// the ETag unmodified.
func Decrypt(key []byte, etag ETag) (ETag, error) {
	const HMACContext = "SSE-etag"

	if !etag.IsEncrypted() {
		return etag, nil
	}
	mac := hmac.New(sha256.New, key)
	mac.Write([]byte(HMACContext))
	decryptionKey := mac.Sum(nil)

	plaintext := make([]byte, 0, 16)
	etag, err := sio.DecryptBuffer(plaintext, etag, sio.Config{
		Key:          decryptionKey,
		CipherSuites: fips.DARECiphers(),