mac.Write(bin[:])
	mac.Sum(partKey[:0])
	return partKey
}

// SealETag seals the etag using the object key.
// It does not encrypt empty ETags because such ETags indicate
// that the S3 client hasn't sent an ETag = MD5(object) and
// the backend can pick an ETag value.
func (key ObjectKey) SealETag(etag []byte) []byte {
	if len(etag) == 0 { // don't encrypt empty ETag - only if client sent ETag = MD5(object)

	}
	return nil
}

func sealETag(encKey crypto.ObjectKey, md5CurrSum []byte) []byte {
	var emptyKey [32]byte
	if bytes.Equal(encKey[:], emptyKey[:]) {
		return md5CurrSum
	}
	return encKey.SealETag(md5CurrSum)
}

func sealETagFn(key crypto.ObjectKey) SealMD5CurrFn {
	fn := func(md5sumcurr []byte) []byte {
		return sealETag(key, md5sumcurr)
	}
	return fn
}