Re: scrypt Internet Draft

[Nick Galbreath <nickg@client9.com>]

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.

* 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.

* 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

* "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

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

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