Merge pull request #4622 from gilles-peskine-arm/default-hashes-curves-2.16

Backport 2.16: Curve and hash selection for X.509 and TLS

include/mbedtls/ssl.h

@@ -2237,7 +2237,7 @@ void mbedtls_ssl_conf_dhm_min_bitlen( mbedtls_ssl_config *conf,
 #if defined(MBEDTLS_ECP_C)
 /**
  * \brief          Set the allowed curves in order of preference.
- *                 (Default: all defined curves.)
+ *                 (Default: all defined curves in order of decreasing size.)
  *
  *                 On server: this only affects selection of the ECDHE curve;
  *                 the curves used for ECDH and ECDSA are determined by the
@@ -2269,7 +2269,9 @@ void mbedtls_ssl_conf_curves( mbedtls_ssl_config *conf,
 #if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED)
 /**
  * \brief          Set the allowed hashes for signatures during the handshake.
- *                 (Default: all available hashes except MD5.)
+ *                 (Default: all SHA-2 hashes, largest first. Also SHA-1 if
+ *                 the compile-time option
+ *                 `MBEDTLS_TLS_DEFAULT_ALLOW_SHA1_IN_KEY_EXCHANGE` is enabled.)
  *
  * \note           This only affects which hashes are offered and can be used
  *                 for signatures during the handshake. Hashes for message

include/mbedtls/x509_crt.h

@@ -229,12 +229,21 @@ typedef void mbedtls_x509_crt_restart_ctx;
 /**
  * Default security profile. Should provide a good balance between security
  * and compatibility with current deployments.
+ *
+ * This profile permits:
+ * - SHA2 hashes.
+ * - All supported elliptic curves.
+ * - RSA with 2048 bits and above.
+ *
+ * New minor versions of Mbed TLS may extend this profile, for example if
+ * new curves are added to the library. New minor versions of Mbed TLS will
+ * not reduce this profile unless serious security concerns require it.
  */
 extern const mbedtls_x509_crt_profile mbedtls_x509_crt_profile_default;
 
 /**
  * Expected next default profile. Recommended for new deployments.
- * Currently targets a 128-bit security level, except for RSA-2048.
+ * Currently targets a 128-bit security level, except for allowing RSA-2048.
  */
 extern const mbedtls_x509_crt_profile mbedtls_x509_crt_profile_next;
 

library/x509_crt.c

@@ -116,9 +116,8 @@ typedef struct {
  */
 #define X509_MAX_VERIFY_CHAIN_SIZE    ( MBEDTLS_X509_MAX_INTERMEDIATE_CA + 2 )
 
-/*
- * Default profile
- */
+/* Default profile. Do not remove items unless there are serious security
+ * concerns. */
 const mbedtls_x509_crt_profile mbedtls_x509_crt_profile_default =
 {
 #if defined(MBEDTLS_TLS_DEFAULT_ALLOW_SHA1_IN_CERTIFICATES)