This document is obsolete. Please
refer to RFC 6931.
Network Working Group D. Eastlake 3rd Request for Comments: 4051 Motorola Laboratories Category: Standards Track April 2005
Additional XML Security Uniform Resource Identifiers (URIs)
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 (2005).
Abstract
A number of Uniform Resource Identifiers (URIs) intended for use with XML Digital Signatures, Encryption, and Canonicalization are defined. These URIs identify algorithms and types of keying information.
XML Digital Signatures, Canonicalization, and Encryption have been standardized by the W3C and the joint IETF/W3C XMLDSIG working group. All of these are now W3C Recommendations and IETF Informational or Standards Track documents. They are available as follows:
IETF level W3C REC Topic ----------- ------- ----- [RFC3275] Draft Std [XMLDSIG] XML Digital Signatures [RFC3076] Info [CANON] Canonical XML - - - - - - [XMLENC] XML Encryption [RFC3741] Info [EXCANON] Exclusive XML Canonicalization
All of these standards and recommendations use URIs [RFC2396] to identify algorithms and keying information types. This document provides a convenient reference list of URIs and descriptions for algorithms in which there is substantial interest, but which cannot or have not been included in the main documents. Note that raising XML digital signature to a Draft Standard in the IETF required removal of any algorithms for which interoperability from the main standards document has not been demonstrated. This required removal of the Minimal Canonicalization algorithm, in which there appears to be a continued interest, to be dropped from the standards track specification. It is included here.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
The URI [RFC2396] being dropped from the standard because of the transition from Proposed Standard to Draft Standard is included in Section 2.4 with its original prefix so as to avoid changing the XMLDSIG standard's namespace.
An "xmldsig-more" URI does not imply any official W3C status for these algorithms or identifiers or that they are only useful in digital signatures. Currently, dereferencing such URIs may or may not produce a temporary placeholder document. Permission to use this URI prefix has been given by the W3C.
An MD5 digest is a 128-bit string. The content of the DigestValue element shall be the base64 [RFC2405] encoding of this bit string viewed as a 16-octet octet stream.
A SHA-224 digest is a 224 bit string. The content of the DigestValue element shall be the base64 [RFC2405] encoding of this string viewed as a 28-octet stream. Because it takes roughly the same amount of effort to compute a SHA-224 message digest as a SHA-256 digest, and terseness is usually not a criteria in an XML application, consideration should be given to the use of SHA-256 as an alternative.
A SHA-384 digest is a 384 bit string. The content of the DigestValue element shall be the base64 [RFC2405] encoding of this string viewed as a 48-octet stream. Because it takes roughly the same amount of effort to compute a SHA-384 message digest as a SHA-512 digest and terseness is usually not a criteria in XML application, consideration should be given to the use of SHA-512 as an alternative.
The HMAC algorithm [RFC2104] takes the truncation length in bits as a parameter; if the parameter is not specified then all the bits of the hash are output. An example of an HMAC-MD5 SignatureMethod element is as follows:
The output of the HMAC algorithm is ultimately the output (possibly truncated) of the chosen digest algorithm. This value shall be base64 [RFC2405] encoded in the same straightforward fashion as the output of the digest algorithms. For example, the SignatureValue element for the HMAC-MD5 digest
The Schema Definition and DTD immediately shown above are taken from [RFC3275].
Although some cryptographic suspicions have recently been cast on MD5 for use in signatures such as RSA-MD5 below, this does not effect use of MD5 in HMAC.
RIPEMD-160 [RIPEMD-160] can also be used in HMAC as described in section 2.2.1 for HMAC-MD5.
2.3. SignatureMethod Public Key Signature Algorithms
These algorithms are distinguished from those in Section 2.2 in that they use public key methods. The verification key is different from and not feasibly derivable from the signing key.
The SignatureValue content for an RSA-MD5 signature is the base64 [RFC2405] encoding of the octet string computed as per [RFC3447], section 8.1.1, signature generation for the RSASSA-PKCS1-v1_5 signature scheme. As specified in the EMSA-PKCS1-V1_5-ENCODE function in [RFC3447, section 9.2.1], the value input to the signature function MUST contain a pre-pended algorithm object identifier for the hash function, but the availability of an ASN.1 parser and recognition of OIDs are not required of a signature verifier. The PKCS#1 v1.5 representation appears as:
CRYPT (PAD (ASN.1 (OID, DIGEST (data))))
Note that the padded ASN.1 will be of the following form:
Vertical bar ("|") represents concatenation. "01", "FF", and "00" are fixed octets of the corresponding hexadecimal value and the asterisk ("*") after "FF" indicates repetition. "hash" is the MD5 digest of the data. "prefix" is the ASN.1 BER MD5 algorithm designator prefix required in PKCS #1 [RFC3447], that is:
This prefix is included to facilitate the use of standard cryptographic libraries. The FF octet MUST be repeated enough times that the value of the quantity being CRYPTed is exactly one octet shorter than the RSA modulus.
