tka,types/key: remove dependency for tailcfg & types/ packages on tka

Following the pattern elsewhere, we create a new tka-specific types package for the types
that need to couple between the serialized structure types, and tka.

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

types/tkatype/tkatype.go

@@ -0,0 +1,34 @@
+// 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 tkatype defines types for working with the tka package.
+//
+// Do not add extra dependencies to this package unless they are tiny,
+// because this package encodes wire types that should be lightweight to use.
+package tkatype
+
+// KeyID references a verification key stored in the key authority. A keyID
+// uniquely identifies a key. KeyIDs are all 32 bytes.
+//
+// For 25519 keys: We just use the 32-byte public key.
+//
+// Even though this is a 32-byte value, we use a byte slice because
+// CBOR-encoded byte slices have a different prefix to CBOR-encoded arrays.
+// Encoding as a byte slice allows us to change the size in the future if we
+// ever need to.
+type KeyID []byte
+
+// MarshaledSignature represents a marshaled tka.NodeKeySignature.
+type MarshaledSignature []byte
+
+// AUMSigHash represents the BLAKE2s digest of an Authority Update
+// Message (AUM), sans any signatures.
+type AUMSigHash [32]byte
+
+// Signature describes a signature over an AUM, which can be verified
+// using the key referenced by KeyID.
+type Signature struct {
+	KeyID     KeyID  `cbor:"1,keyasint"`
+	Signature []byte `cbor:"2,keyasint"`
+}

types/tkatype/tkatype_test.go

@@ -0,0 +1,17 @@
+// 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 tkatype
+
+import (
+	"golang.org/x/crypto/blake2s"
+	"testing"
+)
+
+func TestSigHashSize(t *testing.T) {
+	var sigHash AUMSigHash
+	if len(sigHash) != blake2s.Size {
+		t.Errorf("AUMSigHash is wrong size: got %d, want %d", len(sigHash), blake2s.Size)
+	}
+}