
data supporting the validity of the ES.
Support for ES-C by the verifier is mandated (see clause 6 for
specific conformance requirements).
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An Electronic Signature (ES), with the additional validation data
forming the ES-T and ES-C is illustrated in Figure 1:
+------------------------------------------------------------ES-C-----+
|+--------------------------------------------ES-T-----+ |
||+------Elect.Signature (ES)----------+ +------------+| +-----------+|
|||+---------+ +----------+ +---------+| |Time-Stamp || |Complete ||
||||Signature| | Other | | Digital || |over digital|| |certificate||
||||Policy ID| | Signed | |Signature|| |signature || |and ||
|||| | |Attributes| | || +------------+| |revocation ||
|||+---------+ +----------+ +---------+| | |references ||
||+------------------------------------+ | +-----------+|
|+-----------------------------------------------------+ |
+---------------------------------------------------------------------+
Figure 1: Illustration of an ES, ES-T and ES-C
The verifiers conformance requirements of an ES with a time-stamp of
the digital signature is defined in subclause 6.2.
The ES on its own satisfies the legal requirements for electronic
signatures as defined in the European Directive on electronic
signatures. The signers conformance requirements of an ES are
defined in subclause 6.1, and are met using a structure as indicated
in figure 2:
+------Elect.Signature (ES)-----------|
|+---------+ +----------+ +---------+ |
||Signature| | Other | | Digital | |
||Policy ID| | Signed | |Signature| |
|| | |Attributes| | | |
|+---------+ +----------+ +---------+ |
|+-----------------------------------+|
Figure 2: Illustration of an ES
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Where there are requirements for long term signatures without time-
stamping the digital signature, then a secure record is needed of the
time of verification in association with the electronic signature
(i.e., both must be securely recorded). In addition the certificates
and revocation information used at the time of verification should to
be recorded as indicated in figure 3 as an ES-C(bis).
+-------------------------------------------------------ES-C-----+
| |
| +------Elect.Signature (ES)----------+| +-----------+|
| |+---------+ +----------+ +---------+|| |Complete ||
| ||Signature| | Other | | Digital ||| |certificate||
| ||Policy ID| | Signed | |Signature||| |and ||
| || | |Attributes| | ||| |revocation ||
| |+---------+ +----------+ +---------+|| |references ||
| +------------------------------------+| +-----------+|
| |
+----------------------------------------------------------------+
Figure 3: Illustration of an ES-C(bis)
The verifiers conformance requirements of an ES-C(bis) is defined in
subclause 6.3.
Note: A time-stamp attached to the electronic signature or a secure
time record helps to protect the validity of the signature even if
some of the verification data associated with the signature become
compromised AFTER the signature was generated. The time-stamp or a
secure time record provides evidence that the signature was generated
BEFORE the event of compromise; hence the signature will maintain its
validity status.
The complete validation data (ES-C) described above may be extended
to form an ES with eXtended validation data (ES-X) to meet following
additional requirements.
Firstly, when the verifier does not has access to,
* the signer's certificate,
* all the CA certificates that make up the full certification
path,
* all the associated revocation status information, as referenced
in the ES-C.
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then the values of these certificates and revocation information may
be added to the ES-C. This form of extended validation data is
called a X-Long.
Secondly, if there is a risk that any CA keys used in the certificate
chain may be compromised, then it is necessary to additionally time-
stamp the validation data by either:
* time-stamping all the validation data as held with the ES(ES-
C), this eXtended validation data is called a Type 1 X-Time-
Stamp; or
* time-stamping individual reference data as used for complete
validation.
This form of eXtended validation data is called a Type 2 X-Time-
Stamp.
NOTE: The advantages/drawbacks for Type 1 and Type 2 X-Time-Stamp
are discussed in this document (see clause B.4.6.)
If all the above conditions occur then a combination of the two
formats above may be used. This form of eXtended validation data is
called a X-Long-Time-Stamped.
Support for the extended forms of validation data is optional.
An Electronic Signature (ES) , with the additional validation data
forming the ES-X long is illustrated in Figure 4:
+-------------------------------------------------------- ES-X Long--+
|+---------------------------------------- EC-C --------+ |
||+---- Elect.Signature (ES)----+ +--------+| +--------+ |
|||+-------+-+-------+-+-------+| +----------+|Complete|| |Complete| |
||||Signa- | |Other | |Digital|| |Time-Stamp||certi- || |certi- | |
||||ture | |Signed | |Signa- || |over ||ficate || |ficate | |
||||Policy | |Attri- | |ture || |digital ||and || |and | |
||||ID | |butes | | || |signature ||revoc. || |revoc. | |
|||+-------+ +-------+ +-------+| +----------+|refs || |data | |
||+-----------------------------+ +--------+| +--------+ |
|+------------------------------------------------------+ |
+--------------------------------------------------------------------+
Figure 4: Illustration of an ES and ES-X long.
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An Electronic Signature (ES) , with the additional validation data
forming the eXtended Validation Data - Type 1 is illustrated in
Figure 5:
+----------------------------------------------------------- ES-X 1 -+
|+----------------------------------------- EC-C --------+ |
|| +---- Elect.Signature (ES)----+ +--------+| +-------+ |
|| |+-------+ +-------+ +-------+| +----------+|Complete|| | | |
|| ||Signa- | |Other | |Digital|| |Time-Stamp||certifi-|| | Time- | |
|| ||ture | |Signed | |Signa- || |over ||cate and|| | stamp | |
|| ||Policy | |Attri- | |ture || |digital ||revoc. || | over | |
|| ||ID | |butes | | || |signature ||refs || | CES | |
|| |+-------+ +-------+ +-------+| +----------+| || | | |
|| +-----------------------------+ +--------+| +-------+ |
|+-------------------------------------------------------+ |
+--------------------------------------------------------------------+
Figure 5: Illustration of ES with ES-X Type 1
An Electronic Signature (ES) , with the additional validation data
forming the eXtended Validation Data - Type 2 is illustrated in
Figure 6:
+--------------------------------------------------------- ES-X 2 ---+
|+---------------------------------------- EC-C --------+ |
||+---- Elect.Signature (ES)----+ +--------+| +--------+ |
|||+-------+ +-------+ +-------+| +----------+|Complete|| |Times | |
||||Signa- | |Other | |Digital|| |Time-Stamp||certs || |Stamp | |
||||ture | |Signed | |Signa- || |over ||and || |over | |
||||Policy | |Attri- | |ture || |digital ||revoc. || |Complete| |
||||ID | |butes | | || |signature ||refs || |certs | |
|||+-------+ +-------+ +-------+| +----------+| || |and | |
||+-----------------------------+ +--------+| |revoc. | |
|| | |refs | |
|+------------------------------------------------------+ +--------+ |
+--------------------------------------------------------------------+
Figure 6: Illustration of ES with ES-X Type 2
Before the algorithms, keys and other cryptographic data used at the
time the ES-C was built become weak and the cryptographic functions
become vulnerable, or the certificates supporting previous time-
stamps expires, the signed data, the ES-C and any additional
information (ES-X) should be time-stamped. If possible this should
use stronger algorithms (or longer key lengths) than in the original
time-stamp.
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This additional data and time-stamp is called Archive Validation Data
(ES-A). The Time-Stamping process may be repeated every time the
protection used to time-stamp a previous ES-A become weak. An ES-A
may thus bear multiple embedded time stamps.
An example of an Electronic Signature (ES), with the additional
validation data for the ES-C and ES-X forming the ES-A is illustrated
in Figure 7.
+-------------------------------- ES-A --------- ----------+
| +-------------------- ES-A -----------------+ |
| | +--------- ES-X -------------- + | |
| | |..............................| +-----+ | +-----+ |
| | |..............................| |Time | | |Time | |
| | |..............................| |Stamp| | |Stamp| |
| | | | +-----+ | +-----+ |
| | +----------------------------- + | |
| +-------------------------------------------+ |
+----------------------------------------------------------+
Figure 7: Illustration of ES -A
Support for ES-A is optional.
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The ES-C may be used for arbitration should there be a dispute
between the signer and verifier, provided that:
* a copy of the signature policy referenced by the signer is
available;
* the arbitrator knows where to retrieve the signer's certificate
(if not already present), all the cross-certificates and the
required CRLs and/or OCSPs responses referenced in the ES-C;
* none of the issuing key from the certificate chain have ever
been compromised;
* the cryptography used at the time the ES-C was built has not
been broken at the time the arbitration is performed.
When the second condition is not met, then the plaintiff must provide
an ES-X Long.
When it is known by some external means that the third condition is
not met, then the plaintiff must provide an ES-X Time-Stamped.
When the two previous conditions are not met, the plaintiff must
provide the two above information (i.e., an ES-X Time-Stamped and
Long).
When the last condition is not met, the plaintiff must provide an
ES-A.
It should be noticed that a verifier may need to get two time stamps
at two different instants of time: one soon after the generation of
the ES and one soon after some grace period allowing any entity from
the certification chain to declare a key compromise.
The Validation Process validates an electronic signature in
accordance with the requirements of the signature policy. The output
status of the validation process can be:
* valid;
* invalid;
* incomplete verification.
A Valid response indicates that the signature has passed verification
and it complies with the signature validation policy.
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A signature validation policy is a part of the signature policy which
specifies the technical requirements on the signer in creating a
signature and verifier when validating a signature.
An Invalid response indicates that either the signature format is
incorrect or that the digital signature value fails verification
(e.g., the integrity checks on the digital signature value fails or
any of the certificates on which the digital signature verification
depends is known to be invalid or revoked).
An Incomplete Validation response indicates that the format and
digital signature verifications have not failed but there is
insufficient information to determine if the electronic signature is
valid under the signature policy. This can include situations where
additional information, which does not effect the validity of the
digital signature value, may be available but is invalid.
In the case of Incomplete Validation, it may be possible to request
that the electronic signature be checked again at a later date when
additional validation information might become available. Also, in
the case of incomplete validation, additional information may be made
available to the application or user, thus allowing the application
or user to decide what to do with partially correct electronic
signatures.
The validation process may also output validation data:
* a signature time-stamp;
* the complete validation data;
* the archive validation data.
Figure 8, and subsequent description, describes how the validation
process may build up a complete electronic signature over time.
