Re: scrypt Internet Draft

[Simon Josefsson <simon@josefsson.org>]

Nick Galbreath <nickg@client9.com> writes:

> Thanks Simon.
>
> And now that I can read it... ;-)
>
> * need more detail on salt and format and allowed characters.  For
> some reason the man page on crypt-256/512 is very specific on the
> allowed salt alphabet, etc.

Sure.

> * need reference to base64, most likely  http://tools.ietf.org/html/rfc3548
>  As I learned the hard way there several, and crypt-256/512 uses a
> very custom one.

RFC 4648 even. :-)

> * looking at the name/value pairs, if a single variable, in one order,
> case sensitive isn't so bad.  However, I'll re-read Solr's other ideas

Yeah, I think the N= etc stuff wasn't baked, so other ideas are probably
better.

> * "where N, r and p are unsigned decimal numbers"  this probably needs
> more details on allowable ranges and types, e.g. "positive integers"
> Copying the spec isn't bad here, but I need to think how this can be
> simplified.
>
>             N       CPU/Memory cost parameter, must be larger than 1,
>
>                     a power of 2 and less than 2^(128 * r / 8).
>
>             r       Block size parameter.
>
>             p       Parallelization parameter, a positive integer
>
>                     less than or equal to ((2^32-1) * hLen) / MFLen
>
>                     where hLen is 32 and MFlen is 128 * r.
>
> * needs something on happens on error if parameters are misformed or
> incorrect or out of range.
>
> * I'd add a references section

Yep.

> Just made a gitorious account under 'ngalbreath'  I'm happy to make
> these changes.

You have permissions, please go ahead!

/Simon

> thanks!
>
> nickg
>
>
> On Tue, Sep 18, 2012 at 11:05 AM, Simon Josefsson <simon@josefsson.org> wrote:
>> Sorry, the repository was renamed...  see here instead:
>>
>> https://www.gitorious.org/scrypt/scrypt-unix-crypt/blobs/master/unix-scrypt.txt
>>
>> /Simon
>>
>> Nick Galbreath <nickg@client9.com> writes:
>>
>>>  https://www.gitorious.org/scrypt/scrypt/blobs/master/unix-scrypt.txt
>>>
>>> has vanished!  (or I get a 404)
>>>
>>>
>>>
>>> On Tue, Sep 18, 2012 at 5:10 AM, Simon Josefsson <simon@josefsson.org> wrote:
>>>> 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