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Network Working Group                                         P. GutmannRequest for Comments: 3274                        University of AucklandCategory: Standards Track                                      June 2002Compressed Data Content Type forCryptographic 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],section10.2.5.      compressionAlgorithm is a compression algorithm identifier, as      defined insection 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 insection 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 insection 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 insection 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 examplesection 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 20023. 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",RFC2633, June 1999.Gutmann                     Standards Track                     [Page 4]

RFC 3274          Compressed Data Content Type for CMS         June 2002Appendix 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 }   ENDAuthor Address   Peter Gutmann   University of Auckland   Private Bag 92019   Auckland, New Zealand   EMail: pgut001@cs.auckland.ac.nzGutmann                     Standards Track                     [Page 5]

RFC 3274          Compressed Data Content Type for CMS         June 2002Full 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]