# 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](https://github.com/lowRISC/opentitan/issues).

## 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](https://docs.opentitan.org/doc/ug/install_instructions/#software-development).
For more details about the toolchain, see the [user
guide](https://docs.opentitan.org/doc/ug/otbn_sw)).

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

```sh
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.

### 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 basic checks on the data. The
binutils-based toolchain described above uses this information. Other
users include:

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

  - `util/otbn-rig`: A random instruction generator for OTBN. See
    util/rig/README.md for further 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:

```sh
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:

```sh
# 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.

```sh
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.

### Run OT earlgrey simulation with the OTBN model, rather than the RTL design

For simulation targets, the OTBN block can be built with both the RTL
implementation and the Python-based instruction set simulator (hereafter called
the ISS) compiled in. When running the simulation the plusarg `OTBN_USE_MODEL`
can be used to switch between the RTL implementation and the model, without
recompiling the simulation.

The Verilator simulation of Earl Grey (`lowrisc:systems:top_earlgrey_verilator`)
builds the model by default when compiling the simulation and nothing else needs
to be done. For other simulation targets, set the `OTBN_BUILD_MODEL` define,
e.g. by passing `--OTBN_BUILD_MODEL` to fusesoc.

To run the simulation against the OTBN ISS pass `+OTBN_USE_MODEL=1` to the
simulation run, e.g.

```sh
build/lowrisc_systems_top_earlgrey_verilator_0.1/sim-verilator/Vtop_earlgrey_verilator \
  --meminit=rom,build-bin/sw/device/boot_rom/boot_rom_sim_verilator.elf  \
  --meminit=flash,build-bin/sw/device/tests/dif_otbn_smoketest_sim_verilator.elf \
  +UARTDPI_LOG_uart0=- \
  +OTBN_USE_MODEL=1
```

The simulation communicates with the model by creating a directory in
the temporary directory (/tmp or TMPDIR) and filling it with files.
Normally, it cleans up after itself. If something goes wrong and you'd
like to look at these files, set the `OTBN_MODEL_KEEP_TMP` environment
variable to `1`.

### Run the ISS on its own

There are currently two versions of the ISS and they can be found in
`dv/otbnsim`. The easiest to use is `dv/otbnsim/standalone.py`. This
takes an OTBN binary as an ELF file (as produced by the standard
linker script for `otbn-ld`) and can dump the resulting DMEM if given
the `--dmem-dump` argument. To see an instruction trace, pass the
`--verbose` flag.

There is also `dv/otbnsim/otbnsim.py`. This takes flat binary files
with the contents of IMEM and DMEM and, when finished, generates a
cycle count and dumps DMEM contents. This is used to implement the
model inside of simulation, but is probably not very convenient for
command-line use otherwise.

## Test the ISS

The ISS has a simple test suite, which runs various instructions and
makes sure they behave as expected. You can find the tests in
`dv/otbnsim/test` and can run them with `make -C dv/otbnsim test`.


## Update the RISC-V part of the ISS

The OTBN model is an instruction set simulator written in Python. It lives
in the `opentitan` repository in the `util/otbnsim` directory, but builds on top
of the `riscv-model` Python package. The code for this package can be found at
https://github.com/wallento/riscv-python-model.

To update the model to the latest recommended version, run `pip`.

```sh
# Ensure you have the latest Python dependencies installed
pip3 install --user -U python-requirements.txt
```