RFC4051
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.
2.2. SignatureMethod Message Authentication Code Algorithms.. 4
Contents
- 1 Introduction
- 2 Algorithms
- 3 KeyInfo
- 4 IANA Considerations
- 5 Security Considerations
Introduction
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.
Algorithms
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.
http://www.w3.org/2000/09/xmldsig#
Additional algorithms are given URIs that start with:
http://www.w3.org/2001/04/xmldsig-more#
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.
DigestMethod Algorithms
These algorithms are usable wherever a DigestMethod element occurs.
MD5
Identifier:
http://www.w3.org/2001/04/xmldsig-more#md5
The MD5 algorithm RFC1321 takes no explicit parameters. An example of an MD5 DigestAlgorithm element is:
<DigestAlgorithm
Algorithm="http://www.w3.org/2001/04/xmldsig-more#md5"/>
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.
SHA-224
Identifier:
http://www.w3.org/2001/04/xmldsig-more#sha224
The SHA-224 algorithm [FIPS-180-2change, RFC3874] takes no explicit parameters. An example of a SHA-224 DigestAlgorithm element is:
<DigestAlgorithm
Algorithm="http://www.w3.org/2001/04/xmldsig-more#sha224" />
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.
SHA-384
Identifier:
http://www.w3.org/2001/04/xmldsig-more#sha384
The SHA-384 algorithm [FIPS-180-2] takes no explicit parameters. An example of a SHA-384 DigestAlgorithm element is:
<DigestAlgorithm
Algorithm="http://www.w3.org/2001/04/xmldsig-more#sha384" />
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.
SignatureMethod Message Authentication Code Algorithms
Note: Some text in this section is duplicated from RFC3275 for the convenience of the reader. RFC 3275 is normative in case of conflict.
HMAC-MD5
Identifier:
http://www.w3.org/2001/04/xmldsig-more#hmac-md5
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:
<SignatureMethod
Algorithm="http://www.w3.org/2001/04/xmldsig-more#hmac-md5"> <HMACOutputLength>112</HMACOutputLength>
</SignatureMethod>
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
9294727A 3638BB1C 13F48EF8 158BFC9D
from the test vectors in RFC2104 would be
kpRyejY4uxwT9I74FYv8nQ==
Schema Definition:
<simpleType name="HMACOutputLength">
<restriction base="integer" />
</simpleType>
DTD:
<!ELEMENT HMACOutputLength (#PCDATA) >
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.
HMAC SHA Variations
Identifiers:
http://www.w3.org/2001/04/xmldsig-more#hmac-sha224 http://www.w3.org/2001/04/xmldsig-more#hmac-sha256 http://www.w3.org/2001/04/xmldsig-more#hmac-sha384 http://www.w3.org/2001/04/xmldsig-more#hmac-sha512
SHA-224, SHA-256, SHA-384, and SHA-512 [FIPS-180-2, FIPS-180-2change, RFC3874] can also be used in HMAC as described in section 2.2.1 for HMAC-MD5.
HMAC-RIPEMD160
Identifier:
http://www.w3.org/2001/04/xmldsig-more#hmac-ripemd160
RIPEMD-160 [RIPEMD-160] can also be used in HMAC as described in section 2.2.1 for HMAC-MD5.
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.
RSA-MD5
Identifier:
http://www.w3.org/2001/04/xmldsig-more#rsa-md5
RSA-MD5 implies the PKCS#1 v1.5 padding algorithm described in RFC3447. An example of use is
<SignatureMethod
Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-md5" />
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:
01 | FF* | 00 | prefix | hash
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:
hex 30 20 30 0c 06 08 2a 86 48 86 f7 0d 02 05 05 00 04 10
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.
RSA-SHA256
Identifier:
http://www.w3.org/2001/04/xmldsig-more#rsa-sha256
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:
<SignatureMethod
Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256" />
RSA-SHA384
Identifier:
http://www.w3.org/2001/04/xmldsig-more#rsa-sha384
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:
<SignatureMethod
Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha384" />
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.
RSA-SHA512
Identifier:
http://www.w3.org/2001/04/xmldsig-more#rsa-sha512
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:
<SignatureMethod
Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha512" />
RSA-RIPEMD160
Identifier:
http://www.w3.org/2001/04/xmldsig-more/rsa-ripemd160
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:
<SignatureMethod
Algorithm="http://www.w3.org/2001/04/xmldsig-more/rsa-ripemd160" />
ECDSA-SHA*
Identifiers
http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha1 http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha224 http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha256 http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha384 http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha512
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].
ESIGN-SHA1
Identifier
http://www.w3.org/2001/04/xmldsig-more#esign-sha1 http://www.w3.org/2001/04/xmldsig-more#esign-sha224 http://www.w3.org/2001/04/xmldsig-more#esign-sha256 http://www.w3.org/2001/04/xmldsig-more#esign-sha384 http://www.w3.org/2001/04/xmldsig-more#esign-sha512
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.
An example of use is:
<SignatureMethod
Algorithm="http://www.w3.org/2001/04/xmldsig-more#esign-sha1" />
Minimal Canonicalization
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.
For reference, its identifier remains:
http://www.w3.org/2000/09/xmldsig#minimal
Transform Algorithms
Note that all CanonicalizationMethod algorithms can also be used as transform algorithms.
