Internet Engineering Task Force (IETF) D. Eastlake 3rd Request for Comments: 6931 Huawei Obsoletes: 4051 April 2013 Category: Standards Track ISSN: 2070-1721
Additional XML Security Uniform Resource Identifiers (URIs)
This document expands, updates, and establishes an IANA registry for the list of URIs intended for use with XML digital signatures, encryption, canonicalization, and key management. These URIs identify algorithms and types of information. This document obsoletes RFC 4051.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
XML digital signatures, canonicalization, and encryption have been standardized by the W3C and by the joint IETF/W3C XMLDSIG working group [W3C]. All of these are now W3C Recommendations and some are also RFCs. They are available as follows:
RFC Status W3C REC Topic ----------- ------- -----
[RFC3275] [XMLDSIG10] XML Digital Signatures Draft Standard
[RFC3741] [XCANON] Exclusive XML Canonicalization 1.0 Informational
All of these documents and recommendations use URIs [RFC3986] to identify algorithms and keying information types. The W3C has subsequently produced updated XML Signature 1.1 [XMLDSIG11], Canonical XML 1.1 [CANON11], and XML Encryption 1.1 [XMLENC11] versions, as well as a new XML Signature Properties specification [XMLDSIG-PROP].
All camel-case element names herein, such as DigestValue, are from these documents.
This document is an updated convenient reference list of URIs and corresponding algorithms in which there is expressed interest. Since the previous list [RFC4051] was issued in 2005, significant new cryptographic algorithms of interest to XML security, for some of which the URI is only specified in this document, have been added. This document obsoletes [RFC4051]. All of the URIs appear in the indexes in Section 4. Only the URIs that were added by [RFC4051] or this document have a subsection in Section 2 or 3, with the exception of Minimal Canonicalization (Section 2.4), for example, use of
Eastlake Standards Track [Page 3]
RFC 6931 Additional XML Security URIs April 2013
SHA-256 is defined in [XMLENC11] and hence there is no subsection on that algorithm here, but its URI is included in the indexes in Section 4.
Specification in this document of the URI representing an algorithm does not imply endorsement of the algorithm for any particular purpose. A protocol specification, which this is not, generally gives algorithm and implementation requirements for the protocol. Security considerations for algorithms are constantly evolving, as documented elsewhere. This specification simply provides some URIs and relevant formatting for when those URIs are used.
Note that progressing XML Digital Signature [RFC3275] along the Standards Track required removal of any algorithms from the original version [RFC3075] for which there was not demonstrated interoperability. This required removal of the Minimal Canonicalization algorithm, in which there appears to be continued interest. The URI for Minimal Canonicalization was included in [RFC4051] and is included here.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
This document is not intended to change the algorithm implementation requirements of any IETF or W3C document. Use of [RFC2119] terminology is intended to be only such as is already stated or implied by other authoritative documents.
In addition, for ease of reference, this document includes in the indexes in Section 4 many cryptographic algorithm URIs from several XML security documents using the namespaces with which they are defined in those documents. For example, 2000/09/xmldsig# for some URIs specified in [RFC3275] and 2001/04/xmlenc# for some URIs specified in [XMLENC10].
An MD5 digest is a 128-bit string. The content of the DigestValue element SHALL be the base64 [RFC2045] encoding of this bit string viewed as a 16-octet stream. See [RFC6151] for MD5 security considerations.
The Whirlpool algorithm [10118-3] takes no explicit parameters. A Whirlpool digest is a 512-bit string. The content of the DigestValue element SHALL be the base64 [RFC2045] encoding of this string viewed as a 64-octet stream.
NIST has recently completed a hash function competition for an alternative to the SHA family. The Keccak-f algorithm was selected [Keccak] [SHA-3]. This hash function is commonly referred to as "SHA-3", and this section is a space holder and reservation of URIs for future information on Keccak use in XML security.
A SHA-3 224, 256, 384, and 512 digest is a 224-, 256-, 384-, and 512-bit string, respectively. The content of the DigestValue element SHALL be the base64 [RFC2045] encoding of this string viewed as a 28-, 32-, 48-, and 64-octet stream, respectively.
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 [RFC2045] encoded in the same straightforward fashion as the output of the digest algorithms. Example: the SignatureValue element for the HMAC-MD5 digest
These algorithms are distinguished from those in Section 2.2 above in that they use public-key methods. That is to say, 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 [RFC2045] encoding of the octet string computed as per [RFC3447], Section 8.2.1, 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, 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 is 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 make it easier to use 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.
