rtl

RISC-V Crypto RTL

Notes on the experimental RTL for implementing proposed instructions.

---

• Experimental Results
  • Component Modules
  • Top Level Modules
  • SBoxes
• Tool Flow

This directory contains experimental Verilog RTL implementing the proposed RISC-V scalar crypto instructions.

Experimental Results

These are the results of running each design through Yosys and ABC to get repeatable synthesis results.

• Yosys has a "Simple CMOS" library, which targets only NAND, NOR, NOT cells. This is used to give a "NAND2 Equivalent" measurement.

• "Longest Topological Path Length" from input to output, measured after mapping to Yosys Abstract Cell representation.

Note that only the Cryptography Extension specific instructions are implemented here. For the shared Bitmanip instructions, see the draft Bitmanip specification.

Component Modules

Each of these modules implements a single instruction group. Some are parameterised by XLEN.

Module Name	NAND2 Cells	LTP
riscv_crypto_fu_lut4 (RV32)	566	6
riscv_crypto_fu_lut4 (RV64)	1938	7
riscv_crypto_fu_saes32	1176	30
riscv_crypto_fu_saes64 (8 Sbox)	8663	28
riscv_crypto_fu_saes64 (4 Sbox)	6277	29
riscv_crypto_fu_ssha256	737	5
riscv_crypto_fu_ssha512 (RV32)	701	6
riscv_crypto_fu_ssha512 (RV64)	1986	4
riscv_crypto_fu_ssm3	474	3
riscv_crypto_fu_ssm4	724	25
riscv_crypto_fu_saes32_ssm4	1436	33

Notes:

• All modules compute their results in a single clock cycle and are completely combinatorial. I.e. they do not register their outputs.

• The riscv_crypto_fu_saes64 module (64-bit scalar AES instructions) implements 8 Forward and Inverse SBoxes, and so is an upper bound on the expected size of the implementaiton.

• The riscv_crypto_fu_saes32_ssm4 module implements both the scalar 32-bit AES instructions, and the scalar SM4 instructions. These can be implemented such that they share a considerable amount of datapath logic. The combined module is smaller than the sum of the two individual modules (riscv_crypto_fu_saes32 and riscv_crypto_fu_ssm4) at the cost of a longer critical path.

Top Level Modules

Two modules are provided as "drop-in" functional units for RV32 and RV64 CPUs.

• Both can be configured to support any combination of the scalar crypto instructions.

• They can also optionally drop support for AES decryption instructions.

• The RV32 core can also optionally use the combined AES+SM4 module, reducing its size but increasing its path length.

• The cores optionally allow gating of inputs to each sub-module. This prevents downstream toggling in logic we are not using for the current instruction and saves power.

Module Name	Combined AES/SM4	Gate Inputs	NAND2 Cells	LTP
riscv_crypto_fu_rv32	No	No	4267	29
riscv_crypto_fu_rv32	Yes	No	3845	34
riscv_crypto_fu_rv32	Yes	Yes	3672	35
riscv_crypto_fu_rv64	N/A	No	13678	29
riscv_crypto_fu_rv64	N/A	Yes	14126	29
RocketCore RV32 MulDiv	N/A	N/A	5167	43
RocketCore RV64 MulDiv	N/A	N/A	12404	68

Notes:

• The numbers reported here are for all instructions being included.

• The RV64 core instances 8 sboxes for the 64-bit AES instructions.

• The results with(out) logic gating of the inputs are a little counter-intuitive. It is possible the synthesis tool is "being clever". I have no way of evaluating the effectivness of the logic gating at the moment, 3'rd party evaluations are very welcome.

• The RocketCore RV32 and RV64 mutliplier/divider units are included for comparison. These numbers are taken from the Bitmanip Draft specification v0.92.

SBoxes

The AES and (to a lesser extent) SM4 SBox implementations can dominate the area and timing of the crypto cores. The implementations used here are those found in Markku's lwaes_isa repository. They use the Boyar-Peralta construction for the AES SBox, and share the non-linear middle layer between the AES and SM4 SBoxes.

Module Name	NAND2 Cells	LTP
riscv_crypto_aes_fwd_sbox	293	15
riscv_crypto_aes_inv_sbox	306	15
riscv_crypto_sm4_sbox	307	17
riscv_crypto_aes_sm4_sbox	742	19

Note: the riscv_crypto_aes_sm4_sbox module combines all three SBox modules into one, using the shared middle non-linear layer.

Tool Flow

You will need Yosys, SymbiYosys and Verilator installed for these flows to work.

• Make sure you have run the project workspace setup script:

  $> bin/conf.sh
  $> cd $REPO_HOME/rtl

• To list the available synthesis targets:

  $> make print-synth-targets

  Or just use tab-completion for make.

• All of the synthesis jobs can be run in one go with:

  $> make synth-all

• Use verilator in linting mode:

  $> make lint-all

• The model checking targets can be run using:

  $> make prove-all

• The results of synthesis and simulation runs are placed in $REPO_BUILD/rtl/*.