XPointer
Identifier:
http://www.w3.org/2001/04/xmldsig-more/xptr
This transform algorithm takes an [XPointer] as an explicit parameter. An example of use is RFC3092:
<Transform
Algorithm="http://www.w3.org/2001/04/xmldsig-more/xptr"> <XPointer xmlns="http://www.w3.org/2001/04/xmldsig-more/xptr"> xpointer(id("foo")) xmlns(bar=http://foobar.example) xpointer(//bar:Zab[@Id="foo"]) </XPointer>
</Transform>
Schema Definition:
<element name="XPointer" type="string">
DTD:
<!ELEMENT XPointer (#PCDATA) >
Input to this transform is an octet stream (which is then parsed into XML).
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.
EncryptionMethod Algorithms
This subsection gives identifiers and information for several EncryptionMethod Algorithms.
ARCFOUR Encryption Algorithm
Identifier:
http://www.w3.org/2001/04/xmldsig-more#arcfour
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
<EncryptionMethod
Algorithm="http://www.w3.org/2001/04/xmldsig-more#arcfour"> <KeySize>40</KeySize>
</EncryptionMethod>
Note that Arcfour makes use of the generic KeySize parameter specified and defined in [XMLENC].
Camellia Block Encryption
Identifiers:
http://www.w3.org/2001/04/xmldsig-more#camellia128-cbc http://www.w3.org/2001/04/xmldsig-more#camellia192-cbc http://www.w3.org/2001/04/xmldsig-more#camellia256-cbc
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:
<EncryptionMethod
Algorithm= "http://www.w3.org/2001/04/xmldsig-more#camellia128-cbc" />
Camellia Key Wrap
Identifiers:
http://www.w3.org/2001/04/xmldsig-more#kw-camellia128 http://www.w3.org/2001/04/xmldsig-more#kw-camellia192 http://www.w3.org/2001/04/xmldsig-more#kw-camellia256
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.
An example of use is:
<EncryptionMethod
Algorithm= "http://www.w3.org/2001/04/xmldsig-more#kw-camellia128" />
PSEC-KEM
Identifier:
http://www.w3.org/2001/04/xmldsig-more#psec-kem
The PSEC-KEM algorithm, specified in [ISO/IEC-18033-2], is a key encapsulation mechanism using elliptic curve encryption.
An example of use is:
<EncryptionMethod
Algorithm="http://www.w3.org/2001/04/xmlenc#psec-kem"> <ECParameters> <Version>version</Version> <FieldID>id</FieldID> <Curve>curve</Curve> <Base>base</Base> <Order>order</Order> <Cofactor>cofactor</Cofactor> </ECParameters>
</EncryptionMethod>
See [ISO/IEC-18033-2] for information on the parameters above.
KeyInfo
In section 3.1 a new KeyInfo element child is specified, while in section 3.2 additional KeyInfo Type values for use in RetrievalMethod are specified.
PKCS #7 Bag of Certificates and CRLs
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:
<foo:PKCS7signedData
xmlns:foo="http://www.w3.org/2001/04/xmldsig-more"> ...
</foo:PKCS7signedData>
Additional RetrievalMethod Type Values
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.
Identifiers:
http://www.w3.org/2001/04/xmldsig-more#KeyValue http://www.w3.org/2001/04/xmldsig-more#RetrievalMethod http://www.w3.org/2001/04/xmldsig-more#KeyName http://www.w3.org/2001/04/xmldsig-more#rawX509CRL http://www.w3.org/2001/04/xmldsig-more#rawPGPKeyPacket http://www.w3.org/2001/04/xmldsig-more#rawSPKISexp http://www.w3.org/2001/04/xmldsig-more#PKCS7signedData http://www.w3.org/2001/04/xmldsig-more#rawPKCS7signedData
IANA Considerations
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#".)
Security Considerations
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.
[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.
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.
[RIPEMD-160] ISO/IEC 10118-3:1998, "Information Technology -
Security techniques - Hash-functions - Part3:
Dedicated hash- functions", ISO, 1998.
[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/>
[XMLENC] "XML Encryption Syntax and Processing", J. Reagle,
D. Eastlake, December 2002.
<http://www.w3.org/TR/2001/RED-xmlenc-core-
20021210/>
[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>
Informative References
[CANON] "Canonical XML Version 1.0", John Boyer.
<http://www.w3.org/TR/2001/REC-xml-c14n-20010315>.
[EXCANON] "Exclusive XML Canonicalization Version 1.0", D.
Eastlake, J. Reagle, 18 July 2002.
<http://www.w3.org/TR/REC-xml-enc-c14n-20020718/>.
RFC3076 Boyer, J., "Canonical XML Version 1.0", RFC 3076,
March 2001.
RFC3092 Eastlake 3rd, D., Manros, C., and E. Raymond,
"Etymology of "Foo"", RFC 3092, 2001.
RFC3741 Boyer, J., Eastlake 3rd, D., and J. Reagle,
"Exclusive XML Canonicalization, Version 1.0", RFC
3741, March 2004.
Author's Address
Donald E. Eastlake 3rd Motorola Laboratories 155 Beaver Street Milford, MA 01757 USA
Phone: +1-508-786-7554 (w)
+1-508-634-2066 (h)
EMail: [email protected]
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