[gnutls-devel] GnuTLS | Disable TLS 1.3 dynamically during handshake if bad KX is enabed in priority (#825)

[Development of GNU's TLS library]

Nikos Mavrogiannopoulos commented:


> Now assume misconfigured client which sends TLS 1.3 + TLS 1.2 + GOST ciphersuite. Should they be able to interoperate?

Let's write the use cases we are interested to handle. As you write above we have:
 1. client which sends TLS 1.3 + TLS 1.2 + GOST ciphersuite
 2. client which sets up `-VER-TLS-ALL:+VER-TLS1.3:+KX-ALL`

For (1) the current behavior would be fine, as it will disable TLS1.3 and use GOST. For (2) all is ok if GOST is not in the `KX-ALL` (current situation). If at some point GOST gets into the `NORMAL` priority then effectively `-VER-TLS-ALL:+VER-TLS1.3:+KX-ALL` will be synonymous to NONE.

So as it is now, to my understanding, the problem with our handling of priority strings is that TLS-1.2 only key exchange (or even ciphers?) cannot be added in the default set. Do we agree on the problem?

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