SCscintill76·Jul 25, 2015
Well, I don't know a lot of the math and crypto, but I would probably seed an RNG from the stretched key and then pull large primes out just as ssh-keygen would. You're right, though -- given my "requirements", it's not much more to assume a convenient place to store a normal, encrypted key. It can be public since I already based my security on the password strength and some obscurity. I wouldn't use either of these ideas for any serious production server.
For the deterministic key, you could "backdoor" ssh-keygen's RNG through something like LD_PRELOAD... at that point it's probably just piping a couple of pre-made shell utils together, which could be more portable and simple than rolling-your-own all the key export stuff.
Actually, this strikes me as potentially useful and secure in some niche cases where you don't trust your RNG and
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/or storage. Use diceware and write your long key down, derive anew for each use. Maybe it's silly, but I would consider something like that (at least the initial deriving part) if I were very paranoid or making a long-lived, deeply-deployed key.
Here's a deterministic Ed25519 SSH key generator that takes a 32-byte seed: https://github.com/mithrandi/ssh-key-generator