[TLS] HSM-friendly Key Computation

[Russ Housley <[e-mail]>]

Section 6.3 of draft-ietf-tls-tls13-05 describes the key calculation from the current master secret (MS).  First a key block is computed:

      key_block = PRF(MS,
                      "key expansion",
                      SecurityParameters.server_random +
                      SecurityParameters.client_random);

Then, the key block is divided into keys and IVs:

      client_write_key[SecurityParameters.enc_key_length]
      server_write_key[SecurityParameters.enc_key_length]
      client_write_IV[SecurityParameters.fixed_iv_length]
      server_write_IV[SecurityParameters.fixed_iv_length]

If one wants to implement the cryptographic functions in a Hardware Security Module (HSM), this structure is far from ideal.  In general I would expect the client_write_key and server_write_key to stay inside the protected boundary of the HSM, but the client_write_IV and server_write_IV do not need this protection.  Both of these coming from the same key_block makes this separation very difficult to implement.

Can we get these values from different invocations of the PRF?  I'd like to see one invocation for values that are expected to remain secret and a separate invocation for values that do not need to remain secret.

Russ