Re: scrypt Internet Draft

[Simon Josefsson <simon@josefsson.org>]

Solar Designer <solar@openwall.com> 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