b1940a76ad
TLS used to prefer larger curves, under the idea that a larger curve has a higher security strength and is therefore harder to attack. However, brute force attacks are not a practical concern, so this was not particularly meaningful. If a curve is considered secure enough to be allowed, then we might as well use it. So order curves by resource usage. The exact definition of what this means is purposefully left open. It may include criteria such as performance and memory usage. Risk of side channels could be a factor as well, although it didn't affect the current choice. The current list happens to exactly correspond to the numbers reported by one run of the benchmark program for "full handshake/s" on my machine. Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
1.8 KiB
Weak curves are now disabled by default for X.509 and TLS
The default X.509 verification profile (mbedtls_x509_crt_profile_default) and the default curve and hash selection have changed. X.509 and TLS now allow the same algorithms by default (except that the X.509 profile only lists curves that support signature verification).
Hashes and curves weaker than 255 bits are no longer accepted by default. The following algorithms have been removed: SHA-1 (formerly only accepted for key exchanges but not for certificate signatures), SHA-224, secp192r1, secp224r1, secp192k1, secp224k1 (weaker hashes were already not accepted).
The compile-time option MBEDTLS_TLS_DEFAULT_ALLOW_SHA1_IN_KEY_EXCHANGE is no longer available.
If you still need to accept certificates signed with algorithms that have been removed from the default profile, call mbedtls_x509_crt_verify_with_profile instead of mbedtls_x509_crt_verify and pass a profile that allows the curves you want. For example, to allow SHA-224:
mbedtls_x509_crt_profile my_profile = mbedtls_x509_crt_profile_default;
my_profile.allowed_mds |= MBEDTLS_X509_ID_FLAG( MBEDTLS_MD_SHA224 );
If you still need to allow hashes and curves in TLS that have been removed from the default configuration, call mbedtls_ssl_conf_sig_hashes() and mbedtls_ssl_conf_curves() with the desired lists.
TLS now favors faster curves over larger curves
The default preference order for curves in TLS now favors resource usage (performance and memory consumption) over size. The exact order is unspecified and may change, but generally you can expect 256-bit curves to be preferred.
If you prefer a different order, call mbedtls_ssl_conf_curves() when configuring a TLS connection.