An optimized implementation of the Data Encrpytion Standard (DES) in C.

To attain optimal efficiency, the algorithm incorporates several optimization techniques proposed by Eli Biham in his paper titled "A Fast New DES Implementation in Software."

The core concept is to parallelize encryption operations. The quantity of encryptions is limited by the hardware employed in the encryption process. Nonetheless, leveraging widely available 64-bit processors enables the simultaneous execution of 64 encryptions. To achieve this, we can represent 64 data blocks as a transposed matrix of bits. This approach facilitates encryption through straightforward bitwise operations, eliminating the need for iterative processing through each bit 64 times.

Furthermore, we obviate the need for expensive permutations. Instead, we directly access individual bits as if they were permuted. This eliminates the need for performing initial permutations or generating subkeys for each round, resulting in significant time and resource savings during the encryption process.

Another notable improvement in performance was achieved by employing logic gate operations within S-boxes as opposed to the traditional approach of using lookup tables. Moreover, each S-box already includes the P permutation and the E expansion in its 64-bit result. The implementation of S-boxes within this algorithm is indeed noteworthy, boasting an impressive average gate count of 56. The code, credited to Matthew Kwan, was obtained from the following website. You can gain further insights by consulting his paper on the subject, titled "Reducing the Gate Count of Bitslice DES." There is an even faster implementation, enhanced by impressive 17%, that is utilized in the John the Ripper password cracker.

To test this implementation, the project provides a simple demo (see main.c):

mkdir build \
    && gcc -O3 -Iinclude -o build/out main.c src/*.c \
    && build/out \
    && rm -rdf build