[gnutls-devel] GnuTLS | support non-NULL-terminated PSKs (!917)

[Development of GNU's TLS library]

Nikos Mavrogiannopoulos started a new discussion on lib/handshake-checks.c: https://gitlab.com/gnutls/gnutls/-/merge_requests/917#note_306933300

>  			return gnutls_assert_val(GNUTLS_E_INTERNAL_ERROR);
>  
>  		if (session->internals.saved_username_set) {
> -			if (strcmp(session->internals.saved_username, username) != 0) {
> +			if (username_length == strlen(session->internals.saved_username) &&
> +			    strncmp(session->internals.saved_username, username, username_length) != 0) {
>  				_gnutls_debug_log("Session's PSK username changed during rehandshake; aborting!\n");
>  				return gnutls_assert_val(GNUTLS_E_SESSION_USER_ID_CHANGED);
>  			}
>  		} else {
> -			size_t len = strlen(username);
> -
> -			memcpy(session->internals.saved_username, username, len);
> -			session->internals.saved_username[len] = 0;
> +			memcpy(session->internals.saved_username, username, username_length);

I did an "enhancement" here to check the username length match before checking with `strncmp`, but I think it is wrong, because we cannot save the username in a null terminated way. We would have to save the username length as well. Maybe we can re-use `saved_username_set` to hold the length (assuming we only allow non-zero usernames). What do you think?

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