tka,types/key: implement NLPrivate glue for tailnet key authority keys

Signed-off-by: Tom DNetto <tom@tailscale.com>

types/key/nl.go

@@ -0,0 +1,74 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package key
+
+import (
+	"crypto/ed25519"
+
+	"go4.org/mem"
+	"tailscale.com/tka"
+	"tailscale.com/types/structs"
+)
+
+const (
+	// nlPrivateHexPrefix is the prefix used to identify a
+	// hex-encoded network-lock key.
+	nlPrivateHexPrefix = "nlpriv:"
+)
+
+// NLPrivate is a node-managed network-lock key, used for signing
+// node-key signatures and authority update messages.
+type NLPrivate struct {
+	_ structs.Incomparable // because == isn't constant-time
+	k [ed25519.PrivateKeySize]byte
+}
+
+// NewNLPrivate creates and returns a new network-lock key.
+func NewNLPrivate() NLPrivate {
+	// ed25519.GenerateKey 'clamps' the key, not that it
+	// matters given we don't do Diffie-Hellman.
+	_, priv, err := ed25519.GenerateKey(nil) // nil == crypto/rand
+	if err != nil {
+		panic(err)
+	}
+
+	var out NLPrivate
+	copy(out.k[:], priv)
+	return out
+}
+
+// MarshalText implements encoding.TextUnmarshaler.
+func (k *NLPrivate) UnmarshalText(b []byte) error {
+	return parseHex(k.k[:], mem.B(b), mem.S(nlPrivateHexPrefix))
+}
+
+// MarshalText implements encoding.TextMarshaler.
+func (k NLPrivate) MarshalText() ([]byte, error) {
+	return toHex(k.k[:], nlPrivateHexPrefix), nil
+}
+
+// Public returns the public component of this key.
+func (k NLPrivate) Public() ed25519.PublicKey {
+	return ed25519.PrivateKey(k.k[:]).Public().(ed25519.PublicKey)
+}
+
+// KeyID returns an identifier for this key.
+func (k NLPrivate) KeyID() tka.KeyID {
+	return tka.Key{
+		Kind:   tka.Key25519,
+		Public: k.Public(),
+	}.ID()
+}
+
+// SignAUM implements tka.UpdateSigner.
+func (k NLPrivate) SignAUM(a *tka.AUM) error {
+	sigHash := a.SigHash()
+
+	a.Signatures = append(a.Signatures, tka.Signature{
+		KeyID:     k.KeyID(),
+		Signature: ed25519.Sign(k.k[:], sigHash[:]),
+	})
+	return nil
+}