ctaes
=====

Simple C module for constant-time AES encryption and decryption.

Features:
* Simple, pure C code without any dependencies.
* No tables or data-dependent branches whatsoever, but using bit sliced approach from https://eprint.iacr.org/2009/129.pdf.
* Very small object code: slightly over 4k of executable code when compiled with -Os.
* Slower than implementations based on precomputed tables or specialized instructions, but can do ~15 MB/s on modern CPUs.

Performance
-----------

Compiled with GCC 5.3.1 with -O3, on an Intel(R) Core(TM) i7-4800MQ CPU, numbers in CPU cycles:

| Algorithm | Key schedule | Encryption per byte | Decryption per byte |
| --------- | ------------:| -------------------:| -------------------:|
| AES-128   |         2.8k |                 154 |                 161 |
| AES-192   |         3.1k |                 169 |                 181 |
| AES-256   |         4.0k |                 191 |                 203 |

Build steps
-----------

Object code:

    $ gcc -O3 ctaes.c -c -o ctaes.o

Tests:

    $ gcc -O3 ctaes.c test.c -o test

Benchmark:

    $ gcc -O3 ctaes.c bench.c -o bench

Review
------

Results of a formal review of the code can be found in http://bitcoin.sipa.be/ctaes/review.zip