Due to increases in computer processor power and advances in cryptography, use of RSA-MD5 is NOT RECOMMENDED.
This implies the PKCS#1 v1.5 padding algorithm [RFC3447] as described in section 2.3.1, but with the ASN.1 BER SHA-256 algorithm designator prefix. An example of use is:
This implies the PKCS#1 v1.5 padding algorithm [RFC3447] as described in section 2.3.1, but with the ASN.1 BER SHA-384 algorithm designator prefix. An example of use is:
Because it takes about the same effort to calculate a SHA-384 message digest as a SHA-512 message digest, it is suggested that RSA-SHA512 be used in preference to RSA-SHA384 where possible.
This implies the PKCS#1 v1.5 padding algorithm [RFC3447] as described in section 2.3.1, but with the ASN.1 BER SHA-512 algorithm designator prefix. An example of use is:
This implies the PKCS#1 v1.5 padding algorithm [RFC3447], as described in section 2.3.1, but with the ASN.1 BER RIPEMD160 algorithm designator prefix. An example of use is:
The Elliptic Curve Digital Signature Algorithm (ECDSA) [FIPS-186-2] is the elliptic curve analogue of the DSA (DSS) signature method. For detailed specifications on how to use it with SHA hash functions and XML Digital Signature, please see [X9.62] and [ECDSA].
The ESIGN algorithm specified in [IEEE-P1363a] is a signature scheme based on the integer factorization problem. It is much faster than previous digital signature schemes so ESIGN can be implemented on smart cards without special co-processors.
Thus far two independent interoperable implementations of Minimal Canonicalization have not been announced. Therefore, when XML Digital Signature was advanced from Proposed Standard [RFC3075] to Draft Standard [RFC3275], Minimal Canonicalization was dropped from the standards track documents. However, there is still interest in Minimal Canonicalization, indicating its possible future use. For its definition, see [RFC3075], Section 6.5.1.
Input to this transform is an octet stream (which is then parsed into XML).
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Output from this transform is a node set; the results of the XPointer are processed as defined in the XMLDSIG specification [RFC3275] for a same document XPointer.
ARCFOUR is a fast, simple stream encryption algorithm that is compatible with RSA Security's RC4 algorithm. An example of the EncryptionMethod element using ARCFOUR is
Camellia is an efficient and secure block cipher with the same interface as the AES [Camellia, RFC3713], that is 128-bit block size and 128, 192, and 256 bit key sizes. In XML Encryption, Camellia is used in the same way as the AES: It is used in the Cipher Block Chaining (CBC) mode with a 128-bit initialization vector (IV). The resulting cipher text is prefixed by the IV. If included in XML output, it is then base64 encoded. An example Camellia EncryptionMethod is as follows:
The Camellia [Camellia, RFC3713] key wrap is identical to the AES key wrap algorithm [RFC3394] specified in the XML Encryption standard with "AES" replaced by "Camellia". As with AES key wrap, the check value is 0xA6A6A6A6A6A6A6A6.
The algorithm is the same regardless of the size of the Camellia key used in wrapping (called the key encrypting key or KEK). The implementation of Camellia is OPTIONAL. However, if it is supported, the same implementation guidelines of which combinations of KEK size and wrapped key size should be required to be supported and which are optional to be supported should be followed as for AES. That is to say, if Camellia key wrap is supported, then wrapping 128-bit keys with a 128-bit KEK and wrapping 256-bit keys with a 256-bit KEK are REQUIRED and all other combinations are OPTIONAL.
A PKCS #7 [RFC2315] "signedData" can also be used as a bag of certificates and/or certificate revocation lists (CRLs). The PKCS7signedData element is defined to accommodate such structures within KeyInfo. The binary PKCS #7 structure is base64 [RFC2405] encoded. Any signer information present is ignored. The following is an example, eliding the base64 data [RFC3092]:
The Type attribute of RetrievalMethod is an optional identifier for the type of data to be retrieved. The result of dereferencing a RetrievalMethod reference for all KeyInfo types with an XML structure is an XML element or document with that element as the root. The various "raw" key information types return a binary value. Thus, they require a Type attribute because they are not unambiguously parseable.
