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Generic hybrid ciphers allow for a consistent treatment of asymmetric ciphers when encrypting data and consist of a key encapsulation algorithm with associated parameters and a data encapsulation algorithm with associated parameters. Further, the key encapsulation algorithms introduced in this specification have attractive security properties.
This document augments XML Encryption Version 1.1 [XMLENC-CORE1] by defining algorithms, XML types and elements necessary to enable use of generic hybrid ciphers in XML Security applications.
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This is a Last Call Working Draft of "XML Security Generic Hybrid Ciphers". Changes from the previous working draft include a revision of the namespace and editorial corrections. Please review differences between the previous and this Working Draft.
This document was published by the XML Security Working Group as a Last Call Working Draft. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, please send them to firstname.lastname@example.org (subscribe, archives). The Last Call period ends 10 June 2010. All feedback is welcome.
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This document specifies an XML syntax and processing rules for generic hybrid ciphers and key encapsulation mechanisms based on [ISO18033-2]. The document augments XML Encryption [XMLENC-CORE1].
This document does not normatively specify when and how generic hybrid ciphers and key encapsulation mechanisms are to be used; rather it focuses on the basis for interoperability, namely the fundamental data types required for use of these algorithms in conjunction with XML Security applications and the meaning of those data types, as well as identification of specific algorithms.
The key words "must" and "optional" in this specification are to be interpreted as described in RFC2119 [RFC2119]:
"They must only be used where it is actually required for interoperation or to limit behavior which has potential for causing harm (e.g., limiting retransmissions)"
Consequently, these capitalized keywords are used to unambiguously specify requirements over protocol and application features and behavior that affect the interoperability and security of implementations. These key words are not used (capitalized) to describe XML grammar; schema definitions unambiguously describe such requirements. For instance, an XML attribute might be described as being "optional."
Note also that this entire specification is optional; hence the keywords apply only when compliance with this specification is claimed.
No provision is made for an explicit version number in this syntax. If a future version is needed, it will use a different namespace. The XML namespace [XML-NAMES] URI that must be used by implementations of this (dated) specification is:
While applications must support XML and XML namespaces, the use of
internal entities or the
ghc XML namespace prefix
is optional; we use
these facilities to provide compact and readable examples.
This specification uses Uniform Resource Identifiers [URI] to identify resources, algorithms, and semantics. Identifiers under the control of this specification are coined within the scope of the above namespace.
The term "generic hybrid cipher" is defined in [ISO18033-2] as an asymmetric cipher that combines both asymmetric and symmetric cryptographic techniques. Generic hybrid ciphers that meet the requirements laid out in [ISO18033-2] have attractive security properties. They are introduced in this document to enable applications to use cryptographic algorithms with tight security proofs.
Generic hybrid ciphers allow for a consistent treatment of asymmetric ciphers when encrypting data and consists of a key encapsulation algorithm with associated parameters and a data encapsulation algorithm with associated parameters. The key encapsulation algorithm results in an encapsulated shared key that is then used with the data encapsulation algorithm, e.g. for encryption.
This section discusses and identifies algorithms to be used
with this specification. Entries contain the identifier to be
used as the value of the
Algorithm attribute of
EncryptionMethod element or other element
representing the role of the algorithm, a reference to the
formal specification, definitions for the representation of
keys and the results of cryptographic operations where
applicable, and general applicability comments.
This specification defines a set of algorithms, their URIs, and requirements for implementation. Levels of requirement specified, such as must or optional, refer to implementation, not use.
Generic-hybrid encryption algorithms combine both asymmetric and symmetric cryptographic techniques. Schema definition:
<element name="GenericHybridCipherMethod" type="ghc:GenericHybridCipherMethodType"/> <complexType name="GenericHybridCipherMethodType"> <sequence> <element name="KeyEncapsulationMethod" type="ghc:KeyEncapsulationMethodType"/> <element name="DataEncapsulationMethod" type="xenc:EncryptionMethodType"/> </sequence> </complexType>
KeyEncapsulationMethod element identifies
the key encapsulation method as well as provides values for
identifies the data encapsulation (encryption) method as well
as provides any parameters associated with the data
The Generic-Hybrid encryption algorithm may be used for a variety of purposes; in particular, when used with a key encapsulation mechanism such as those specified in Section 4.2 and a suitable key wrap algorithm, it can be used for key transport with tight security proofs.
GenericHybridCipherMethod element shall
appear as a child element of the
generic-hybrid is specified as the value of
This document specifies two key encapsulation algorithms, RSAES-KEM and ECIES-KEM, for use with the Generic-Hybrid cipher in key transport scenarios.
<complexType name="KeyEncapsulationMethodType"> <sequence> <element ref="xenc11:KeyDerivationMethod"/> <element name="KeyLen" type="positiveInteger"/> <any namespace="##other" minOccurs="0" maxOccurs="unbounded"/> </sequence> <attribute name="Algorithm" type="anyURI" use="required"/> </complexType>
xenc11:KeyDerivationMethod element of the
KeyEncapsulationMethodType specifies a key
derivation method to use when deriving a key from key
material generated in accordance with the key encapsulation
element is defined in [XMLENC-CORE1]. The
KeyLen element specifies length of the derived
Algorithm attribute identifies the
actual key encapsulation method used.
RSAES-KEM is a key encapsulation algorithm based on the RSA encryption scheme.
Given a recipient's public RSA key (n, e) where n is the RSA modulus and e is the public exponent, the following steps shall be taken to encapsulate a symmetric key when the RSAES-KEM key encapsulation algorithm is used (these are the same steps as specified in Section 11.5.3 of [ISO18033-2]):
Given a recipient's private RSA key (n, d) where n is the RSA modulus and d is the private exponent, the following steps shall be taken to decrypt an encapsulated symmetric key from ciphertext C0 when the RSAES-KEM key encapsulation algorithm is used (these are the same steps as specified in Section 11.5.4 of [ISO18033-2]:
For use of the RSAES-KEM key encapsulation algorithm with Key Transport, see Section 5.
ECIES-KEM is a key encapsulation algorithm based on the Elliptic Curve scheme.
Given a recipient's public EC key h and an elliptic curve E with base point g and order (size) o, the following steps shall be taken to encapsulate a symmetric key when the ECIES-KEM key encapsulation algorithm is used (these are the same steps as specified in Section 10.2.3 of [ISO18033-2]):
Given a recipient's private EC key x and an elliptic curve E with base point g and order (size) o, the following steps shall be taken to decrypt an encapsulated symmetric key from ciphertext C0 when the ECIES-KEM key encapsulation algorithm is used (these are the same steps as specified in Section 10.2.4 of [ISO18033-2]):
For use of the ECIES-KEM key encapsulation algorithm with Key Transport, see Section 5.
When using a Key Encapsulation algorithm such as RSAES-KEM
or ECIES-KEM for key transport, the key K which is one of the
outputs of the KEM algorithm (see RSAES-KEM and ECIES-KEM) is now used as a wrapping key,
encrypting a data-encryption key DEK: C1 = WRAP(K, DEK). The
combined ciphertext C0 | C1 (where C0 is the other output of
the KEM algorithm) is then placed in the
xenc:CipherValue element of the
xenc:CipherData child element of the
element will identify the recipient's public key).
This section is non-normative.
The following is a syntactically correct example of an
xenc:EncryptedKey element using the
Generic-Hybrid method together with the ECIES-KEM algorithm
for key encapsulation and AES-128 KeyWrap for wrapping the
content key using the encapsulated key (the example would
look precisely the same when using the RSAES-KEM algorithm
except for the identification of the Key Encapsulation
Algorithm which instead would have the value
"http://www.w3.org/2010/xmlsec-ghc#rsaes-kem" and the
identification of an RSA public key instead of an ECC key in
<xenc:EncryptedKey xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xenc="http://www.w3.org/2001/04/xmlenc#" xmlns:ds="http://www.w3.org/2000/09/xmldsig#" xmlns:dsig11="http://www.w3.org/2009/xmldsig11#" xmlns:xenc11="http://www.w3.org/2009/xmlenc11#" xmlns:ghc="http://www.w3.org/2010/xmlsec-ghc#" <xenc:EncryptionMethod Algorithm="http://www.w3.org/2010/xmlsec-ghc#generic-hybrid"> <ghc:GenericHybridCipherMethod> <ghc:KeyEncapsulationMethod Algorithm="http://www.w3.org/2010/xmlsec-ghc#ecies-kem"> <xenc11:KeyDerivationMethod Algorithm="http://www.w3.org/2009/xmlenc11#ConcatKDF"> <xenc11:ConcatKDFParams AlgorithmID="0000" PartyUInfo="03D8" PartyVInfo=""> <ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/> </xenc11:ConcatKDFParams> <ghc:KeyLen>16</ghc:KeyLen> </ghc:KeyEncapsulationMethod> <ghc:DataEncapsulationMethod Algorithm="http://www.w3.org/2001/04/xmlenc#kw-aes128"/> </ghc:GenericHybridCipherMethod> </xenc:EncryptionMethod> <ds:KeyInfo> <dsig11:ECKeyValue> <dsig11:NamedCurve URI="urn:oid:1.2.840.10045.3.1.7"/> <dsig11:PublicKey>DEADBEEF</dsig11:PublicKey> </dsig11:ECKeyValue> </ds:KeyInfo> <xenc:CipherData> <xenc:CipherValue>DEADBEEF</xenc:CipherValue> <!-- Is concatenation of originator's ephemeral key (expressed as an octet string) and the wrapped key --> </xenc:CipherData> </xenc:EncryptedKey>
Generic hybrid ciphers with key encapsulation mechanisms as specified in this document provides a high security level assuming key derivation algorithms and other security parameters have been properly chosen. See further [ISO18033-2], Annex B for a deeper security discussion on these constructions.
An implementation is conformant to this specification if it successfully generates syntax according to the schema definitions and satisfies any and all must/required/shall requirements.The full normative grammar is defined by the XSD schema and the normative text in this specification. The standalone XSD schema file is authoritative in case there is any disagreement between it and the XSD schema portions in this specification.
This section is non-normative.
Non-normative RELAX NG schema [RELAXNG-SCHEMA] information is available in a separate document [XMLSEC-RELAXNG].
Dated references below are to the latest known or appropriate edition of the referenced work. The referenced works may be subject to revision, and conformant implementations may follow, and are encouraged to investigate the appropriateness of following, some or all more recent editions or replacements of the works cited. It is in each case implementation-defined which editions are supported.