Encryption Keys

                                    Presentation to the W3C XML Encryption WG

                                    1 March 20001

                                    Blair Dillaway, Microsoft Corp.

Requirements (4 of 4)

• Encrypted key information should be syntactically distinct from encrypted data to reflect semantic differences
• Leverage XML Signature defined KeyInfo
• All encryption key info for an encrypted data must refer to the same key
• Provide a way to associate a ‘name’ with an encryption key
• Accommodate all types of algorithms: block cipher, stream cipher, asymmetric ciphers
• Applies to both encrypted data and keys

   Specifying an Encrypted Key

  <EncryptedKey  name=‘keyname’>
     <EncryptionMethod>*
     <ReferenceList>*  items encrypted using the key encoded in the cipher text
      <ds:KeyInfo>
              (KeyName, KeyValue, EncryptedKeyReference)
               refs to key value used to encrypt the   cipher text
       </ds:KeyInfo>
       <CipherText>encrypted key value</CipherText>
  </EncryptedKey>

   Refs to Encrypted Keys

• <EncryptedKeyReference  URI=‘someuri’>
        <ds:Transforms>*    optional
  </EncryptedKeyReference>

  Binding to Encrypted Data

• <EncryptedData>
        <EncryptedKey>*    Can be multiple
        <ds:KeyInfo>
              (KeyName, EncryptedKeyReference…)  Refs to keys
        </ds:KeyInfo>
        <EncryptionMethod>*
        <CipherText>+
  </EncryptedData>

   Issues (1 of 3)

• Potential confusion due to overuse of term ‘reference’

  KeyReference, DataReference, EncryptedReference, EncryptedKeyReference.

• Should we provide a means of specifying a key by  reference to be used for multiple decryptions?

  <EncryptionInfo>
        <ds:KeyInfo>
              <KeyName>foo</KeyName>
        </ds:KeyInfo>
        <ReferenceList>+
  </EncryptionInfo>

  Issues (2 of 3)

• Should we provide a means of encrypting keys to multiple recipients within a single EncryptedKey element?

  <EncryptedKey>
        <Recipients>
              <Recipient>
                    <EncryptionMethod>*
                    <KeyInfo>+
                    <CipherText>+KeyName>foo</KeyName>
              </Recipient>
        </Recipients>
  </EncryptedKey>

  Issues (3 of 3)

  • Alternate key sharing syntax.

  <EncryptedData>
        <ds:KeyInfo>
              <EncryptedKey>+
        </ds:KeyInfo>
        <CipherText>+
  </EncryptionData>

  and

  <EncryptedKey>
        <ds:KeyInfo>
              <EncryptedKey>+
        </ds:KeyInfo>
  </EncryptionKey>