The default X.509 verification profile (mbedtls_x509_crt_profile_default) and the default curve and hash selection in TLS have changed. They are now aligned, except that the X.509 profile only lists curves that support signature verification.
Hashes and curves weaker than 255 bits (security strength less than 128 bits) are no longer accepted by default. The following hashes have been removed: SHA-1 (formerly only accepted for key exchanges but not for certificate signatures), SHA-224 (weaker hashes were already not accepted). The following curves have been removed: secp192r1, secp224r1, secp192k1, secp224k1.
The compile-time options MBEDTLS_TLS_DEFAULT_ALLOW_SHA1_IN_CERTIFICATES and MBEDTLS_TLS_DEFAULT_ALLOW_SHA1_IN_KEY_EXCHANGE are no longer available.
The curve secp256k1 has also been removed from the default X.509 and TLS profiles. RFC 8422 deprecates it in TLS, and it is very rarely used, although it is not known to be weak at the time of writing.
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 and hashes 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.
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 over larger curves.
If you prefer a different order, call mbedtls_ssl_conf_curves() when configuring a TLS connection.