package sshd import ( "crypto/sha256" "encoding/base64" "errors" "net" "golang.org/x/crypto/ssh" ) // Auth is used to authenticate connections based on public keys. type Auth interface { // Whether to allow connections without a public key. AllowAnonymous() bool // Given address and public key, return if the connection should be permitted. Check(net.Addr, ssh.PublicKey) (bool, error) } // MakeAuth makes an ssh.ServerConfig which performs authentication against an Auth implementation. func MakeAuth(auth Auth) *ssh.ServerConfig { config := ssh.ServerConfig{ NoClientAuth: false, // Auth-related things should be constant-time to avoid timing attacks. PublicKeyCallback: func(conn ssh.ConnMetadata, key ssh.PublicKey) (*ssh.Permissions, error) { ok, err := auth.Check(conn.RemoteAddr(), key) if !ok { return nil, err } perm := &ssh.Permissions{Extensions: map[string]string{ "pubkey": string(key.Marshal()), }} return perm, nil }, KeyboardInteractiveCallback: func(conn ssh.ConnMetadata, challenge ssh.KeyboardInteractiveChallenge) (*ssh.Permissions, error) { if !auth.AllowAnonymous() { return nil, errors.New("public key authentication required") } _, err := auth.Check(conn.RemoteAddr(), nil) return nil, err }, } return &config } // MakeNoAuth makes a simple ssh.ServerConfig which allows all connections. // Primarily used for testing. func MakeNoAuth() *ssh.ServerConfig { config := ssh.ServerConfig{ NoClientAuth: false, // Auth-related things should be constant-time to avoid timing attacks. PublicKeyCallback: func(conn ssh.ConnMetadata, key ssh.PublicKey) (*ssh.Permissions, error) { perm := &ssh.Permissions{Extensions: map[string]string{ "pubkey": string(key.Marshal()), }} return perm, nil }, KeyboardInteractiveCallback: func(conn ssh.ConnMetadata, challenge ssh.KeyboardInteractiveChallenge) (*ssh.Permissions, error) { return nil, nil }, } return &config } // Fingerprint performs a SHA256 BASE64 fingerprint of the PublicKey, similar to OpenSSH. // See: https://anongit.mindrot.org/openssh.git/commit/?id=56d1c83cdd1ac func Fingerprint(k ssh.PublicKey) string { hash := sha256.Sum256(k.Marshal()) return "SHA256:" + base64.StdEncoding.EncodeToString(hash[:]) }