What version of Go are you using (go version)?

$ go version
go version go1.13.6 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

Applies to all OSes.

What did you do?

N/A

What did you expect to see?

N/A

What did you see instead?

N/A

Proposal

x/crypto/ssh now has methods to parse private keys using OpenSSH format, but there's no way to write those keys so they can be used by OpenSSH. My proposal is to add the following methods to the x/crypto/ssh package:

MarshalPrivateKey(key crypto.PrivateKey, comment string) (*pem.Block, error) 

MarshalPrivateKeyWithPassphrase(key crypto.PrivateKey, comment string, passphrase []byte) (*pem.Block, error)

OpenSSH supports the PKCS#1 and EC private keys, but only supports Ed25519 using the OpenSSH format, but newer versions are already using the new format for all types of keys.

I've already coded those methods and will push a contribution asap.