From my naive understanding of ssh, the server shares its public key ( /etc/shh/ssh_host_rsa_key.pub ) with the client, who then adds it to the list of known hosts ( /user/.ssh/known_hosts ). So, I would expect if I look in those two files to see a long identical string in both (the "key"). When I compare the text in these two files I can see a string in both which is the same at the beginning but then becomes different. Why can't I see the same long string in both files (which I would assume is the key)?

  • Maybe the client only stores a hash where the server obviously would need the full key. Aug 13 '16 at 4:25
  • An OpenSSH server normally has 3-4 keys, of different types to support possibly varying clients, but a client normally gets and stores in known_hosts only one of them -- make sure you are comparing the host key for the type stated in the client file. PS: it's not shh, although I do sometimes wish it were quieter. Mar 27 at 22:19

For privacy.

With modern versions of OpenSSH, the known_hosts file on the client contains a hash of the server name (or IP address), rather than the name directly. The reason for only storing a hash is that if someone obtains a copy of this file (e.g. leaked backup), they can't discover which servers are recorded, i.e. they can't discover which sites you connected to. They can verify guesses, of course: that's unavoidable since the legitimate client must be able to do this.

The ssh-keygen utility has a few options to manipulate the known_hosts entries, but of course can't convert a hash to the non-hash format. You can set HashKnownHosts no in your client configuration (~/.ssh/config) to turn off hashing, then the entries will be in a format where the public key is clearly visible.

  • I've never seen a version of OpenSSH that hashes the keyblobs, only the servernames -- and that not by default (at least in upstream, an installation or distro might change the default). Mar 27 at 22:17

For me, it contains the same string. the server has: ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBLmw2JjbKMO5LXTcJ67et6TBZeLff1WghM6koKjiHGh+gBbZzHrhDj20MuTxTB1kaTYh7f9T2G/zmhVpFMyUUoQ=

and the client has

|1|some_base64|more_base64 ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBLmw2JjbKMO5LXTcJ67et6TBZeLff1WghM6koKjiHGh+gBbZzHrhDj20MuTxTB1kaTYh7f9T2G/zmhVpFMyUUoQ=.

man sshd says, in part:


Each line in these files contains the following fields: markers (optional), hostnames, bits, exponent, modulus, comment. The fields are separated by spaces.

hostnames may be stored in a hashed form which hides host names and addresses should the file's contents be disclosed. Hashed hostnames start with a ‘|’ character.

Bits, exponent, and modulus are taken directly from the RSA host key; they can be obtained, for example, from /etc/ssh/ssh_host_key.pub. The optional comment field continues to the end of the line, and is not used.

  • 1
    For me they are not the same, only the first 19 characters are identical (of the string starting with "AAAAE2....").
    – kotozna
    Jun 19 '15 at 17:37
  • 3
    can you decode the base64 and parse them according to this guide to see the difference? security.stackexchange.com/questions/42268/…
    – Z.T.
    Jun 19 '15 at 18:20
  • FYI: your key is elliptic curve, but you quoted a section of the man page about rsa. Is there a more applicable section to quote? Apr 15 '16 at 3:12
  • 1
    @MikeOunsworth actually that section of the manpage was and still is out of date. For SSH version 1, which is obsolete and broken and RSA only, .pub files and known_hosts and authorized_keys (all) used three decimal numbers. For v2 all use a string identifying the keytype/algorithm and a base64 blob, for RSA DSA several variants of ECDSA and ED255519 (although OpenSSH since 7.0 deprecates DSA). The authorized_keys format section of the manpage describes both v1 and v2 formats, but the known_hosts section doesn't. Jun 14 '16 at 4:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.