path: root/2021/osmodevcall-pysim_shell/pysim_shell.adoc

pySim-shell - next genreation SIM configuration tool
====================================================
:author:	Harald Welte <laforge@gnumonks.org>
:copyright:	2021 by Harald Welte (License: CC-BY-SA)
:backend:	slidy
:max-width:	45em

== why program your own SIM cards?

* because you want to operate  your own cellular network (2G, 3G, 4G, 5G)
** whether FOSS (Osmocom CNI, srsLTE, nextepc/open5gs, free5gc, OpenBTS, yateBTS, ...)
** but also proprietary (e.g. Amarisoft)
* 2G can use random 3rd party cards, but it's not really convenient in most cases
* Anything >= 3G requires you to know the card-specific secrete keys
** in reality that translates to having your own programmable SIM cards

== some smartcard terminology

* card filesystem
** *MF* (Master File): The root directory
** *DF* (Dedicated File): A subdirectory
** *ADF* (Application Dedicated File): Directory of an application (like USIM, ISIM)
** *EF* (Entry File): A regulare file
*** *Transparent EF*: An unstructured file
*** *Linear Fixed EF*: An file consisting of fixed-length records

== specs vs. proprietary

* SIM cards are fully specified by a combination of ISU, ETSI and 3GPP specs
* this covers only the operation after the card has been issued, e.g.
** reading and writing files accessible without PIN auth, after PIN auth
** performing GSM authentication or UMTS AKA
* it does _not_ cover how the card is issued/provisioned
** secret key material (Ki / K / OP / OPc) is not readable from the card
** it is an implementation detail on how the card manufacturer writes those to the card
* this leads to the need for card-specific support / code in software like pySim

== pySim "classic"

* originally developed in 2009 by Sylvain Munaut
* Linux/FOSS tool for configuration of SuperSIM/MagicSIM
* replicated what proprietary vendor tools did to configure IMSI, Ki, ...
* those ultra-cheap Chinese SIM cards at the time (from the SIM card cloning days of COMP128v1) shipped with binary-only windows programs

== pySim "classic" fundamentals

* library code
** _transport_ - how to exchange APDUs with card
** _commands_ - encoding of ISO7816-4 / TS 51.011 / ETS 102 221 commands
** _cards_ - classes implementing various different programmable SIM cards
* executable programs
** `pySim-prog` for card programming
** `pySim-read` for card reading

== pySim transports

* transport - how to exchange APDUs with card
** _serial_ transport for serial readers (UART Rx+Tx connected to I/O)
** _pcsc_ transport for pcsc-lite supported readers (e.g. USB CCID)
* some more exotic options added later on by Vadim Yanitskiy:
** _calypso_ for using L1CTL to talk to OsmocomBB on Ti Calypso phone
** _modem_atcmd_ for using AT+CSIM commands talking to a cellular modem

== advanced pySim "classic" features

* batch programming of cards, using CSV file input
* even with ability to execute external tool for driving motorized card readers

== Limitations of pySim "classic"

* supports only very few files/parameters that can be programmed
** IMSI, MSISDN, Ki and SMSP was sufficient for making classic GSM SIMs work on your own network
** today, people want to configure VoLTE/IMS parameters, 5G SUCI stuff, etc.
* adding more fields to pySim-classic makes it unusable: Who wants to specify 250 different fields as command line arguments?

== Usability of pySim "classic"

* it was designed to program a batch of cards with defaults and random keys
* there are all kinds of 'automagic' bits happening if not all fields are specified
* pySim at least originally assumed that you program a full card, and not that you perform incremental small changes
* no user manual, very limited RADME and wiki instructions

== the road to pySim-shell

What to do about the user interface?

* GUIs are bad, they are not really scriptable/automatizable
* GUIs are hard to portable, test automatically, etc.
* people setting up FOSS cellular programs most likely use bash/zsh/tcsh on daily basis, so they are familiar with the concept of interactive shells / CLIs
* so let's do a CLI / shell / REPL!

== pySim-shell and `cmd2`

* uses `cmd2` python module for the shell
** extremely powerful framework for building CLI/shell type interfaces
** allows us to focus on implementing logic (like individual commands), not the shell itself
* supports output redirection to local files
* supports piping output through external programs ("| grep foo")
* supports `argparse` to define commands and their options
* allows to dynamically register and unregister commands
* allows full control over tab completion items. 

