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Gnuk - An Implementation of USB Cryptographic Token for GnuPG

							 Version 1.2.15
							   Niibe Yutaka
				      Free Software Initiative of Japan

Release Notes

This is the release of Gnuk, version 1.2.15, which has major
incompatible changes to Gnuk 1.0.x.  Specifically, it now supports
overriding key import, but importing keys (or generating keys) results
password reset.  Also, you need to import private keys before changing
your password.  Please update your documentation for Gnuk Token, so
that the instruction of importing keys won't cause any confusion.

It has supports of EdDSA, ECDSA (with NIST P256 and secp256k1), and
ECDH (with X25519, NIST P256 and secp256k1), but this ECC feature is
somehow experimental, and it requires modern GnuPG 2.2 with libgcrypt
1.7.0 or later.

It also supports RSA-4096, but users should know that it takes more
than 8 seconds to sign/decrypt.  Key generation of RSA-4096 just fails,
because the device doesn't have enough memory.

It supports new KDF-DO feature.  Please note that this is
experimental.  To use the feature, you need to use newer GnuPG (2.2.6
or later).  You need to prepare the KDF-DO on your token by the
card-edit/kdf-setup command.

With FST-01SZ, experimental ack button support is available for test.

What's Gnuk?

Gnuk is an implementation of USB cryptographic token for GNU Privacy
Guard.  Gnuk supports OpenPGP card protocol version 3, and it runs on
STM32F103 processor (and its compatible).

I wish that Gnuk will be a developer's soother who uses GnuPG.  I have
been nervous of storing secret key(s) on usual secondary storage.
There is a solution with OpenPGP card, but it is not the choice for
me, as card reader is not common device.  With Gnuk, this issue will
be solved by a USB token.

Please look at the graphics of "gnuk.svg" for the software name.  My
son used to be with his NUK(R), always, everywhere.  Now, I am with a
USB Cryptographic Token by "Gnuk", always, everywhere.


Q0: How Gnuk USB Token is superior than other solutions (OpenPGP
    card 2.0, YubiKey, etc.) ?
A0: Good points of Gnuk are:
    * If you have skill of electronics and like DIY, you can build
      Gnuk Token cheaper (see Q8-A8).
    * You can study Gnuk to modify and to enhance.  For example, you
      can implement your own authentication method with some sensor
      such as an acceleration sensor.
    * It is "of Free Software"; Gnuk is distributed under GPLv3+,
	    "by Free Software"; Gnuk development requires only Free Software
				(GNU Toolchain, Python, etc.), 
	    "for Free Software"; Gnuk supports GnuPG.

Q1: What kind of key algorithm is supported?
A1: Gnuk version 1.0 only supports RSA-2048.
    Gnuk version 1.2.x supports 255-bit EdDSA, as well as RSA-4096.
    (Note that it takes long time to sign with RSA-4096.)

Q2: How long does it take for digital signing?
A2: It takes a second and a half or so for RSA-2048. 
    It takes more than 8 seconds for RSA-4096.

Q3: What's your recommendation for target board?
A3: Orthodox choice is Olimex STM32-H103.
    FST-01 (Flying Stone Tiny 01) is available for sale, and it is a
    kind of the best choice, hopefully.
    If you have a skill of electronics, STM32 Nucleo F103 is the best
    choice for experiment.

Q4: What's version of GnuPG are you using?
A4: In Debian GNU/Linux system, I use GnuPG modern 2.2.12.

Q5: What's version of pcscd and libccid are you using?
A5: I don't use them, pcscd and libccid are optional, you can use Gnuk
    Token without them.
    I tested pcscd 1.5.5-4 and libccid 1.3.11-2 which were in Debian

Q6: What kinds of hardware is required for development?
A6: You need a target board plus a JTAG/SWD debugger.  If you just
    want to test Gnuk for target boards with DfuSe, JTAG debugger is
    not the requirement.  Note that for real use, you need JTAG/SWD
    debugger to enable flash ROM protection.

Q7: How much does it cost?
A7: Olimex STM32-H103 plus ARM-USB-TINY-H cost 70 Euro or so.

Q8: How much does it cost for DIY version?
A8: STM32 Nucleo F103 costs about $10 USD.

