Go Version

1.8.1 stable

kris-nova:crypto kris$ go version
go version go1.8 darwin/amd64

Environment

Darwin AMD64

What did you do?

While attempting to authenticate with x/crypto/ssh I was able to return an empty slice of ssh.Signers. My code compiled, but upon running recieved a nil pointer dereference as one would expect.

import "golang.org/x/crypto/ssh"
//snip
	publicKeyCallback := func() ([]cssh.Signer, error) {
		var signers []cssh.Signer
		return signers, nil
	})

Expected

I would have expected the library to behave in the same way the open-ssh and other SSH tools handle signing attempts, which is to try attempts in the hopes that one will eventually succeed. I think this is the intent of the library given the note here.

In this exact case, we would have no signing methods to try, and the authentication would immediately run out of possible methods to attempt. Thus returning an error like:

ssh: handshake failed: ssh: unable to authenticate, attempted methods [none publickey], no supported methods remain

Actual

nil pointer dereference 😞

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x139851a]

goroutine 1 [running]:
github.com/kris-nova/klone/vendor/golang.org/x/crypto/ssh.publicKeyCallback.auth(0x0, 0xc42017cc40, 0x20, 0x20, 0x0, 0x0, 0x1853260, 0xc420441080, 0x184cda0, 0xc420019500, ...)
	/go/src/github.com/kris-nova/klone/vendor/golang.org/x/crypto/ssh/client_auth.go:187 +0x3a