Update comments in ECHDE-PSK part of ssl_write_client_key_exchange()
@@ -3091,12 +3091,17 @@ ecdh_calc_secret:
ssl->out_msg[header_len] = (unsigned char) own_pubkey_len;
content_len = own_pubkey_len + 1;
- /* The ECDH secret is the premaster secret used for key derivation. */
+ /* As RFC 5489 section 2, the premaster secret is formed as follows:
+ * - a uint16 containing the length (in octets) of the ECDH computation
+ * - the octet string produced by the ECDH computation
+ * - a uint16 containing the length (in octets) of the PSK
+ * - the PSK itself
+ */
unsigned char *p = ssl->handshake->premaster;
unsigned char *p_end = p + sizeof( ssl->handshake->premaster );
size_t zlen = 0;
- /* Compute ECDH shared secret. */
+ /* Perform ECDH computation after the uint16 reserved for the length */
status = psa_raw_key_agreement( PSA_ALG_ECDH,
handshake->ecdh_psa_privkey,
handshake->ecdh_psa_peerkey,
@@ -3111,6 +3116,7 @@ ecdh_calc_secret:
if( status != PSA_SUCCESS || destruction_status != PSA_SUCCESS )
return( MBEDTLS_ERR_SSL_HW_ACCEL_FAILED );
+ /* Write the ECDH computation length before the ECDH computation */
MBEDTLS_PUT_UINT16_BE( zlen, p, 0 );
p += 2 + zlen;
@@ -3131,12 +3137,14 @@ ecdh_calc_secret:
return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
}
+ /* Write the PSK length as uint16 */
MBEDTLS_PUT_UINT16_BE( psk_len, p, 0 );
p += 2;
if( p_end < p || (size_t)( p_end - p ) < psk_len )
return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
+ /* Write the PSK itself */
memcpy( p, psk, psk_len );
p += psk_len;
