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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).

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 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.

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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.

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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

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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

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