| Solution |
On FortiGate running firmware v7.0.12 or firmware v7.2.5, when attempting to perform SSH from an SSH tool to FortiGate firmware v7.0.12 or v7.2.5, FortiGate offers keys ssh-rsa and ssh-ed25519 as the server host keys algorithms.
ssh-rsa will no longer be offered as the server key host algorithm after upgrading to FortiOS v7.0.13 or FortiOS 7.2.6 and only offer the ssh-ed25519 algorithm.
The same key host algorithm offering can be verified in the SSH debugs:
2023-10-18 10:52:20 SSH: Proposal: 0, Ciphers: 'diffie-hellman-group-exchange-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521'
2023-10-18 10:52:20 SSH: Proposal: 2, Ciphers: 'chacha20-poly1305@openssh.com,aes256-ctr,aes256-gcm@openssh.com'
2023-10-18 10:52:20 SSH: Proposal: 3, Ciphers: 'chacha20-poly1305@openssh.com,aes256-ctr,aes256-gcm@openssh.com'
2023-10-18 10:52:20 SSH: Proposal: 4, Ciphers: 'hmac-sha2-256,hmac-sha2-256-etm@openssh.com,hmac-sha2-512,hmac-sha2-512-etm@openssh.com'
2023-10-18 10:52:20 SSH: Proposal: 5, Ciphers: 'hmac-sha2-256,hmac-sha2-256-etm@openssh.com,hmac-sha2-512,hmac-sha2-512-etm@openssh.com'
2023-10-18 10:52:20 SSH: list_hostkey_types: ssh-rsa,ssh-ed25519
2023-10-18 10:52:20 SSH: SSH2_MSG_KEXINIT sent
2023-10-18 10:52:20 SSH: SSH2_MSG_KEXINIT received
2023-10-18 10:52:20 SSH: kex_parse_kexinit: diffie-hellman-group-exchange-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521
2023-10-18 10:52:20 SSH: kex_parse_kexinit: ssh-rsa,ssh-ed25519
2023-10-18 10:52:20 SSH: kex_parse_kexinit: chacha20-poly1305@openssh.com,aes256-ctr,aes256-gcm@openssh.com
2023-10-18 10:52:20 SSH: kex_parse_kexinit: chacha20-poly1305@openssh.com,aes256-ctr,aes256-gcm@openssh.com
2023-10-18 10:52:20 SSH: kex_parse_kexinit: hmac-sha2-256,hmac-sha2-256-etm@openssh.com,hmac-sha2-512,hmac-sha2-512-etm@openssh.com
2023-10-18 10:52:20 SSH: kex_parse_kexinit: hmac-sha2-256,hmac-sha2-256-etm@openssh.com,hmac-sha2-512,hmac-sha2-512-etm@openssh.com
FortiGate versions 7.0.13 and 7.2.6 offer only the ssh-ed25519 host key algorithm.
In the SSH debugs, the key host algorithm offering can be checked:
2023-10-18 10:07:52 SSH: Proposal: 0, Ciphers: 'diffie-hellman-group-exchange-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp
521'
2023-10-18 10:07:52 SSH: Proposal: 2, Ciphers: 'chacha20-poly1305@openssh.com,aes256-ctr,aes256-gcm@openssh.com'
2023-10-18 10:07:52 SSH: Proposal: 3, Ciphers: 'chacha20-poly1305@openssh.com,aes256-ctr,aes256-gcm@openssh.com'
2023-10-18 10:07:52 SSH: Proposal: 4, Ciphers: 'hmac-sha2-256,hmac-sha2-256-etm@openssh.com,hmac-sha2-512,hmac-sha2-512-etm@openssh.com'
2023-10-18 10:07:52 SSH: Proposal: 5, Ciphers: 'hmac-sha2-256,hmac-sha2-256-etm@openssh.com,hmac-sha2-512,hmac-sha2-512-etm@openssh.com'
2023-10-18 10:07:52 SSH: list_hostkey_types: ssh-ed25519
2023-10-18 10:07:52 SSH: SSH2_MSG_KEXINIT sent
2023-10-18 10:07:52 SSH: SSH2_MSG_KEXINIT received
2023-10-18 10:07:52 SSH: kex_parse_kexinit: diffie-hellman-group-exchange-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521
2023-10-18 10:07:52 SSH: kex_parse_kexinit: ssh-ed25519
2023-10-18 10:07:52 SSH: kex_parse_kexinit: chacha20-poly1305@openssh.com,aes256-ctr,aes256-gcm@openssh.com
2023-10-18 10:07:52 SSH: kex_parse_kexinit: chacha20-poly1305@openssh.com,aes256-ctr,aes256-gcm@openssh.com
2023-10-18 10:07:52 SSH: kex_parse_kexinit: hmac-sha2-256,hmac-sha2-256-etm@openssh.com,hmac-sha2-512,hmac-sha2-512-etm@openssh.com
2023-10-18 10:07:52 SSH: kex_parse_kexinit: hmac-sha2-256,hmac-sha2-256-etm@openssh.com,hmac-sha2-512,hmac-sha2-512-etm@openssh.com
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