RFC3274 - Compressed Data Content Type for Cryptographic Message Syntax (CMS)

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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].

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 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.

3. Security Considerations

This RFCis 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, RFC2119, March 1997.

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

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

[RFC2630] Housley, R., "Cryptographic Message Syntax", RFC2630, June
1999.

[RFC2633] Rmasdell, B., "S/MIME Version 3 Message Specification", RFC
2633, 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

Author Address

Peter Gutmann
University of Auckland
Private Bag 92019
Auckland, New Zealand

EMail: pgut001@cs.auckland.ac.nz

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