Add helper function for calculation of TLS 1.3 PSK binder
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/library/ssl_tls13_keys.c b/library/ssl_tls13_keys.c
index ce551f6..950d32c 100644
--- a/library/ssl_tls13_keys.c
+++ b/library/ssl_tls13_keys.c
@@ -565,4 +565,136 @@
return( 0 );
}
+static int ssl_tls1_3_calc_finished_core( mbedtls_md_type_t md_type,
+ unsigned char const *base_key,
+ unsigned char const *transcript,
+ unsigned char *dst )
+{
+ const mbedtls_md_info_t* const md_info = mbedtls_md_info_from_type( md_type );
+ size_t const md_size = mbedtls_md_get_size( md_info );
+ unsigned char finished_key[MBEDTLS_MD_MAX_SIZE];
+ int ret;
+
+ /* We should never call this function with an unknown hash,
+ * but add an assertion anyway. */
+ if( md_info == 0 )
+ return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
+
+ /* TLS 1.3 Finished message
+ *
+ * struct {
+ * opaque verify_data[Hash.length];
+ * } Finished;
+ *
+ * verify_data =
+ * HMAC( finished_key,
+ * Hash( Handshake Context +
+ * Certificate* +
+ * CertificateVerify* )
+ * )
+ *
+ * finished_key =
+ * HKDF-Expand-Label( BaseKey, "finished", "", Hash.length )
+ */
+
+ ret = mbedtls_ssl_tls1_3_hkdf_expand_label(
+ md_type, base_key, md_size,
+ MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( finished ),
+ NULL, 0,
+ finished_key, md_size );
+ if( ret != 0 )
+ goto exit;
+
+ ret = mbedtls_md_hmac( md_info, finished_key, md_size, transcript, md_size, dst );
+ if( ret != 0 )
+ goto exit;
+
+exit:
+
+ mbedtls_platform_zeroize( finished_key, sizeof( finished_key ) );
+ return( ret );
+}
+
+int mbedtls_ssl_tls1_3_create_psk_binder( mbedtls_ssl_context *ssl,
+ const mbedtls_md_type_t md_type,
+ unsigned char const *psk, size_t psk_len,
+ int psk_type,
+ unsigned char const *transcript,
+ unsigned char *result )
+{
+ int ret = 0;
+ unsigned char binder_key[MBEDTLS_MD_MAX_SIZE];
+ unsigned char early_secret[MBEDTLS_MD_MAX_SIZE];
+ mbedtls_md_info_t const *md_info = mbedtls_md_info_from_type( md_type );
+ size_t const md_size = mbedtls_md_get_size( md_info );
+
+ /* We should never call this function with an unknown hash,
+ * but add an assertion anyway. */
+ if( md_info == 0 )
+ return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
+
+ /*
+ * 0
+ * |
+ * v
+ * PSK -> HKDF-Extract = Early Secret
+ * |
+ * +-----> Derive-Secret(., "ext binder" | "res binder", "")
+ * | = binder_key
+ * v
+ */
+
+ ret = mbedtls_ssl_tls1_3_evolve_secret( md_type,
+ NULL, /* Old secret */
+ psk, psk_len, /* Input */
+ early_secret );
+ if( ret != 0 )
+ {
+ MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_tls1_3_evolve_secret", ret );
+ goto exit;
+ }
+
+ if( psk_type == MBEDTLS_SSL_TLS1_3_PSK_RESUMPTION )
+ {
+ ret = mbedtls_ssl_tls1_3_derive_secret( md_type,
+ early_secret, md_size,
+ MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( res_binder ),
+ NULL, 0, MBEDTLS_SSL_TLS1_3_CONTEXT_UNHASHED,
+ binder_key, md_size );
+ MBEDTLS_SSL_DEBUG_MSG( 4, ( "Derive Early Secret with 'res binder'" ) );
+ }
+ else
+ {
+ ret = mbedtls_ssl_tls1_3_derive_secret( md_type,
+ early_secret, md_size,
+ MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( ext_binder ),
+ NULL, 0, MBEDTLS_SSL_TLS1_3_CONTEXT_UNHASHED,
+ binder_key, md_size );
+ MBEDTLS_SSL_DEBUG_MSG( 4, ( "Derive Early Secret with 'ext binder'" ) );
+ }
+
+ if( ret != 0 )
+ {
+ MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_tls1_3_derive_secret", ret );
+ goto exit;
+ }
+
+ /*
+ * The binding_value is computed in the same way as the Finished message
+ * but with the BaseKey being the binder_key.
+ */
+
+ ret = ssl_tls1_3_calc_finished_core( md_type, binder_key, transcript, result );
+ if( ret != 0 )
+ goto exit;
+
+ MBEDTLS_SSL_DEBUG_BUF( 3, "psk binder", result, md_size );
+
+exit:
+
+ mbedtls_platform_zeroize( early_secret, sizeof( early_secret ) );
+ mbedtls_platform_zeroize( binder_key, sizeof( binder_key ) );
+ return( ret );
+}
+
#endif /* MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL */