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Multiplication in Z2m[x] on Cortex-M4

This code package contains the software accompanying the paper "Faster multiplication in Z2m[x] on the Cortex-M4 to speed up NIST PQC candidates". The paper is available at

Large parts of the benchmarking code in this package and this README document are based on PQM4.

The implementations of the schemes, Kindi, NTRU-HRSS, NTRUEncrypt, RLizard and Saber, are included as they were as the time of writing. They will not be maintained here. The schemes are purely included for demonstration purposes and to keep the results described in the paper easily verifiable. We instead refer to PQM4 for up-to-date Cortex-M4 implementations.

Setup and installation

Our target platform is the STM32F4 Discovery board featuring an ARM Cortex-M4 CPU, 1 MiB of Flash, and 192 KiB of RAM. Connecting the development to the host computer requires a mini-USB cable and a USB-TTL converter together with a 2-pin dupont / jumper cable.

We rely on Python version 3.6 or newer for benchmarking scripts and generation of the assembly code.

Installing the ARM toolchain

The build system assumes that you have the arm-none-eabi toolchain toolchain installed. On most Linux systems, the correct toolchain gets installed when you install the arm-none-eabi-gcc (or gcc-arm-none-eabi) package. On some Linux distributions, you will also have to explicitly install libnewlib-arm-none-eabi .

Installing stlink

To flash binaries onto the development board, we are using stlink. Depending on your operating system, stlink may be available in your package manager -- if not, please refer to the stlink GitHub page for instructions on how to compile it from source (in that case, be careful to use libusb-1.0.0-dev, not libusb-0.1).

Installing pyserial

The host-side Python code requires the pyserial module. Your package repository might offer python-serial or python-pyserial directly (as of writing, this is the case for Ubuntu, Debian and Arch). Alternatively, this can be easily installed from PyPA by calling pip install -r requirements.txt (or pip3, depending on your system). If you do not have pip installed yet, you can typically find it as python3-pip using your package manager.

Connecting the board to the host

Connect the board to your host machine using the mini-USB port. This provides it with power, and allows you to flash binaries onto the board. It should show up in lsusb as STMicroelectronics ST-LINK/V2.

If you are using a UART-USB connector that has a PL2303 chip on board (which appears to be the most common), the driver should be loaded in your kernel by default. If it is not, it is typically called pl2303. On macOS, you will still need to install it (and reboot). When you plug in the device, it should show up as Prolific Technology, Inc. PL2303 Serial Port when you type lsusb.

Using dupont / jumper cables, connect the TX/TXD pin of the USB connector to the PA3 pin on the board, and connect RX/RXD to PA2. Depending on your setup, you may also want to connect the GND pins.

Run hostside/ to receive and print output from the board.


We rely on the libopencm3 firmware to ease development for the STM32F4 Discovery board. It is included as a submodule. After cloning the repository, initialize it using git submodule update --init. Then, compile it by calling make in the libopencm3 directory.

Testing and benchmarking full schemes

To generate the testing and benchmarking binaries for all schemes run make. This will generate optimized polynomial multiplication using the optimal method for each scheme. For each of the schemes {saber, kindi256342, ntruhrss, ntru-kem-743, rlizard-1024} this will build

  • a test-{scheme}.bin which runs key generation, encapsulation, and decapsulation and checks if the obtained keys are the same. For each of the schemes this should print OK KEYS
  • a benchmarks-{scheme}.bin which prints the cycle counts spent in key generation, encapsulation, and decapsulation
  • a stack-{scheme}.bin which prints the stack usage of key generation, encapsulation, and decapsulation

These binaries can be flashed to the board using st-flash write {binary} 0x8000000.

To run all benchmarks for all schemes, we also provide

Testing and benchmarking polynomial multiplication

The optimized polynomial multiplication procedures can also be tested and benchmarked individually. For this you need to supply the multiplication method, the size of your input polynomials n, and the schoolbook threshold t. For dimensions ≤ t, schoolbook multiplication will be used. Our code was tested for 1 ≤ n ≤ 1024. We support four multiplication methods which come with restrictions on the modulus q

method possible q test / benchmark binary
notoom ≤ 216 benchmark-karatsuba_{n}_{t}.bin
toom3 ≤ 215 benchmark-toom3_{n}_{t}.bin
toom4 ≤ 213 benchmark-toom4_{n}_{t}.bin
toom4toom3 ≤ 211 benchmark-toom4toom3_{n}_{t}.bin

To test and benchmark these multiplications, you need to specify the method, n, and t as given in the table above, e.g. make benchmark-toom4_1024_16.bin. This also validates that the result is actually correct.

We provide scripts for running these benchmarks for all supported n and methods:,,, and

Generating stand-alone polymul_asm

The scripts can also be used to generate optimized polynomial multiplication assembly independent of the five schemes analyzed here. The signature of the generated polymul_asm routine is

void polymul_asm(uint16_t r[2*n-1],  const uint16_t a[n], const uint16_t b[n]);

To generate the polymul_asm corresponding to a specific multiplication method, an input polynomial degree n, and the schoolbook threshold t, run make mult_{method}_{n}_{t}.s, e.g. make mult_toom4_256_16.s.

Benchmark results

Refer to the paper for a more detailed analysis of the benchmarks. For convenience, we include benchmarks for the small schoolbook multiplications, below. The cycle counts include an overhead of approximately 50 cycles for benchmarking.

n cycles n cycles n cycles n cycles
1 60 13 247 25 996 37 2 112
2 66 14 268 26 1 164 38 2 113
3 72 15 359 27 1 160 39 2 106
4 79 16 360 28 1 287 40 2 107
5 89 17 506 29 1 283 41 2 478
6 97 18 503 30 1 285 42 2 863
7 110 19 549 31 1 350 43 2 868
8 117 20 551 32 1 351 44 2 864
9 135 21 679 33 1 569 45 3 038
10 144 22 677 34 1 701 46 3 039
11 175 23 725 35 1 697 47 3 032
12 184 24 727 36 1 699 48 3 033


All files, except the source code in pqm4/, fall under the CC0 Public Domain dedication, either as a result of licensing through this project or as a consequence of the CC0 Public Domain dedication of PQM4 or the eXtended Keccak code package. See the license details of PQM4 for more specific information on the licenses that apply to the individual schemes.


Code accompanying the paper "Faster multiplication in Z2m[x] on Cortex-M4 to speed up NIST PQC candidates"




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