Network Working Group N. Haller
Request for Comments: 2289 Bellcore
Obsoletes: 1938 C. Metz
Category: Standards Track Kaman Sciences Corporation
P. Nesser
Nesser & Nesser Consulting
M. Straw
Bellcore
February 1998
A One-Time Password System
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document describes a one-time password authentication system
(OTP). The system provides authentication for system access (login)
and other applications requiring authentication that is secure
against passive attacks based on replaying captured reusable
passwords. OTP evolved from the S/KEY (S/KEY is a trademark of
Bellcore) One-Time Password System that was released by Bellcore and
is described in references [3] and [5].
One form of attack on networked computing systems is eavesdropping on
network connections to obtain authentication information such as the
login IDs and passwords of legitimate users. Once this information is
captured, it can be used at a later time to gain access to the
system. One-time password systems are designed to counter this type
of attack, called a "replay attack" [4].
The authentication system described in this document uses a secret
pass-phrase to generate a sequence of one-time (single use)
passwords. With this system, the user's secret pass-phrase never
needs to cross the network at any time such as during authentication
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RFC 2289 A One-Time Password System February 1998
or during pass-phrase changes. Thus, it is not vulnerable to replay
attacks. Added security is provided by the property that no secret
information need be stored on any system, including the server being
protected.
The OTP system protects against external passive attacks against the
authentication subsystem. It does not prevent a network eavesdropper
from gaining access to private information and does not provide
protection against either "social engineering" or active attacks [9].
There are two entities in the operation of the OTP one-time password
system. The generator must produce the appropriate one-time password
from the user's secret pass-phrase and from information provided in
the challenge from the server. The server must send a challenge that
includes the appropriate generation parameters to the generator, must
verify the one-time password received, must store the last valid
one-time password it received, and must store the corresponding one-
time password sequence number. The server must also facilitate the
changing of the user's secret pass-phrase in a secure manner.
The OTP system generator passes the user's secret pass-phrase, along
with a seed received from the server as part of the challenge,
through multiple iterations of a secure hash function to produce a
one-time password. After each successful authentication, the number
of secure hash function iterations is reduced by one. Thus, a unique
sequence of passwords is generated. The server verifies the one-time
password received from the generator by computing the secure hash
function once and comparing the result with the previously accepted
one-time password. This technique was first suggested by Leslie
Lamport [1].
In this document, the words that are used to define the significance
of each particular requirement are usually capitalized. These words
are:
- MUST
This word or the adjective "REQUIRED" means that the item is an
absolute requirement of the specification.
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RFC 2289 A One-Time Password System February 1998
- SHOULD
This word or the adjective "RECOMMENDED" means that there might
exist valid reasons in particular circumstances to ignore this
item, but the full implications should be understood and the case
carefully weighed before taking a different course.
- MAY
This word or the adjective "OPTIONAL" means that this item is
truly optional. One vendor might choose to include the item
because a particular marketplace requires it or because it
enhances the product, for example; another vendor may omit the
same item.
The security of the OTP system is based on the non-invertability of a
secure hash function. Such a function must be tractable to compute in
the forward direction, but computationally infeasible to invert.
The interfaces are currently defined for three such hash algorithms,
MD4 [2] and MD5 [6] by Ronald Rivest, and SHA [7] by NIST. All
conforming implementations of both server and generators MUST support
MD5. They SHOULD support SHA and MAY also support MD4. Clearly, the
generator and server must use the same algorithm in order to
interoperate. Other hash algorithms may be specified for use with
this system by publishing the appropriate interfaces.
The secure hash algorithms listed above have the property that they
accept an input that is arbitrarily long and produce a fixed size
output. The OTP system folds this output to 64 bits using the
algorithms in the Appendix A. 64 bits is also the length of the one-
time passwords. This is believed to be long enough to be secure and
short enough to be entered manually (see below, Form of Output) when
necessary.
This section describes the generation of the one-time passwords.