The Elliptic Curve Digital Signature Algorithm (ECDSA) [FIPS180-4] is the elliptic curve analogue of the Digital Signature Algorithm (DSA) signature method, i.e., the Digital Signature Standard (DSS). It takes no explicit parameters. For detailed specifications of how to use it with SHA hash functions and XML Digital Signature, please see [X9.62] and [RFC4050]. The #ecdsa-ripemd160 and #ecdsa-whirlpool fragments in the new namespace identifies a signature method processed in the same way as specified by the #ecdsa-sha1 fragment of this namespace, with the exception that RIPEMD160 or Whirlpool is used instead of SHA-1.
The output of the ECDSA algorithm consists of a pair of integers usually referred by the pair (r, s). The signature value consists of the base64 encoding of the concatenation of two octet streams that respectively result from the octet-encoding of the values r and s in that order. Conversion from integer to octet stream must be done according to the I2OSP operation defined in the [RFC3447] specification with the l parameter equal to the size of the base point order of the curve in bytes (e.g., 32 for the P-256 curve and 66 for the P-521 curve [FIPS186-3]).
For an introduction to elliptic curve cryptographic algorithms, see [RFC6090] and note the errata (Errata ID 2773-2777).
The ESIGN algorithm specified in [IEEEP1363a] 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.
As in the definition of the RSA-SHA1 algorithm in [XMLDSIG11], the designator "RSA" means the RSASSA-PKCS1-v1_5 algorithm as defined in [RFC3447]. When identified through the #rsa-whirlpool fragment identifier, Whirlpool is used as the hash algorithm instead. Use of the ASN.1 BER Whirlpool algorithm designator is implied. That designator is hex 30 4e 30 0a 06 06 28 cf 06 03 00 37 05 00 04 40 as an explicit octet sequence. This corresponds to OID 1.0.10184.108.40.206 defined in [10118-3].
These identifiers imply the PKCS#1 EMSA-PSS encoding algorithm [RFC3447]. The RSASSA-PSS algorithm takes the digest method (hash function), a mask generation function, the salt length in bytes (SaltLength), and the trailer field as explicit parameters.
Algorithm identifiers for hash functions specified in XML encryption [XMLENC11] [XMLDSIG11] and in Section 2.1 are considered to be valid algorithm identifiers for hash functions. According to [RFC3447], the default value for the digest function is SHA-1, but due to the discovered weakness of SHA-1 [RFC6194], it is recommended that SHA-256 or a stronger hash function be used. Notwithstanding [RFC3447], SHA-256 is the default to be used with these SignatureMethod identifiers if no hash function has been specified.
The default salt length for these SignatureMethod identifiers if the SaltLength is not specified SHALL be the number of octets in the hash value of the digest method, as recommended in [RFC4055]. In a parameterized RSASSA-PSS signature the ds:DigestMethod and the SaltLength parameters usually appear. If they do not, the defaults make this equivalent to http://www.w3.org/2007/05/xmldsig-more#sha256-rsa-MGF1 (see Section 2.3.10). The TrailerField defaults to 1 (0xBC) when omitted.
[RFC3447] currently specifies only one mask generation function MGF1 based on a hash function. Although [RFC3447] allows for parameterization, the default is to use the same hash function as the digest method function. Only this default approach is supported by this section; therefore, the definition of a mask generation function type is not needed yet. The same applies to the trailer field. There is only one value (0xBC) specified in [RFC3447]. Hence, this default parameter must be used for signature generation. The default salt length is the length of the hash function.
Thus far, two independent interoperable implementations of Minimal Canonicalization have not been announced. Therefore, when XML Digital Signature was advanced along the Standards Track from [RFC3075] to [RFC3275], Minimal Canonicalization was dropped. However, there is still interest. For its definition, see Section 6.5.1 of [RFC3075].
Camellia is a block cipher with the same interface as the AES [Camellia] [RFC3713]; it has a 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:
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 whatever the size of the Camellia key used in wrapping, called the "key encrypting key" or "KEK". If Camellia is supported, it is particularly suggested that wrapping 128-bit keys with a 128-bit KEK and wrapping 256-bit keys with a 256-bit KEK be supported.
SEED [RFC4269] is a 128-bit block size with 128-bit key sizes. In XML Encryption, SEED can be 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.
Key wrapping with SEED is identical to Section 2.2.1 of [RFC3394] with "AES" replaced by "SEED". The algorithm is specified in [RFC4010]. The implementation of SEED is optional. The default initial value is 0xA6A6A6A6A6A6A6A6.
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 [RFC2045] encoded. Any signer information present is ignored. The following is an example [RFC3092], eliding the base64 data:
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 parsable.
