Hack and / - Password Cracking with GPUs, Part III: Tune Your Attack

Jul 09, 2012 By Kyle Rankin  inHOW-TOsSecurity You've built the hardware, installed the software and cracked somepasswords. Now find out how to fine-tune your attacks.

In the first two parts of this series, I explained what hardware to getand then described how to use the hashcat software suite to performdictionary and brute-force attacks. If you have been following along,by this point, you should have had plenty of time to build your ownpassword-cracking hardware and experiment with oclhashcat. As I mentionedin my last column, password cracking is a pretty dense subject. In thisarticle, I finish the series by describing how to tune andrefine your attacks further so they can be more effective.

Use More GPU Cycles

The first area where you can fine-tune your attacks is to put more or lessload on your GPU. The -n option, when passed to oclhashcat, changes howmuch of your GPU will be used for an attack. The documentation says thatthis value is set to 80 by default; however, on my computer, it seemed likethe default was set closer to 40. When I first ran a brute-force attack,the output told me I was using around 70–80% of my GPU. Once I added-n 80to my oclhashcat command, I noticed I was using between 96–98% of my GPU andhad added an extra 40,000 comparisons per second:/path/to/mp32.bin -1 ?d?l?u ?1?1?1?1?1?1

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