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/configs/baremetal.h b/configs/baremetal.h
index 2e92e76..89f3771 100644
--- a/configs/baremetal.h
+++ b/configs/baremetal.h
@@ -57,6 +57,10 @@
 #define MBEDTLS_ECP_MAX_BITS   256
 #define MBEDTLS_MPI_MAX_SIZE    32 // 256 bits is 32 bytes
 
+#define MBEDTLS_SSL_CONF_SINGLE_EC
+#define MBEDTLS_SSL_CONF_SINGLE_EC_GRP_ID MBEDTLS_ECP_DP_SECP256R1
+#define MBEDTLS_SSL_CONF_SINGLE_EC_TLS_ID 23
+
 /* Key exchanges */
 #define MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA_ENABLED
 #define MBEDTLS_SSL_CIPHERSUITES MBEDTLS_TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8
diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index 735a1e4..509cfa4 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -87,6 +87,12 @@
 #error "MBEDTLS_CMAC_C defined, but not all prerequisites"
 #endif
 
+#if defined(MBEDTLS_SSL_CONF_SINGLE_EC) &&           \
+    ( !defined(MBEDTLS_SSL_CONF_SINGLE_EC_TLS_ID) || \
+      !defined(MBEDTLS_SSL_CONF_SINGLE_EC_GRP_ID) )
+#error "MBEDTLS_SSL_CONF_SINGLE_EC defined, but not all prerequesites"
+#endif
+
 #if defined(MBEDTLS_USE_TINYCRYPT) && defined(MBEDTLS_NO_64BIT_MULTIPLICATION)
 #error "MBEDTLS_USE_TINYCRYPT defined, but it cannot be defined with MBEDTLS_NO_64BIT_MULTIPLICATION"
 #endif
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
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 169e054..9937b30 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -1070,7 +1070,9 @@
 #endif
 
 #if defined(MBEDTLS_ECP_C)
+#if !defined(MBEDTLS_SSL_CONF_SINGLE_EC)
     const mbedtls_ecp_group_id *curve_list; /*!< allowed curves             */
+#endif /* MBEDTLS_SSL_CONF_SINGLE_EC */
 #endif
 
 #if defined(MBEDTLS_DHM_C)
@@ -2785,6 +2787,7 @@
 #endif /* MBEDTLS_DHM_C && MBEDTLS_SSL_CLI_C */
 
 #if defined(MBEDTLS_ECP_C)
+#if !defined(MBEDTLS_SSL_CONF_SINGLE_EC)
 /**
  * \brief          Set the allowed curves in order of preference.
  *                 (Default: all defined curves.)
@@ -2808,12 +2811,17 @@
  * \note           This list should be ordered by decreasing preference
  *                 (preferred curve first).
  *
+ * \note           On highly constrained systems, the support for a single
+ *                 fixed elliptic curve can be configured at compile time
+ *                 through the option MBEDTLS_SSL_CONF_SINGLE_EC.
+ *
  * \param conf     SSL configuration
  * \param curves   Ordered list of allowed curves,
  *                 terminated by MBEDTLS_ECP_DP_NONE.
  */
 void mbedtls_ssl_conf_curves( mbedtls_ssl_config *conf,
                               const mbedtls_ecp_group_id *curves );
+#endif /* !MBEDTLS_SSL_CONF_SINGLE_EC */
 #endif /* MBEDTLS_ECP_C */
 
 #if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED)
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index f7ae625..9259955 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -1484,6 +1484,8 @@
 
 #endif /* MBEDTLS_SSL_CONF_SINGLE_CIPHERSUITE */
 
+#if !defined(MBEDTLS_SSL_CONF_SINGLE_EC)
+
 #define MBEDTLS_SSL_BEGIN_FOR_EACH_SUPPORTED_EC_TLS_ID( TLS_ID_VAR )    \
     {                                                                   \
         mbedtls_ecp_group_id const *__gid;                              \
@@ -1513,4 +1515,24 @@
         }                                                               \
     }
 
+#else /* !MBEDTLS_SSL_CONF_SINGLE_EC */
+
+#define MBEDTLS_SSL_BEGIN_FOR_EACH_SUPPORTED_EC_TLS_ID( TLS_ID_VAR )    \
+    {                                                                   \
+        uint16_t TLS_ID_VAR = MBEDTLS_SSL_CONF_SINGLE_EC_TLS_ID;        \
+        ((void) ssl);
+
+#define MBEDTLS_SSL_END_FOR_EACH_SUPPORTED_EC_TLS_ID                    \
+    }
+
+#define MBEDTLS_SSL_BEGIN_FOR_EACH_SUPPORTED_EC_GRP_ID( EC_ID_VAR )         \
+    {                                                                       \
+        mbedtls_ecp_group_id EC_ID_VAR = MBEDTLS_SSL_CONF_SINGLE_EC_GRP_ID; \
+        ((void) ssl);
+
+#define MBEDTLS_SSL_END_FOR_EACH_SUPPORTED_EC_GRP_ID                    \
+    }
+
+#endif /* MBEDTLS_SSL_CONF_SINGLE_EC */
+
 #endif /* ssl_internal.h */
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index d669fe7..5b67d62 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -8616,6 +8616,7 @@
 #endif /* MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED */
 