This process consists of an initial step in which all inputs are
combined, a computation step where the secure hash function is
applied a specified number of times, and an output function where the
64 bit one-time password is converted to a human readable form.
Appendix C contains examples of the outputs given a collection of
inputs. It provides implementors with a means of verification the
use of these algorithms.
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RFC 2289 A One-Time Password System February 1998
Initial Step
In principle, the user's secret pass-phrase may be of any length. To
reduce the risk from techniques such as exhaustive search or
dictionary attacks, character string pass-phrases MUST contain at
least 10 characters (see Form of Inputs below). All implementations
MUST support a pass-phrases of at least 63 characters. The secret
pass-phrase is frequently, but is not required to be, textual
information provided by a user.
In this step, the pass phrase is concatenated with a seed that is
transmitted from the server in clear text. This non-secret seed
allows clients to use the same secret pass-phrase on multiple
machines (using different seeds) and to safely recycle their secret
pass-phrases by changing the seed.
The result of the concatenation is passed through the secure hash
function and then is reduced to 64 bits using one of the function
dependent algorithms shown in Appendix A.
Computation Step
A sequence of one-time passwords is produced by applying the secure
hash function multiple times to the output of the initial step
(called S). That is, the first one-time password to be used is
produced by passing S through the secure hash function a number of
times (N) specified by the user. The next one-time password to be
used is generated by passing S though the secure hash function N-1
times. An eavesdropper who has monitored the transmission of a one-
time password would not be able to generate the next required
password because doing so would mean inverting the hash function.
Form of Inputs
The secret pass-phrase is seen only by the OTP generator. To allow
interchangeability of generators, all generators MUST support a
secret pass-phrase of 10 to 63 characters. Implementations MAY
support a longer pass-phrase, but such implementations risk the loss
of interchangeability with implementations supporting only the
minimum.
The seed MUST consist of purely alphanumeric characters and MUST be
of one to 16 characters in length. The seed is a string of characters
that MUST not contain any blanks and SHOULD consist of strictly
alphanumeric characters from the ISO-646 Invariant Code Set. The
seed MUST be case insensitive and MUST be internally converted to
lower case before it is processed.
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RFC 2289 A One-Time Password System February 1998
The sequence number and seed together constitute a larger unit of
data called the challenge. The challenge gives the generator the
parameters it needs to calculate the correct one-time password from
the secret pass-phrase. The challenge MUST be in a standard syntax so
that automated generators can recognize the challenge in context and
extract these parameters. The syntax of the challenge is:
otp-<algorithm identifier> <sequence integer> <seed>
The three tokens MUST be separated by a white space (defined as any
number of spaces and/or tabs) and the entire challenge string MUST be
terminated with either a space or a new line. The string "otp-" MUST
be in lower case. The algorithm identifier is case sensitive (the
existing identifiers are all lower case), and the seed is case
insensitive and converted before use to lower case. If additional
algorithms are defined, appropriate identifiers (short, but not
limited to three or four characters) must be defined. The currently
defined algorithm identifiers are:
md4 MD4 Message Digest
md5 MD5 Message Digest
sha1 NIST Secure Hash Algorithm Revision 1
An example of an OTP challenge is: otp-md5 487 dog2
Form of Output
The one-time password generated by the above procedure is 64 bits in
length. Entering a 64 bit number is a difficult and error prone
process. Some generators insert this password into the input stream
and some others make it available for system "cut and paste." Still
other arrangements require the one-time password to be entered
manually. The OTP system is designed to facilitate this manual entry
without impeding automatic methods. The one-time password therefore
MAY be converted to, and all servers MUST be capable of accepting it
as, a sequence of six short (1 to 4 letter) easily typed words that
only use characters from ISO-646 IVCS. Each word is chosen from a
dictionary of 2048 words; at 11 bits per word, all one-time passwords
may be encoded.
The two extra bits in this encoding are used to store a checksum.
