Appendix B. Two Layers of Recipient Information

All of the currently defined recipient algorithm classes only use two layers of the COSE structure. The first layer (COSE_Encrypt) is the message content, and the second layer (COSE_Recipient) is the content key encryption. However, if one uses a recipient algorithm such as the RSA Key Encapsulation Mechanism (RSA-KEM) (see Appendix A of RSA- KEM [RFC5990]), then it makes sense to have two layers of the COSE_Recipient structure.

These layers would be:

Layer 0: The content encryption layer. This layer contains the payload of the message.
Layer 1: The encryption of the CEK by a KEK.
Layer 2: The encryption of a long random secret using an RSA key and a key derivation function to convert that secret into the KEK.

This is an example of what a triple-layer message would look like. To make it easier to read, it is presented using the extended CBOR diagnostic notation (defined in [RFC8610]) rather than as a binary dump. The message has the following layers:

Layer 0: Has content encrypted with AES-GCM using a 128-bit key.
Layer 1: Uses the AES Key Wrap algorithm with a 128-bit key.
Layer 2: Uses ECDH Ephemeral-Static direct to generate the Layer 1 key.

In effect, this example is a decomposed version of using the ECDH- ES+A128KW algorithm.

Size of binary file is 183 bytes

   96(
     [ / COSE_Encrypt /
       / protected h'a10101' / &lt;&lt; {
           / alg / 1:1 / AES-GCM 128 /
         } >>,
       / unprotected / {
         / iv / 5:h'02d1f7e6f26c43d4868d87ce'
       },
       / ciphertext / h'64f84d913ba60a76070a9a48f26e97e863e2852948658f0
   811139868826e89218a75715b',
       / recipients / [
         [ / COSE_Recipient /
           / protected / h'',
           / unprotected / {
             / alg / 1:-3 / A128KW /
           },
           / ciphertext / h'dbd43c4e9d719c27c6275c67d628d493f090593db82
   18f11',
           / recipients / [
             [ / COSE_Recipient /
               / protected h'a1013818' / &lt;&lt; {
                   / alg / 1:-25 / ECDH-ES + HKDF-256 /
                 } >> ,
               / unprotected / {
                 / ephemeral / -1:{
                   / kty / 1:2,
                   / crv / -1:1,
                   / x / -2:h'b2add44368ea6d641f9ca9af308b4079aeb519f11
   e9b8a55a600b21233e86e68',
                   / y / -3:false
                 },
                 / kid / 4:'[email protected]'
               },
               / ciphertext / h''
             ]
           ]
         ]
       ]
     ]
   )