The following subsections provide an index by URI and by fragment identifier (the portion of the URI after "#") of the algorithm and KeyInfo URIs defined in this document and in the standards (plus the one KeyInfo child element name defined in this document). The "Sec/Doc" column has the section of this document or, if not specified in this document, the document where the item is specified. See also [XMLSECXREF].
An "xmldsig-more" URI does not imply any official W3C or IETF status for these algorithms or identifiers nor does it imply that they are only useful in digital signatures. Currently, dereferencing such URIs may or may not produce a temporary placeholder document. Permission to use these URI prefixes has been given by the W3C.
IANA has established a registry entitled "XML Security URIs". The initial contents correspond to Section 4.2 of this document with each section number in the "Sec/Doc" column augmented with a reference to this RFC (for example, "2.6.4" means "[RFC6931], Section 2.6.4").
New entries, including new Types, will be added based on Expert Review [RFC5226]. Criterion for inclusion are (1) documentation sufficient for interoperability of the algorithm or data type and the XML syntax for its representation and use and (2) sufficient importance as normally indicated by inclusion in (2a) an approved W3C Note, Proposed Recommendation, or Recommendation or (2b) an approved IETF Standards Track document. Typically, the registry will reference a W3C or IETF document specifying such XML syntax; that document will either contain a more abstract description of the algorithm or data type or reference another document with a more abstract description.
This RFC is concerned with documenting the URIs that designate algorithms and some data types used in connection with XML security. The security considerations vary widely with the particular algorithms, and the general security considerations for XML security are outside of the scope of this document but appear in [XMLDSIG11], [XMLENC11], [CANON10], [CANON11], and [GENERIC].
[RFC6151] should be consulted before considering the use of MD5 as a DigestMethod or RSA-MD5 as a SignatureMethod.
See [RFC6194] for SHA-1 security considerations and [RFC6151] for MD5 security considerations.
Additional security considerations are given in connection with the description of some algorithms in the body of this document.
Implementers should be aware that cryptographic algorithms become weaker with time. As new cryptoanalysis techniques are developed and computing performance improves, the work factor to break a particular cryptographic algorithm will reduce. Therefore, cryptographic implementations should be modular, allowing new algorithms to be readily inserted. That is, implementers should be prepared for the set of mandatory-to-implement algorithms to change over time.
The contributions to this document by the following people, listed in alphabetic order, are gratefully acknowledged: Benoit Claise, Adrian Farrel, Stephen Farrell, Ernst Giessmann, Frederick Hirsch, Bjoern Hoehrmann, Russ Housley, Satoru Kanno, Charlie Kaufman, Konrad Lanz, HwanJin Lee, Barry Leiba, Peter Lipp, Subramanian Moonesamy, Thomas Roessler, Hanseong Ryu, Peter Saint-Andre, and Sean Turner.
The following contributors to [RFC4051], on which this document is based, are gratefully acknowledged: Glenn Adams, Merlin Hughs, Gregor Karlinger, Brian LaMachia, Shiho Moriai, Joseph Reagle, Russ Housley, and Joel Halpern.
9. Added numerous URIs that are specified in W3C XML Security documents to the Indexes. These do not have sections in the body of this document -- for example, those for dsa-sha256, mgf1sha*, decrypt#XML, and xmldsig-filter2.
[Camellia] Aoki, K., Ichikawa, T., Matsui, M., Moriai, S., Nakajima, J., and T. Tokita, "Camellia: A 128-bit Block Cipher Suitable for Multiple Platforms - Design and Analysis", 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.
[IEEEP1363a] IEEE, "Standard Specifications for Public Key Cryptography- Amendment 1: Additional Techniques", IEEE 1363a-2004, 2004.
[RC4] Schneier, B., "Applied Cryptography: Protocols, Algorithms, and Source Code in C", Second Edition, John Wiley and Sons, New York, NY, 1996.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997.
Eastlake Standards Track [Page 31]
RFC 6931 Additional XML Security URIs April 2013
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2315] Kaliski, B., "PKCS #7: Cryptographic Message Syntax Version 1.5", RFC 2315, March 1998.
[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.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005.
[RFC4050] 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.
[RFC4055] Schaad, J., Kaliski, B., and R. Housley, "Additional Algorithms and Identifiers for RSA Cryptography for use in the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 4055, June 2005.
[RFC4269] Lee, H., Lee, S., Yoon, J., Cheon, D., and J. Lee, "The SEED Encryption Algorithm", RFC 4269, December 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.
Eastlake Standards Track [Page 32]
RFC 6931 Additional XML Security URIs April 2013
[X9.62] American National Standards Institute, Accredited Standards Committee X9, "Public Key Cryptography for the Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)", ANSI X9.62:2005, 2005.