The 64 bits of key are broken down into pairs of bits, then these
pairs are summed together. The two least significant bits of this sum
are encoded in the last two bits of the six word sequence with the
least significant bit of the sum as the last bit encoded. All OTP
generators MUST calculate this checksum and all OTP servers MUST
verify this checksum explicitly as part of the operation of decoding
this representation of the one-time password.
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RFC 2289 A One-Time Password System February 1998
Generators that produce the six-word format MUST present the words in
upper case with single spaces used as separators. All servers MUST
accept six-word format without regard to case and white space used as
a separator. The two lines below represent the same one-time
password. The first is valid as output from a generator and as input
a server, the second is valid only as human input to a server.
OUST COAT FOAL MUG BEAK TOTE
oust coat foal mug beak tote
Interoperability requires that all OTP servers and generators use
the same dictionary. The standard dictionary was originally
specified in the "S/KEY One Time Password System" that is described
in RFC 1760 [5]. This dictionary is included in this document as
Appendix D.
To facilitate the implementation of smaller generators, hexadecimal
output is an acceptable alternative for the presentation of the
one-time password. All implementations of the server software MUST
accept case-insensitive hexadecimal as well as six-word format. The
hexadecimal digits may be separated by white space so servers are
REQUIRED to ignore all white space. If the representation is
partitioned by white space, leading zeros must be retained.
Examples of hexadecimal format are:
Representation Value
3503785b369cda8b 0x3503785b369cda8b
e5cc a1b8 7c13 096b 0xe5cca1b87c13096b
C7 48 90 F4 27 7B A1 CF 0xc74890f4277ba1cf
47 9 A68 28 4C 9D 0 1BC 0x479a68284c9d01bc
In addition to accepting six-word and hexadecimal encodings of the
64 bit one-time password, servers SHOULD accept the alternate
dictionary encoding described in Appendix B. The six words in this
encoding MUST not overlap the set of words in the standard
dictionary. To avoid ambiguity with the hexadecimal representation,
words in the alternate dictionary MUST not be comprised solely of
the letters A-F. Decoding words thus encoded does not require any
knowledge of the alternative dictionary used so the acceptance of
any alternate dictionary implies the acceptance of all alternate
dictionaries. Words in the alternative dictionaries are case
sensitive. Generators and servers MUST preserve the case in the
processing of these words.
In summary, all conforming servers MUST accept six-word input that
uses the Standard Dictionary (RFC 1760 and Appendix D), MUST accept
hexadecimal encoding, and SHOULD accept six-word input that uses the
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RFC 2289 A One-Time Password System February 1998
Alternative Dictionary technique (Appendix B). As there is a remote
possibility that a hexadecimal encoding of a one-time password will
look like a valid six-word standard dictionary encoding, all
implementations MUST use the following scheme. If a six-word
encoded one-time password is valid, it is accepted. Otherwise, if
the one-time password can be interpreted as hexadecimal, and with
that decoding it is valid, then it is accepted.
An application on the server system that requires OTP authentication
is expected to issue an OTP challenge as described above. Given the
parameters from this challenge and the secret pass-phrase, the
generator can compute (or lookup) the one-time password that is
passed to the server to be verified.
The server system has a database containing, for each user, the
one-time password from the last successful authentication or the
first OTP of a newly initialized sequence. To authenticate the user,
the server decodes the one-time password received from the generator
into a 64-bit key and then runs this key through the secure hash
function once. If the result of this operation matches the stored
previous OTP, the authentication is successful and the accepted
one-time password is stored for future use.
Because the number of hash function applications executed by the
generator decreases by one each time, at some point the user must
reinitialize the system or be unable to authenticate.
Although some installations may not permit users to initialize
remotely, implementations MUST provide a means to do so that does
not reveal the user's secret pass-phrase. One way is to provide a
means to reinitialize the sequence through explicit specification
of the first one-time password.
When the sequence of one-time passwords is reinitialized,
implementations MUST verify that the seed or the pass-phrase is
changed. Installations SHOULD discourage any operation that sends
the secret pass-phrase over a network in clear-text as such practice
defeats the concept of a one-time password.