Q9: I got an error like "gpg: selecting openpgp failed: ec=6.108", what's up?
A9: Older GnuPG's SCDaemon has problems for handling insertion/removal of
    card/reader.  When your newly inserted token is not found by
    GnuPG, try killing scdaemon and let it to be invoked again.  I do:

	 $ gpg-connect-agent "SCD KILLSCD" "SCD BYE" /bye

    and confirm scdaemon doesn't exist, then,

	 $ gpg-connect-agent learn /bye

Qa: With GNOME 2, I can't use Gnuk Token for SSH.  How can we use it for SSH?
Aa: You need to deactivate seahorse-agent and gnome-keyring, but use
    gpg-agant for the role of ssh-agent.  For gnome-keyring please do:

      $ gconftool-2 --type bool --set /apps/gnome-keyring/daemon-components/ssh false

Qb: With GNOME 3.0, I can't use Gnuk Token at all.  Why?
Ab: That's because gnome-keyring-daemon interferes GnuPG.  Type:

      $ gnome-session-properties

    and at the tab of "Startup Programs", disable check buttons for
    "GPG Password Agent" and "SSH Key Agent".

Qc: With GNOME 3.x (x >= 8?), I can't use Gnuk Token at all.  Why?
Ac: That's because gnome-keyring-daemon interferes GnuPG.  Please
    disable the invocation of gnome-keyring-daemon.  In Debian
    wheezy, it's in the files /etc/xdg/autostart/gnome-keyring-ssh.desktop
    and /etc/xdg/autostart/gnome-keyring-gpg.desktop.
    We have a line something like:


    Please edit this line to:


Qd: Do you know a good SWD debugger to connect FST-01 or something?
Ad: ST-Link/V2 is cheap one.  We have a tool/ as flash ROM
    writer program.  STM32 Nucleo F103 comes with the valiant of
    ST-Link/V2.  However, the firmware of ST-Link/V2 is proprietary.
    Now, I develop BBG-SWD, SWD debugger by BeagleBone Green.

Tested features

Gnuk is tested by test suite.  Please see the test directory.

	* Personalization of the card
	  * Changing Login name, URL, Name, Sex, Language, etc.
	* Password handling (PW1, RC, PW3)
	* Key import for three types:
	  * key for digital signing
	  * key for decryption
	  * key for authentication
	* PSO: Digital Signature
	* PSO: Decipher
	* Changing value of password status bytes (0x00C4): forcesig
	* Verify with pin pad
	* Modify with pin pad
	* Card holder certificate (read)
	* Removal of keys
	* Key generation on device side for RSA-2048
	* Overriding key import

Original features of Gnuk, tested manually lightly:

	* OpenPGP card serial number setup
	* Card holder certificate (write by UPDATE BINARY)
	* Upgrading with "EXTERNAL AUTHENTICATE" by reGNUal


We use Olimex STM32-H103 board and Flying Stone Tiny 01 (FST-01).

With DfuSe support, STBee is also our targets.  But this target with
DfuSe is for experiment only, because it is impossible for DfuSe to
disable read from flash.  For real use, please consider killing DfuSe
and enabling read protection using JTAG debugger.

For experimental PIN-pad support, I connect a consumer IR receive
module to FST-01, and use controller for TV.  PIN verification is
supported by this configuration.  Yes, it is not secure at all, since
it is very easy to monitor IR output of the controllers.  It is just
an experiment.  Note that hardware needed for this experiment is only
a consumer IR receive module which is as cheap as 50 JPY.

Note that you need pinpad support for GnuPG to use PIN-pad enabled
Gnuk.  The pinpad support for GnuPG is only available in version 2.

Build system and Host system

Makefile is written for GNU make.  You need Bash 4.x for configure.

If your bash is not installed as /bin/bash, you need to run configure
script prepending 'bash' before './configure'.

Some tools are written in Python.  If your Python is not installed as
/usr/bin/python, please prepend 'python' for your command invocation.
I use Python 3.7 and PyUSB 1.0.0.

Source code

Gnuk source code is under src/ directory.

Note that SHA-2 hash function implementation, src/sha256.c, is based
on the original implementation by Dr. Brian Gladman.  See:
(was at:


It is distributed under GNU General Public Licence version 3 or later
(GPLv3+).  Please see src/COPYING.

Please note that it is distributed with external source code too.
Please read relevant licenses for external source code as well.

The author(s) of Gnuk expect users of Gnuk will be able to access the
source code of Gnuk, so that users can study the code and can modify
if needed.  This doesn't mean person who has a Gnuk Token should be
able to access everything on the Token, regardless of its protections.
Private keys, and other information should be protected properly.

External source code

Gnuk is distributed with external source code.

* chopstx/  -- Chopstx 1.18

  We use Chopstx as the kernel for Gnuk.

  Chopstx is distributed under GPLv3+ (with a special exception).

* polarssl/  -- based on PolarSSL 1.2.10 (now mbedTLS)

  Souce code taken from:

  We use PolarSSL for RSA computation, and AES encryption/decryption.

  PolarSSL is distributed under GPLv2+.  We use PolarSSL under GPLv3
  as our options.