Soon after receiving the electronic signature (ES) from the signer
(1), the digital signature value may be checked, the validation
process must at least add a time-stamp (2), unless the signer has
provided one which is trusted by the verifier. The validation
process may also validate the electronic signature, as required under
the identified signature policy, using additional data (e.g.,
certificates, CRL, etc.) provided by trusted service providers. If
the validation process is not complete then the output from this
stage is the ES-T.
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When all the additional data (e.g., the complete certificate and
revocation information) necessary to validate the electronic
signature first becomes available, then the validation process:
* obtains all the necessary additional certificate and revocation
status information;
* completes all the validation checks on the ES, using the
complete certificate and revocation information (if a time-
stamp is not already present, this may be added at the same
stage combining ES-T and ES-C process);
* records the complete certificate and revocation references (3);
* indicates the validity status to the user (4).
+----------------------------------------- ES-C ----------+
|+----------------------------- ES-T --------+ |
||+--- Elect.Signature (ES) ----+ | +--------+ |
|||+-------+ +-------+ +-------+|+----------+| |Complete| |
||||Signa- | |Other | |Digital|||Time-Stamp|| |certifi-| |
||||ture | |Signed | |Signa- |||over || |cate and| |
||||Policy | |Attri- | |ture |||digital || |revoca- | |
||||ID | |butes | | |||signature || |tion | |
|||+-------+ +-------+ +-------+|+----------+| |referen-| |
||+------------\----------------+ ^ | |ces | |
|| \ | | +--------+ |
|| \ 1 / | ^ |
|+----------------\----------------/---------+ | |
+------------------\--------------/--------------- /------+
\ /2 ----3------/
+----------+ | / /
| Signed |\ v / |
|User data | \ +--------------------+ +------------+
+----------+ \--->| Validation Process |---> |- Valid |
+---|--^-------|--^--+ 4 |- Invalid |
| | | | |- Validation|
v | v | | Incomplete|
+---------+ +--------+ +------------+
|Signature| |Trusted |
| Policy | |Service |
| Issuer | |Provider|
+---------+ +--------+
Figure 8: Illustration of an ES with Complete validation data (ES-C)
Pinkas, et al. Informational [Page 17]
RFC 3126 Electronic Signature Formats September 2001
At the same time as the validation process creates the ES-C, the
validation process may provide and/or record the values of
certificates and revocation status information used in ES-C, called
the ES-X Long (5). This is illustrated in figure 9:
+----------------------------------------------------- ES-X ---------+
|+---------------------------------------- ES-C --------+ +--------+ |
||+--- Elect.Signature (ES) ----+ +--------+ | |Complete| |
|||+-------+ +-------+ +-------+|+----------+|Complete| | |certifi-| |
||||Signa- | |Other | |Digital|||Time-Stamp||certifi-| | |cate | |
||||ture | |Signed | |Signa- |||over ||cate and| | |and | |
||||Policy | |Attri- | |ture |||digital ||revoca- | | |revoca- | |
||||ID | |butes | | |||signature ||tion | | |tion | |
|||+-------+ +---|---+ +-------+|+----------+|referen-| | |Data | |
||+--------------\--------------+ ^ |ces | | +--------+ |
|| \ | +--------+ | ^ |
|| \ 1 2/ ^ | | |
|+------------------\--------------/------------|-------+ / |
+--------------------\------------/------------/-------------/-------+
\ / ---3----/ /
+----------+ | / / ------------5-----/
| Signed |\ v | | /
|User data | \ +--------------------+ +-----------+
+----------+ \--->| Validation Process |---> | - Valid |
+---|--^-------|--^--+ 4 | - Invalid |
| | | | +-----------+
v | v |
+---------+ +--------+
|Signature| |Trusted |
| Policy | |Service |
| Issuer | |Provider|
+---------+ +--------+
Figure 9: Illustration ES with eXtended validation data (Long)
When the validation process creates the ES-C it may also create
extended forms of validation data. A first alternative is to time-
stamp all data forming the Type 1 X-Time-Stamp (6). This is
illustrated in figure 10:
Pinkas, et al. Informational [Page 18]
RFC 3126 Electronic Signature Formats September 2001
+----------------------------------------------------- ES-X -------+
|+---------------------------------------- ES-C --------+ +------+ |
||+--- Elect.Signature (ES) ----+ +--------+ | |Time- | |
|||+-------+ +-------+ +-------+|+----------+|Complete| | |Stamp | |
||||Signa- | |Other | |Digital|||Time-Stamp||certifi-| | |over | |
||||ture | |Signed | |Signa- |||over ||cate and| | |CES | |
||||Policy | |Attri- | |ture |||digital ||revoca- | | +------+ |
||||ID | |butes | | |||signature ||tion | | ^ |
|||+-------+ +--|----+ +-------+|+----------+|referen-| | | |
||+-------------|---------------+ ^ |ces | | | |
|| | | +--------+ | | |
|| \ 1 2/ ^ | | |
|+----------------\------------------/----------|-------+ | |
+------------------\----------------/-----------/-------------/----+
\ / ----3---/ /
+----------+ | / / ---------------6---/
| Signed |\ v | | /
|User data | \ +--------------------+ +-----------+
+----------+ \--->| Validation Process |---> | - Valid |
+---|--^-------|--^--+ 4 | - Invalid |
| | | | +-----------+
v | v |
+---------+ +--------+
|Signature| |Trusted |
| Policy | |Service |
| Issuer | |Provider|
+---------+ +--------+
Figure 10: Illustration of ES with eXtended validation data -
Type 1 X-Time-Stamp
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Another alternative is to time-stamp the certificate and revocation
information references used to validate the electronic signature (but
not the signature) (6'); this is called Type 2 X-Time-Stamped. This
is illustrated in figure 11:
+----------------------------------------------------- ES-X -----------+
|+---------------------------------------- ES-C --------+ +----------+ |
||+--- Elect.Signature (ES) ----+ +--------+ | |Time-Stamp| |
|||+-------+ +-------+ +-------+|+----------+|Complete| | |over | |
||||Signa- | |Other | |Digital|||Time-Stamp||certifi-| | |Complete | |
||||ture | |Signed | |Signa- |||over ||cate and| | |Certifi- | |
||||Policy | |Attri- | |ture |||digital ||revoc. | | |cate and | |
||||ID | |butes | | |||signature ||refs | | |revoc. | |
|||+-------+ +---^---+ +-------+|+----^-----++---^----+ | |refs | |
||+--------------\--------------+ | | | +----------+ |
|+----------------\------------------/-----------|------+ ^ |
+----------------1-\----------------/-----------/--------------|-------+
\ / -----3---/ |
+----------+ | 2/ / ---------------6'-----/
| Signed |\ v | | /
|User data | \ +--------------------+ +-----------+
+----------+ \--->| Validation Process |---> | - Valid |
+---|--^-------|--^--+ 4 | - Invalid |
| | | | +-----------+
v | v |
+---------+ +--------+
|Signature| |Trusted |
| Policy | |Service |
| Issuer | |Provider|
+---------+ +--------+
Figure 11: Illustration of ES with eXtended validation data -
Type 2 X-Time-Stamp
Before the algorithms used in any of electronic signatures become or
are likely, to be compromised or rendered vulnerable in the future,
it is necessary to time-stamp the entire electronic signature,
including all the values of the validation and user data as an ES
with Archive validation data (ES-A)
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An ES-A is illustrated in figure 12:
-------------------------------------------- ES-A --------------------+
----------------------------------------------------------------+ |
+------------------------------- EC-C --------++-----+ | |
| ||Time-| | |
|+-- Elect.Signature (ES) -+ +--------+||Stamp| +-------+ |
||+------++-------++-------|+------+|Complete|||over | Complete| |
|||Signa-||Other ||Digital||Time- ||certifi-|||CES | |certi- |+----|
|||ture ||Signed ||Signa- ||Stamp ||cate and||+-----+ |ficate |Arch-|
|||Policy||Attri- ||ture ||over ||revoca- ||+------+ |and |ive |
|||ID ||butes || ||digit.||tion |||Time- | |revoca-|Time |
||+------++---|---++-------||signa-||referen-|||Stamp-| |tion |stamp|
|+------------|------------+|ture ||ces |||over | |data |+----|
| | +------++--------+|Complete\+-------+ ^ |
| | ^ ^ ||cert. | | | |
+-------------|----------------|---------|----+|and rev| | | |
\ | / |refs. | | | |
\ | / +-------+ | | |
-----------------\-------------|-------/------------------------+ | |
+----------+ \ | / / |
| Signed | \2 |3 / /--------------7-------/ |
|User data | \ | | / |
+-------\--+ \ | | / |
---------\------------|--------|----|---/-----------------------------+
\ v | | |
1\ +--------------------+ +-----------+
\------>| Validation Process |---> | - Valid |
+---|--^-------|--^--+ 4 | - Invalid |
| | | | +-----------+
v | v |
+---------+ +--------+
|Signature| |Trusted |
| Policy | |Service |
| Issuer | |Provider|
+---------+ +--------+
Figure 12: Illustration of an ES with Archive validation data (ES-A)
This document also defines additional optional features of an
electronic signature to:
* indicate a commitment type being made by the signer;
* indicate the role under which a signature was created;
* support multiple signatures.
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This clause uses and builds upon the Cryptographic Message Syntax
(CMS), as defined in RFC 2630 [CMS], and Enhanced Security Services
(ESS), as defined in RFC 2634 [ESS]. The overall structure of
Electronic Signature is as defined in [CMS]. The Electronic
Signature (ES) uses attributes defined in [CMS], [ESS] and this
document. This document defines in full the ES attributes which it
uses and are not defined elsewhere.
The mandated set of attributes and the digital signature value is
defined as the minimum Electronic Signature (ES) required by this
document. A signature policy MAY mandate other signed attributes to
be present.
The data content type of the ES is as defined in [CMS].
The data content type is intended to refer to arbitrary octet
strings, such as ASCII text files; the interpretation is left to the
application. Such strings need not have any internal structure
(although they could have their own ASN.1 definition or other
structure).
The Signed-data content type of the ES is as defined in [CMS].
The signed-data content type consists of a content of any type and
zero or more signature values. Any number of signers in parallel can
sign any type of content. The typical application of the signed-data
content type represents one signer's digital signature on content of
the data content type.
To make sure that the verifier uses the right certificate, this
document mandates that the hash of the signers certificate is always
included in the Signing Certificate signed attribute.
The syntax of the SignedData type of the ES is as defined in [CMS].
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The fields of type SignedData have the meanings defined [CMS] except
that:
* version is the syntax version number. The value of version
must be 3.
