5.3.1 Trading Security for Interoperability

If you connect to a server and use GnuTLS' functions to verify the certificate chain, and get a GNUTLS_CERT_INSECURE_ALGORITHM validation error (see section Verifying X.509 Certificate Paths), it means that somewhere in the certificate chain there is a certificate signed using RSA-MD2 or RSA-MD5. These two digital signature algorithms are considered broken, so GnuTLS fail when attempting to verify the certificate. In some situations, it may be useful to be able to verify the certificate chain anyway, assuming an attacker did not utilize the fact that these signatures algorithms are broken. This section will give help on how to achieve that.

First, it is important to know that you do not have to enable any of the flags discussed here to be able to use trusted root CA certificates signed using RSA-MD2 or RSA-MD5. The only attack today is that it is possible to generate certificates with colliding signatures (collision resistance); you cannot generate a certificate that has the same signature as an already existing signature (2nd preimage resistance).

If you are using gnutls_certificate_verify_peers2 to verify the certificate chain, you can call gnutls_certificate_set_verify_flags with the GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD2 or GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5 flag, as in:

  gnutls_certificate_set_verify_flags (x509cred,
                                       GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5);

This will tell the verifier algorithm to enable RSA-MD5 when verifying the certificates.

If you are using gnutls_x509_crt_verify or gnutls_x509_crt_list_verify, you can pass the GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5 parameter directly in the flags parameter.

If you are using these flags, it may also be a good idea to warn the user when verification failure occur for this reason. The simplest is to not use the flags by default, and only fall back to using them after warning the user. If you wish to inspect the certificate chain yourself, you can use gnutls_certificate_get_peers to extract the raw server's certificate chain, then use gnutls_x509_crt_import to parse each of the certificates, and then use gnutls_x509_crt_get_signature_algorithm to find out the signing algorithm used for each certificate. If any of the intermediary certificates are using GNUTLS_SIGN_RSA_MD2 or GNUTLS_SIGN_RSA_MD5, you could present a warning.