types/key: init HardwareAttestionKey implementation (#16867)

Define the HardwareAttestionKey interface describing a platform-specific
hardware backed node identity attestation key. Clients will register the
key type implementations for their platform.

Updates tailscale/corp#31269

Signed-off-by: Patrick O'Doherty <patrick@tailscale.com>

types/key/hardware_attestation.go

@@ -0,0 +1,68 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package key
+
+import (
+	"crypto"
+	"encoding/json"
+	"fmt"
+)
+
+var ErrUnsupported = fmt.Errorf("key type not supported on this platform")
+
+// HardwareAttestationKey describes a hardware-backed key that is used to
+// identify a node. Implementation details will
+// vary based on the platform in use (SecureEnclave for Apple, TPM for
+// Windows/Linux, Android Hardware-backed Keystore).
+// This key can only be marshalled and unmarshalled on the same machine.
+type HardwareAttestationKey interface {
+	crypto.Signer
+	json.Marshaler
+	json.Unmarshaler
+}
+
+// emptyHardwareAttestationKey is a function that returns an empty
+// HardwareAttestationKey suitable for use with JSON unmarshalling.
+var emptyHardwareAttestationKey func() HardwareAttestationKey
+
+// createHardwareAttestationKey is a function that creates a new
+// HardwareAttestationKey for the current platform.
+var createHardwareAttestationKey func() (HardwareAttestationKey, error)
+
+// HardwareAttestationKeyFn is a callback function type that returns a HardwareAttestationKey
+// and an error. It is used to register platform-specific implementations of
+// HardwareAttestationKey.
+type HardwareAttestationKeyFn func() (HardwareAttestationKey, error)
+
+// RegisterHardwareAttestationKeyFns registers a hardware attestation
+// key implementation for the current platform.
+func RegisterHardwareAttestationKeyFns(emptyFn func() HardwareAttestationKey, createFn HardwareAttestationKeyFn) {
+	if emptyHardwareAttestationKey != nil {
+		panic("emptyPlatformHardwareAttestationKey already registered")
+	}
+	emptyHardwareAttestationKey = emptyFn
+
+	if createHardwareAttestationKey != nil {
+		panic("createPlatformHardwareAttestationKey already registered")
+	}
+	createHardwareAttestationKey = createFn
+}
+
+// NewEmptyHardwareAttestationKey returns an empty HardwareAttestationKey
+// suitable for JSON unmarshalling.
+func NewEmptyHardwareAttestationKey() (HardwareAttestationKey, error) {
+	if emptyHardwareAttestationKey == nil {
+		return nil, ErrUnsupported
+	}
+	return emptyHardwareAttestationKey(), nil
+}
+
+// NewHardwareAttestationKey returns a newly created HardwareAttestationKey for
+// the current platform.
+func NewHardwareAttestationKey() (HardwareAttestationKey, error) {
+	if createHardwareAttestationKey == nil {
+		return nil, ErrUnsupported
+	}
+	return createHardwareAttestationKey()
+}