* The identification of signer's certificate used to create the
signature is always present as a signed attribute.
* The degenerate case where there are no signers is not valid in
this document.
The syntax of the EncapsulatedContentInfo a type of the ES is as
defined in [CMS].
For the purpose of long term validation as defined by this document,
it is advisable that either the eContent is present, or the data
which is signed is archived in such as way as to preserve the any
data encoding. It is important that the OCTET STRING used to generate
the signature remains the same every time either the verifier or an
arbitrator validates the signature.
The degenerate case where there are no signers is not valid in this
document.
The syntax of the SignerInfo a type of the ES is as defined in [CMS].
Per-signer information is represented in the type SignerInfo. In the
case of multiple independent signatures, there is an instance of this
field for each signer.
The fields of type SignerInfo have the meanings defined in [CMS]
except that signedAttributes must, as a minimum, contain the
following attributes:
* ContentType as defined in clause 3.7.1.
* MessageDigest as defined in clause 3.7.2.
* SigningTime as defined in clause 3.7.3.
* SigningCertificate as defined in clause 3.8.1.
* SignaturePolicyId as defined in clause 3.9.1.
The message digest calculation process is as defined in [CMS].
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The procedures for CMS signed data validation are as defined in [CMS]
and enhanced in this document.
The input to the signature verification process includes the signer's
public key verified as correct using either the ESS Signing
Certificate attribute or the Other Signing Certificate attribute.
The syntax of the message-digest attribute type of the ES is as
defined in [CMS] and further qualified by this document.
The signing-time attribute type specifies the time at which the
signer claims to have performed the signing process.
This present document recommends the use of GeneralizedTime.
One, and only one, of the following two alternative attributes MUST
be present with the signed-data defined by this document to identify
the signing certificate. Both attributes include an identifier and a
hash of the signing certificate. The first, which is adopted in
existing standards, may be only used with the SHA-1 hashing
algorithm. The other shall be used when other hashing algorithms are
to be supported.
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The signing certificate attribute is designed to prevent the simple
substitution and re-issue attacks, and to allow for a restricted set
of authorization certificates to be used in verifying a signature.
The syntax of the signing certificate attribute type of the ES is as
defined in [ESS], and further qualified and profile in this document.
The ESS signing certificate attribute must be a signed attribute.
This document mandates the presence of this attribute as a signed CMS
attribute, and the sequence must not be empty. The certificate used
to verify the signature must be identified in the sequence, the
Signature Validation Policy may mandate other certificate references
to be present, that may include all the certificates up to the point
of trust. The encoding of the ESSCertID for this certificate must
include the issuerSerial field.
The issuerAndSerialNumber present in the SignerInfo must be
consistent with issuerSerial field. The certificate identified must
be used during the signature verification process. If the hash of
the certificate does not match the certificate used to verify the
signature, the signature must be considered invalid.
The sequence of policy information field is not used in this
document.
NOTE: Where an attribute certificate is used by the signer to
associate a role, or other attributes of the signer, with the
electronic signature this is placed in the Signer Attribute attribute
as defined in clause 3.12.3.
The following attribute is identical to the ESS SigningCertificate
defined above except that this attribute can be used with hashing
algorithms other than SHA-1.
This attribute must be used in the same manner as defined above for
the ESS SigningCertificate attribute.
The following object identifier identifies the signing certificate
attribute:
id-aa-ets-otherSigCert OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) id-aa(2) 19 }
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The signing certificate attribute value has the ASN.1 syntax
OtherSigningCertificate
OtherSigningCertificate ::= SEQUENCE {
certs SEQUENCE OF OtherCertID,
policies SEQUENCE OF PolicyInformation OPTIONAL
-- NOT USED IN THIS DOCUMENT
}
OtherCertID ::= SEQUENCE {
otherCertHash OtherHash,
issuerSerial IssuerSerial OPTIONAL
}
OtherHash ::= CHOICE {
sha1Hash OtherHashValue, -- This contains a SHA-1 hash
otherHash OtherHashAlgAndValue
}
OtherHashValue ::= OCTET STRING
OtherHashAlgAndValue ::= SEQUENCE {
hashAlgorithm AlgorithmIdentifier,
hashValue OtherHashValue
}
This document mandates that a reference to the signature policy, is
included in the signedData, this reference is either explicitly
identified or implied by the semantics of the signed content and
other external data. A signature policy defines the rules for
creation and validation of an electronic signature, is included as a
signed attribute with every signature. The signature policy
identifier must be a signed attribute.
The following object identifier identifies the signature policy
identifier attribute:
id-aa-ets-sigPolicyId OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) id-aa(2) 15 }
Signature-policy-identifier attribute values have ASN.1 type
SignaturePolicyIdentifier.
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SignaturePolicyIdentifier ::= CHOICE{
SignaturePolicyId SignaturePolicyId,
SignaturePolicyImplied SignaturePolicyImplied }
SignaturePolicyId ::= SEQUENCE {
sigPolicyIdentifier SigPolicyId,
sigPolicyHash SigPolicyHash,
sigPolicyQualifiers SEQUENCE SIZE (1..MAX) OF
SigPolicyQualifierInfo OPTIONAL
}
SignaturePolicyImplied ::= NULL
The presence of the NULL type indicates that the signature policy is
implied by the semantics of the signed data and other external data.
The sigPolicyId field contains an object-identifier which uniquely
identifies a specific version of the signature policy. The syntax of
this field is as follows:
SigPolicyId ::= OBJECT IDENTIFIER
The sigPolicyHash field contains the identifier of the hash algorithm
and the hash of the value of the signature policy.
If the signature policy is defined using a computer processable
notation like ASN.1, then the hash is calculated on the value without
the outer type and length fields and the hashing algorithm must be as
specified in the field signPolicyHshAlg.
If the signature policy is defined using another structure, the type
of structure and the hashing algorithm must be either specified as
part of the signature policy, or indicated using a signature policy
qualifier.
SigPolicyHash ::= OtherHashAlgAndValue
A signature policy identifier may be qualified with other information
about the qualifier. The semantics and syntax of the qualifier is as
associated with the object-identifier in the sigPolicyQualifierId
field. The general syntax of this qualifier is as follows:
SigPolicyQualifierInfo ::= SEQUENCE {
sigPolicyQualifierId SigPolicyQualifierId,
sigQualifier ANY DEFINED BY sigPolicyQualifierId
}
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This document specifies the following qualifiers:
* spuri: This contains the web URI or URL reference to the
signature policy
* spUserNotice: This contains a user notice which should be
displayed whenever the signature is validated.
-- sigpolicyQualifierIds defined in this document
SigPolicyQualifierId ::= OBJECT IDENTIFIER
id-spq-ets-uri OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) id-spq(5) 1 }
SPuri ::= IA5String
id-spq-ets-unotice OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) id-spq(5) 2 }
SPUserNotice ::= SEQUENCE {
noticeRef NoticeReference OPTIONAL,
explicitText DisplayText OPTIONAL
}
NoticeReference ::= SEQUENCE {
organization DisplayText,
noticeNumbers SEQUENCE OF INTEGER
}
DisplayText ::= CHOICE {
visibleString VisibleString (SIZE (1..200)),
bmpString BMPString (SIZE (1..200)),
utf8String UTF8String (SIZE (1..200))
}
The following attributes MAY be present with the signed-data defined
by this document. The attributes are defined in ref [CMS] and are
imported into this specification and were appropriate qualified and
profiling by this document.
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The following attributes MAY be present with the signed-data defined
by this document. The attributes are defined in ref [ESS] and are
imported into this specification and were appropriate qualified and
profiling by this document.
The content reference attribute is a link from one SignedData to
another. It may be used to link a reply to the original message to
which it refers, or to incorporate by reference one SignedData into
another.
The content reference attribute MUST be used as defined in [ESS].
The content reference MUST be a signed attribute.
The syntax of the content reference attribute type of the ES is as
defined in [ESS].
The content identifier attribute provides an identifier for the
signed content for use when reference may be later required to that
content, for example in the content reference attribute in other
signed data sent later.
The content identifier must be a signed attribute.
The syntax of the content identifier attribute type of the ES is as
defined in [ESS].
The minimal signedContentIdentifier should contain a concatenation of
user-specific identification information (such as a user name or
public keying material identification information), a GeneralizedTime
string, and a random number.
The content hints attribute provides information that describes the
format of the signed content. It may be used by the signer to
indicate to a verifier the precise format that MUST be used to
Pinkas, et al. Informational [Page 29]
RFC 3126 Electronic Signature Formats September 2001
present the data (e.g., text, voice, video) to a verifier. This
attribute MUST be present when it is mandatory to present the signed
data to human users on verification.
The syntax of the content hints attribute type of the ES is as
defined in ESS (RFC 2634, section 2.9 [9]).
When used to indicate the precise format of the data to be presented
to the user the following rules apply:
The contentType (defined in RFC 2630 [8]) indicates the type of the
associated content. It is an object identifier (i.e., a unique
string of integers) assigned by an authority that defines the content
type.
The UTF8String shall define the presentation format. The format may
be defined by MIME types as indicated below.
Note 1: The contentType can be id-data defined in CMS (RFC 2630 [8]).
The UTF8String can be used to indicate the encoding of the data, like
MIME type. RFC 2045 [25] provides a common structure for encoding a
range of electronic documents and other multi-media types, see annex
B for further information, a system supporting verification of
electronic signature may present information to users in the form
identified by the MIME type.
id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs7(7) 1 }
There may be situation were a signer wants to explicitly indicate to
a verifier that by signing the data, it illustrates a type of
commitment on behalf of the signer. The commitmentTypeIndication
attribute conveys such information.
The commitmentTypeIndication attribute must be a signed attribute.
The commitment type may be:
* defined as part of the signature policy, in which case the
commitment type has precise semantics that is defined as part
of the signature policy.
Pinkas, et al. Informational [Page 30]
RFC 3126 Electronic Signature Formats September 2001
* be a registered type, in which case the commitment type has
precise semantics defined by registration, under the rules of
the registration authority. Such a registration authority may
be a trading association or a legislative authority.
The signature policy specifies a set of attributes that it
"recognizes". This "recognized" set includes all those commitment
types defined as part of the signature policy as well as any
externally defined commitment types that the policy may choose to
recognize. Only recognized commitment types are allowed in this
field.
