DEV Community

Cover image for Implementing SHA2 (256/512) algorithm with Rust const generics

Implementing SHA2 (256/512) algorithm with Rust const generics

dandyvica profile image Alain Viguier ・4 min read

The const generics feature is landing in stable Rust from version 1.51. This feature has been longly-awaited, and you can get an idea of its possibilities here: Rust 1.51 release notes

I wanted to give it a try and thought that SHA2 message digest algorithms were a good test bed.

SHA2 algorithm

I'm not going to dig into the details of the SHA message digest algorithm. It's a NIST standard (National Institute of Standards and Technology) labelled FIPS-180. You can get the detailed description in the NIST US government web site: FIPS-180-4

In a nutshell, following is the general algorithm:

  • initialize the hash value and constants
  • pad the massage to be a multiple of 64 or 128 (i.e. 512 or 1024 bits)
  • for each block of data:
    • prepare the message schedule using scrambling functions
    • for each of the 64 or 80 rounds, compute intermediate hash value
  • output hash value

I'm only dealing here with the hash of byte-oriented messages.

What can be noticed are the similarities between SHA224/256 and SHA384/512 in terms of functions, initial hash values,
constants, padding:

Tables SHA224/256 SHA384/512
block size 64 bytes 128 bytes
padding 56 bytes boundary 112 bytes boundary
Initialization vector 8 4-byte values 8 8-byte values
K-constants 64 4-byte values 80 8-byte values
rounds 64 80
functions see below see below

Rust implementation

Taking into account all those common features, we can define a SHA hash as:

// hash is either 256 or 512 bits but always 8 u32 or u64 integers
// T is either u32 or u64
pub struct Hash<T, const BLOCKSIZE: usize, const ROUNDS: usize> {
    pub k_constants: [T; ROUNDS],   // ROUNDS = 64 or 80
    pub hash: [T; 8],
    pub scramble_funcs: ScramblePool<T>, // scrambling functions sigma etc
    pub block: [u8; BLOCKSIZE], // BLOCKSIZE = 64 or 128
Enter fullscreen mode Exit fullscreen mode

Scrambling functions

This terminology is not found in any NIST paper but I found it useful to name those functions. These are a combination of logical or, and, xor, bit rotation or bit shifting. But they are very similar:

  • Ch(x,y,z) = (x & y) ^ (!x & z)
  • Maj(x,y,z) = (x & y) ^ (x & z) ^ (y & z)
  • Σ0 and Σ1 are the same, but with different shift values
  • σ0 and σ1 are the same, but with different shift values

Using const generics, we can define those function generically, for u32 or u64 types but also for shifting values:

    // SIGMA(X) = RotR(X,A) ⊕ RotR(X,B) ⊕ RotR(X,C)
    pub fn SIGMA<const A: u8, const B: u8, const C: u8>(x: T) -> T
        T: BitAnd<Output = T>,
        T: BitXor<Output = T>,
        T: Shifter<T>,
        T: Copy,
        x.right_rotate(A) ^ x.right_rotate(B) ^ x.right_rotate(C)

    // sigma(X) = RotR(X,A) ⊕ RotR(X,B) ⊕ X >> C
    pub fn sigma<const A: u8, const B: u8, const C: u8>(x: T) -> T
        T: Shr<Output = T>,
        T: BitXor<Output = T>,
        T: Shifter<T>,
        T: Copy,
        x.right_rotate(A) ^ x.right_rotate(B) ^ x.right_shift(C)
Enter fullscreen mode Exit fullscreen mode

where right_rotate() and right_shift() are trait functions implemented for u32 and u64.

The scrambling functions are then gathered into a generic structure:

// Rust doesn't allow yet utf-8 symbol for variables
pub struct ScramblePool<T> {
    pub ch: FnScramble3<T>,
    pub maj: FnScramble3<T>,
    pub sigma0: FnScramble1<T>,
    pub sigma1: FnScramble1<T>,
    pub SIGMA0: FnScramble1<T>,
    pub SIGMA1: FnScramble1<T>,
Enter fullscreen mode Exit fullscreen mode

SHA2 algorithm

Because, of all this genericity, we can implement the SHA2 algorithm, bring either SHA256 or SHA512, totally generically. You can browse the code here:


Depending of which algorithm you want to use, it's easy to define an alias for a specific type:

pub type Sha256 = Hash<u32, 64, 64>;
pub type Sha512 = Hash<u64, 128, 80>;
Enter fullscreen mode Exit fullscreen mode

and implement the generic Hash for both u32 and u64:

// example for u32
impl Hash<u32, 64, 64> {
    pub fn new() -> Self {
        let iv: [u32; 8] = [
            0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab,
        Hash {
            hash: iv,
            k_constants: Self::k_constants(),
            scramble_funcs: ScramblePool::<u32> {
                ch: Scramble::<u32>::Ch,
                maj: Scramble::<u32>::Maj,
                sigma0: Scramble::<u32>::sigma::<7, 18, 3>,
                sigma1: Scramble::<u32>::sigma::<17, 19, 10>,
                SIGMA0: Scramble::<u32>::SIGMA::<2, 13, 22>,
                SIGMA1: Scramble::<u32>::SIGMA::<6, 11, 25>,
            block: [0u8; 64],

    pub fn k_constants() -> [u32; 64] {
            0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b, 0x59f111f1, 0x923f82a4,
            0xab1c5ed5, 0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3, 0x72be5d74, 0x80deb1fe,
            0x9bdc06a7, 0xc19bf174, 0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc, 0x2de92c6f,
            0x4a7484aa, 0x5cb0a9dc, 0x76f988da, 0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7,
            0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967, 0x27b70a85, 0x2e1b2138, 0x4d2c6dfc,
            0x53380d13, 0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85, 0xa2bfe8a1, 0xa81a664b,
            0xc24b8b70, 0xc76c51a3, 0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070, 0x19a4c116,
            0x1e376c08, 0x2748774c, 0x34b0bcb5, 0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3,
            0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208, 0x90befffa, 0xa4506ceb, 0xbef9a3f7,

Enter fullscreen mode Exit fullscreen mode

Testing with SHA2 test vectors

The NIST site give a list of files to be tested, for integration test. When running cargo t, those tests are also run and sha digests verified.

I didn't implement SHA224 or SHA384 but is left as an exercice 😀

Hope this help !

Photo by Conscious Design on Unsplash

Discussion (0)

Editor guide