Extend PSA_ALG_TLS12_PSK_TO_MS alg (add #PSA_KEY_DERIVATION_INPUT_OTHER_SECRET input)
* This key derivation algorithm uses the following inputs, which must be
* passed in the order given here:
* - #PSA_KEY_DERIVATION_INPUT_SEED is the seed.
+ * - #PSA_KEY_DERIVATION_INPUT_OTHER_SECRET is the other secret for the
+ * computation of the premaster secret. This input is optional;
+ * if omitted, it defaults to a string of null bytes with the same length
+ * as the secret (PSK) input.
* - #PSA_KEY_DERIVATION_INPUT_SECRET is the secret key.
* - #PSA_KEY_DERIVATION_INPUT_LABEL is the label.
*
* For the application to TLS-1.2, the seed (which is
* forwarded to the TLS-1.2 PRF) is the concatenation of the
* ClientHello.Random + ServerHello.Random,
- * and the label is "master secret" or "extended master secret".
+ * the label is "master secret" or "extended master secret" and
+ * the other secret depends on the key exchange specified in the cipher suite:
+ * - for a plain PSK cipher suite (RFC 4279, Section 2), omit
+ * PSA_KEY_DERIVATION_INPUT_OTHER_SECRET
+ * - for a DHE-PSK (RFC 4279, Section 3) or ECDHE-PSK cipher suite
+ * (RFC 5489, Section 2), the other secret should be the output of the
+ * PSA_ALG_FFDH or PSA_ALG_ECDH key agreement performed with the peer.
+ * The recommended way to pass this input is to use a key derivation
+ * algorithm constructed as
+ * PSA_ALG_KEY_AGREEMENT(ka_alg, PSA_ALG_TLS12_PSK_TO_MS(hash_alg))
+ * and to call psa_key_derivation_key_agreement(). Alternatively,
+ * this input may be an output of `psa_raw_key_agreement()` passed with
+ * psa_key_derivation_input_bytes(), or an equivalent input passed with
+ * psa_key_derivation_input_bytes() or psa_key_derivation_input_key().
+ * - for a RSA-PSK cipher suite (RFC 4279, Section 4), the other secret
+ * should be the 48-byte client challenge (the PreMasterSecret of
+ * (RFC 5246, Section 7.4.7.1)) concatenation of the TLS version and
+ * a 46-byte random string chosen by the client. On the server, this is
+ * typically an output of psa_asymmetric_decrypt() using
+ * PSA_ALG_RSA_PKCS1V15_CRYPT, passed to the key derivation operation
+ * with `psa_key_derivation_input_bytes()`.
*
* For example, `PSA_ALG_TLS12_PSK_TO_MS(PSA_ALG_SHA256)` represents the
* TLS-1.2 PSK to MasterSecret derivation PRF using HMAC-SHA-256.
@@ -2454,6 +2478,16 @@ static inline int mbedtls_svc_key_id_is_null( mbedtls_svc_key_id_t key )
*/
#define PSA_KEY_DERIVATION_INPUT_PASSWORD ((psa_key_derivation_step_t)0x0102)
+/** A high-entropy additional secret input for key derivation.
+ *
+ * This is typically the shared secret resulting from a key agreement obtained
+ * via `psa_key_derivation_key_agreement()`. It may alternatively be a key of
+ * type `PSA_KEY_TYPE_DERIVE` passed to `psa_key_derivation_input_key()`, or
+ * a direct input passed to `psa_key_derivation_input_bytes()`.
+ */
+#define PSA_KEY_DERIVATION_INPUT_OTHER_SECRET \
+ ((psa_key_derivation_step_t)0x0103)
+
/** A label for key derivation.
*
* This should be a direct input.
