//     before calling Format.
//     S3 clients expect that the ETag of an SSE-S3 encrypted
//     single-part object is equal to the object's content MD5.
//     Formatting the SSE-S3 ETag before decryption will result
//     in a random-looking ETag which an S3 client will not accept.
//
// Hence, a caller has to check:
//
//	if method == SSE-S3 {
//	   ETag, err := Decrypt(key, ETag)
//	   if err != nil {