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Versions: 00 01 02 03 04 05 06 RFC 3274

Internet Draft                                      Editor: Peter Gutmann
draft-ietf-smime-compression-06.txt                 University of Auckland
August 30, 2001
Expires February 2002

                  Compressed Data Content Type for CMS

Status of this memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

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Abstract

The Cryptographic Message Syntax data format doesn't currently contain
any provisions for compressing data before processing it. 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).  This document defines a format for using compressed data as a
CMS content type.

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].  Future implementations of
CMS should include this extension.

The format of the messages are described in ASN.1 [ASN1].

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).

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 SHOULD include ZLIB [RFC1950] [RFC1951], which is
free of any intellectual property restrictions and 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 implemenation
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.

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.

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 the 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.

Author Address

Peter Gutmann
University of Auckland
Private Bag 92019
Auckland, New Zealand
pgut001@cs.auckland.ac.nz

References

  ASN1  CCITT Recommendation X.208: Specification of Abstract Syntax
        Notation One (ASN.1), 1988.

  RFC2119 Key Words for Use in RFC's to Indicate Requirement Levels,
        S.Bradner, March 1997.

  RFC1950 ZLIB Compressed Data Format Specification version 3.3,
        P.Deutsch and J-L Gailly, May 1996.

  RFC1951 DEFLATE Compressed Data Format Specification version 1.3,
        P.Deutsch, May 1996.

  RFC2630 Cryptographic Message Syntax, R.Housley, June 1999.

  RFC2633 S/MIME Version 3 Message Specification, B.Ramsdell, June
        1999.

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

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