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Deka - an OpenCL A5/1 cracker

Deka is a fast, free and portable A5/1 (that's the cipher used in mobile phones) cracker written in OpenCL. Thanks to efficient use of vector instructions and hard-drive NCQ, the Kc key on a real-world GSM network can usually be recovered in 5-60 seconds with 2 minutes RTT (i.e., cracking many keys in parallel) depending on network security, signal quality etc. (test machine is a high-end desktop: 8 core AMD FX-8150, 32 GB RAM, 3x ATI HD 7970, 4x ADATA SX900)

Deka started as an attempt to port Kraken, the first A5/1 cracker available, to AMD GCN architecture, and resulted in a complete rewrite. Deka is binary compatible with Kraken, allowing easy evaluation and seamless switch.


  • Deka is portable. It runs on every platform where Python, GCC and OpenCL work.
  • Deka is fast. The kernel is fully vectorized, including the final challenge lookup, and we ship kernels for architectures supporting 128- and 256-bit vectors. The table lookup is using NCQ and stripping unnecessary syscalls to minimize computing overhead.
  • Deka is clusterable. Work is distributed over TCP. You can run more nodes to speed cracking up.
  • Deka is modular. You can, for example, write A5/1 chain generator in VHDL, load it to a FPGA, and you just need to change a few lines of code to make it work with deka.
  • Deka is, we hope, easier to understand. For example you can enjoy comments in the source code :)


To be honest, there are some.

  • Deka is more difficult to set up than Kraken, as there is no automated tool to configure tables (yet).
  • Deka is still dependent on Kraken (with Kraken's unclear licensing terms), as we do not have our own table converter and A5/1 backclocker.
  • Deka is burst-oriented, which worsens round-trip-time for real-time cracking by approximately a factor of two.


Nice to have features:

  • “cancel” command to cancel processing of a given burst. This is useful when the key is successfully recovered and so there is no use to continue computing.

Alternative features:

  • does not use barriers/conditional variables, but busy waiting
  • leaks memory – reportqs are not freed when the client disconnects

Attack in a nutshell

TL;DR You can go through the A5/1 keyspace and save some “distinguished points”. When you want to recover the key, you reconstruct the keyspace from the nearest distinguished point. (I want to know more!)

Deka listens on a TCP socket, waits for a keystream and once you submit one, it finds a secret state that resulted in this keystream. Hence you need some GSM sniffer, keystream guesser, TCP client and secret state processor. gsmtk implements exactly that.

Recommended configuration:

We need to do an equivalent of 5 billion A5/1 encryptions and read 200k pseudorandom 4KiB blocks from disk to crack a key on an insecure network (multiply with 10 on secure network).

  • Computer running a recent Linux distribution (Deka has been developed on Debian Jessie with HD7970 cards, but definitely should work on other distributions and probably on other UNIX systems too), 64bit (we need a 64bit system as we allocate lots of memory)
  • CPU power which depends on the rest of the system. For example an AMD FX-8150 can process 120 kfrag/s.
  • 8 or better 16 GB RAM to fit the table index, track bursts and have some space for block cache
  • OpenCL capable card, or several of them
    • Deka has been tested on AMD HD7970 and nVidia GTX 610M (which is of course slow).
  • 1.7TB of fast storage - SSDs at best. And probably some SATA controller that does not have problems with lots of random reads.


Getting deka


A5/1 cracking turned out to be a complicated task, at least for some. Unfortunately, I can't provide support with basic Linux and programming skills. These things include for example:
  • Ability to read the installation manual and comments in configuration files.
  • Understanding the concept of “files”, “directories”, “devices” and “addresses”.
  • Understanding the concepts of “Makefile”, “compiler” and “JIT”.

Please don't take this as some meanness, I just started getting tons of emails from people who obviously don't follow. If you have found a real bug, have some improvement, or are just interested in technical discussion, you are welcome.


  • Řehoř Gölöncséryi (niekt0) and Tomash (sysop) for introducing me to GSM security and the GSMstack toolkit and advice regarding how wrong it is and what should I do better.
  • Kraken team, as there is very few documentation available - reading their sources was a great help in understanding the attack.


Deka (n.):

  • blanket (cz)
  • a method of torture particularly popular among soldiers in former Czechoslovakia

Vankúš is Slovak for a small rodent gopher; the userspace OpenCL library called oclvankus may resemble another cracker, oclhashcat.

Paplón is Slovak for goose.

project/gsm/deka/start.txt · Last modified: 2022/03/25 19:42 by jenda