 #if defined(MBEDTLS_ECP_C)
+#if !defined(MBEDTLS_SSL_CONF_SINGLE_EC)
 /*
  * Set the allowed elliptic curves
  */
@@ -8624,6 +8625,7 @@
 {
     conf->curve_list = curve_list;
 }
+#endif /* MBEDTLS_SSL_CONF_SINGLE_EC */
 #endif /* MBEDTLS_ECP_C */
 
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
@@ -10840,7 +10842,7 @@
 };
 #endif
 
-#if defined(MBEDTLS_ECP_C)
+#if defined(MBEDTLS_ECP_C) && !defined(MBEDTLS_SSL_CONF_SINGLE_EC)
 static mbedtls_ecp_group_id ssl_preset_suiteb_curves[] = {
 #if defined(MBEDTLS_ECP_DP_SECP256R1_ENABLED)
     MBEDTLS_ECP_DP_SECP256R1,
@@ -10983,8 +10985,10 @@
 #endif
 
 #if defined(MBEDTLS_ECP_C)
+#if !defined(MBEDTLS_SSL_CONF_SINGLE_EC)
             conf->curve_list = ssl_preset_suiteb_curves;
 #endif
+#endif
             break;
 
         /*
@@ -11024,8 +11028,10 @@
 #endif
 
 #if defined(MBEDTLS_ECP_C)
+#if !defined(MBEDTLS_SSL_CONF_SINGLE_EC)
             conf->curve_list = mbedtls_ecp_grp_id_list();
 #endif
+#endif
 
 #if defined(MBEDTLS_DHM_C) && defined(MBEDTLS_SSL_CLI_C)
             conf->dhm_min_bitlen = 1024;
diff --git a/programs/ssl/query_config.c b/programs/ssl/query_config.c
index 574d4d7..b9271ea 100644
--- a/programs/ssl/query_config.c
+++ b/programs/ssl/query_config.c
@@ -2778,6 +2778,30 @@
     }
 #endif /* MBEDTLS_SSL_CONF_SINGLE_CIPHERSUITE */
 
+#if defined(MBEDTLS_SSL_CONF_SINGLE_EC)
+    if( strcmp( "MBEDTLS_SSL_CONF_SINGLE_EC", config ) == 0 )
+    {
+        MACRO_EXPANSION_TO_STR( MBEDTLS_SSL_CONF_SINGLE_EC );
+        return( 0 );
+    }
+#endif /* MBEDTLS_SSL_CONF_SINGLE_EC */
+
+#if defined(MBEDTLS_SSL_CONF_SINGLE_EC_TLS_ID)
+    if( strcmp( "MBEDTLS_SSL_CONF_SINGLE_EC_TLS_ID", config ) == 0 )
+    {
+        MACRO_EXPANSION_TO_STR( MBEDTLS_SSL_CONF_SINGLE_EC_TLS_ID );
+        return( 0 );
+    }
+#endif /* MBEDTLS_SSL_CONF_SINGLE_EC_TLS_ID */
+
+#if defined(MBEDTLS_SSL_CONF_SINGLE_EC_GRP_ID)
+    if( strcmp( "MBEDTLS_SSL_CONF_SINGLE_EC_GRP_ID", config ) == 0 )
+    {
+        MACRO_EXPANSION_TO_STR( MBEDTLS_SSL_CONF_SINGLE_EC_GRP_ID );
+        return( 0 );
+    }
+#endif /* MBEDTLS_SSL_CONF_SINGLE_EC_GRP_ID */
+
     /* If the symbol is not found, return an error */
     return( 1 );
 }
diff --git a/programs/ssl/ssl_client2.c b/programs/ssl/ssl_client2.c
index 205f27a..897c828 100644
--- a/programs/ssl/ssl_client2.c
+++ b/programs/ssl/ssl_client2.c
@@ -215,7 +215,7 @@
 #define USAGE_ALPN ""
 #endif /* MBEDTLS_SSL_ALPN */
 
-#if defined(MBEDTLS_ECP_C)
+#if defined(MBEDTLS_ECP_C) && !defined(MBEDTLS_SSL_CONF_SINGLE_EC)
 #define USAGE_CURVES \
     "    curves=a,b,c,d      default: \"default\" (library default)\n"  \
     "                        example: \"secp521r1,brainpoolP512r1\"\n"  \
@@ -764,7 +764,7 @@
 #if defined(MBEDTLS_SSL_ALPN)
     const char *alpn_list[ALPN_LIST_SIZE];
 #endif
-#if defined(MBEDTLS_ECP_C)
+#if defined(MBEDTLS_ECP_C) && !defined(MBEDTLS_SSL_CONF_SINGLE_EC)
     mbedtls_ecp_group_id curve_list[CURVE_LIST_SIZE];
     const mbedtls_ecp_curve_info *curve_cur;
 #endif
@@ -1102,8 +1102,10 @@
                 default: goto usage;
             }
         }
+#if !defined(MBEDTLS_SSL_CONF_SINGLE_EC)
         else if( strcmp( p, "curves" ) == 0 )
             opt.curves = q;
+#endif /* !MBEDTLS_SSL_CONF_SINGLE_EC */
         else if( strcmp( p, "etm" ) == 0 )
         {
             switch( atoi( q ) )
@@ -1404,7 +1406,7 @@
     }
 #endif /* MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED */
 
