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I have setup an AlmaLinux9 server with SSHD. I have also generated a priv+pub key on my workstation, and when I check the algorithm used by my key it shows:

ssh-keygen -l -f myprivatekey.pem
2048 SHA256:p8qjW7xUG6CrB7I0edKIGoQarzHtTgqoHf7I32S7LcE testkey (RSA)

When I use this key to login from another AlmaLinux 9 workstation to the server, I can successfully login. When I try to use this same key from putty on a Windows client, the client says "Server refused our key", and the server secure log shows:

userauth_pubkey: key type ssh-rsa not in PubkeyAcceptedAlgorithms [preauth]

What exactly is using ssh-rsa algorithm? If my key algorithm is SHA256, which I think is rsa-sha2-256 (which is a modern and secure algorithm), why is the server complaining about "ssh-rsa". I assume putty is using a seperate algorithm for the session, and my putty settings show:

these ciphers

This post explains that putty uses a seperate algorithm for the session, but where can I see what algorithm it is using? I think my screenshot from putty settings > advanced > ssh settings > encryption shows acceptable algorithms, but I don't see reference to ssh-rsa. So why is putting using ssh-rsa?

In case it's relevant, I'm using "extraPUTTY v0.30"

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    This question is similar to: Understanding ssh-rsa not in PubkeyAcceptedAlgorithms. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem.
    – Ja1024
    Commented Sep 23 at 16:06
  • Question updated to focus on how to determine which algorithm putty is using
    – TSG
    Commented Sep 23 at 16:18
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    ExtraPuTTY 0.30 is over 9 years old, and it looks like the project has been abandoned entirely. You definitely need to update to a recent OpenSSH client.
    – Ja1024
    Commented Sep 23 at 16:34
  • Is there a way to see what encryption cipher ExtraPutty is using? I tried PuTTY 0.81 and it logs in fine, so there is some encryption algorithm being used by putty/extraputty, but the "Encryption Cipher Policy" on both looks identical. I can't figure out where I can find the encryption putty is using for the session
    – TSG
    Commented Sep 23 at 17:07
  • The encryption cypher policy is specifying the semitic cypher to use for encrypting the tunnel once established. This has no5hing to do with your issue
    – user1937198
    Commented Sep 23 at 19:12

1 Answer 1

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There's lots of good pieces of information in them comments that I think create a complete answer to my question (and more). I'll summarize them here so any other newbies can understand the key concepts:

  • The public/private keys are encrypted with an encryption algorithm (in my case SHA256 with 2048 bit key)
  • The ssh client program authenticates to the sshd server with a key exchange algorithm (should be rsa-sha2-256 or rsa-sha2-512, older ssh-rsa may be refused and that was my problem)
  • Once the ssh client program has authenticated, it encrypts it's communications with the sshd server using a cipher (as shown in the screenshot above), and this encrypted channel is the 'tunnel'

If I made a mistake/used the wrong term (cipher vs algorithm vs encryption) just comment and I'll revise. I think this answer will add some context for people still lost after reading this post

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    How the private key is encrypted locally and the key exchange algorithm don’t matter in this case. In the context of keys, the identifier ssh-rsa stands for a raw RSA key pair. In the OpenSSH protocol, the key is used to calculate signatures. This requires the public-key algorithm like RSA to be combined with a hash algorithm. In the past, OpenSSH supported the combination of RSA with the SHA-1 hash algorithm and called this ssh-rsa as well. Since this combination is now obsolete, you instead have to use SHA-256 (rsa-sha2-256) or SHA-512 (rsa-sha2-512) as the hash algorithm.
    – Ja1024
    Commented Sep 24 at 17:19
  • It’s very unfortunate that OpenSSH uses the same name ssh-rsa for two different things: RSA public keys (which are perfectly fine) and RSA signatures based on SHA-1 (which is insecure and now obsolete). The latter should probably have been named rsa-sha1 instead to explicitly name the hash algorithm and make the identifier future-proof, but the OpenSSH designers/developers didn’t make that decision in the past.
    – Ja1024
    Commented Sep 24 at 17:20

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