As it is easy for people to construct their own unique URIs [RFC2396] and possibly obtain a URI from the W3C if appropriate, it is not intended that any additional "http://www.w3.org/2001/04/xmldsig- more#" URIs be created beyond those enumerated in this document. (W3C Namespace stability rules prohibit the creation of new URIs under "http://www.w3.org/2000/09/xmldsig#".)
Due to computer speed and cryptographic advances, the use of MD5 as a DigestMethod and the use of MD5 in the RSA-MD5 SignatureMethod is NOT RECOMMENDED. The concerned cryptographic advances do not effect the security of HMAC-MD5; however, there is little reason not to use one of the SHA series of algorithms.
Acknowledgements
Glenn Adams, Merlin Hughs, Gregor Karlinger, Brian LaMachia, Shiho Moriai, Joseph Reagle, Russ Housley, and Joel Halpern.
Normative References
[Camellia] "Camellia: A 128-bit Block Cipher Suitable for Multiple Platforms - Design and Analysis -", K. Aoki, T. Ichikawa, M. Matsui, S. Moriai, J. Nakajima, T. Tokita, In Selected Areas in Cryptography, 7th Annual International Workshop, SAC 2000, August 2000, Proceedings, Lecture Notes in Computer Science 2012, pp. 39-56, Springer- Verlag, 2001.
[ECDSA] Blake-Wilson, S., Karlinger, G., Kobayashi, T., and Y. Wang, "Using the Elliptic Curve Signature Algorithm (ECDSA) for XML Digital Signatures", RFC 4050, April 2005.
[FIPS-180-2] "Secure Hash Standard", (SHA-1/256/384/512) US Federal Information Processing Standard, 1 August 2002.
[FIPS-180-2change] "FIPS 180-2, Secure Hash Standard Change Notice 1", adds SHA-224 to [FIPS 180-2], 25 February 2004.
[FIPS-186-2] "Digital Signature Standard", National Institute of Standards and Technology, 2000.
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[IEEE-P1363a] "Standard Specifications for Public Key Cryptography: Additional Techniques", October 2002.
[ISO/IEC-18033-2] "Information technology -- Security techniques -- Encryption algorithms -- Part 3: Asymmetric ciphers", CD, October 2002.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm ", RFC 1321, April 1992.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2396] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
[RFC2405] Madson, C. and N. Doraswamy, "The ESP DES-CBC Cipher Algorithm With Explicit IV", RFC 2405, November 1998.
[RFC2315] Kaliski, B., "PKCS #7: Cryptographic Message Syntax Version 1.5", RFC 2315, March 1998.
[RFC3075] Eastlake 3rd, D., Reagle, J., and D. Solo, "XML- Signature Syntax and Processing", RFC 3075, March 2001. (RFC 3075 was obsoleted by RFC 3275 but is referenced in this document for its description of Minimal Canonicalization which was dropped in RFC 3275.)
[RFC3275] Eastlake 3rd, D., Reagle, J., and D. Solo, "(Extensible Markup Language) XML-Signature Syntax and Processing", RFC 3275, March 2002.
[RFC3394] Schaad, J. and R. Housley, "Advanced Encryption Standard (AES) Key Wrap Algorithm", RFC 3394, September 2002.
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[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003.
[RFC3713] Matsui, M., Nakajima, J., and S. Moriai, "A Description of the Camellia Encryption Algorithm", RFC 3713, April 2004.
[RFC3874] Housley, R., "A 224-bit One-way Hash Function: SHA-224", RFC 3874, September 2004.
[X9.62] X9.62-200X, "Public Key Cryptography for the Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)", Accredited Standards Committee X9, American National Standards Institute.
[XMLDSIG] "XML-Signature Syntax and Processing", D. Eastlake 3rd, J. Reagle, & D. Solo, 12 February 2002. <http://www.w3.org/TR/xmldsig-core/>
[XPointer] "XML Pointer Language (XPointer) Version 1.0", W3C working draft, Steve DeRose, Eve Maler, Ron Daniel Jr., January 2001. <http://www.w3.org/TR/2001/WD-xptr-20010108>
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