Allow hardcoding single supported elliptic curve
This commit introduces the option MBEDTLS_SSL_CONF_SINGLE_EC
which can be used to register a single supported elliptic curve
at compile time. It replaces the runtime configuration API
mbedtls_ssl_conf_curves() which allows to register a _list_
of supported elliptic curves.
In contrast to other options used to hardcode configuration options,
MBEDTLS_SSL_CONF_SINGLE_EC isn't a numeric option, but instead it's
only relevant if it's defined or not. To actually set the single
elliptic curve that should be supported, numeric options
MBEDTLS_SSL_CONF_SINGLE_EC_TLS_ID
MBEDTLS_SSL_CONF_SINGLE_EC_GRP_ID
must both be defined and provide the TLS ID and the Mbed TLS internal
ID and the chosen curve, respectively.
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 8290c51..9443c92 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -3652,6 +3652,33 @@
*/
//#define MBEDTLS_SSL_CONF_SINGLE_CIPHERSUITE MBEDTLS_SUITE_TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8
+/* Enable support a single elliptic curve fixed
+ * at compile-time, at the benefit of code-size.
+ *
+ * On highly constrained systems which large control
+ * over the configuration of the connection endpoints,
+ * this option can be used to hardcode the choice of
+ * a single elliptic curve to use for all elliptic
+ * curve operations during the handshake.
+ *
+ * If this is set, you must also define the following:
+ * - MBEDTLS_SSL_CONF_SINGLE_EC_GRP_ID
+ * This must resolve to the Mbed TLS group ID for the elliptic
+ * curve to use (e.g. MBEDTLS_ECP_DP_SECP256R1_ENABLED); see
+ * ::mbedtls_ecp_group_id in mbedtls/ecp.h for a complete list
+ * of curve identifiers.
+ * - MBEDTLS_SSL_CONF_SINGLE_EC_TLS_ID
+ * This must resolve to the identifier for the elliptic curve
+ * to use according to the IANA NamedCurve registry:
+ * https://tools.ietf.org/html/rfc4492#section-5.1
+ *
+ * If defined, this option overwrites the effect of the
+ * runtime configuration API mbedtls_ssl_conf_curves().
+ */
+//#define MBEDTLS_SSL_CONF_SINGLE_EC
+//#define MBEDTLS_SSL_CONF_SINGLE_EC_TLS_ID
+//#define MBEDTLS_SSL_CONF_SINGLE_EC_GRP_ID
+
/* \} SECTION: Compile-time SSL configuration */
/* Target and application specific configurations