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XML Encryption, XML Signature, and Derived Keys: Suggestion For a Minor Addition

XML Encryption, XML Signature, and Derived Keys: Suggestion For a Minor Addition. Magnus Nyström RSA. Background. RSA Laboratories PKCS #5 deals with “password-based cryptography” I.e., how to derive keys from shared secrets such as passwords

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XML Encryption, XML Signature, and Derived Keys: Suggestion For a Minor Addition

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  1. XML Encryption, XML Signature, and Derived Keys: Suggestion For a Minor Addition Magnus Nyström RSA

  2. Background • RSA Laboratories PKCS #5 deals with “password-based cryptography” • I.e., how to derive keys from shared secrets such as passwords • These keys are then used for encryption or message authentication • PKCS #5 syntax originally in ASN.1 • Natural for use with S/MIME, etc. • XML syntax published in 2007 • http://www.rsa.com/rsalabs/node.asp?id=2127

  3. PKCS #5 XML Syntax (snippet) <xs:complexType name="PBES2ParameterType"><xs:sequence> <xs:element name="KeyDerivationFunc“ type="AlgorithmIdentifierType"/> <xs:element name="EncryptionScheme“ type="xenc:EncryptionMethodType"/></xs:sequence></xs:complexType> • For use in xenc:EncryptionMethod • <xenc:EncryptionMethod Algorithm = rsa.com…./pkcs-5#pbes2)<pkcs-5:PBES2-params> <KeyDerivationFunc Algorithm=“http://www.rsasecurity.com/.../pkcs-5#pbkdf2”> … </KeyDerivationFunc> <EncryptionScheme Algorithm=“http://www.w3.org/2001/04/xmlenc#aes128-cbc”> </EncryptionScheme></pkcs-5:PBES2-params></xenc:EncryptionMethod>

  4. What’s Missing? • An ability to inform a recipient that she should use a key derived from a known pass-phrase (or other shared secret) for multiple encrypted data (or authenticated data) instances • A single encrypted (authenticated) data works with current approach (PBES2/PBMAC1) • WS-I also recommends forward cross-referencing in this case • It was felt this should be an extension to XML Enc/ XML Dsig rather than PKCS • Too generic – Derived Key • The current gap causes some issues – e.g. in IETF KEYPROV that leverages PKCS #5 • Had to define their own Derived Key key type

  5. One (out of many!) Possible Way to Do It • Modeled after <xenc:EncryptedKeyType> • <element name="DerivedKey“ type="xmlsec:DerivedKeyType"/> • <complexType name="DerivedKeyType"> <sequence> <element name="KeyDerivationMethod" type="xmlsec:KeyDerivationMethodType" minOccurs="0"/> <element ref="xenc:ReferenceList" minOccurs="0"/> <element name="CarriedKeyName" type="string" minOccurs="0"/> </sequence> <attribute name="Id" type="ID" use="optional"/> <attribute name="Type" type="anyURI" use="optional"/></complexType>

  6. Summary • There are use cases for a “Derived Key” key type • They are not currently covered by XML Enc, XML Dsig (or by PKCS #5) • XML Security Group could be natural place to introduce this • Would like to contribute in this area of work • Happy to take on editing responsibility in this regard

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