Hacker News Reader

Comment by gizzlon | Hacker News Reader

Doesn't this open you up to replay attacks though? Since you can't store that a token was already used..

I failed to say that your token context should have a "time based expiration", in that a new token is reissued periodically as defined by you and your needs. I would refer to the ASP.NET Forms Auth mechanism with its sliding expiration.

Sure you can. Just include a timestamp, and expire the token, at, say, time + 90 seconds, or whatever makes sense for the application.

I get that you can expire it, and that helps, but it's not the same as use-once. Of course, just using a timeout is probably fine in many cases, especially if it's used with SSL. But replay attacks are still possible since there's a windows where it can be re-used.

Replay attacks are always gonna be possible unless you use a one time token or signature, thems the break's..., unless you wish to get into the something you have and something you know model. How can you do a use once token making concurrent requests without a strong authentication mechanism client side such as issuing private keys to clients....and all the PKI admin overhead. I think its safe to say, that a restful api should be stateless, and bottlenecks such as session state are not necessary.

I'll take that as a "yes" ;)

AFAIK, neither signatures or "something you have, know" alone fixes replay attacks. Since this is a well known problem in cryptography, many solutions exists. All of which are probably overkill for this use.

At least with the use of a digital signature and nonce you can guarantee that the request hasn't been tampered with!

Reply on news.ycombinator.com