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Network Working Group P. Gutmann Request for Comments: 3274 University of Auckland Category: Standards Track June 2002 Compressed Data Content Type for Cryptographic Message Syntax (CMS) 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 (2002). All Rights Reserved. Abstract This document defines a format for using compressed data as a Cryptographic Message Syntax (CMS) content type. Compressing data before transmission provides a number of advantages, including the elimination of data redundancy which could help an attacker, speeding up processing by reducing the amount of data to be processed by later steps (such as signing or encryption), and reducing overall message size. Although there have been proposals for adding compression at other levels (for example at the MIME or SSL level), these don't address the problem of compression of CMS content unless the compression is supplied by an external means (for example by intermixing MIME and CMS).

1. Introduction

This document describes a compressed data content type for CMS. This is implemented as a new ContentInfo type and is an extension to the types currently defined in CMS [RFC2630]. CMS implementations SHOULD include support for the CompressedData content type. The format of the messages are described in ASN.1 [ASN1]. Gutmann Standards Track [Page 1]
RFC 3274 Compressed Data Content Type for CMS June 2002 The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

1.1 Compressed Data Content Type

The compressed-data content type consists of content of any type, compressed using a specified algorithm. The following object identifier identifies the compressed-data content type: id-ct-compressedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) ct(1) 9 } The compressed-data content type shall have ASN.1 type CompressedData: CompressedData ::= SEQUENCE { version CMSVersion, compressionAlgorithm CompressionAlgorithmIdentifier, encapContentInfo EncapsulatedContentInfo } The fields of type CompressedData have the following meanings: version is the syntax version number. It MUST be 0. Details of the CMSVersion type are discussed in CMS [RFC2630], section 10.2.5. compressionAlgorithm is a compression algorithm identifier, as defined in section 2. encapContentInfo is the content which is compressed. Details of the EncapsulatedContentInfo type are discussed in CMS [RFC2630], section 5.2. Implementations SHOULD use the SMIMECapabilities attribute to indicate their ability to process compressed content types. Details of SMIMECapabilities are discussed in MSG [RFC2633], section 2.5.2. A compression SMIMECapability consists of the AlgorithmIdentifier for the supported compression algorithm. In the case of the algorithm specified in this document, this is id-alg-zlibCompression, as specified in section 2. Alternatively, the use of compression may be handled by prior arrangement (for example as part of an interoperability profile). Gutmann Standards Track [Page 2]
RFC 3274 Compressed Data Content Type for CMS June 2002 The SMIMECapability SEQUENCE representing the ability to process content compressed with the algorithm identified by id-alg- zlibCompression MUST be DER-encoded as the following hexadecimal string: 30 0D 06 0B 2A 86 48 86 F7 0D 01 09 10 03 08 (but see also the implementation note in section 2.1).

2. Compression Types

CMS implementations that support the CompressedData content type MUST include support for the ZLIB compression algorithm [RFC1950] [RFC1951], which has a freely-available, portable and efficient reference implementation. The following object identifier identifies ZLIB: id-alg-zlibCompress OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 8 } This algorithm has no parameters. The parameters field SHOULD be encoded as omitted, but MAY be encoded as NULL (see the implementation note in section 2.1).

2.1. Implementation notes

ZLIB allows for a number of compression levels ranging from good but slow compression, to less good but fast compression. The compression level is always compatible with the decompression algorithm, so there is no need to specify the compression level as an algorithm parameter. There are two possible encodings for the ZLIB null parameters field which arise from the fact that when the 1988 syntax for AlgorithmIdentifier was translated into the 1997 syntax, the OPTIONAL associated with the AlgorithmIdentifier parameters got lost. Later it was recovered via a defect report, but by then, everyone thought that algorithm parameters were mandatory. Because of this, some implementations will encode null parameters as an ASN.1 NULL element and some will omit them entirely (see for example section 12 of CMS [RFC2630]). Although the correct encoding is to omit the parameters field, implementations may encounter encodings which use an ASN.1 NULL element for the parameters. Gutmann Standards Track [Page 3]
RFC 3274 Compressed Data Content Type for CMS June 2002

3. Security Considerations

This RFC is not concerned with security, except for the fact that compressing data before encryption can enhance the security provided by other processing steps by reducing the quantity of known plaintext available to an attacker. However, implementations should be aware of possible security threats of combining security sensitive material with possibly untrusted data before the compression and encryption. This is because information about the sensitive data may be inferred from knowing the untrusted data and the compression ratio.

4. IANA Considerations

The CompressedData content type and compression algorithms are identified by object identifiers (OIDs). OIDs were assigned from an arc contributed to the S/MIME Working Group by RSA Security. Should additional compression algorithms be introduced, the advocates for such algorithms are expected to assign the necessary OIDs from their own arcs. No action by the IANA is necessary for this document or any anticipated updates. References [ASN1] CCITT Recommendation X.208: Specification of Abstract Syntax Notation One (ASN.1), 1988. [RFC2119] Bradner, S., "Key Words for Use in RFC's to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC1950] Deutsch, P. and J-L Gailly, "ZLIB Compressed Data Format Specification version 3.3", RFC 1950, May 1996. [RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification version 1.3", RFC 1951, May 1996. [RFC2630] Housley, R., "Cryptographic Message Syntax", RFC 2630, June 1999. [RFC2633] Rmasdell, B., "S/MIME Version 3 Message Specification", RFC 2633, June 1999. Gutmann Standards Track [Page 4]
RFC 3274 Compressed Data Content Type for CMS June 2002 Appendix A: ASN.1 Module CompressedDataContent { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) compress(11) } DEFINITIONS IMPLICIT TAGS ::= BEGIN IMPORTS CMSVersion, EncapsulatedContentInfo FROM CryptographicMessageSyntax { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1) } AlgorithmIdentifier FROM AuthenticationFramework { joint-iso-itu-t ds(5) module(1) authenticationFramework(7) 3 }; CompressedData ::= SEQUENCE { version CMSVersion, -- Always set to 0 compressionAlgorithm CompressionAlgorithmIdentifier, encapContentInfo EncapsulatedContentInfo } CompressionAlgorithmIdentifier ::= AlgorithmIdentifier -- Algorithm Identifiers id-alg-zlibCompress OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 8 } -- Content Type Object Identifiers id-ct-compressedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) ct(1) 9 } END Author Address Peter Gutmann University of Auckland Private Bag 92019 Auckland, New Zealand EMail: pgut001@cs.auckland.ac.nz Gutmann Standards Track [Page 5]
RFC 3274 Compressed Data Content Type for CMS June 2002 Full Copyright Statement Copyright (C) The Internet Society (2002). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Gutmann Standards Track [Page 6]