ssh keys as seed entropy for key derivation

[hitchho]

ssh keys today are used directly as signing/encryption keys. this works but limits what you can derive from them.

proposal: use the ssh key as entropy input to a domain-separated kdf, then derive purpose-specific keys from that root. the ssh key becomes a seed, not a leaf.

root = hkdf(ssh_private_key_bytes, "zcli-v1")
  ├── signing   = hkdf(root, "ed25519-signing")
  ├── license   = hkdf(root, "zpro-license")
  ├── ring-vrf  = hkdf(root, "bandersnatch-ring")
  ├── encryption = hkdf(root, "x25519-encryption")
  └── ...future purposes without touching the ssh key again

backwards compat: if the derived signing key doesn't match existing on-chain state, fall back to direct ssh key usage. new users get the kdf path by default.

why:

• one ssh key -> unlimited purpose-specific keys
• domain separation prevents cross-protocol attacks
• ring vrf needs bandersnatch keys, not ed25519 - kdf handles this naturally
• license identity needs a stable non-rotating key - another derivation path
• no new secrets to manage. ssh-agent already protects the root.