types/key: add ChallengePublic, ChallengePrivate, NewChallenge

Updates #5972

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

types/key/chal.go

@@ -0,0 +1,78 @@
+// 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 (
+	"errors"
+
+	"tailscale.com/types/structs"
+)
+
+const (
+	// chalPublicHexPrefix is the prefix used to identify a
+	// hex-encoded challenge public key.
+	//
+	// This prefix is used in the control protocol, so cannot be
+	// changed.
+	chalPublicHexPrefix = "chalpub:"
+)
+
+// ChallengePrivate is a challenge key, used to test whether clients control a
+// key they want to prove ownership of.
+//
+// A ChallengePrivate is ephemeral and not serialized to the disk or network.
+type ChallengePrivate struct {
+	_ structs.Incomparable // because == isn't constant-time
+	k [32]byte
+}
+
+// NewChallenge creates and returns a new node private key.
+func NewChallenge() ChallengePrivate {
+	return ChallengePrivate(NewNode())
+}
+
+// Public returns the ChallengePublic for k.
+// Panics if ChallengePublic is zero.
+func (k ChallengePrivate) Public() ChallengePublic {
+	pub := NodePrivate(k).Public()
+	return ChallengePublic(pub)
+}
+
+// MarshalText implements encoding.TextMarshaler, but by returning an error.
+// It shouldn't need to be marshalled anywhere.
+func (k ChallengePrivate) MarshalText() ([]byte, error) {
+	return nil, errors.New("refusing to marshal")
+}
+
+// SealToChallenge is like SealTo, but for a ChallengePublic.
+func (k NodePrivate) SealToChallenge(p ChallengePublic, cleartext []byte) (ciphertext []byte) {
+	return k.SealTo(NodePublic(p), cleartext)
+}
+
+// OpenFrom opens the NaCl box ciphertext, which must be a value
+// created by NodePrivate.SealToChallenge, and returns the inner cleartext if
+// ciphertext is a valid box from p to k.
+func (k ChallengePrivate) OpenFrom(p NodePublic, ciphertext []byte) (cleartext []byte, ok bool) {
+	return NodePrivate(k).OpenFrom(p, ciphertext)
+}
+
+// ChallengePublic is the public portion of a ChallengePrivate.
+type ChallengePublic struct {
+	k [32]byte
+}
+
+// String returns the output of MarshalText as a string.
+func (k ChallengePublic) String() string {
+	bs, err := k.MarshalText()
+	if err != nil {
+		panic(err)
+	}
+	return string(bs)
+}
+
+// MarshalText implements encoding.TextMarshaler.
+func (k ChallengePublic) MarshalText() ([]byte, error) {
+	return toHex(k.k[:], chalPublicHexPrefix), nil
+}