Sep 04, 2015

Parsing OpenSSH Ed25519 keys for fun and profit

Adding key derivation to git-nerps from OpenSSH keys, needed to get the actual "secret" or something deterministically (plus in an obvious and stable way) derived from it (to then feed into some pbkdf2 and get the symmetric key).

Idea is for liteweight ad-hoc vms/containers to have a single "master secret", from which all others (e.g. one for git-nerps' encryption) can be easily derived or decrypted, and omnipresent, secure, useful and easy-to-generate ssh key in ~/.ssh/id_ed25519 seem to be the best candidate.

Unfortunately, standard set of ssh tools from openssh doesn't seem to have anything that can get key material or its hash - next best thing is to get "fingerprint" or such, but these are derived from public keys, so not what I wanted at all (as anyone can derive that, having public key, which isn't secret).

And I didn't want to hash full openssh key blob, because stuff there isn't guaranteed to stay the same when/if you encrypt/decrypt it or do whatever ssh-keygen does.

What definitely stays the same is the values that openssh plugs into crypto algos, so wrote a full parser for the key format (as specified in PROTOCOL.key file in openssh sources) to get that.

While doing so, stumbled upon fairly obvious and interesting application for such parser - to get really and short easy to backup, read or transcribe string which is the actual secret for Ed25519.

I.e. that's what OpenSSH private key looks like:

-----BEGIN OPENSSH PRIVATE KEY-----
b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAMwAAAAtzc2gtZW
QyNTUxOQAAACDaKUyc/3dnDL+FS4/32JFsF88oQoYb2lU0QYtLgOx+yAAAAJi1Bt0atQbd
GgAAAAtzc2gtZWQyNTUxOQAAACDaKUyc/3dnDL+FS4/32JFsF88oQoYb2lU0QYtLgOx+yA
AAAEAc5IRaYYm2Ss4E65MYY4VewwiwyqWdBNYAZxEhZe9GpNopTJz/d2cMv4VLj/fYkWwX
zyhChhvaVTRBi0uA7H7IAAAAE2ZyYWdnb2RAbWFsZWRpY3Rpb24BAg==
-----END OPENSSH PRIVATE KEY-----

And here's the only useful info in there, enough to restore whole blob above from, in the same base64 encoding:

HOSEWmGJtkrOBOuTGGOFXsMIsMqlnQTWAGcRIWXvRqQ=

Latter, of course, being way more suitable for tried-and-true "write on a sticker and glue at the desk" approach. Or one can just have a file with one host key per line - also cool.

That's the 32-byte "seed" value, which can be used to derive "ed25519_sk" field ("seed || pubkey") in that openssh blob, and all other fields are either "none", "ssh-ed25519", "magic numbers" baked into format or just padding.

So rolled the parser from git-nerps into its own tool - ssh-keyparse, which one can run and get that string above for key in ~/.ssh/id_ed25519, or do some simple crypto (as implemented by djb in ed25519.py, not me) to recover full key from the seed.

From output serialization formats that tool supports, especially liked the idea of Douglas Crockford's Base32 - human-readable one, where all confusing l-and-1 or O-and-0 chars are interchangeable, and there's an optional checksum (one letter) at the end:

% ssh-keyparse test-key --base32
3KJ8-8PK1-H6V4-NKG4-XE9H-GRW5-BV1G-HC6A-MPEG-9NG0-CW8J-2SFF-8TJ0-e

% ssh-keyparse test-key --base32-nodashes
3KJ88PK1H6V4NKG4XE9HGRW5BV1GHC6AMPEG9NG0CW8J2SFF8TJ0e

base64 (default) is still probably most efficient for non-binary (there's --raw otherwise) backup though.

[ssh-keyparse code link]

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