🔨 Restore curve25519-sha256 Key Exchange/remove SSH-Audit references in documentation

[sinclairpaul]

Proposed Changes

The current Dev version of SSH-Audit warns about the curve25519-sha256 and curve25519-sha256@libssh.org keys with the statement:

[warn] does not provide protection against post-quantum attacks

These have then been removed from the configuration as the documentation states that the SSH configuration meets all SSH-Audit warnings.

Unfortunately this renders a number of SSH Clients unusable, leading to Compatibility Mode being used.

As these warnings are referencing advances in Quantum Computing and future breakthroughs in decryption https://en.wikipedia.org/wiki/Harvest_now,_decrypt_later, it would seem these would be excessive for the addon in it's current setting.

To avoid confusion also removed the statements around meeting all SSH-Audit warnings.

Related Issues

Fixes #904

Summary by CodeRabbit

• Documentation

  • Removed references to passing all ssh-audit checks and the related image from the documentation.
  • Deleted links and visual evidence associated with ssh-audit results.

• Refactor

  • Updated SSH configuration by removing curve25519 key exchange algorithms from the list of supported algorithms.

[coderabbitai]

Walkthrough

The changes remove references in documentation to passing all ssh-audit checks and delete the associated ssh-audit result image. In the SSH server configuration, the curve25519 key exchange algorithms are removed from the KexAlgorithms setting, leaving other algorithms unchanged.

Changes

File(s)	Change Summary
ssh/.README.j2, ssh/DOCS.md	Removed documentation statements and image claiming ssh-audit compliance; deleted related references.
ssh/rootfs/etc/ssh/sshd_config	Removed curve25519-sha256 and curve25519-sha256@libssh.org from KexAlgorithms.

Sequence Diagram(s)

sequenceDiagram
    participant User
    participant SSH_Client
    participant SSH_Server

    User->>SSH_Client: Initiate SSH connection
    SSH_Client->>SSH_Server: Propose supported KexAlgorithms
    SSH_Server->>SSH_Client: Responds with allowed KexAlgorithms (without curve25519 variants)
    SSH_Client->>SSH_Server: Proceed with mutually supported algorithm
    SSH_Server-->>User: Connection established (if compatible)

Loading

Assessment against linked issues

Objective	Addressed	Explanation
Relax KexAlgorithms to not be overly restrictive for Windows SSH clients (#904)	✅
Remove documentation and claims about ssh-audit compliance (#904)	✅

Possibly related PRs

• Fix ssh-audit hardenings #897: Also modifies the KexAlgorithms line, directly related to the changes in this PR.

Suggested labels

documentation, enhancement

Suggested reviewers

• frenck

Poem

A whisk of code, a gentle tweak,
Curve25519 takes a seat,
Docs now hum a softer tune,
No audit badge, but none too soon.
SSH doors swing open wide,
For Windows friends to step inside!
🐇✨

---

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[coderabbitai]

Actionable comments posted: 1

📜 Review details

Configuration used: CodeRabbit UI
Review profile: CHILL
Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between a12c089 and 0535d52.

⛔ Files ignored due to path filters (1)

• images/ssh-audit.png is excluded by !**/*.png

📒 Files selected for processing (3)

• ssh/.README.j2 (0 hunks)
• ssh/DOCS.md (0 hunks)
• ssh/rootfs/etc/ssh/sshd_config (1 hunks)

💤 Files with no reviewable changes (2)

• ssh/.README.j2
• ssh/DOCS.md

⏰ Context from checks skipped due to timeout of 90000ms (1)

• GitHub Check: workflows / Verify

🔇 Additional comments (1)

ssh/rootfs/etc/ssh/sshd_config (1)

26-26: Curve25519 KEX algorithms are still absent – contradicts PR goal
The PR description says we are restoring curve25519-sha256 (and its libssh.org variant) for client compatibility, yet the KexAlgorithms line still omits them. Users relying on these widely-supported curves will continue to fall back to compatibility mode.

-KexAlgorithms -diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521
+KexAlgorithms -diffie-hellman-group-exchange-sha256,curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521

Please update the line (or remove the explicit list entirely and rely on the OpenSSH default) so the code matches the stated objective.

Likely an incorrect or invalid review comment.

[sinclairpaul]

Output from SSH-Audit

# key exchange algorithms
(kex) mlkem768x25519-sha256               -- [info] available since OpenSSH 9.9
                                          `- [info] default key exchange since OpenSSH 10.0
                                          `- [info] hybrid key exchange based on post-quantum resistant algorithm and proven conventional X25519 algorithm
(kex) sntrup761x25519-sha512              -- [info] available since OpenSSH 9.9
                                          `- [info] default key exchange in OpenSSH 9.9
                                          `- [info] hybrid key exchange based on post-quantum resistant algorithm and proven conventional X25519 algorithm
(kex) sntrup761x25519-sha512@openssh.com  -- [info] available since OpenSSH 8.5
                                          `- [info] default key exchange from OpenSSH 9.0 to 9.8
                                          `- [info] hybrid key exchange based on post-quantum resistant algorithm and proven conventional X25519 algorithm
(kex) curve25519-sha256                   -- [warn] does not provide protection against post-quantum attacks
                                          `- [info] available since OpenSSH 7.4, Dropbear SSH 2018.76
                                          `- [info] default key exchange from OpenSSH 7.4 to 8.9
(kex) curve25519-sha256@libssh.org        -- [warn] does not provide protection against post-quantum attacks
                                          `- [info] available since OpenSSH 6.4, Dropbear SSH 2013.62
                                          `- [info] default key exchange from OpenSSH 6.5 to 7.3
(kex) ext-info-s                          -- [info] available since OpenSSH 9.6
                                          `- [info] pseudo-algorithm that denotes the peer supports RFC8308 extensions
(kex) kex-strict-s-v00@openssh.com        -- [info] pseudo-algorithm that denotes the peer supports a stricter key exchange method as a counter-measure to the Terrapin attack (CVE-2023-48795)

[frenck]

Thanks, @sinclairpaul 👍

../Frenck

[VlastiBroucek]

Shouldn't be closed until the update docker is available for install... Which is not!

[sinclairpaul]

When it is released, if you want to run the edge version you can.