-#if defined(MBEDTLS_ECP_C)
+#if defined(MBEDTLS_ECP_C) && !defined(MBEDTLS_SSL_CONF_SINGLE_EC)
     if( opt.curves != NULL )
     {
         p = (char *) opt.curves;
@@ -1458,7 +1460,7 @@
             curve_list[i] = MBEDTLS_ECP_DP_NONE;
         }
     }
-#endif /* MBEDTLS_ECP_C */
+#endif /* MBEDTLS_ECP_C && !MBEDTLS_SSL_CONF_SINGLE_EC */
 
 #if defined(MBEDTLS_SSL_ALPN)
     if( opt.alpn_string != NULL )
@@ -1829,12 +1831,14 @@
 #endif
 
 #if defined(MBEDTLS_ECP_C)
+#if !defined(MBEDTLS_SSL_CONF_SINGLE_EC)
     if( opt.curves != NULL &&
         strcmp( opt.curves, "default" ) != 0 )
     {
         mbedtls_ssl_conf_curves( &conf, curve_list );
     }
-#endif
+#endif /* !MBEDTLS_SSL_CONF_SINGLE_EC */
+#endif /* MBEDTLS_ECP_C */
 
 #if defined(MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED)
     if( ( ret = mbedtls_ssl_conf_psk( &conf, psk, psk_len,
diff --git a/programs/ssl/ssl_server2.c b/programs/ssl/ssl_server2.c
index 160997a..cfb3d6a 100644
--- a/programs/ssl/ssl_server2.c
+++ b/programs/ssl/ssl_server2.c
@@ -381,7 +381,7 @@
 #define USAGE_ECJPAKE ""
 #endif
 
-#if defined(MBEDTLS_ECP_C)
+#if defined(MBEDTLS_ECP_C) && !defined(MBEDTLS_SSL_CONF_SINGLE_EC)
 #define USAGE_CURVES \
     "    curves=a,b,c,d      default: \"default\" (library default)\n"  \
     "                        example: \"secp521r1,brainpoolP512r1\"\n"  \
@@ -1412,7 +1412,7 @@
 #if defined(SNI_OPTION)
     sni_entry *sni_info = NULL;
 #endif
-#if defined(MBEDTLS_ECP_C)
+#if defined(MBEDTLS_ECP_C) && !defined(MBEDTLS_SSL_CONF_SINGLE_EC)
     mbedtls_ecp_group_id curve_list[CURVE_LIST_SIZE];
     const mbedtls_ecp_curve_info * curve_cur;
 #endif
@@ -1694,8 +1694,10 @@
             }
             opt.force_ciphersuite[1] = 0;
         }
+#if !defined(MBEDTLS_SSL_CONF_SINGLE_EC)
         else if( strcmp( p, "curves" ) == 0 )
             opt.curves = q;
+#endif /* !MBEDTLS_SSL_CONF_SINGLE_EC */
         else if( strcmp( p, "version_suites" ) == 0 )
             opt.version_suites = q;
         else if( strcmp( p, "renegotiation" ) == 0 )
@@ -2152,7 +2154,7 @@
     }
 #endif /* MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED */
 
-#if defined(MBEDTLS_ECP_C)
+#if defined(MBEDTLS_ECP_C) && !defined(MBEDTLS_SSL_CONF_SINGLE_EC)
     if( opt.curves != NULL )
     {
         p = (char *) opt.curves;
@@ -2206,7 +2208,7 @@
             curve_list[i] = MBEDTLS_ECP_DP_NONE;
         }
     }
-#endif /* MBEDTLS_ECP_C */
+#endif /* MBEDTLS_ECP_C && !MBEDTLS_SSL_CONF_SINGLE_EC */
 
 #if defined(MBEDTLS_SSL_ALPN)
     if( opt.alpn_string != NULL )
@@ -2824,12 +2826,14 @@
 #endif
 
 #if defined(MBEDTLS_ECP_C)
+#if !defined(MBEDTLS_SSL_CONF_SINGLE_EC)
     if( opt.curves != NULL &&
         strcmp( opt.curves, "default" ) != 0 )
     {
         mbedtls_ssl_conf_curves( &conf, curve_list );
     }
-#endif
+#endif /* !MBEDTLS_SSL_CONF_SINGLE_EC */
+#endif /* MBEDTLS_ECP_C */
 
 #if defined(MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED)
     if( strlen( opt.psk ) != 0 && strlen( opt.psk_identity ) != 0 )