The following object identifier identifies the commitment type
indication attribute:
id-aa-ets-commitmentType OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 16}
Commitment-Type-Indication attribute values have ASN.1 type
CommitmentTypeIndication.
CommitmentTypeIndication ::= SEQUENCE {
commitmentTypeId CommitmentTypeIdentifier,
commitmentTypeQualifier SEQUENCE SIZE (1..MAX) OF
CommitmentTypeQualifier OPTIONAL
}
CommitmentTypeIdentifier ::= OBJECT IDENTIFIER
CommitmentTypeQualifier ::= SEQUENCE {
commitmentTypeIdentifier CommitmentTypeIdentifier,
qualifier ANY DEFINED BY
commitmentTypeIdentifier
}
The use of any qualifiers to the commitment type is outside the scope
of this document.
The following generic commitment types are defined in this document:
id-cti-ets-proofOfOrigin OBJECT IDENTIFIER ::= { iso(1) member-
body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
cti(6) 1}
id-cti-ets-proofOfReceipt OBJECT IDENTIFIER ::= { iso(1) member-
body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
cti(6) 2}
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id-cti-ets-proofOfDelivery OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) cti(6) 3}
id-cti-ets-proofOfSender OBJECT IDENTIFIER ::= { iso(1) member-
body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
cti(6) 4}
id-cti-ets-proofOfApproval OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) cti(6) 5}
id-cti-ets-proofOfCreation OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) cti(6) 6}
These generic commitment types have the following meaning:
Proof of origin indicates that the signer recognizes to have created,
approved and sent the message.
Proof of receipt indicates that signer recognizes to have received
the content of the message.
Proof of delivery indicates that the TSP providing that indication
has delivered a message in a local store accessible to the recipient
of the message.
Proof of sender indicates that the entity providing that indication
has sent the message (but not necessarily created it).
Proof of approval indicates that the signer has approved the content
of the message.
Proof of creation indicates that the signer has created the message
(but not necessarily approved, nor sent it).
The signer-location attribute is an attribute which specifies a
mnemonic for an address associated with the signer at a particular
geographical (e.g., city) location. The mnemonic is registered in
the country in which the signer is located and is used in the
provision of the Public Telegram Service (according to ITU-T
Recommendation F.1 [PTS]).
The signer-location attribute must be a signed attribute.
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The following object identifier identifies the signer-location
attribute:
id-aa-ets-signerLocation OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 17}
Signer-location attribute values have ASN.1 type SignerLocation.
SignerLocation ::= SEQUENCE {
-- at least one of the following must be present
countryName [0] DirectoryString OPTIONAL,
-- as used to name a Country in X.500
localityName [1] DirectoryString OPTIONAL,
-- as used to name a locality in X.500
postalAdddress [2] PostalAddress OPTIONAL
}
PostalAddress ::= SEQUENCE SIZE(1..6) OF DirectoryString
The signer-attributes attribute is an attribute which specifies
additional attributes of the signer (e.g., role).
It may be either:
* claimed attributes of the signer; or
* certified attributes of the signer;
The signer-attributes attribute must be a signed attribute.
The following object identifier identifies the signer-attribute
attribute:
id-aa-ets-signerAttr OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 18}
signer-attribute attribute values have ASN.1 type SignerAttribute.
SignerAttribute ::= SEQUENCE OF CHOICE {
claimedAttributes [0] ClaimedAttributes,
certifiedAttributes [1] CertifiedAttributes
}
ClaimedAttributes ::= SEQUENCE OF Attribute
CertifiedAttributes ::= AttributeCertificate
-- as defined in X.509 : see section 10.3
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RFC 3126 Electronic Signature Formats September 2001
NOTE: The claimed and certified attribute are imported from ITU-T
Recommendations X.501 [16] and ITU-T Recommendation X.509:Draft
Amendment on Certificate Extensions, October 1999.
The content time-stamp attribute is an attribute which is the time-
stamp of the signed data content before it is signed.
The content time-stamp attribute must be a signed attribute.
The following object identifier identifies the signer-attribute
attribute:
id-aa-ets-contentTimestamp OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) id-aa(2) 20}
Content time-stamp attribute values have ASN.1 type ContentTimestamp:
ContentTimestamp::= TimeStampToken
The value of messageImprint field within TimeStampToken must be a
hash of the value of eContent field within encapContentInfo within
the signedData.
For further information and definition of TimeStampToken see [TSP].
Multiple independent signatures are supported by independent
SignerInfo from each signer.
Each SignerInfo must include all the attributes required under this
document and must be processed independently by the verifier.
Multiple embedded signatures are supported using the counter-
signature unsigned attribute (see clause 3.10.1). Each counter
signature is carried in Countersignature held as an unsigned
attribute to the SignerInfo to which the counter-signature is
applied.
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This clause specifies the validation data structures which builds on
the electronic signature specified in clause 3. This includes:
* Time-Stamp applied to the electronic signature value.
* Complete validation data which comprises the time-stamp of the
signature value, plus references to all the certificates and
revocation information used for full validation of the
electronic signature.
The following optional eXtended forms of validation data are also
defined:
* X-timestamp: There are two types of time-stamp used in extended
validation data defined by this document.
- Type 1 -Time-Stamp which comprises a time-stamp over the ES
with Complete validation data (ES-C).
- Type 2 X-Time-Stamp which comprises of a time-stamp over the
certification path references and the revocation information
references used to support the ES-C.
* X-Long: This comprises a Complete validation data plus
the actual values of all the certificates and revocation
information used in the ES-C.
* X-Long-Time-Stamp: This comprises a Type 1 or Type 2 X-
Timestamp plus the actual values of all the certificates
and revocation information used in the ES-C.
This clause also specifies the data structures used in Archive
validation data:
* Archive validation data comprises a Complete validation data,
the certificate and revocation values (as in a X-Long
validation data), any other existing X-timestamps, plus the
Signed User data and an additional archive time-stamp over all
that data. An archive time-stamp may be repeatedly applied
after long periods to maintain validity when electronic
signature and timestamping algorithms weaken.
The additional data required to create the forms of electronic
signature identified above is carried as unsigned attributes
associated with an individual signature by being placed in the
Pinkas, et al. Informational [Page 35]
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unsignedAttrs field of SignerInfo. Thus all the attributes defined
in clause 4 are unsigned attributes.
NOTE: Where multiple signatures are to be supported, as described in
clause 3.13, each signature has a separate SignerInfo. Thus, each
signature requires its own unsigned attribute values to create ES-T,
ES-C etc.
An Electronic Signature with Timestamp is an Electronic Signature for
which part, but not all, of the additional data required for
validation is available (e.g., some certificates and revocation
information is available but not all).
The minimum structure Timestamp validation data is the Signature
Timestamp Attribute as defined in clause 4.1.1 over the ES signature
value.
The Signature Timestamp attribute is timestamp of the signature
value. It is an unsigned attribute. Several instances of this
attribute from different TSAs may occur with an electronic signature.
The Signature Validation Policy specifies, in the
signatureTimestampDelay field of TimestampTrustConditions, a maximum
acceptable time difference which is allowed between the time
indicated in the signing time attribute and the time indicated by the
Signature Timestamp attribute. If this delay is exceeded then the
electronic signature must be considered as invalid.
The following object identifier identifies the Signature Timestamp
attribute:
id-aa-signatureTimeStampToken OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
id-aa(2) 14}
The Signature timestamp attribute value has ASN.1 type
SignatureTimeStampToken.
SignatureTimeStampToken ::= TimeStampToken
The value of messageImprint field within TimeStampToken must be a
hash of the value of signature field within SignerInfo for the
signedData being timestamped.
Pinkas, et al. Informational [Page 36]
RFC 3126 Electronic Signature Formats September 2001
For further information and definition of TimeStampToken see [TSP].
An electronic signature with complete validation data is an
Electronic Signature for which all the additional data required for
validation (i.e., all certificates and revocation information) is
available. Complete validation data (ES-C) build on the electronic
signature Time-Stamp as defined above.
The minimum structure of a Complete validation data is:
* the Signature Time-Stamp Attribute, as defined in clause 4.1.1;
* Complete Certificate Refs, as defined in clause 4.2.1;
* Complete Revocation Refs, as defined in clause 4.2.2.
The Complete validation data MAY also include the following
additional information, forming a X-Long validation data, for use if
later validation processes may not have access to this information:
* Complete Certificate Values, as defined in clause 4.2.3;
* Complete Revocation Values, as defined in clause 4.2.4.
The Complete validation data MAY also include one of the following
additional attributes, forming a X-Time-Stamp validation data, to
provide additional protection against later CA compromise and provide
integrity of the validation data used:
* ES-C Time-Stamp, as defined in clause 4.2.5; or
* Time-Stamped Certificates and CRLs references, as defined in
clause 4.2.6.
NOTE 1: As long as the CA's are trusted such that these keys cannot
be compromised or the cryptography used broken, the ES-C provides
long term proof of a valid electronic signature.
A valid electronic signature is an electronic signature which passes
validation according to a signature validation policy.
NOTE 2: The ES-C provides the following important property for long
standing signatures; that is having been found once to be valid, must
continue to be so months or years later. Long after the validity
period of the certificates have expired, or after the user key has
been compromised.
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The Complete Certificate Refs attribute is an unsigned attribute. It
references the full set of CA certificates that have been used to
validate a ES with Complete validation data (ES-C) up to (but not
including) the signer's certificate. Only a single instance of this
attribute must occur with an electronic signature.
Note: The signer's certified is referenced in the signing certificate
attribute (see clause 3.1).
id-aa-ets-certificateRefs OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 21}
The complete certificate refs attribute value has the ASN.1 syntax
CompleteCertificateRefs.
CompleteCertificateRefs ::= SEQUENCE OF OTHERCertID
OTHERCertID is defined in clause 3.8.2.
The IssuerSerial that must be present in OTHERCertID. The certHash
must match the hash of the certificate referenced.
NOTE: Copies of the certificate values may be held using the
Certificate Values attribute defined in clause 4.3.1.
The Complete Revocation Refs attribute is an unsigned attribute.
Only a single instance of this attribute must occur with an
electronic signature. It references the full set of the CRL or OCSP
responses that have been used in the validation of the signer and CA
certificates used in ES with Complete validation data.
The following object identifier identifies the CompleteRevocationRefs
attribute:
id-aa-ets-revocationRefs OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 22}
The complete revocation refs attribute value has the ASN.1 syntax
CompleteRevocationRefs.
