# OpenTitan Big Number Accelerator (OTBN)

This directory contains the implementation of the OpenTitan Big Number Accelerator (OTBN). OTBN is a coprocessor for asymmetric cryptographic operations like RSA or Elliptic Curve Cryptography (ECC).

See https://docs.opentitan.org/hw/ip/otbn/doc/index.html for documentation on the current version of OTBN; documentation matching the code in this directory can be found in the doc directory.

OTBN is currently in early development. Please ask questions and report issues through the GitHub issue tracker.

## Develop OTBN

### Build OTBN software

Note the toolchain is still under active development. Full OTBN ISA support isn't complete

An assembler, linker and disassembler for OTBN can be found in hw/ip/otbn/util. These are wrappers around a RISC-V GCC and binutils toolchain so one must be available (see the OpenTitan documentation on obtaining a toolchain. For more details about the toolchain, see the user guide).

hw/ip/otbn/util/build.sh provides a simple script to build a single OTBN assembly files using the toolchain:

hw/ip/otbn/util/build.sh prog.S prog_bin/prog


Will assemble and link prog.S and produce various outputs using prog_bin/prog as a prefix for all output filenames. Run ./hw/ip/otbn/util/build.sh without arguments for more information.

### Run the standalone simulation

Note that OTBN is still in the early development stages so this simulation does not support the full ISA

A standalone environment to run OTBN alone in verilator is included. Build it with fusesoc as follows:

fusesoc --cores-root=. run --target=sim --setup --build lowrisc:ip:otbn_top_sim


It includes functionality to set the initial DMem and IMem contents. The start address is hard coded to 0. Modify the ImemStartAddr parameter in ./dv/verilator/otbn_top_sim.sv to change this. Combined with the build script described above, a single assembly file can be built and run on the simulation as follows:

# Create directory for build outputs
mkdir otbn_build
# Build smoke test
hw/ip/otbn/util/build.sh ./hw/ip/otbn/dv/smoke/smoke_test.S ./otbn_build/smoke

# Run the resulting binary on the OTBN standalone simulation
./build/lowrisc_ip_otbn_top_sim_0.1/sim-verilator/Votbn_top_sim -t \
--meminit=imem,./otbn_build/smoke_imem.elf \
--meminit=dmem,./otbn_build/smoke_dmem.elf


This will initialise the IMem with ./otbn_build/smoke_imem.elf and the DMem with ./otbn_build/smoke_dmem.elf. The -t argument enables tracing. The simulation automatically halts on an ecall instruction and prints the final register values.

### Run the smoke test

A smoke test which exercises some functionality of OTBN can be found, together with its expected outputs (in the form of final register values), in ./hw/ip/otbn/dv/smoke. The test can be run using a script.

hw/ip/otbn/dv/smoke/run_smoke.sh


This will build the standalone simulation, build the smoke test binary, run it and check the results are as expected.

### Build the RTL implementation in OT earlgrey simulation

To build the RTL implementation of OTBN in an earlgrey simulation, run fusesoc without passing the OTBN_MODEL flag. For example, the Verilator simulation can be compiled as follows.

fusesoc --cores-root=. run --target=sim --setup --build lowrisc:systems:top_earlgrey_verilator


### Work with the ISA

The instruction set is described in machine readable form in data/insns.yml. This is parsed by Python code in util/insn_yaml.py, which runs various sanity checks on the data. Current tooling that uses this information:

• util/yaml_to_doc.py: Generates a Markdown snippet which is included in the OTBN specification.