Encode public/private key in OpenSSH format

[ggrandes]

How encode EdDSAPublicKey / EdDSAPrivateKey for later usage in OpenSSH?

Acording to draft-bjh21-ssh-ed25519-00 & draft-josefsson-eddsa-ed25519-03. I am trying to extract the values for encoding... but i don't known where to get these.

[str4d]

Both of those drafts are long-expired. You should use https://tools.ietf.org/html/draft-ietf-curdle-ssh-ed25519-00 instead, which is also expired but references the key encoding draft that became RFC 8032. Specifically:

4.  Public Key Format

   The "ssh-ed25519" key format has the following encoding:

      string    "ssh-ed25519"
      string    key

   Here 'key' is the 32-octet public key described by
   [I-D.irtf-cfrg-eddsa], Section 5.1.5.

Thus the public key should be in this encoding. You can get that from this library using EdDSAPublicKey.toByteArray() or EdDSAPrivateKey.toByteArray() (both should return the same value).

I don't see any specification for private keys in OpenSSH, so I can't really help there, although you can get the encoding of the private key which would be used (if anywhere) via EdDSAPrivateKey.getSeed().