CompleteRevocationRefs ::= SEQUENCE OF CrlOcspRef
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CrlOcspRef ::= SEQUENCE {
crlids [0] CRLListID OPTIONAL,
ocspids [1] OcspListID OPTIONAL,
otherRev [2] OtherRevRefs OPTIONAL
}
CompleteRevocationRefs must contain one CrlOcspRef for the signing
certificate, followed by one for each OTHERCertID in the
CompleteCertificateRefs attribute. The second and subsequent
CrlOcspRef fields must be in the same order as the OTHERCertID to
which they relate. At least one of CRLListID or OcspListID or
OtherRevRefs should be present for all but the "trusted" CA of the
certificate path.
CRLListID ::= SEQUENCE {
crls SEQUENCE OF CrlValidatedID}
CrlValidatedID ::= SEQUENCE {
crlHash OtherHash,
crlIdentifier CrlIdentifier OPTIONAL}
CrlIdentifier ::= SEQUENCE {
crlissuer Name,
crlIssuedTime UTCTime,
crlNumber INTEGER OPTIONAL
}
OcspListID ::= SEQUENCE {
ocspResponses SEQUENCE OF OcspResponsesID}
OcspResponsesID ::= SEQUENCE {
ocspIdentifier OcspIdentifier,
ocspRepHash OtherHash OPTIONAL
}
OcspIdentifier ::= SEQUENCE {
ocspResponderID ResponderID,
-- As in OCSP response data
producedAt GeneralizedTime
-- As in OCSP response data
}
When creating an crlValidatedID, the crlHash is computed over the
entire DER encoded CRL including the signature. The crlIdentifier
would normally be present unless the CRL can be inferred from other
information.
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RFC 3126 Electronic Signature Formats September 2001
The crlIdentifier is to identify the CRL using the issuer name and
the CRL issued time which must correspond to the time "thisUpdate"
contained in the issued CRL. The crlListID attribute is an unsigned
attribute. In the case that the identified CRL is a Delta CRL then
references to the set of CRLs to provide a complete revocation list
must be included.
The OcspIdentifier is to identify the OSCP response using the issuer
name and the time of issue of the OCSP response which must correspond
to the time "producedAt" contained in the issued OCSP response.
Since it may be needed to make the difference between two OCSP
responses received within the same second, then the hash of the
response contained in the OcspResponsesID may be needed to solve the
ambiguity.
NOTE: Copies of the CRL and OCSP responses values may be held using
the Revocation Values attribute defined in clause 4.3.2.
OtherRevRefs ::= SEQUENCE {
otherRevRefType OtherRevRefType,
otherRevRefs ANY DEFINED BY otherRevRefType
}
OtherRevRefType ::= OBJECT IDENTIFIER
The syntax and semantics of other revocation references is outside
the scope of this document. The definition of the syntax of the
other form of revocation information is as identified by
OtherRevRefType.
The Certificate Values attribute is an unsigned attribute. Only a
single instance of this attribute must occur with an electronic
signature. It holds the values of certificates referenced in the
CompleteCertificateRefs attribute.
Note: If an Attribute Certificate is used, it is not provided in this
structure but must be provided by the signer as a signer-attributes
attribute (see clause 12.3).
The following object identifier identifies the CertificateValues
attribute:
id-aa-ets-certValues OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 23}
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The certificate values attribute value has the ASN.1 syntax
CertificateValues.
CertificateValues ::= SEQUENCE OF Certificate
Certificate is defined in RFC2459 and ITU-T Recommendation X.509 [1])
The Revocation Values attribute is an unsigned attribute. Only a
single instance of this attribute must occur with an electronic
signature. It holds the values of CRLs and OCSP referenced in the
CompleteRevocationRefs attribute.
The following object identifier identifies the Revocation Values
attribute:
id-aa-ets-revocationValues OBJECT IDENTIFIER ::= { iso(1) member-
body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
id-aa(2) 24}
The revocation values attribute value has the ASN.1 syntax
RevocationValues.
RevocationValues ::= SEQUENCE {
crlVals [0] SEQUENCE OF CertificateList OPTIONAL,
ocspVals [1] SEQUENCE OF BasicOCSPResponse OPTIONAL,
otherRevVals [2] OtherRevVals
}
OtherRevVals ::= SEQUENCE {
otherRevValType OtherRevValType,
otherRevVals ANY DEFINED BY otherRevValType
}
OtherRevValType ::= OBJECT IDENTIFIER
The syntax and semantics of the other revocation values is outside
the scope of this document. The definition of the syntax of the
other form of revocation information is as identified by
OtherRevRefType.
CertificateList is defined in RFC 2459 [RFC2459] and in ITU-T
Recommendation X.509 [X509]).
BasicOCSPResponse is defined in RFC 2560 [OCSP].
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This attribute is used for the Type 1 X-Time-Stamped validation data.
The ES-C Time-Stamp attribute is an unsigned attribute. It is time-
stamp of a hash of the electronic signature and the complete
validation data (ES-C). It is a special purpose TimeStampToken
Attribute which time-stamps the ES-C. Several instances instance of
this attribute may occur with an electronic signature from different
TSAs.
The following object identifier identifies the ES-C Time-Stamp
attribute:
id-aa-ets-escTimeStamp OBJECT IDENTIFIER ::= { iso(1) member-
body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
id-aa(2) 25}
The ES-C time-stamp attribute value has the ASN.1 syntax
ESCTimeStampToken.
ESCTimeStampToken ::= TimeStampToken
The value of messageImprint field within TimeStampToken must be a
hash of the concatenated values (without the type or length encoding
for that value) of the following data objects as present in the ES
with Complete validation data (ES-C):
* signature field within SignerInfo;
* SignatureTimeStampToken attribute;
* CompleteCertificateRefs attribute;
* CompleteRevocationRefs attribute.
For further information and definition of the Time Stamp Token see
[TSP].
This attribute is used for the Type 2 X-Time-Stamp validation data.
A TimestampedCertsCRLsRef attribute is an unsigned attribute. It is
a list of referenced certificates and OCSP responses/CRLs which are
been time-stamped to protect against certain CA compromises. Its
syntax is as follows:
The following object identifier identifies the
TimestampedCertsCRLsRef attribute:
Pinkas, et al. Informational [Page 42]
RFC 3126 Electronic Signature Formats September 2001
id-aa-ets-certCRLTimestamp OBJECT IDENTIFIER ::= { iso(1) member-
body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
id-aa(2) 26}
The attribute value has the ASN.1 syntax TimestampedCertsCRLs.
TimestampedCertsCRLs ::= TimeStampToken
The value of messageImprint field within TimeStampToken must be a
hash of the concatenated values (without the type or length encoding
for that value) of the following data objects as present in the ES
with Complete validation data (ES-C):
* CompleteCertificateRefs attribute;
* CompleteRevocationRefs attribute.
Where an electronic signature is required to last for a very long
time, and a the time-stamp on an electronic signature is in danger of
being invalidated due to algorithm weakness or limits in the validity
period of the TSA certificate, then it may be required to time-stamp
the electronic signature several times. When this is required an
archive time-stamp attribute may be required. This time-stamp may be
repeatedly applied over a period of time.
The Archive Time-Stamp attribute is time-stamp of the user data and
the entire electronic signature. If the Certificate values and
Revocation Values attributes are not present these attributes must be
added to the electronic signature prior to the time-stamp. The
Archive Time-Stamp attribute is an unsigned attribute. Several
instances of this attribute may occur with on electronic signature
both over time and from different TSAs.
The following object identifier identifies the Nested Archive Time-
Stamp attribute:
id-aa-ets-archiveTimestamp OBJECT IDENTIFIER ::= { iso(1) member-
body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
id-aa(2) 27}
Archive time-stamp attribute values have the ASN.1 syntax
ArchiveTimeStampToken
ArchiveTimeStampToken ::= TimeStampToken
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The value of messageImprint field within Time-StampToken must be a
hash of the concatenated values (without the type or length encoding
for that value) of the following data objects as present in the
electronic signature:
* encapContentInfo eContent OCTET STRING;
* signedAttributes;
* signature field within SignerInfo;
* SignatureTimeStampToken attribute;
* CompleteCertificateRefs attribute;
* CompleteRevocationData attribute;
* CertificateValues attribute
(If not already present this information must be included in
the ES-A);
* RevocationValues attribute
(If not already present this information must be included in
the ES-A);
* ESCTimeStampToken attribute if present;
* TimestampedCertsCRLs attribute if present;
* any previous ArchiveTimeStampToken attributes.
For further information and definition of TimeStampToken see [TSP]
The time-stamp should be created using stronger algorithms (or longer
key lengths) than in the original electronic signatures.
The security of the electronic signature mechanism defined in this
document depends on the privacy of the signer's private key.
Implementations must take steps to ensure that private keys cannot be
compromised.
Implementers should be aware that cryptographic algorithms become
weaker with time. As new cryptoanalysis techniques are developed and
computing performance improves, the work factor to break a particular
cryptographic algorithm will reduce. Therefore, cryptographic
algorithm implementations should be modular allowing new algorithms
to be readily inserted. That is, implementers should be prepared for
the set of mandatory to implement algorithms to change over time.
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RFC 3126 Electronic Signature Formats September 2001
This document only defines conformance requirements up to a ES with
Complete validation data (ES-C). This means that none of the
extended and archive forms of Electronic Signature (ES-X, ES-A) need
to be implemented to get conformance to this standard.
This document mandates support for elements of the signature policy.
A system supporting signers according to this document must, at a
minimum, support generation of an electronic signature consisting of
the following components:
* The general CMS syntax and content type as defined in RFC 2630
(see clauses 4.1 and 4.2).
* CMS SignedData as defined in RFC 2630 with version set to 3 and
at least one SignerInfo must be present (see clauses 4.3, 4.4,
4.5, 4.6).
* The following CMS Attributes as defined in RFC 2630:
- ContentType; This must always be present
(see clause 3.7.1);
- MessageDigest; This must always be present
(see clause 3.7.2);
- SigningTime; This must always be present
(see clause 3.7.3).
* The following ESS Attributes as defined in RFC 2634:
- SigningCertificate: This must be set as defined in clauses
3.8.1 and 3.8.2.
* The following Attributes as defined in clause 3.9:
- SignaturePolicyIdentifier; This must always be present.
* Public Key Certificates as defined in ITU-T Recommendation
X.509 [1] and profiled in RFC 2459 [7] (see clause 9.1).