== pySim-shell UI fundamentals

* some global commands for things like PIN/CHV verification, enabling, disabling, unblcoking
* you `select` your way through the card filesystem, like 'cd' on bash/zsh/...
** tab-completion for [known] file names

.Selecting a file
----
pySIM-shell (MF/ADF.USIM)> select EF.IMSI
{
    "file_descriptor": {
        "shareable": true,
        "file_type": "working_ef",
        "structure": "transparent"
    },
    "file_identifier": "6F07",
    "proprietary_info": {
        "proprietary_D0": "20",
        "proprietary_D2": "0F"
    },
    "life_cycle_status_int": "operational_activated",
    "security_attrib_ref_expanded": "6F0603",
    "file_size": 9,
    "short_file_id": "38"
}
pySIM-shell (MF/ADF.USIM/EF.IMSI)>
----

== pySim-shell reading/writing

* context-sensitive 'action' commands depend on the type of file selected
** `read_binary`, `update_binary`, ... for transparent EF
** `read_record`, `update_record`, ... for linear fixed EF

.Reading and updating a transparent EF
----
pySIM-shell (MF/ADF.USIM/EF.IMSI)> read_binary
089910070000400300
pySIM-shell (MF/ADF.USIM/EF.IMSI)> update_binary 089910070000400301
pySIM-shell (MF/ADF.USIM/EF.IMSI)> read_binary
089910070000400301
----

== pySim-shell decoding/encoding

* working with hex-encoded binary data is not very convenient
* each specified file in the filesystem is represented by a python class
** class can provide encoder + decoder methods to go from binary to JSON and vice-versa
* some shell commands combine reading a file from the card + decoding / encoding
** `read_binary_decoded`, `write_binary_decoded`, ... for transparent EF
** `read_record_decoded`, `write_record_decoded`, ... for linear fixed EF

.Reading and updating a decoded transparent EF
----
pySIM-shell (MF/ADF.USIM/EF.IMSI)> read_binary_decoded
{
    "imsi": "901700000043000"
}
pySIM-shell (MF/ADF.USIM/EF.IMSI)> update_binary_decoded '{"imsi": "901700000043999"}'
pySIM-shell (MF/ADF.USIM/EF.IMSI)> read_binary_decoded
{
    "imsi": "901700000043999"
}
----

== pySim-shell editing of single parameters

* many files contain a lot of parameters
* entering all of them as a single line in `update_binary_decoded` becomes rather cumbersome
* `edit_binary_decoded` and `edit_record_decoded` to the rescue
** reads data from the card
** decodes it to JSON
** spawns whatever is your favorite text editor
** you do your editing, save + exit
** re-encodes the modified JSON
** writes the result to the card

== pySim-shell `export`

* one-shot command to iterate over all [known] files below the current DF and dump them
* output format is structured in a way that you can use it as a pySim-shell script
* results in easy method to backup and restore a card

NOTE:: only the ETSI/3GPP standard files that are covered by pySim classes are exported.  There may be any
number of proprietary additional files on the card.  Particularly K/OP/OPc or OTA keys are not exported this
way

== pySim-shell advanced cmd2 usage

* `run_script`: execute any text file as script inside the pySim-shell
* `run_pyscript`: execute a python script with access to all of the state / classes / methods
* `ipy`: run an interactive IPython session
* `py`: run an interactive python interpreter
** `card` represents the `pySim.cards.Card` instance
** `rs` represents the `pySim.filesystem.RuntimeState`

.Using interactive python shell with access to pySim-shell state
----
pySIM-shell (MF)> py
Python 3.9.2 (default, Feb 28 2021, 17:03:44)
[GCC 10.2.1 20210110] on linux
Type "help", "copyright", "credits" or "license" for more information.

End with `Ctrl-D` (Unix) / `Ctrl-Z` (Windows), `quit()`, `exit()`.
Non-Python commands can be issued with: app("your command")

>>> rs.selected_file
<pySim.filesystem.CardMF object at 0x7f97952f3070>
>>> rs.selected_file.desc
'Master File (directory root)'
----


== Other recent pySim developments

* introduction of `sphinx` for generating documentation
* adding API documentation at least for all of the *new* classes/methods that are used to build pySim-shell,
  and which you need to extend to cover more files
* automatic generation of pySim-shell command reference from `argparse` instances

== Call for Contributions

* pySim-shell is already very usable, but needs your help!
** most files still need encoder + decoder methods to be implemented
** we need automatic testing of encoders + decoders
** we want JSON schema for the JSON representation of each file
** we want to have batch programming in pySim-shell

So if you know a bit of python and care about SIM cards, please send your patches!

== Further Reading

* https://osmocom.org/projects/pysim/wiki[pySim project page on osmocom.org]
* https://git.osmocom.org/pysim/about/[pySim source code / git repository]
* https://media.ccc.de/v/36c3-10737-sim_card_technology_from_a-z[Video of talk "SIM card technology from A-Z"]


== EOF

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