[gnutls-devel] GnuTLS | gnutls-serv and gnutls-client fail with "Detected downgrade to TLS 1.2 from TLS 1.3" (#837)

[Development of GNU's TLS library]

Daiki Ueno commented:


I would say this is the correct behavior according to the [spec](https://tools.ietf.org/html/rfc8446#section-4.1.3):
```
   TLS 1.3 has a downgrade protection mechanism embedded in the server's
   random value.  TLS 1.3 servers which negotiate TLS 1.2 or below in
   response to a ClientHello MUST set the last 8 bytes of their Random
   value specially in their ServerHello.

   If negotiating TLS 1.2, TLS 1.3 servers MUST set the last 8 bytes of
   their Random value to the bytes:

     44 4F 57 4E 47 52 44 01

   [...]

   TLS 1.3 clients receiving a ServerHello indicating TLS 1.2 or below
   MUST check that the last 8 bytes are not equal to either of these
   values.  [...]
   If a match is found, the client MUST abort the handshake with
   an "illegal_parameter" alert.
```
If you enable TLS 1.3 in the priority string, the peer behaves as a TLS 1.3 server or client so the downgrade protection will be in effect.

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