Re: scrypt Internet Draft

[Solar Designer <solar@openwall.com>]

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.

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.

Thanks,

Alexander