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Date: Thu, 5 Jul 2012 17:09:32 -0400 (EDT)
From: "Brad Tilley" <brad@...ystems.com>
To: john-users@...ts.openwall.com
Subject: Re: John the Ripper 1.7.9-jumbo-6

>> It's just a basic lack of understanding. Perhaps a wiki page would be
appropriate to help user expectations be inline with reality? If that's
an
>> acceptable idea, I'd be glad to write a rough first draft provided one
does not already exist.
>
> Please feel free to write that wiki page.  We don't have one yet.
>
> That would be a welcome contribution.
>
> Thank you!
>
> Alexander

OK... here's a rough first draft I wrote during lunch today. Feel free to
add it to the wiki (I don't know where the most appropriate spot for it
is, otherwise I would add it myself). I can edit/improve it later as
needed:

* Title

Differences Between Fast Hashes and Slow Hashes

* Summary

This article is intended for new John users who have not had a lot of
experience cracking different types of hashes. The goal is to explain the
differences between fast hashes and slow hashes so that new users will
have realistic expectations when using John to crack passwords.

* Two Basic Hash Types

1. Fast Hashes
2. Slow Hashes

Fast hashes are typically cryptographic hashes. These hashes have several
design requirements, one of which is that they must be easy to compute. In
other words, they must be fast and efficient to calculate. Why would their
designers want them to be fast?

The main reason for the efficiency requirement is that cryptographic
hashes are used for such things as message authentication codes (MACs),
digital signatures, and other authentication systems as well as for
checksums to ensure data integrity during file transfers, etc. For these
reasons (and several others) software and hardware that calculate these
hashes must be able to do so very quickly with almost no noticeable
latency to the end-users of these systems. The faster they are, the
better. Some examples of fast hashes include (but are not
limited to) MD4, MD5 and SHA-1. These hashes are used extensively by many
software and hardware systems and are in general very common.

One advantage to fast hashes is that they are very portable and can be
found in programming libraries for most every major programming language.
Thus, developers using PHP, C++, Java, C, Ruby, etc. can easily use a fast
hash such as MD5 to store user passwords. And, if the developers switch
from one language to another, they can do so with the knowledge that MD5
is just a library call away.

Slow hashes, on the other hand, have different design goals. They are
expected to be copied and subsequently attacked by crackers. Thus, they
are designed to be inefficient and more difficult to calculate. Some
examples of these slow hashes are bcrypt, PBKDF2 and scrypt. Also, slow
hashes are not as widely available and not as simple to implement as fast
hashes.

Both fast and slow hashes are in use today for password storage so you
should expect to see both when auditing systems in heterogeneous
environments. Microsoft's Active Directory Servers store user passwords as
fast MD4 hashes while many Linux and BSD systems use slower bcrypt hashes.
And, if you've never cracked a slow hash type, it may come as somewhat of
a shock to you to see how many cracks per minute you get when attacking
the slower brcrypt hashes. Here's an example that demonstrates the
difference:

john-1.7.9-jumbo-6/run/john --test --format=nt2
Benchmarking: NT MD4 [128/128 SSE2 intrinsics 12x]... DONE
Raw:	30302K c/s real, 30302K c/s virtual

john-1.7.9-jumbo-6/run/john --test --format=bf
Benchmarking: OpenBSD Blowfish (x32) [32/64 X2]... (6xOMP) DONE
Raw:	5724 c/s real, 947 c/s virtual

As you can see, the performance difference is huge. Even with OpenMP, the
bcrypt hashes are still far more difficult to attack.

* Other Password Cracking Software

Many of the new GPU based password crackers only work on the fast, easy
hashes. They can crack hundreds of millions of these hashes each second.
They are very impressive and exciting to watch as the cracked passwords
scroll by on the console. If you are new to password cracking and you have
only used one of these newer GPU based password crackers to audit simple,
fast hashes, you may expect that all password hashes crack that easily. If
you hold that expectation, you will be disappointed when you use John to
crack OpenBSD Blowfish hashes. You may even think that John is not as good
as those other password crackers, but if you do, you would be mistaken.

John works with the fast, easy hashes as well as the hard, slow hashes and
adds new hashes each release. The developers of John don't back away from
hard, slow hashes. In fact, they find it a challenge to crack them! They
understand that in the real-world many security aware organizations
implement systems that use slow, hard hashes and thus need the ability to
audit their passwords too, and they should be able to do that.


* Conclusion

I typically limit my password auditing sessions to 48 hours and I audit
both the fast and the slow hashes and I would encourage you to do the
same. It's been my experience that 48 hours is a reasonable time period in
which to crack most of the weaker passwords. During this time, I may crack
50% or more of Active Directory hashes, but only 20% to 30% of bcrypt
hashes. However, there are exceptions to this. On one occasion, I cracked
close to 70% of the bcrypt hashes using John, but that was only because
there was a very common, simple pattern used throughout most of the
hashes. That's by far the most bcrypt hashes I've ever cracked while
auditing. It was a shame to see such a good, strong hash wasted on such
weak passwords.

In conclusion, now that you know what sort of performance to expect when
cracking fast and slow hashes, you should be more effective when
performing password auditing.


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