tka: implement AUM and Key types

This is the first in a series of PRs implementing the internals for the
Tailnet Key Authority. This PR implements the AUM and Key types, which
are used by pretty much everything else. Future PRs:

 - The State type & related machinery
 - The Tailchonk (storage) type & implementation
 - The Authority type and sync implementation

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

tka/key_test.go

@@ -0,0 +1,64 @@
+// 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 tka
+
+import (
+	"bytes"
+	"crypto/ed25519"
+	"encoding/binary"
+	"math/rand"
+	"testing"
+)
+
+// returns a random source based on the test name + extraSeed.
+func testingRand(t *testing.T, extraSeed int64) *rand.Rand {
+	var seed int64
+	if err := binary.Read(bytes.NewBuffer([]byte(t.Name())), binary.LittleEndian, &seed); err != nil {
+		panic(err)
+	}
+	return rand.New(rand.NewSource(seed + extraSeed))
+}
+
+// generates a 25519 private key based on the seed + test name.
+func testingKey25519(t *testing.T, seed int64) (ed25519.PublicKey, ed25519.PrivateKey) {
+	pub, priv, err := ed25519.GenerateKey(testingRand(t, seed))
+	if err != nil {
+		panic(err)
+	}
+	return pub, priv
+}
+
+func TestVerify25519(t *testing.T) {
+	pub, priv := testingKey25519(t, 1)
+	key := Key{
+		Kind:   Key25519,
+		Public: pub,
+	}
+
+	aum := AUM{
+		MessageKind: AUMRemoveKey,
+		KeyID:       []byte{1, 2, 3, 4},
+		// Signatures is set to crap so we are sure its ignored in the sigHash computation.
+		Signatures: []Signature{{KeyID: []byte{45, 42}}},
+	}
+	sigHash := aum.SigHash()
+	aum.Signatures = []Signature{
+		{
+			KeyID:     key.ID(),
+			Signature: ed25519.Sign(priv, sigHash[:]),
+		},
+	}
+
+	if err := aum.Signatures[0].Verify(aum.SigHash(), key); err != nil {
+		t.Errorf("signature verification failed: %v", err)
+	}
+
+	// Make sure it fails with a different public key.
+	pub2, _ := testingKey25519(t, 2)
+	key2 := Key{Kind: Key25519, Public: pub2}
+	if err := aum.Signatures[0].Verify(aum.SigHash(), key2); err == nil {
+		t.Error("signature verification with different key did not fail")
+	}
+}