  The file include/polarssl/bn_mul.h is heavily modified for ARM

  The function rsa_private in polarssl/library/rsa.c is modified so
  that it doesn't check T against N.  The function rsa_pkcs1_sign is
  modified to avoid warnings in case of !POLARSSL_PKCS1_V21.

  The functions rsa_pkcs1_verify and rsa_rsassa_pkcs1_v15_verify in
  include/polarssl/rsa.h and polarssl/library/rsa.c are modified
  (fixed) for last argument SIG, as the memory at SIG aren't modified
  by those routines.

  The constant POLARSSL_MPI_MAX_SIZE in include/polarssl/bignum.h is
  modified for 2048-bit keys only Gnuk.

  The function mpi_mul_hlp in library/bignum.c is modified for more
  optimization for ARM Cortex-M3.  Functions mpi_montred, mpi_sub_hlp,
  mpi_sub_abs, mpi_mul_mpi, mpi_montmul, and mpi_exp_mod are modified
  to avoid side channel attacks.  Note that we don't use RSA-blinding
  technique for Gnuk.  Function mpi_gen_prime and mpi_is_prime are
  modified to use Fouque-Tibouchi method.  Function mpi_exp_mod is
  modified to use new function mpi_montsqr for speed up.

  The file library/aes.c is modified so that some constants can
  go to .sys section.

  The file include/polarssl/config.h are modified not to define
  POLARSSL_HAVE_LONGLONG to avoid linking libgcc, to define
  POLARSSL_AES_ROM_TABLES to have AES tables, not to define

  And polarssl/library/bignum.c is modified to work on 64-bit machine.

  Aurelien Jarno also modified:


  See ChangeLog (and/or history of git) for detail.

USB vendor ID and product ID (USB device ID)

When you have a vendor ID and assign a product ID for Gnuk, edit the
file GNUK_USB_DEVICE_ID and add an entry for yours.  In this case,
please contact Niibe, so that it is listed to the file in the official
release of the source code.

When you are modifing Gnuk and installing the binary to device, you
should replace the vendor string and serial number to yours (in the
file GNUK_USB_DEVICE_ID and SERIALNO of the script of src/configure),
so that users can see it's not by original vendor, and it is modified

FSIJ allows you to use USB device ID of FSIJ (234b:0000) for devices
with Gnuk under one of following conditions:

  * For everyone for experimental purpose:

    - You must not distribute a binary with FSIJ's USB device ID, but
      must use the binary by yourself only for your experiment.  Note
      that "Distributing binary" includes distributing a device which
      holds the binary.

  * For general individuals:

    - You must use your Gnuk device with a card serial number which is
      *not* by FSIJ.  Easy one would be a card serial number generated
      by chip unique ID.

  * For individuals with explicit permission from FSIJ.

    - You should have an assigned card serial number by FSIJ,
      please use that number for your device.
      (There a file 'GNUK_SERIAL_NUMBER' in the official release.)

FSIJ could give companies or business entities "second source
manufacturer" license to use USB device ID of FSIJ for devices with
unmodified version of Gnuk, provided they support Free Software and
respect users' freedom for computing.  Please ask FSIJ for the

Otherwise, companies which want to distribute Gnuk devices, please use
your own USB vendor ID and product ID.  Please replace vendor string
and possibly product string to yours, when you modify Gnuk.

Host Requirements

For GNU/Linux, PC/SC service is an option, you can use GnuPG's
internal CCID driver instead.  If you chose using PC/SC service,
libccid version >= 1.3.11 is recommended for GNU/Linux.

How to compile

You need GNU toolchain and newlib for 'arm-none-eabi' target.

On Debian we can install the packages of gcc-arm-none-eabi,
gdb-arm-none-eabi and its friends.  I'm using:

	binutils-arm-none-eabi	2.31.1-12+11
	gcc-arm-none-eabi 	15:7-2018-q2-6
	gdb-multiarch 		8.2.1-1

Or else, see for preparation of
GNU Toolchain for 'arm-none-eabi' target.

Change directory to `src':

  $ cd gnuk-VERSION/src

Then, run `configure':

  $ ./configure --vidpid=<VID:PID>

Here, you need to specify USB vendor ID and product ID.  For FSIJ's,
it's: --vidpid=234b:0000 .  Please read section 'USB vendor ID and
product ID' above.

Then, type:

  $ make

Then, we will have "gnuk.elf" under src/build directory.

