Keys & ssh-keygen compatibility

Whisper’s identity is an ordinary OpenSSH key pair. The public key you share in Whisper is the same line you would put in authorized_keys.

Supported

Every key type ssh-keygen can produce whose security is still sound:

ssh-ed25519 — the default and the best choice: small keys, short messages and the fastest operations.
ecdsa-sha2-nistp256, -nistp384, -nistp521
ssh-rsa, 2048 bits or more — works, but makes messages about 1 KB longer, which matters for links sent by SMS.

Import

Settings → My key → Import from ssh-keygen accepts OpenSSH private keys, unencrypted or passphrase-protected, using the bcrypt KDF with any of:

aes128-ctr · aes192-ctr · aes256-ctr · aes128-cbc · aes192-cbc · aes256-cbc · aes128-gcm@openssh.com · aes256-gcm@openssh.com · chacha20-poly1305@openssh.com

PEM-format keys are rejected with the command to convert them: ssh-keygen -p -f <keyfile>.

Export

Export backup writes your key as an OpenSSH private key — encrypted with aes256-ctr if you set a passphrase — which ssh and ssh-keygen accept. The test suite imports real ssh-keygen keys of every type and cipher, compares fingerprints with ssh-keygen -l, and checks that exported keys are accepted by ssh-keygen.

Not supported, on purpose

Key	Why not
`sk-ssh-ed25519@openssh.com`, `sk-ecdsa-…`	The private key never leaves the FIDO security key, and those devices can sign but cannot decrypt.
RSA under 2048 bits, DSA	No longer considered safe (OpenSSH itself disabled DSA).
3DES-protected key files	Re-encrypt with `ssh-keygen -p -Z aes256-ctr -f <keyfile>`.

Fingerprints

Contacts are identified by their SHA256: fingerprint, exactly as ssh-keygen -l prints it, and you can search the address book by fingerprint.