Pinkas, et al. Informational [Page 45]
RFC 3126 Electronic Signature Formats September 2001
A system supporting verifiers according to this document with time-
stamping facilities must, at a minimum, support:
* Verification of the mandated components of an electronic
signature, as defined in clause 5.1.
* Signature Time-Stamp attribute, as defined in clause 4.1.1.
* Complete Certificate Refs attribute, as defined in clause
4.2.1.
* Complete Revocation Refs Attribute, as defined in clause
4.2.2.
* Public Key Certificates, as defined in ITU-T Recommendation
X.509 and profiled in RFC 2459.
* Either of:
- Certificate Revocation Lists, as defined in ITU-T
Recommendation X.509 [1] and profiled in RFC 2459 [7]; or
- On-line Certificate Status Protocol responses, as defined in
RFC 2560.
A system supporting verifiers according to the present document
shall, at a minimum, support:
* Verification of the mandated components of an electronic
signature, as defined in subclause 5.1.
* Complete Certificate Refs attribute, as defined in subclause
4.2.1.
* Complete Revocation Refs Attribute, as defined in subclause
9.2.2.
* A record shall be maintained, which cannot be undetectably
modified, of the electronic signature and the time when the
signature was first validated using the referenced certificates
and revocation information.
* Public Key Certificates, as defined in ITU-T Recommendation
X.509 [1] and profiled in RFC 2459 [7] (see subclause 10.1).
Pinkas, et al. Informational [Page 46]
RFC 3126 Electronic Signature Formats September 2001
* Either of:
- Certificate Revocation Lists, as defined in ITU-T
Recommendation X.509 [1] and profiled in RFC 2459 [7] Or
- On-line Certificate Status Protocol, as defined in RFC 2560
[8] (see subclause 10.3).
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[ESS] Hoffman, P., "Enhanced Security Services for S/MIME", RFC
2634, June 1999.
[CMS] Housley, R., "Cryptographic Message Syntax", RFC 2630,
June 1999.
[OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S. and C.
Adams, "On-line Status Certificate Protocol", RFC 2560,
June 1999.
[TSP] Adams, C., Cain, P., Pinkas, D. and R. Zuccherato,
"Internet X.509 Public Key Infrastructure Time-Stamp
Protocol (TSP)", RFC 3161, August 2001.
[PTS] Public Telegram Service. ITU-T Recommendation F1.
[RFC2459] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
X.509 Public Key Infrastructure, Certificate and CRL
Profile", RFC 2459, January 1999.
[PKCS9] RSA Laboratories, "The Public-Key Cryptography Standards
(PKCS)", RSA Data Security Inc., Redwood City, California,
November 1993 Release.
[ISONR] ISO/IEC 10181-5: Security Frameworks in Open Systems.
Non-Repudiation Framework. April 1997.
[TS101733] ETSI Standard TS 101 733 V.1.2.2 (2000-12) Electronic
Signature Formats. Note: copies of ETSI TS 101 733 can be
freely downloaded from the ETSI web site www.etsi.org.
Pinkas, et al. Informational [Page 47]
RFC 3126 Electronic Signature Formats September 2001
This Informational RFC has been produced in ETSI TC-SEC.
ETSI
F-06921 Sophia Antipolis, Cedex - FRANCE
650 Route des Lucioles - Sophia Antipolis
Valbonne - France
Tel: +33 4 92 94 42 00 Fax: +33 4 93 65 47 16
secretariat@etsi.fr
http://www.etsi.org
Contact Point
Harri Rasilainen
ETSI
650 Route des Lucioles
F-06921 Sophia Antipolis, Cedex
FRANCE
EMail: harri.rasilainen@etsi.fr
Denis Pinkas
Integris
68, Route de Versailles
78434 Louveciennes CEDEX
FRANCE
EMail: Denis.Pinkas@bull.net
John Ross
Security & Standards
192 Moulsham Street
Chelmsford, Essex
CM2 0LG
United Kingdom
EMail: ross@secstan.com
Nick Pope
Security & Standards
192 Moulsham Street
Chelmsford, Essex
CM2 0LG
United Kingdom
EMail: pope@secstan.com
Pinkas, et al. Informational [Page 48]
RFC 3126 Electronic Signature Formats September 2001
Annex A (normative): ASN.1 Definitions
This annex provides a summary of all the ASN.1 syntax definitions for
new syntax defined in this document.
The signature policy is a set of rules for the creation and
validation of an electronic signature, under which the signature can
be determined to be valid. A given legal/contractual context may
recognize a particular signature policy as meeting its requirements.
A signature policy may be issued, for example, by a party relying on
the electronic signatures and selected by the signer for use with
that relying party. Alternatively, a signature policy may be
established through an electronic trading association for use amongst
its members. Both the signer and verifier use the same signature
policy.
The signature policy may be explicitly identified or may be implied
by the semantics of the data being signed and other external data
like a contract being referenced which itself refers to a signature
policy.
An explicit signature policy has a globally unique reference, which
is bound to an electronic signature by the signer as part of the
signature calculation.
Pinkas, et al. Informational [Page 66]
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The signature policy needs to be available in human readable form so
that it can be assessed to meet the requirements of the legal and
contractual context in which it is being applied. To facilitate the
automatic processing of an electronic signature the parts of the
signature policy which specify the electronic rules for the creation
and validation of the electronic signature also needs to be in a
computer processable form.
The signature policy thus includes the following:
* Information about the signature policy that can be displayed to
the signer or the verifiers.
* Rules, which apply to functionality, covered by this document
(referred to as the Signature Validation Policy).
* Rules which may be implied through adoption of Certificate
Policies that apply to the electronic signature (e.g., rules
for ensuring the secrecy of the private signing key).
* Rules, which relate to the environment used by the signer,
e.g., the use of an agreed CAD (Card Accepting Device) used in
conjunction with a smart card.
An explicit Signature Validation Policy may be structured so that it
can be computer processable. Any format of the signature validation
policy is allowed by this document. However, for a given explicit
signature policy there must be one definitive form that has a unique
binary encoded value.
The Signature Validation Policy includes rules regarding use of TSPs
(CA, Attribute Authorities, Time Stamping Authorities) as well as
rules defining the components of the electronic signature that must
be provided by the signer with data required by the verifier to
provide long term proof.
The information being signed may be defined as a MIME-encapsulated
message which can be used to signal the format of the content in
order to select the right display or application. It can be composed
of formatted text (e.g., EDIFACT), free text or of fields from an
electronic form (e-form). For example, the Adobe(tm) format "pdf"
may be used or the eXtensible Mark up Language (XML).
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The definition of electronic signature includes: "a commitment has
been explicitly endorsed under a "Signature policy", at a given time,
by a signer under an identifier, e.g., a name or a pseudonym, and
optionally a role".
When two independent parties want to evaluate an electronic
signature, it is fundamental that they get the same result. To meet
this requirement same signature policy must be used by the signer and
verifier.
The signature policy may be explicitly identified or may be implied
by the semantics of the data being signed and other external data
which designate the signature policy to be used.
By signing over the signature policy identifier the signer explicitly
indicates that he or she has applied the signature policy in creating
the signature. Thus, undertakes any explicit or implied commitments.
In order to unambiguously identify an explicit signature policy that
is to be used to verify the signature an identifier and hash of the
"Signature policy" shall be part of the signed data. Additional
information about the explicit policy (e.g., web reference to the
document) may be carried as "qualifiers" to the signature policy
identifier.
When the signature policy not explicitly identified, but is implied
by the semantics of the data being signed, then the signature will
include a signature policy identifier that indicates that the
signature policy is implied. In this case the verification rules
must be determined by using other external data which will designate
the signature policy to be used. If it may be determined from the
context that all the documents to be verified refer to the same
signature policy, then that policy may be predetermined or fixed
within the application.
In order to identify unambiguously the "Signature Validation Policy"
to be used to verify the signature an identifier and hash of the
"Signature policy" must be part of the signed data. Additional
information about the policy (e.g., web reference to the document)
may be carried as "qualifiers" to the signature policy identifier.
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RFC 3126 Electronic Signature Formats September 2001
The definition of electronic signature includes: "a commitment has
been explicitly endorsed under a signature policy, at a given time,
by a signer under an identifier, e.g., a name or a pseudonym, and
optionally a role".
The commitment type can be indicated in the electronic signature
either:
* explicitly using a "commitment type indication" in the
electronic signature;
* implicitly or explicitly from the semantics of the signed data.
If the indicated commitment type is explicit using a "commitment type
indication" in the electronic signature, acceptance of a verified
signature implies acceptance of the semantics of that commitment
type. The semantics of explicit commitment types indications must be
specified either as part of the signature policy or may be registered
for generic use across multiple policies.
If a signature includes a commitment type indication other than one
of those recognized under the signature policy the signature must be
treated as invalid.
How commitment is indicated using the semantics of the data being
signed is outside the scope of this document.
NOTE: Examples of commitment indicated through the semantics of the
data being signed, are:
* An explicit commitment made by the signer indicated by the type
of data being signed over. Thus, the data structure being
signed can have an explicit commitment within the context of
the application (e.g., EDIFACT purchase order).
* An implicit commitment which is a commitment made by the signer
because the data being signed over has specific semantics
(meaning) which is only interpretable by humans, (i.e., free
text).
The definition of the ETSI electronic signature includes: "a
commitment has been explicitly endorsed under a signature policy, at
a given time, by a signer under an identifier, e.g., a name or a
pseudonym, and optionally a role."
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In many real life environments users will be able to get from
different CAs or even from the same CA, different certificates
containing the same public key for different names. The prime
advantage is that a user can use the same private key for different
purposes. Multiple use of the private key is an advantage when a
smart card is used to protect the private key, since the storage of a
smart card is always limited. When several CAs are involved, each
different certificate may contain a different identity, e.g., as a
national or as an employee from a company. Thus when a private key
is used for various purposes, the certificate is needed to clarify
the context in which the private key was used when generating the
signature. Where there is the possibility of multiple use of private
keys it is necessary for the signer to indicate to the verifier the
precise certificate to be used.
Many current schemes simply add the certificate after the signed data
and thus are subject to various substitution attacks. An example of
a substitution attack is a "bad" CA that would issue a certificate to
someone with the public key of someone else. If the certificate from
the signer was simply appended to the signature and thus not
protected by the signature, any one could substitute one certificate
by another and the message would appear to be signed by some one
else.
