Solar Designer <[email protected]> writes: > On Tue, Sep 18, 2012 at 08:51:06AM +0200, Simon Josefsson wrote: >> We could start it as a parallel effort though. Would you like to help >> work on this? I started a document here: >> >> https://www.gitorious.org/scrypt/scrypt/blobs/master/unix-scrypt.txt > > FWIW, I am planning to do some research/testing/benchmarking of scrypt > for this kind of uses very soon. Chances are that I'll want to make > modifications to scrypt proper as a result - probably at least have an > optional time-memory tradeoff defeater (a fourth parameter) as briefly > discussed with Colin on the crypt-dev list. Naturally, I expect some > healthy resistance to any proposed modifications to scrypt, now that > it's been around for 3 years and is about to get standardized. Yet I > think this is something to discuss and consider. > > There are also some difficulties with using scrypt as a crypt(3) > password hash type. As discussed on crypt-dev, scrypt at <= 1 MB (yes, > misuse of it) is not a good replacement for bcrypt, whereas scrypt at > much larger memory settings (proper use) should better be used with > concurrency limits (not currently found inside crypt(3) implementations, > nor in many crypt(3)-using daemons). So the issue is a bit non-trivial.
Yes selecting parameters is difficult. I'm also concerned that too small parameters end up being weaker than PBKDF2/bcrypt. Generally, I'm not entirely sure how one would use scrypt for authentication services -- probably the best is to reserve a chunk of memory and setup a scrypt computation service. You would then have no issues up until some pre-determined number of authentications/second, that you could rate-limit per-user on. > Speaking of the encoding syntax, I think the key=value,... style of > syntax is probably a bad idea. It complicates parsing and brings up > unnecessary questions such as whether a parser is supposed to handle > keys in the one standard order only or in any order, etc. IIRC, the > "rounds=..." thing first appeared in SunMD5, then was reused for > SHA-crypt, and well, there were some parsing ambiguities with them. It > might be better to just allocate a fixed number of base-64 characters at > the start of the string (right after the $7$ or whatever hash type > prefix) to correspond to the parameters. And if we need to add an extra > parameter later, we just pick a new prefix (call it e.g. $7a$). I used > a similar approach in phpass "portable hashes", where the character > right after the $P$ prefix holds base-2 logarithm of the iteration > count. This is trivial to parse and encode, and there's just one valid > encoding. So I suggest that we try not to make things more flexible > than we actually need them to be. Excellent, this was the kind of feedback I was hoping for. I agree. If you have a gitorious account and want to help with the document, I'll add you. /Simon
