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Cryptnox Python Communication Library

Warning: This is a beta release of the software. It is released for development purposes. Use at your own risk.

A Python3 library to use the Cryptnox smartcard applet. It provides high level functions to send instructions with the Cryptnox and to manage its lifecycle. The core module is CryptnoxPy which provides a Connection class to establish a channel of communication that can be used to initialize a card instance through the factory method.

To buy NFC enabled cards that are supported by this library go to: https://www.cryptnox.com/

License

The library is available under dual licensing. You can use the library under the conditions of GNU LESSER GENERAL PUBLIC LICENSE 3.0+ or contact us to ask about commercial licensing.

Documentation

API documentation can be found in HTML format in the docs folder It is generated using Sphynx from the code and can be generated in other formats too.

Installation and requirements

Requires :

  • Python 3.6-3.9
  • PCSCd on Linux

Ubuntu / Debian

(sudo) apt-get install python3-pip python3-setuptools pcscd

Fedora / CentOS / RHEL

yum install python3-pip python3-setuptools pcsc-lite-ccid

On some Linux, starts PCSCd service

(sudo) systemctl start pcscd
(sudo) systemctl enable pcscd

Installation of this library

Make sure that you installed the required packages for your system. See the “Requires” section above.

Install with pip:

pip install cryptnoxpy

Install from source:

Download and run in its directory:

pip install .

or:

pip install git+ssh://git@github.com/Cryptnox-Software/cryptnoxpy.git

This might require sudo on some systems.

Remove:

pip uninstall cryptnoxpy

Installation issues

If the Linux system doesn't have Python 3.6, 3.7, 3.8 nor 3.9, install Python 3.7 with the following recipe (Debian like):

sudo apt-get install -y make build-essential libssl-dev zlib1g-dev swig libpcsclite-dev
sudo apt-get install -y libbz2-dev libreadline-dev libsqlite3-dev wget curl llvm git
sudo apt-get install -y libncurses5-dev libncursesw5-dev xz-utils tk-dev pcscd opensc
wget https://www.python.org/ftp/python/3.7.8/Python-3.7.8.tgz
tar xf Python-3.7.8.tgz
cd Python-3.7.8
./configure --enable-optimizations
make -j8 build_all
sudo make -j8 altinstall

sudo pip3.7 install git+ssh://git@gitlab.com/cryptnox-phase2/cryptnoxpy.git

# or (if issue about agent forwarding with sudo) :

cd ~
git clone git@gitlab.com:cryptnox-phase2/cryptnoxpy.git
cd cryptnoxpy
sudo pip3.7 install .

In case of pyscard can't be installed automatically with pip:

  1. Try to pip3 install with sudo or root: sudo pip install .
  2. If still a failure, install the following packages: Needed if pyscard can't be installed from package manager sudo apt install python3-dev swig libpcsclite-dev then retry sudo pip install ..

If you use contactless readers on Linux, the RFID modules need to be disabled :

sudo rmmod pn533_usb
sudo rmmod pn533
sudo rm -r /lib/modules/*/kernel/drivers/nfc/pn533

Update issues

In case you just want to update the package, with old pip version on some Linux, it is better to remove and reinstall the package:

sudo pip uninstall cryptnoxpy
sudo pip install .

Library use

To get the card a connection has to be established with the reader’s index. The connection can then be passed to the factory that will initialize an object for the card in the reader from the correct class for the card type and version.

import cryptnoxpy

try:
    connection = cryptnoxpy.Connection(0)
except cryptnoxpy.ReaderException:
    print("Reader not found on index")
else:
    try:
        card = cryptnoxpy.factory.get_card(connection)
    except cryptnoxpy.CryptnoxException as error:
        # There is an issue with loading the card
        # CryptnoxException is the base exception class for module
        print(error)
    else:
        # Card is loaded and can be used
        print(f"Card serial number: {card.serial_number}")

The factory will:

  • connect to the card
  • select the applet
  • read the applet parameters
  • select class to handle the card

The card contains basic information:

  • card.serial_number : Integer : Card/applet instance Unique ID
  • card.applet_version : 3 integers list : Applet version (ex. 1.2.2)

Initialization and pairing

Right after the installation, the applet is not initialized, and the user needs to send some parameters to use the card. The initialization can be executed once. Any change of the base parameters requires a full applet reinstallation (except PIN/PUK change).

After the initialization, the card and the PC must share a common secret to be used as authenticated secure channel. This secret is required any time further, to communicate with the card (using a secure channel). The registration of this common secret is done during the init phase.

