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. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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 Full Copyright Statement Copyright (C) The Internet Society 2001. 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