types/key: add IsZero methods

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

types/key/key.go

@@ -10,6 +10,9 @@ import "golang.org/x/crypto/curve25519"
 // Private represents a curve25519 private key.
 type Private [32]byte
 
+// Private reports whether p is the zero value.
+func (p Private) IsZero() bool { return p == Private{} }
+
 // B32 returns k as the *[32]byte type that's used by the
 // golang.org/x/crypto packages. This allocates; it might
 // not be appropriate for performance-sensitive paths.
@@ -18,6 +21,9 @@ func (k Private) B32() *[32]byte { return (*[32]byte)(&k) }
 // Public represents a curve25519 public key.
 type Public [32]byte
 
+// Public reports whether p is the zero value.
+func (p Public) IsZero() bool { return p == Public{} }
+
 // B32 returns k as the *[32]byte type that's used by the
 // golang.org/x/crypto packages. This allocates; it might
 // not be appropriate for performance-sensitive paths.