If you are not the authorized vendor, please never distribute this
file of "gnuk.elf", which includes VID:PID in the image.  If you would
like to distribute the image (for example, to check if it's
reproducible or not), the file "gnuk-no-vidpid.elf" is the one with no

How to install

Olimex STM32-H103 board

If you are using Olimex JTAG-Tiny, type following to invoke OpenOCD
and write "gnuk.elf" to Flash ROM:

  $ openocd -f interface/ftdi/olimex-jtag-tiny.cfg \
            -f board/olimex_stm32_h103.cfg \
            -c "program build/gnuk.elf verify reset exit"

Command invocation is assumed in src/ directory.

Flying Stone Tiny 01

If you are using Flying Stone Tiny 01, you need a SWD writer.

OpenOCD 0.9.0 now supports ST-Link/V2.  We can use it like:

  $ openocd -f interface/stlink-v2.cfg -f target/stm32f1x.cfg \
            -c "program build/gnuk.elf verify reset exit"


Note that this is only for your experiment; Your private key materials
on the board can be accessed by DfuSe.

Reset the board with "USER" switch pushed.  Type following to write
to flash:

  # cd ../tool
  # ./ ../src/build/gnuk.hex

Then, reset the board.

How to protect flash ROM

To protect, invoke OpenOCD like (for FST-01):

  $ openocd -f interface/stlink-v2.cfg -f target/stm32f1x.cfg \
            -c init -c "reset halt" -c "stm32f1x lock 0" -c reset -c exit

After power-off / power-on sequence, the contents of flash ROM cannot
be accessible from JTAG debugger.

Unprotecting is:

  $ openocd -f interface/stlink-v2.cfg -f target/stm32f1x.cfg \
            -c init -c "reset halt" -c "stm32f1x unlock 0" -c reset -c exit

Upon unprotection, flash is erased.

Note that it would be still possible for some implementation of DfuSe
to access the contents, even if it's protected.  If you really want to
protect, killing DfuSe and accessing by JTAG debugger is recommended.

(Optional) Configure serial number and X.509 certificate

This is completely optional.

For this procedure, you need python and pyscard (python-pyscard
package in Debian) or PyUSB (python-usb package in Debian).

(1) [pyscard] Stop scdaemon
    [PyUSB] Stop the pcsc daemon.

If scdaemon is running, please kill it, or you will get "Smartcard
Exception" by "Sharing violation".

  $ gpg-connect-agent "SCD KILLSCD" "SCD BYE" /bye

In case of PyUSB tool, you need to stop pcscd.

  # systemctl stop pcscd

(2) [Optional] Write fixed serial number

If you use fixed serial number in the file 'GNUK_SERIAL_NUMBER', you can do:

  Writing serial number

(3) [Optional] Write card holder certificate

If you have card holder certificate binary file, you can do:

  $ ../tool/ ../../<YOUR-CERTIFICATE>.bin
  Updating card holder certificate

How to run

Debug enabled

If you compiled with --enable-debug option, Gnuk has two interfaces
(one is CCID/ICCD device and another is virtual COM port).  Open
virtual COM port by:

  $ cu -l /dev/ttyACM0

and you will see debug output of Gnuk.

Testing Gnuk

Type following command to see Gnuk runs:

  $ gpg --card-status

Besides, there is a functionality test under tests/ directory.  See

Personalize the Token, import keys, and change the password

You can personalize the token, putting your information like: Name,
Login name, Sex, Languages, URL.  To do so, GnuPG command is:

  $ gpg --card-edit

Note that the factory setting of user password is "123456" and admin
password is "12345678" as the specification.

It is recommended to create your keys on your computer, and import
them to Gnuk Token.  After you create your keys (they must be 2048-bit
RSA), you can import them.

Gnuk supports key generation, but this feature is young and should be
considered experimental.

For detail, please see documentation under doc/.  You can see the HTML
version at:

How to debug

We can use GDB.

  $ arm-none-eabi-gdb gnuk.elf

Inside GDB, we can connect OpenOCD by:

  (gdb) target remote localhost:3333


  (gdb) target extended-remote localhost:3333

You can see the output of PCSCD:

  # /etc/init.d/pcscd stop
  # LIBCCID_ifdLogLevel=7 /usr/sbin/pcscd --debug --foreground

You can observe the traffic of USB using "usbmon".  See the file:

Firmware update

See doc/note/firmware-update.

Git Repositories

Please use:

You can get it by:
    $ git clone

It's also available at:
You can browse at:

I put Chopstx as a submodule of Git.  Please do this:

    $ git submodule update --init

Information on the Web

For more information, please visit:

Please see the FST-01 support pages:

Please consider to join Gnuk-users mailing list:

Your Contributions

FSIJ welcomes your contributions.  Please assign your copyright
to FSIJ (if possible), as I do.

Foot note

* NUK(R) is a registered trademark owend by MAPA GmbH, Germany.


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