ssl_tls12_server: export/import PK parsed key in TLS side

Instead of setting both algorithm and enrollement algorithm in the
PK module when parsing the key:

- for Weierstrass keys we only set ECDSA algorithm,
- for Montgomery keys we don't set any algorithm.

Reasons:
- PK module can only do ECDSA and not ECDH
- ECDH is only used in TLS
- Montgomery keys cannot be used to do ECDSA, while Weierstrass ones
  can do both ECDSA and ECDH.

So the idea is that once TLS needs the key to do ECDH (either Weierstrass
and Montgomery), it exports the one parsed from the PK module and then
re-imports it setting proper algorithm and flags. In this way the TLS
module will own the new key so it will be its duty to clear it on
exit.

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>

library/ssl_tls12_server.c

@@ -2597,12 +2597,12 @@ static int ssl_get_ecdh_params_from_cert(mbedtls_ssl_context *ssl)
     mbedtls_pk_context *pk;
     mbedtls_pk_type_t pk_type;
     psa_key_attributes_t key_attributes = PSA_KEY_ATTRIBUTES_INIT;
+    unsigned char buf[PSA_KEY_EXPORT_ECC_KEY_PAIR_MAX_SIZE(PSA_VENDOR_ECC_MAX_CURVE_BITS)];
+    size_t key_len;
 #if !defined(MBEDTLS_PK_USE_PSA_EC_DATA)
     uint16_t tls_id = 0;
     psa_key_type_t key_type = PSA_KEY_TYPE_NONE;
-    size_t key_len;
     mbedtls_ecp_group_id grp_id;
-    unsigned char buf[PSA_KEY_EXPORT_ECC_KEY_PAIR_MAX_SIZE(PSA_VENDOR_ECC_MAX_CURVE_BITS)];
     mbedtls_ecp_keypair *key;
 #endif /* !MBEDTLS_PK_USE_PSA_EC_DATA */
 
@@ -2625,22 +2625,41 @@ static int ssl_get_ecdh_params_from_cert(mbedtls_ssl_context *ssl)
                 return MBEDTLS_ERR_SSL_PK_TYPE_MISMATCH;
             }
 
-            ssl->handshake->xxdh_psa_privkey = pk->priv_id;
+            /* Get the attributes of the key previously parsed by PK module in
-
+             * order to extract its type and length (in bits). */
-            /* Key should not be destroyed in the TLS library */
+            status = psa_get_key_attributes(pk->priv_id, &key_attributes);
-            ssl->handshake->xxdh_psa_privkey_is_external = 1;
-
-            status = psa_get_key_attributes(ssl->handshake->xxdh_psa_privkey,
-                                            &key_attributes);
             if (status != PSA_SUCCESS) {
-                ssl->handshake->xxdh_psa_privkey = MBEDTLS_SVC_KEY_ID_INIT;
+                ret = PSA_TO_MBEDTLS_ERR(status);
-                return PSA_TO_MBEDTLS_ERR(status);
+                goto exit;
             }
-
             ssl->handshake->xxdh_psa_type = psa_get_key_type(&key_attributes);
             ssl->handshake->xxdh_psa_bits = psa_get_key_bits(&key_attributes);
 
-            psa_reset_key_attributes(&key_attributes);
+            /* Now export and then re-import the same key with proper flags
+             * and algorithm. We also set key's type and bits that we just got
+             * above. */
+            key_attributes = psa_key_attributes_init();
+            psa_set_key_usage_flags(&key_attributes, PSA_KEY_USAGE_DERIVE);
+            psa_set_key_algorithm(&key_attributes, PSA_ALG_ECDH);
+            psa_set_key_type(&key_attributes,
+                             PSA_KEY_TYPE_ECC_KEY_PAIR(ssl->handshake->xxdh_psa_type));
+            psa_set_key_bits(&key_attributes, ssl->handshake->xxdh_psa_bits);
+
+            status = psa_export_key(pk->priv_id, buf, sizeof(buf), &key_len);
+            if (status != PSA_SUCCESS) {
+                ret = PSA_TO_MBEDTLS_ERR(status);
+                goto exit;
+            }
+            status = psa_import_key(&key_attributes, buf, key_len,
+                                    &ssl->handshake->xxdh_psa_privkey);
+            if (status != PSA_SUCCESS) {
+                ret = PSA_TO_MBEDTLS_ERR(status);
+                goto exit;
+            }
+
+            /* Set this key as owned by the TLS library: it will be its duty
+             * to clear it exit. */
+            ssl->handshake->xxdh_psa_privkey_is_external = 0;
 
             ret = 0;
             break;
@@ -2696,6 +2715,10 @@ static int ssl_get_ecdh_params_from_cert(mbedtls_ssl_context *ssl)
             ret = MBEDTLS_ERR_SSL_PK_TYPE_MISMATCH;
     }
 
+exit:
+    psa_reset_key_attributes(&key_attributes);
+    mbedtls_platform_zeroize(buf, sizeof(buf));
+
     return ret;
 }
 #else /* MBEDTLS_USE_PSA_CRYPTO */