cryptol/examples/SuiteB_FFI

SuiteB_FFI

SuiteB_FFI is a drop-in replacement for the builtin SuiteB module, using the FFI to implement AES operations with hardware AES-NI instructions when available, and falling back to the SuiteB software implementation in Cryptol and Haskell otherwise.

When AES-NI is available, it is several times faster than SuiteB; on a Linux machine with an Intel i7-10510U it is about 5.5x faster. Note that AES-NI is only available on modern x86-64 processors, but you could modify this to support similar instructions for other architectures or bind to a portable C library instead.

The actual AES implementation is written with x86-64 intrinsics in C and based on Intel's AES-NI whitepaper but with modifications to support how the SuiteB module's API is structured. In particular, the representation of round keys as [4][32] in SuiteB means that we need to do a byte swap every time we access memory to correct for endianness.

Maintaining compatibility with SuiteB actually severely limits the speedup that we can achieve: right now about 96% of the time spent calling the encrypt function in Cryptol is spent doing the Cryptol evaluation surrounding the FFI call and the actual AES computation only takes up less than 4% of the total time. This is because the encrypt function in SuiteB only encrypts one block, so if you were to create a foreign function that encrypted many blocks at once, the overhead would be much smaller relative to the work done in the foreign function and you would achieve a much greater speedup.

The implementation passes the AES tests from the Cryptol SuiteB test suite (aes-mct-ecb and aes-vectors) but has not been verified beyond that.

Usage

Run make to build the C code. The code checks for hardware AES-NI support at runtime with cpuid when any AES function is first used. On non-x86-64 platforms, running make will build a dummy implementation that always falls back to software SuiteB. On x86-64, make uses -march=native to build an optimized version for running on the current machine. To build a (somewhat) portable version that will run on any x86-64 machine with AES-NI and SSSE3, use make PORTABLE=1. Once it is built, loading the SuiteB_FFI Cryptol module will automatically load the shared library.

Run make test to test correctness. This runs the tests in the tests/ directory which are copied from Cryptol's /tests/suiteb/ but using the SuiteB_FFI module instead of SuiteB.

Run make perf-test to test performance on the aes-mct-ecb test. This prints out the time it takes to run the test with the FFI version and the Cryptol version.

Run make perf-bench to test the performance of the individual AES-128 functions with Cryptol's :time benchmarking command. This prints out the benchmarked time for the key expansion and encryption functions for both implementations, and the speedup in both cases.

Run make clean to delete generated files.