[A51] Bit position , table structure and gpu in A51 tables

fritchkornia at keemail.me fritchkornia at keemail.me
Fri Mar 18 15:31:44 CET 2016


If there is a "bit offset" this means that the "distinguish end point 
computing" before disk lookup will be done in fact 51 times as 64 consecutive 
bits can fit 51 times in 114bits ?
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18. Mar 2016 11:39 by jenda at yakumo.hrach.eu:


> https://brmlab.cz/project/gsm/deka/attack-theory
> https://brmlab.cz/project/gsm/deka/attack-implementation
>
>> I tought after a quick reading that the plaintext equivalent  of the 
>> tables was the LFSRs state after 122 clocking and the output the 64 first 
>> bit of keystream (after 64 further clocking) with a simple reduction 
>> function  output = new plaintext
>
> No, it's the last 64 bits of keystream after 99+64 clockings. The reduction 
> functions are XORed constants generated using A5/1 itself (does anyone know 
> if there is any special purpose in this, or if it is just convenient?)
>
> See > 
> http://jenda.hrach.eu/gitweb/?p=deka;a=blob;f=genkernel32.sh;h=96ec2fc0a6b540101f6f6e8fa418c7b385b460d8;hb=HEAD> 
>  for the actual chain computation (which is unfortunately a bit hard to 
> read, as it is a hand-vectorized code).
>
>> However while testing there is a Bit position that poped up . Does it mean 
>> that the compute function differ for each steps or is it just different in 
>> each tables ?
>
> By bit position, you probably mean the offset if the 64bit sample in the 
> 114bit burst.
>
>> Finally what is the purpose of the GPU when finding new KC ? Is it just 
>> for the compute/reduction step ?
>
> The chain is computed to the nearest endpoint on the GPU, then the 
> corresponding startpoint is looked up on disk, and then the chain is 
> reconstructed from that startpoint on the GPU again.
>
>
> On 18.3.2016 10:33, > fritchkornia at keemail.me>  wrote:
>>
>> Hi
>>
>> I have a few questions regarding the A5/1 tables. I tought after a quick 
>> reading that the plaintext equivalent  of the tables was the LFSRs state 
>> after 122 clocking and the output the 64 first bit of keystream (after 64 
>> further clocking) with a simple reduction function  output = new plaintext
>>
>> However while testing there is a Bit position that poped up . Does it mean 
>> that the compute function differ for each steps or is it just different in 
>> each tables ?
>>
>> IS there a simple guide to understand the table structure somehow .
>>
>> Finally what is the purpose of the GPU when finding new KC ? Is it just 
>> for the compute/reduction step ?
>>
>>
>> Best Regards
>>
>>
>>
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>>
>
>
> --
> Jan Hrach | > http://jenda.hrach.eu
> GPG CD98 5440 4372 0C6D 164D A24D F019 2F8E 6527 282E
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