In order to counter this kind of attack, the identifier of the signer
has to be protected by the digital signature from the signer.
Although it does not provide the same advantages as the previous
technique, another technique to counter that threat has been
identified. It requires all CAs to perform a Proof Of Possession of
the private key at the time of registration. The problem with that
technique is that it does not provide any guarantee at the time of
verification and only some proof "after the event" may be obtained,
if and only if the CA keeps the Proof Of Possession in audit trail.
In order to identify unambiguously the certificate to be used for the
verification of the signature an identifier of the certificate from
the signer must be part of the signed data.
The definition of electronic signature includes: "a commitment has
been explicitly endorsed under a non repudiation security policy, at
a given time, by a signer under an identifier, e.g., a name or a
pseudonym, and optionally a role."
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While the name of the signer is important, the position of the signer
within a company or an organization can be even more important. Some
contracts may only be valid if signed by a user in a particular role,
e.g., a Sales Director. In many cases whom the sales Director really
is, is not that important but being sure that the signer is empowered
by his company to be the Sales Director is fundamental.
This document defines two different ways for providing this feature:
* by placing a claimed role name in the CMS signed attributes
field;
* by placing a attribute certificate containing a certified role
name in the CMS signed attributes field.
NOTE: Another possible approach would have been to use additional
attributes containing the roles name(s) in the signer's certificate.
However, it was decided not to follow this approach as it breaks the
basic philosophy of the certificate being issued for one primary
purpose. Also, by using separate certificates for management of the
signer's identity certificate and management of additional roles can
simplify the management, as new identity keys need not be issued if a
use of role is to be changed.
The signer may be trusted to state his own role without any
certificate to corroborate this claim. In which case the claimed
role can be added to the signature as a signed attribute.
Unlike public key certificates that bind an identifier to a public
key, Attribute Certificates bind the identifier of a certificate to
some attributes, like a role. An Attribute Certificate is NOT issued
by a CA but by an Attribute Authority (AA). The Attribute Authority
will be most of the time under the control of an organization or a
company that is best placed to know which attributes are relevant for
which individual.
The Attribute Authority may use or point to public key certificates
issued by any CA, provided that the appropriate trust may be placed
in that CA. Attribute Certificates may have various periods of
validity. That period may be quite short, e.g., one day. While this
requires that a new Attribute Certificate is obtained every day,
valid for that day, this can be advantageous since revocation of such
certificates may not be needed. When signing, the signer will have
to specify which Attribute Certificate it selects. In order to do
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so, a reference to the Attribute Certificate will have to be included
in the signed data in order to be protected by the digital signature
from the signer.
In order to identify unambiguously the attribute certificate(s) to be
used for the verification of the signature an identifier of the
attribute certificate(s) from the signer must be part of the signed
data.
In some transactions the purported location of the signer at the time
he or she applies his signature may need to be indicated. For this
reason an optional location indicator must be able to be included.
In order to provide indication of the location of the signer at the
time he or she applied his signature a location attribute may be
included in the signature.
The definition of electronic signature includes: "a commitment has
been explicitly endorsed under a signature policy, at a given time,
by a signer under an identifier, e.g., a name or a pseudonym, and
optionally a role."
There are several ways to address this problem. The solution adopted
in this document is to sign over a time which the signer claims is
the signing time (i.e., claimed signing time) and to require a
trusted time stamp to be obtained when building a ES with Time-Stamp.
When a verifier accepts a signature, the two times must be within
acceptable limits.
The solution that is adopted in this document offers the major
advantage that electronic signatures can be generated without any
on-line connection to a trusted time source (i.e., they may be
generated off-line).
Thus two dates and two signatures are required:
* a signing time indicated by the signer and which is part of the
data signed by the signer (i.e., part of the basic electronic
signature);
* a time indicated by a Time-Stamping Authority (TSA) which is
signed over the digital signature value of the basic electronic
signature. The signer, verifier or both may obtain the TSA
time-stamp.
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In order for an electronic signature to be valid under a signature
policy, it must be time-stamped by a TSA where the signing time as
indicated by the signer and the time of time stamping as indicated by
a TSA must be "close enough" to meet the requirements of the
signature validation policy.
"Close enough" means a few minutes, hours or even days according to
the "Signature Validation Policy".
NOTE: The need for Time-Stamping is further explained in clause
B.4.5. A further optional attribute is defined in this document to
time-stamp the content, to provide proof of the existence of the
content, at the time indicated by the time-stamp.
Using this optional attribute a trusted secure time may be obtained
before the document is signed and included under the digital
signature. This solution requires an on-line connection to a trusted
time-stamping service before generating the signature and may not
represent the precise signing time, since it can be obtained in
advance. However, this optional attribute may be used by the signer
to prove that the signed object existed before the date included in
the time-stamp (see 3.12.3, Content Time-Stamp).
Also, the signing time should be between the time indicated by this
time-stamp and time indicated by the ES-T time-stamp.
When presenting signed data to a human user it may be important that
there is no ambiguity as to the presentation of the signed
information to the relying party. In order for the appropriate
representation (text, sound or video) to be selected by the relying
party a content hint may be indicated by the signer. If a relying
party system does not use the format specified in the content hints
to present the data to the relying party, the electronic signature
may not be valid.
A verifier will have to prove that the certificate of the signer was
valid at the time of the signature. This can be done by either:
* using Certificate Revocation Lists (CRLs);
* using responses from an on-line certificate status server (for
example; obtained through the OCSP protocol).
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When using CRLs to get revocation information, a verifier will have
to make sure that he or she gets at the time of the first
verification the appropriate certificate revocation information from
the signer's CA. This should be done as soon as possible to minimize
the time delay between the generation and verification of the
signature. This involves checking that the signer certificate serial
number is not included in the CRL. The signer, the verifier or any
other third party may obtain either this CRL. If obtained by the
signer, then it must be conveyed to the verifier. It may be
convenient to archive the CRL for ease of subsequent verification or
arbitration.
Alternatively, provided the CRL is archived elsewhere which is
accessible for the purpose of arbitration, then the serial number of
the CRL used may be archived together with the verified electronic
signature.
It may happen that the certificate serial number appears in the CRL
but with the status "suspended" (i.e., on hold). In such a case, the
electronic signature is not yet valid, since it is not possible to
know whether the certificate will or will not be revoked at the end
of the suspension period. If a decision has to be taken immediately
then the signature has to be considered as invalid. If a decision
can wait until the end of the suspension period, then two cases are
possible:
* the certificate serial number has disappeared from the list and
thus the certificate can be considered as valid and that CRL
must be captured and archived either by the verifier or
elsewhere and be kept accessible for the purpose of
arbitration.
* the certificate serial number has been maintained on the list
with the status definitively revoked and thus the electronic
signature must be considered as invalid and discarded.
At this point the verifier may be convinced that he or she got a
valid signature, but is not yet in a position to prove at a later
time that the signature was verified as valid. Before addressing
this point, an alternative to CRL is to use OCSP responses.
When using OCSP to get revocation information , a verifier will have
to make sure that he or she gets at the time of the first
verification an OCSP response that contains the status "valid". This
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should be done as soon as possible after the generation of the
signature. The signer, the verifier or any other third party may
fetch this OCSP response. Since OSCP responses are transient and thus
are not archived by any TSP including CA, it is the responsibility of
every verifier to make sure that it is stored in a safe place. The
simplest way is to store them associated with the electronic
signature. An alternative would be to store them in some storage so
that they can then be easily retrieved.
In the same way as for the case of the CRL, it may happen that the
certificate is declared as invalid but with the secondary status
"suspended".
In such a case, the electronic signature is not yet valid, since it
is not possible to know whether the certificate will or will not be
revoked at the end of the suspension period. If a decision has to be
taken immediately then the electronic signature has to be considered
as invalid. If a decision can wait until the end of the suspension
period, then two cases are possible:
* An OCSP response with a valid status is obtained at a later
date and thus the certificate can be considered as valid and
that OCSP response must be captured.
* An OCSP response with an invalid status is obtained with a
secondary status indicating that the certificate is
definitively revoked and thus the electronic signature must be
considered as invalid and discarded.
As in the CRL case, at this point, the verifier may be convinced that
he or she got a valid signature, but is not yet in a position to
prove at a later time that the signature was verified as valid.
A verifier will have to prove that the certification path was valid,
at the time of the signature, up to a trust point according to the
naming constraints and the certificate policy constraints from the
"Signature Validation Policy". It will be necessary to capture all
the certificates from the certification path, starting with those
from the signer and ending up with those of the self-signed
certificate from one trusted root of the "Signature Validation
Policy". In addition, it will be necessary to capture the Authority
Revocation Lists (ARLs) to prove than none of the CAs from the chain
was revoked at the time of the signature.
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As in the OCSP case, at this point, the verifier may be convinced
that he or she got a valid signature, but is not yet in a position to
prove at a later time that the signature was verified as valid.
An important property for long standing signatures is that a
signature, having been found once to be valid, must continue to be so
months or years later.
A signer, verifier or both may be required to provide on request,
proof that a digital signature was created or verified during the
validity period of the all the certificates that make up the
certificate path. In this case, the signer, verifier or both will
also be required to provide proof that all the user and CA
certificates used were not revoked when the signature was created or
verified.
It would be quite unacceptable, to consider a signature as invalid
even if the keys or certificates were later compromised. Thus there
is a need to be able to demonstrate that the signature keys was valid
around the time that the signature was created to provide long term
evidence of the validity of a signature.
It could be the case that a certificate was valid at the time of the
signature but revoked some time later. In this event, evidence must
be provided that the document was signed before the signing key was
revoked.
Time-Stamping by a Time Stamping Authority (TSA) can provide such
evidence. A time stamp is obtained by sending the hash value of the
given data to the TSA. The returned "time-stamp" is a signed
document that contains the hash value, the identity of the TSA, and
the time of stamping. This proves that the given data existed before
the time of stamping. Time-Stamping a digital signature (by sending
a hash of the signature to the TSA) before the revocation of the
signer's private key, provides evidence that the signature has been
created before the key was revoked.
If a recipient wants to hold a valid electronic signature he will
have to ensure that he has obtained a valid time stamp for it, before
that key (and any key involved in the validation) is revoked. The
sooner the time-stamp is obtained after the signing time, the better.
