func macSHA1(key []byte) hash.Hash {
	h := sha1.New
	// The BoringCrypto SHA1 does not have a constant-time
	// checksum function, so don't try to use it.
	if !boring.Enabled {
		h = newConstantTimeHash(h)
	}
	return hmac.New(h, key)
}

// macSHA256 returns a SHA-256 based MAC. This is only supported in TLS 1.2 and
// is currently only used in disabled-by-default cipher suites.