I'm trying to connect to a new system with PuTTY and am seeing the following:
Fatal error: Couldn't agree on host key algorithm (available: rsa-sha2-512,rsa-sha2-256,ecdsa-sha2-nistp256)
Checking available algos on the server:
acoder@client ~ $ nmap --script ssh2-enum-algos -sV -p 22 server.name.here Starting Nmap 7.01 ( ) at 2021-04-16 10:33 EDT Nmap scan report for server.name.here (1.1.1.1) Host is up (0.018s latency). PORT STATE SERVICE VERSION 22/tcp open ssh OpenSSH 8.0 (protocol 2.0) | ssh2-enum-algos: | kex_algorithms: (7) | ecdh-sha2-nistp256 | ecdh-sha2-nistp384 | ecdh-sha2-nistp521 | diffie-hellman-group-exchange-sha256 | diffie-hellman-group14-sha256 | diffie-hellman-group16-sha512 | diffie-hellman-group18-sha512 | server_host_key_algorithms: (3) | rsa-sha2-512 | rsa-sha2-256 | ecdsa-sha2-nistp256 | encryption_algorithms: (6) | | aes256-ctr | aes256-cbc | | aes128-ctr | aes128-cbc | mac_algorithms: (6) | | | | hmac-sha2-256 | hmac-sha1 | hmac-sha2-512 | compression_algorithms: (2) | none |_ Here's what's available on my client machine:
PubkeyAcceptedKeyTypes
acoder@client ~ $ ssh -Q key ssh-ed25519 ssh-rsa ssh-dss ecdsa-sha2-nistp256 ecdsa-sha2-nistp384 ecdsa-sha2-nistp521 Ciphers
acoder@client ~ $ ssh -Q cipher 3des-cbc blowfish-cbc cast128-cbc arcfour arcfour128 arcfour256 aes128-cbc aes192-cbc aes256-cbc aes128-ctr aes192-ctr aes256-ctr MACs
acoder@client ~ $ ssh -Q mac hmac-sha1 hmac-sha1-96 hmac-sha2-256 hmac-sha2-512 hmac-md5 hmac-md5-96 hmac-ripemd160 KexAlgorithms
acoder@client ~ $ ssh -Q kex diffie-hellman-group1-sha1 diffie-hellman-group14-sha1 diffie-hellman-group-exchange-sha1 diffie-hellman-group-exchange-sha256 ecdh-sha2-nistp256 ecdh-sha2-nistp384 ecdh-sha2-nistp521 What am I overlooking here?
01 Answer
You're overlooking that PuTTY and ssh (OpenSSH) are two completely separate programs. They don't share any code; they don't use any common crypto library. The ssh -Q lists don't tell you anything about what PuTTY can support.
I think the problem is that your PuTTY version is too old. Out of the signature methods offered by the server, ECDSA signatures (ecdsa-sha2-nistp256) need at least PuTTY version 0.68, while the RSA-SHA2 methods (rsa-sha2-*) will only be available in the next PuTTY release (0.75).
The cause of this is that OpenSSH servers have disabled support for the old SHA1-based ssh-rsa signature algorithm very recently (they still use the same RSA keys, but only through SHA2-based signatures), while support for DSA keys has been removed several years ago.