It is important to note that signatures may be generated "off-line"
and time-stamped at a later time by anyone, for example by the signer
or any recipient interested in the value of the signature. The time
stamp can thus be provided by the signer together with the signed
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document, or obtained by the recipient following receipt of the
signed document.
The time stamp is NOT a component of the Electronic Signature, but
the essential component of the ES with Time-Stamp.
It is required in this document that signer's digital signature value
is time-stamped by a trusted source, known as a Time-Stamping
Authority.
This document requires that the signer's digital signature value is
time-stamped by a trusted source before the electronic signature can
become a ES with Complete validation data (ES-C). The acceptable
TSAs are specified in the Signature Validation Policy.
Should both the signer and verifier be required to time-stamp the
signature value to meet the requirements of the signature policy, the
signature policy MAY specify a permitted time delay between the two
time stamps.
Time-Stamped extended electronic signatures are needed when there is
a requirement to safeguard against the possibility of a CA key in the
certificate chain ever being compromised. A verifier may be required
to provide on request, proof that the certification path and the
revocation information used a the time of the signature were valid,
even in the case where one of the issuing keys or OCSP responder keys
is later compromised.
The current document defines two ways of using time-stamps to protect
against this compromise:
* Time-Stamp the ES with Complete validation data, when an OCSP
response is used to get the status of the certificate from the
signer.
* Time-Stamp only the certification path and revocation
information references when a CRL is used to get the status of
the certificate from the signer.
NOTE: the signer, verifier or both may obtain the time-stamp.
When an OCSP response is used, it is necessary to time stamp in
particular that response in the case the key from the responder would
be compromised. Since the information contained in the OCSP response
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is user specific and time specific, an individual time stamp is
needed for every signature received. Instead of placing the time
stamp only over the certification path references and the revocation
information references, which include the OCSP response, the time
stamp is placed on the ES-C. Since the certification path and
revocation information references are included in the ES with
Complete validation data they are also protected. For the same
cryptographic price, this provides an integrity mechanism over the ES
with Complete validation data. Any modification can be immediately
detected. It should be noticed that other means of
protecting/detecting the integrity of the ES with Complete Validation
Data exist and could be used.
Although the technique requires a time stamp for every signature, it
is well suited for individual users wishing to have an integrity
protected copy of all the validated signatures they have received.
By time-stamping the complete electronic signature, including the
digital signature as well as the references to the certificates and
revocation status information used to support validation of that
signature, the time-stamp ensures that there is no ambiguity in the
means of validating that signature.
This technique is referred to as ES with eXtended validation data
(ES-X), type 1 Time-Stamped in this document.
NOTE: Trust is achieved in the references by including a hash of the
data being referenced.
If it is desired for any reason to keep a copy of the additional data
being referenced, the additional data may be attached to the
electronic signature, in which case the electronic signature becomes
a ES-X Long as defined by this document.
A ES-X Long Time-Stamped is simply the concatenation of a ES-X Time-
Stamped with a copy of the additional data being referenced.
References Time-Stamping each ES with Complete validation data as
defined above may not be efficient, particularly when the same set of
CA certificates and CRL information is used to validate many
signatures.
Time-Stamping CA certificates will stop any attacker from issuing
bogus CA certificates that could be claimed to existing before the CA
key was compromised. Any bogus time-stamped CA certificates will
show that the certificate was created after the legitimate CA key was
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compromised. In the same way, time-stamping CA CRLs, will stop any
attacker from issuing bogus CA CRLs which could be claimed to
existing before the CA key was compromised.
Time-Stamping of commonly used certificates and CRLs can be done
centrally, e.g., inside a company or by a service provider. This
method reduces the amount of data the verifier has to time-stamp, for
example it could reduce to just one time stamp per day (i.e., in the
case were all the signers use the same CA and the CRL applies for the
whole day). The information that needs to be time stamped is not the
actual certificates and CRLs but the unambiguous references to those
certificates and CRLs.
To comply with extended validation data, type 2 Time-stamped, this
document requires the following:
* All the CA certificates references and revocation information
references (i.e., CRLs) used in validating the ES-C are covered
by one or more time-stamp.
Thus a ES-C with a time-stamp signature value at time T1, can be
proved valid if all the CA and CRL references are time-stamped at
time T1+.
Advances in computing increase the probability of being able to break
algorithms and compromise keys. There is therefore a requirement to
be able to protect electronic signatures against this probability.
Over a period of time weaknesses may occur in the cryptographic
algorithms used to create an electronic signature (e.g., due to the
time available for cryptoanalysis, or improvements in
cryptoanalytical techniques). Before this such weaknesses become
likely, a verifier should take extra measures to maintain the
validity of the electronic signature. Several techniques could be
used to achieve this goal depending on the nature of the weakened
cryptography. In order to simplify, a single technique, called
Archive validation data, covering all the cases is being used in this
document.
Archive validation data consists of the Complete validation data and
the complete certificate and revocation data, time stamped together
with the electronic signature. The Archive validation data is
necessary if the hash function and the crypto algorithms that were
used to create the signature are no longer secure. Also, if it
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cannot be assumed that the hash function used by the Time Stamping
Authority is secure, then nested time-stamps of Archived Electronic
Signature are required.
The potential for Trusted Service Provider (TSP) key compromise
should be significantly lower than user keys, because TSP(s) are
expected to use stronger cryptography and better key protection. It
can be expected that new algorithms (or old ones with greater key
lengths) will be used. In such a case, a sequence of time-stamps
will protect against forgery. Each time-stamp needs to be affixed
before either the compromise of the signing key or of the cracking of
the algorithms used by the TSA. TSAs (Time-Stamping Authorities)
should have long keys (e.g., which at the time of drafting this
document was 2048 bits for the signing RSA algorithm) and/or a "good"
or different algorithm.
Nested time-stamps will also protect the verifier against key
compromise or cracking the algorithm on the old electronic
signatures.
The process will need to be performed and iterated before the
cryptographic algorithms used for generating the previous time stamp
are no longer secure. Archive validation data may thus bear multiple
embedded time stamps.
Using type 1 or 2 of Time-Stamped extended validation data verifiers
still needs to keep track of all the components that were used to
validate the signature, in order to be able to retrieve them again
later on. These components may be archived by an external source
like a trusted service provider, in which case referenced information
that is provided as part of the ES with Complete validation data
(ES-C) is adequate. The actual certificates and CRL information
reference in the ES-C can be gathered when needed for arbitration.
In some business scenarios both the signer and the verifier need to
time-stamp their own copy of the signature value. Ideally the two
time-stamps should be as close as possible to each other.
Example: A contract is signed by two parties A and B representing
their respective organizations, to time-stamp the signer and verifier
data two approaches are possible:
* under the terms of the contract pre-defined common "trusted"
TSA may be used;
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* if both organizations run their own time-stamping services, A
and B can have the transaction time-stamped by these two time-
stamping services. In the latter case, the electronic
signature will only be considered as valid, if both time-stamps
were obtained in due time (i.e., there should not be a long
delay between obtaining the two time-stamps). Thus, neither A
nor B can repudiate the signing time indicated by their own
time-stamping service.
Therefore, A and B do not need to agree on a common "trusted" TSA to
get a valid transaction.
It is important to note that signatures may be generated "off-line"
and time-stamped at a later time by anyone, e.g., by the signer or
any recipient interested in validating the signature. The time-stamp
over the signature from the signer can thus be provided by the signer
together with the signed document, and /or obtained by the verifier
following receipt of the signed document.
The business scenarios may thus dictate that one or more of the
long-term signature time-stamping methods describe above be used.
This will need to be part of a mutually agreed the Signature
Validation Policy with is part of the overall signature policy under
which digital signature may be used to support the business
relationship between the two parties.
TSA servers should be built in such a way that once the private
signature key is installed, that there is minimal likelihood of
compromise over as long as possible period. Thus the validity period
for the TSA's keys should be as long as possible.
Both the ES-T and the ES-C contain at least one time stamp over the
signer's signature. In order to protect against the compromise of
the private signature key used to produce that time-stamp, the
Archive validation data can be used when a different Time-Stamping
Authority key is involved to produce the additional time-stamp. If
it is believed that the TSA key used in providing an earlier time-
stamp may ever be compromised (e.g., outside its validity period),
then the ES-A should be used. For extremely long periods this may be
applied repeatedly using new TSA keys.
Some electronic signatures may only be valid if they bear more than
one signature. This is the case generally when a contract is signed
between two parties. The ordering of the signatures may or may not
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be important, i.e., one may or may not need to be applied before the
other. Several forms of multiple and counter signatures may need to
be supported, which fall into two basic categories:
* independent signatures;
* embedded signatures.
Independent signatures are parallel signatures where the ordering of
the signatures is not important. The capability to have more than
one independent signature over the same data must be provided.
Embedded signatures are applied one after the other and are used
where the order the signatures are applied is important. The
capability to sign over signed data must be provided.
These forms are described in clause 3.13. All other multiple
signature schemes, e.g., a signed document with a countersignature,
double countersignatures or multiple signatures, can be reduced to
one or more occurrence of the above two cases.
Annex C (informative): Identifiers and roles
The name used by the signer, held as the subject in the signer's
certificate, must uniquely identify the entity. The name must be
allocated and verified on registration with the Certification
Authority, either directly or indirectly through a Registration
Authority, before being issued with a Certificate.
This document places no restrictions on the form of the name. The
subject's name may be a distinguished name, as defined in [RFC2459],
held in the subject field of the certificate, or any other name form
held in the X.509 subjectAltName certificate extension field. In the
case that the subject has no distinguished name, the subject name can
be an empty sequence and the subjectAltName extension must be
critical.
All TSP name forms (Certification Authorities, Attribute Authorities
and Time-Stamping Authorities) must be in the form of a distinguished
name held in the subject field of the certificate.
The TSP name form must include the legal jurisdiction (i.e., country)
under which it operates and an identification for the organization
providing the service.
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Where a signer signs as an individual but wishes to also identify
him/herself as acting on behalf of an organization, it may be
necessary to provide two independent forms of identification. The
first identity, with is directly associated with the signing key
identifies him/her as an individual. The second, which is managed
independently, identifies that person acting as part of the
organization, possibly with a given role.
In this case the first identity is carried in the
subject/subjectAltName field of the signer's certificate as described
above.
This document supports the following means of providing a second form
of identification:
* by placing a secondary name field containing a claimed role in
the CMS signed attributes field;
* by placing an attribute certificate containing a certified role
in the CMS signed attributes field.
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Full Copyright Statement
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