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Neil
1236ba11fb
Three ways a line could be trusted by us and invisible to the user's own ssh — or, worse, raise a CHANGED alarm from an entry ssh drops. Buffer.from does not fail on bad base64, it SKIPS invalid characters, so `<key>!!!` and a blob with `@@` spliced into it both decoded to the correct key and matched. Verified live against OpenSSH 10.2p1 on 127.0.0.1:2224: the unmodified control reached authentication and all three malformed variants produced "No ED25519 host key is known". A re-encode-and-compare makes us agree. `<key>AAAA` is the interesting one, and the reason the first fix was not enough: 68 characters plus 4 is still legal base64, and the algorithm header still reads ssh-ed25519, so neither the base64 check nor the existing header check sees anything wrong. ssh parses the whole key structure. We decoded 54 bytes where an ed25519 key is 51 and reported `mismatch` — a man-in-the-middle warning caused by a typo in a file ssh silently ignores. So the blob is now walked as what it is: a run of length-prefixed fields that must consume it exactly. Algorithm-agnostic on purpose, so a key type we do not model is checked as well as one we do. It also rejects a length prefix that overruns the buffer, which readHostKeyType only checked for the first field. And ssh's extract_salt demands exactly one SHA1 digest — "expected salt len 20, got 16" — where we accepted any non-empty salt. A short salt is still a usable HMAC key for us, so a hand-crafted line could match for us and be a parse error for ssh. ssh-keygen -H always writes 20 bytes, so refusing loses no real entry. Worth recording that ssh-keygen -F cannot answer any of this: it matches host names and prints lines without ever decoding the key, so it reports "found" for all four blobs. The real client was the only instrument that worked. Found by a parity review; the base64 finding as reported was right about the behaviour and wrong about the mechanism for the padded case, which is what led to the structural check. 1,533 SSH tests pass.