Merge pull request #4621 from gilles-peskine-arm/default-hashes-curves-2.x

Backport 2.x: Curve and hash selection for X.509 and TLS

ChangeLog.d/ecp-window-size.txt

@@ -0,0 +1,3 @@
+Changes
+   * Reduce the default value of MBEDTLS_ECP_WINDOW_SIZE. This reduces RAM usage
+     during ECC operations at a negligible performance cost.

include/mbedtls/config.h

@@ -3617,7 +3617,7 @@
 
 /* ECP options */
 //#define MBEDTLS_ECP_MAX_BITS             521 /**< Maximum bit size of groups */
-//#define MBEDTLS_ECP_WINDOW_SIZE            6 /**< Maximum window size used */
+//#define MBEDTLS_ECP_WINDOW_SIZE            4 /**< Maximum window size used */
 //#define MBEDTLS_ECP_FIXED_POINT_OPTIM      1 /**< Enable fixed-point speed-up */
 
 /* Entropy options */

include/mbedtls/ecp.h

@@ -258,7 +258,8 @@ mbedtls_ecp_group;
 #if !defined(MBEDTLS_ECP_WINDOW_SIZE)
 /*
  * Maximum "window" size used for point multiplication.
- * Default: 6.
+ * Default: a point where higher memory usage yields disminishing performance
+ *          returns.
  * Minimum value: 2. Maximum value: 7.
  *
  * Result is an array of at most ( 1 << ( MBEDTLS_ECP_WINDOW_SIZE - 1 ) )
@@ -275,7 +276,7 @@ mbedtls_ecp_group;
  *      224       475     475     453     398     342
  *      192       640     640     633     587     476
  */
-#define MBEDTLS_ECP_WINDOW_SIZE    6   /**< The maximum window size used. */
+#define MBEDTLS_ECP_WINDOW_SIZE    4   /**< The maximum window size used. */
 #endif /* MBEDTLS_ECP_WINDOW_SIZE */
 
 #if !defined(MBEDTLS_ECP_FIXED_POINT_OPTIM)
@@ -466,8 +467,7 @@ mbedtls_ecp_curve_type mbedtls_ecp_get_type( const mbedtls_ecp_group *grp );
 
 /**
  * \brief           This function retrieves the information defined in
- *                  mbedtls_ecp_curve_info() for all supported curves in order
- *                  of preference.
+ *                  mbedtls_ecp_curve_info() for all supported curves.
  *
  * \note            This function returns information about all curves
  *                  supported by the library. Some curves may not be

include/mbedtls/ssl.h

@@ -2987,7 +2987,9 @@ 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,
+ *                 except that Montgomery curves come last. This order
+ *                 is likely to change in a future version.)
  *
  *                 On server: this only affects selection of the ECDHE curve;
  *                 the curves used for ECDH and ECDSA are determined by the
@@ -3019,7 +3021,9 @@ 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

@@ -263,12 +263,21 @@ 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

@@ -91,9 +91,8 @@ 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)