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].









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  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: [email protected]







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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
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  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
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Acknowledgement

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



















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