ssh: implement strict KEX protocol changes

Implement the "strict KEX" protocol changes, as described in section
1.9 of the OpenSSH PROTOCOL file (as of OpenSSH version 9.6/9.6p1).

Namely this makes the following changes:
  * Both the server and the client add an additional algorithm to the
    initial KEXINIT message, indicating support for the strict KEX mode.
  * When one side of the connection sees the strict KEX extension
    algorithm, the strict KEX mode is enabled for messages originating
    from the other side of the connection. If the sequence number for
    the side which requested the extension is not 1 (indicating that it
    has already received non-KEXINIT packets), the connection is
  * When strict kex mode is enabled, unexpected messages during the
    handshake are considered fatal. Additionally when a key change
    occurs (on the receipt of the NEWKEYS message) the message sequence
    numbers are reset.

Thanks to Fabian Bäumer, Marcus Brinkmann, and Jörg Schwenk from Ruhr
University Bochum for reporting this issue.

Fixes CVE-2023-48795
Fixes golang/go#64784

Change-Id: I96b53afd2bd2fb94d2b6f2a46a5dacf325357604
Reviewed-by: Nicola Murino <>
Reviewed-by: Tatiana Bradley <>
TryBot-Result: Gopher Robot <>
Run-TryBot: Roland Shoemaker <>
Reviewed-by: Damien Neil <>
LUCI-TryBot-Result: Go LUCI <>
3 files changed
tree: c9b72524f94ca6c058ed9f39c72188f02ff08804
  1. acme/
  2. argon2/
  3. bcrypt/
  4. blake2b/
  5. blake2s/
  6. blowfish/
  7. bn256/
  8. cast5/
  9. chacha20/
  10. chacha20poly1305/
  11. cryptobyte/
  12. curve25519/
  13. ed25519/
  14. hkdf/
  15. internal/
  16. md4/
  17. nacl/
  18. ocsp/
  19. openpgp/
  20. otr/
  21. pbkdf2/
  22. pkcs12/
  23. poly1305/
  24. ripemd160/
  25. salsa20/
  26. scrypt/
  27. sha3/
  28. ssh/
  29. tea/
  30. twofish/
  31. x509roots/
  32. xtea/
  33. xts/
  34. .gitattributes
  35. .gitignore
  36. codereview.cfg
  38. go.mod
  39. go.sum

Go Cryptography

Go Reference

This repository holds supplementary Go cryptography libraries.


The easiest way to install is to run go get -u You can also manually git clone the repository to $GOPATH/src/

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