The init parameters required are :

  • Name (up to 20 chars string)
  • Email (up to 60 chars string)
  • PIN (9 digits string)
  • PUK (15 digits string)
  • optional : the first Paring Secret (32 bytes bytearray)
pairing_key = card.init(name, email, pin, puk, pairing_secret)

The returned data is the first PairingKey (32 bytes byte-array) and its index (0) : 0x00 + ParingKeySlot0

During the initialization phase, until the user public key for authentication registration is allowed, the set_pairing_key command is also allowed. Then set_pairing_key needs the applet to have the signature unlocked.

After getting the pairing_key, the user needs to store it in a safe place. In the case the client would communicate with several cards, the user needs to associate the pairing_key with the instance serial number of the card, so that the user client can keep track of multiple cards, and use the right one with the right card. The pairing_key must be saved in a file to reconnect the next time to this card. It should be saved with the serial number of card in order to associate this card with this key.

A common hardcoded PairingKey can be used.

After this init phase, the secure channel must be used with all communications with the card. A secure channel is an encrypted and 2-ways authenticated link layer with the card using standards APDU messages. Many applet commands require a secure channel.

PIN

The PIN chosen during the initialization needs to be provided after each card reset, and a secure channel is opened.

To test a PIN string, simply use:

card.verify_pin(pin)

Seed administration

The applet manages a 256 bits master secret called the “seed”. This is the BIP32 Master Seed, and can be externally computed from a mnemonic to a binary seed using BIP39. The key pairs used for ECDSA are then computationally derived from this seed using BIP32 derivation scheme.

Seed generation

The seed can be generated in the card using the random number generator in the java chip system (AIS 20 class DRG.3). Doing this way, the seed secret never escapes the card protection.

The method to generate a new seed key is:

card.generate_seed(pin)

The card can also randomly generate BIP39 mnemonics words list. But in this case, the query answer is only output and not used internally by the card. It is administrator responsibility to get a mnemonic using the GENERATE MNEMONIC command and then eventually compute the corresponding seed, which can be uploaded in the card using RECOVER KEY command. We don’t recommend doing so, this is very insecure, as the seed is exposed in clear and full in the user’s system.

Recovery

The Cryptnox applet can load binary seed.

The seed is loaded in the card using this method:

card.load_seed(seed, pin)

Seed is 32 bytes.

Once this seed is loaded in the card using the load_seed method, this card now behaves like were (or the one) it was backup. Be aware that key derivation paths are not backup, and must be identical to retrieve the same key pairs. See derivation and key system just below for more details.

For more details about the recovery, see load_seed operation in the API documentation.

Derivation and keys system

The card applet is fully compliant with BIP32, except the maximum depth of derivation from the master key is 8 levels. It can be turned on for the card to return extended public keys for use in applications requiring it.

The card stores the present key pair (and its parent), used for signature. This can be changed using the derive method, and also during a signature command, giving a relative path (from the present key pair), or in an absolute path (from the master key pair). See derive method in the API documentation.

Any derivation aborts any opened signing sessions and resets the authentications for signature. The generated key is used for all subsequent sign sessions.

The ability to start derivation from the parent keys allows to more efficiently switch between children of the same key. Note however that only the immediate parent of the current key is cached so one cannot use this to go back in the keys hierarchy.

For ease of use, the user can derive from the root master node key pair (absolute path) at each card startup, or even before each signature. This takes a couple of seconds. So this is better to store intermediate public keys hash and check the status to observe the current key pair in use. This off-card complex key management is not needed if the signatures volume is below one thousand per day.

See derive and sign methods in the API documentation.

EC Signature

The derivation of the key pair node can be also possible using the signature command (relative or absolute).

The card applet can sign any 256 bits hash provided, using ECDSA with 256k1 EC parameters. Most of the blockchain system used SHA2-256 to hash the message, but this card applet is agnostic from this point, since the signature is performed on a hash provided by the user. Note that this hash needs to be confirmed by the users beforehand, when they provide their EC384 signature of this hash.

The code to sign with the EC current key node is:

signature = card.sign(data_hash, cryptnoxpy.Derivation.CURRENT_KEY)

data_hash is a byte-array containing the EC hash to sign using ECDSA ecp256k1:

The signature a byte array, encoded as an ASN1 DER sequence of two INTEGER values, r and s.

See the sign method in the API documentation for more information.

Changelog

All notable changes to this project will be documented in this file.

The format is based on Keep a Changelog, and this project adheres to Semantic Versioning.

1.0.4 - 2021-09-09

Changed

  • Improvements in setup

1.0.3 - 2021-09-07

Changed

  • Documentation changed to rst
  • Version number stored in the module instead of getting it from pbr

Removed

  • PBR dependency

Fixed

  • PyPI doesn’t install dependencies

1.0.0 - 2021-08-20

Added

  • Card operations
  • Pipfile and requirements for setting up environment
  • Setup file to install the library

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