Use illegal_parameter instead of decode_error for invalid key shares

[justsmth]

Cherry-picked commit

• google/boringssl@72a6050

Original commit message

This is extremely silly and a huge waste of everyone's time (I deeply regret disambiguating illegal_parameter and decode_error for RFC 8446), but so it goes. Technically these errors pass the TLS syntax and are an invalid value, so they should be illegal_parameter.

Note: TLS defines two generic alerts (see Section 6) to use upon
failure to parse a message. Peers which receive a message which
cannot be parsed according to the syntax (e.g., have a length
extending beyond the message boundary or contain an out-of-range
length) MUST terminate the connection with a "decode_error" alert.
Peers which receive a message which is syntactically correct but
semantically invalid (e.g., a DHE share of p - 1, or an invalid enum)
MUST terminate the connection with an "illegal_parameter" alert.

Update-Note: The error sent on invalid key share is now more correct. This does not change which connections do or do not fail, only which of two practically identical alert codes is sent to the other side.

Change-Id: If0ddf511d6cf23383c6134ad30e3ae080c4f2769
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/71627
Auto-Submit: David Benjamin davidben@google.com
Reviewed-by: Bob Beck bbe@google.com
Commit-Queue: Bob Beck bbe@google.com

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.68%. Comparing base (54fc7b7) to head (a643953).

Files with missing lines	Patch %	Lines
ssl/ssl_key_share.cc	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1923      +/-   ##
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- Coverage   78.68%   78.68%   -0.01%     
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  Files         585      585              
  Lines      100836   100836              
  Branches    14293    14291       -2     
==========================================
- Hits        79344    79343       -1     
- Misses      20855    20857       +2     
+ Partials      637      636       -1     

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