WIP / RFC for pysim 'next generation;' aka pysim-shell

Tomcsanyi, Domonkos domi at tomcsanyi.net
Wed Apr 7 07:23:18 UTC 2021


Hi Mychaela,

Without looking at the code I assume the way it works is that based on the service table the tool knows what files are supposed to be on the card and queries for them - just like an “ordinary phone” does. The point of the ST is exactly to avoid unnecessary bruteforcing/lookups of files that don’t exist.
However it is correct to say that finding a file hidden on purpose (i.e. missing entry from the ST) could only be done via bruteforce imho.

In case I am wrong sorry, didn’t mean to mislead anybody.

Cheers,
Domi

> 07.04.2021 dátummal, 2:41 időpontban Mychaela Falconia <mychaela.falconia at gmail.com> írta:
> 
> Harald Welte wrote:
> 
>> * we now have a 'tree' command to list the filesystem hierarchy
> 
> And just how do you get the card to tell you what selectable file IDs
> exist?  I haven't seen anything like an ls operation in either the
> classic GSM 11.11 SIM protocol or the UICC protocol, thus the only way
> (that I know of) to find out what selectable file IDs exist is to do a
> brute force search of the 16-bit file ID space at every directory
> level.  First select MF, then try selecting every possible 16-bit file
> ID from 0000 to FFFF (only skipping 3F00 for MF itself), and note
> which return something other than "not found" error.  Follow up with a
> GET RESPONSE command for every SELECT which succeeded, parse the
> response, and report the findings.  For all found file IDs which turn
> out to be DFs when the response is parsed, note those DF file IDs, and
> then repeat the brute force search inside every found DF - and then in
> any found nested DFs too.
> 
> This brute force search is implemented in fc-simtool and fc-uicc-tool
> programs in my fc-sim-tools suite, my competitor to pySim:
> 
> https://www.freecalypso.org/hg/fc-sim-tools/
> 
> As one would naturally expect, such brute force searches are painfully
> slow - IIRC, bfsearch-mf of sysmoISIM-SJA2 (just the MF tree, ADF trees
> have to be searched separately with bfsearch-adf) took about an hour,
> using HID Omnikey 3121 card reader, same model as the one currently
> sold in Sysmocom webshop - using an o'scope, I observed that it clocks
> the card at 4.8 MHz, almost up to the spec limit of 5 MHz.
> 
> Because these brute force searches are so slow, I collect the captures
> and check them into my source repository under the data directory - so
> if you are curious to see what undocumented proprietary files exist on
> both Sysmocom and Grcard SIMs (whose existence cannot be discovered in
> any other way than this bfsearch), just look in the repository linked
> above. :-)
> 
> I am not able to run pySim-shell on my Slackware system without
> expending more effort than I can currently justify, but I have glanced
> at the Python code, and I don't see anything like the just described
> brute force search - nor do I see it issuing any kind of secret
> undocumented ls-type APDU commands to the card - thus I am guessing
> that this 'tree' command displays nothing more than the tool's
> hard-coded knowledge of what files "should" exist at each given
> directory level, rather than what is actually found to exist.  If I
> got this part wrong, then someone please explain what this command
> *actually* returns, and how it obtains this knowledge - I don't know
> of any way other than a brute force search.
> 
> M~


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