Implementations MAY use the following technique for
[re]initialization:
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RFC 2289 A One-Time Password System February 1998
o The user picks a new seed and hash count (default values may
be offered). The user provides these, along with the
corresponding generated one-time password, to the host system.
o The user MAY also provide the corresponding generated one
time password for count-1 as an error check.
o The user SHOULD provide the generated one-time password for
the old seed and old hash count to protect an idle terminal
or workstation (this implies that when the count is 1, the
user can login but cannot then change the seed or count).
In the future a specific protocol may be defined for
reinitialization that will permit smooth and possibly automated
interoperation of all hosts and generators.
All conforming server implementations MUST protect against the race
condition described in this section. A defense against this attack
is outlined; implementations MAY use this approach or MAY select an
alternative defense.
It is possible for an attacker to listen to most of a one-time
password, guess the remainder, and then race the legitimate user to
complete the authentication. Multiple guesses against the last word
of the six-word format are likely to succeed.
One possible defense is to prevent a user from starting multiple
simultaneous authentication sessions. This means that once the
legitimate user has initiated authentication, an attacker would be
blocked until the first authentication process has completed. In
this approach, a timeout is necessary to thwart a denial of service
attack.
This entire document discusses an authentication system that
improves security by limiting the danger of eavesdropping/replay
attacks that have been used against simple password systems [4].
The use of the OTP system only provides protections against passive
eavesdropping/replay attacks. It does not provide for the privacy
of transmitted data, and it does not provide protection against
active attacks such as session hijacking that are known to be
present in the current Internet [9]. The use of IP Security
(IPsec), see [10], [11], and [12] is recommended to protect against
TCP session hijacking.
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RFC 2289 A One-Time Password System February 1998
The success of the OTP system to protect host systems is dependent
on the non-invertability of the secure hash functions used. To our
knowledge, none of the hash algorithms have been broken, but it is
generally believed [6] that MD4 is not as strong as MD5. If a
server supports multiple hash algorithms, it is only as secure as
the weakest algorithm.
The idea behind OTP authentication was first proposed by Leslie
Lamport [1]. Bellcore's S/KEY system, from which OTP is derived, was
proposed by Phil Karn, who also wrote most of the Bellcore reference
implementation.
[1] Leslie Lamport, "Password Authentication with Insecure
Communication", Communications of the ACM 24.11 (November
1981), 770-772
[2] Rivest, R., "The MD4 Message-Digest Algorithm", RFC 1320,
April 1992.
[3] Neil Haller, "The S/KEY One-Time Password System", Proceedings
of the ISOC Symposium on Network and Distributed System
Security, February 1994, San Diego, CA
[4] Haller, N., and R. Atkinson, "On Internet Authentication",
RFC 1704, October 1994.
[5] Haller, N., "The S/KEY One-Time Password System",
RFC 1760, February 1995.
[6] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[7] National Institute of Standards and Technology (NIST),
"Announcing the Secure Hash Standard", FIPS 180-1, U.S.
Department of Commerce, April 1995.
[8] International Standard - Information Processing -- ISO 7-bit
coded character set for information interchange (Invariant Code
Set), ISO-646, International Standards Organization, Geneva,
Switzerland, 1983
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RFC 2289 A One-Time Password System February 1998
[9] Computer Emergency Response Team (CERT), "IP Spoofing and
Hijacked Terminal Connections", CA-95:01, January 1995.
Available via anonymous ftp from info.cert.org in
/pub/cert_advisories.
[10] Atkinson, R., "Security Architecture for the Internet Protocol",
RFC 1825, August 1995.
[11] Atkinson, R., "IP Authentication Header", RFC 1826, August
1995.
[12] Atkinson, R., "IP Encapsulating Security Payload (ESP)", RFC
1827, August 1995.
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RFC 2289 A One-Time Password System February 1998