[gnutls-devel] GnuTLS | psk_ke_modes_recv_params() wrongly sets HSK_PSK_KE_MODE_INVALID (#1303)

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Tim Kosse created an issue: https://gitlab.com/gnutls/gnutls/-/issues/1303



## Description of problem:

If the preferred side (as per session->internals.priorities->server_precedence) only supports one algorithm and it is not the first in the other side's list of algorithms, psk_ke_modes_recv_params wrongly sets session->internals.hsk_flags to HSK_PSK_KE_MODE_INVALID.

Observed in GnuTLS 3.7.2 compiled from the official source tarball without any special configure arguments.

The issue was discovered while analyzing https://forum.filezilla-project.org/viewtopic.php?t=54333

## How to reproduce:

You can easily reproduce this issue:
* Applying the attached [reproducer.diff](/uploads/e553e1da4aacce21eccca71f898fbe3f/reproducer.diff), which just swaps the algorithms in the psk_key_exchange_modes extension Client Hello. 
* Run `gnutls-serv -d9999`
* Connect to it with `gnutls-cli 127.0.0.1 -p 5556`
* In the output of gnutls-serv look for `|<3>| ASSERT: psk_ke_modes.c[psk_ke_modes_recv_params]:192` which is printed when HSK_PSK_KE_MODE_INVALID is set.

In this scenario the following happens in psk_ke_modes.c:
* Line 156 is reached.
* By line 174, the following values are held:
  * session->internals.priorities->server_precedence is false
  * dhpsk_pos is 0
  * psk_pos is MAX_POS
  * cli_dhpsk_pos is 1
  * cli_psk_pos is 0
* As result, neither mode is set in session->internals.hsk_flags and line 191 is reached.

## Proposed patch:

I have attached a simple fix for the issue in [psk_ke_modes_send_params.diff](/uploads/305bdcc93e5af97051116813e06dfbaa/psk_ke_modes_send_params.diff)

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