Add config.h option MBEDTLS_ECP_NO_INTERNAL_RNG

No effect so far, except on dependency checking, as the feature it's meant to
disable isn't implemented yet (so the descriptions in config.h and the
ChangeLog entry are anticipation for now).

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

include/mbedtls/check_config.h

@@ -123,6 +123,14 @@
 #error "MBEDTLS_ECP_C defined, but not all prerequisites"
 #endif
 
+#if defined(MBEDTLS_ECP_C) && !(            \
+    defined(MBEDTLS_ECP_ALT) ||             \
+    defined(MBEDTLS_CTR_DRBG_C) ||          \
+    defined(MBEDTLS_HMAC_DRBG_C) ||         \
+    defined(MBEDTLS_ECP_NO_INTERNAL_RNG))
+#error "MBEDTLS_ECP_C requires a DRBG module unless MBEDTLS_ECP_NO_INTERNAL_RNG is defined or an alternative implementation is used"
+#endif
+
 #if defined(MBEDTLS_PK_PARSE_C) && !defined(MBEDTLS_ASN1_PARSE_C)
 #error "MBEDTLS_PK_PARSE_C defined, but not all prerequesites"
 #endif

include/mbedtls/config.h

@@ -618,6 +618,28 @@
  */
 #define MBEDTLS_ECP_NIST_OPTIM
 
+/**
+ * \def MBEDTLS_ECP_NO_INTERNAL_RNG
+ *
+ * When this option is disabled, mbedtls_ecp_mul() will make use of an
+ * internal RNG when called with a NULL \c f_rng argument, in order to protect
+ * against some side-channel attacks.
+ *
+ * This protection introduces a dependency of the ECP module on one of the
+ * DRBG modules. For very constrained implementations that don't require this
+ * protection (for example, because you're only doing signature verification,
+ * so not manipulating any secret, or because local/physical side-channel
+ * attacks are outside your threat model), it might be desirable to get rid of
+ * that dependency.
+ *
+ * \warning Enabling this option makes some uses of ECP vulnerable to some
+ * side-channel attacks. Only enable it if you know that's not a problem for
+ * your use case.
+ *
+ * Uncomment this macro to disable some counter-measures in ECP.
+ */
+//#define MBEDTLS_ECP_NO_INTERNAL_RNG
+
 /**
  * \def MBEDTLS_ECDSA_DETERMINISTIC
  *