Rust’s const evaluation system is a game-changer for compile-time cryptography. I’ve been exploring this fascinating area, and I’m excited to share what I’ve learned.

At its core, const evaluation allows us to perform computations during compilation, baking the results directly into our binary. This is incredibly powerful for cryptography, as we can implement complex algorithms that run with zero runtime overhead.

Let’s start with a simple example. Imagine we want to create a lookup table for a substitution cipher. Traditionally, we’d define this at runtime, but with const evaluation, we can do it at compile-time:

const fn generate_sbox() -> [u8; 256] {
    let mut sbox = [0u8; 256];
    let mut i = 0;
    while i < 256 {
        sbox[i] = (i * 17 + 13) % 256;
        i += 1;
    }
    sbox
}

const SBOX: [u8; 256] = generate_sbox();

This SBOX is now baked into our binary, ready for use without any runtime initialization.

But we can go much further. Let’s implement a basic symmetric encryption algorithm entirely at compile-time. We’ll use a simple XOR cipher for demonstration:

const fn xor_encrypt(plaintext: &[u8], key: &[u8]) -> [u8; 64] {
    let mut ciphertext = [0u8; 64];
    let mut i = 0;
    while i < plaintext.len() && i < 64 {
        ciphertext[i] = plaintext[i] ^ key[i % key.len()];
        i += 1;
    }
    ciphertext
}

const ENCRYPTED_MESSAGE: [u8; 64] = xor_encrypt(b"This is a secret message", b"key");

Now we have a message encrypted at compile-time, hardcoded into our binary. This can be incredibly useful for embedding secrets or default configurations securely.

One of the most exciting applications of compile-time cryptography is in generating cryptographically secure random numbers. While true randomness can’t be achieved at compile-time, we can create deterministic yet unpredictable sequences:

const fn const_rand(seed: u64) -> u64 {
    let a = 6364136223846793005u64;
    let c = 1442695040888963407u64;
    seed.wrapping_mul(a).wrapping_add(c)
}

const fn generate_random_array() -> [u64; 10] {
    let mut arr = [0u64; 10];
    let mut seed = 12345; // Choose a seed
    let mut i = 0;
    while i < 10 {
        seed = const_rand(seed);
        arr[i] = seed;
        i += 1;
    }
    arr
}

const RANDOM_ARRAY: [u64; 10] = generate_random_array();

This RANDOM_ARRAY is now a fixed part of our binary, yet its values are hard to predict without knowing the seed and algorithm.

Compile-time cryptography isn’t limited to simple operations. We can implement more complex algorithms too. Here’s a basic SHA-256 hash function implemented entirely as const functions:

const fn ch(x: u32, y: u32, z: u32) -> u32 {
    (x & y) ^ (!x & z)
}

const fn maj(x: u32, y: u32, z: u32) -> u32 {
    (x & y) ^ (x & z) ^ (y & z)
}

const fn rotate_right(n: u32, d: u32) -> u32 {
    (n >> d) | (n << (32 - d))
}

const fn sha256_transform(state: &mut [u32; 8], block: &[u8; 64]) {
    const K: [u32; 64] = [
        0x428a2f98, 0x71374491, /* ... more constants ... */
    ];

    let mut w = [0u32; 64];
    let mut i = 0;
    while i < 16 {
        w[i] = u32::from_be_bytes([
            block[i * 4],
            block[i * 4 + 1],
            block[i * 4 + 2],
            block[i * 4 + 3],
        ]);
        i += 1;
    }

    while i < 64 {
        let s0 = rotate_right(w[i - 15], 7) ^ rotate_right(w[i - 15], 18) ^ (w[i - 15] >> 3);
        let s1 = rotate_right(w[i - 2], 17) ^ rotate_right(w[i - 2], 19) ^ (w[i - 2] >> 10);
        w[i] = w[i - 16]
            .wrapping_add(s0)
            .wrapping_add(w[i - 7])
            .wrapping_add(s1);
        i += 1;
    }

    let mut a = state[0];
    let mut b = state[1];
    let mut c = state[2];
    let mut d = state[3];
    let mut e = state[4];
    let mut f = state[5];
    let mut g = state[6];
    let mut h = state[7];

    i = 0;
    while i < 64 {
        let s1 = rotate_right(e, 6) ^ rotate_right(e, 11) ^ rotate_right(e, 25);
        let ch = ch(e, f, g);
        let temp1 = h
            .wrapping_add(s1)
            .wrapping_add(ch)
            .wrapping_add(K[i])
            .wrapping_add(w[i]);
        let s0 = rotate_right(a, 2) ^ rotate_right(a, 13) ^ rotate_right(a, 22);
        let maj = maj(a, b, c);
        let temp2 = s0.wrapping_add(maj);

        h = g;
        g = f;
        f = e;
        e = d.wrapping_add(temp1);
        d = c;
        c = b;
        b = a;
        a = temp1.wrapping_add(temp2);

        i += 1;
    }

    state[0] = state[0].wrapping_add(a);
    state[1] = state[1].wrapping_add(b);
    state[2] = state[2].wrapping_add(c);
    state[3] = state[3].wrapping_add(d);
    state[4] = state[4].wrapping_add(e);
    state[5] = state[5].wrapping_add(f);
    state[6] = state[6].wrapping_add(g);
    state[7] = state[7].wrapping_add(h);
}

const fn sha256(input: &[u8]) -> [u8; 32] {
    let mut state = [
        0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab,
        0x5be0cd19,
    ];

    let mut i = 0;
    while i + 64 <= input.len() {
        let mut block = [0u8; 64];
        let mut j = 0;
        while j < 64 {
            block[j] = input[i + j];
            j += 1;
        }
        sha256_transform(&mut state, &block);
        i += 64;
    }

    let mut result = [0u8; 32];
    i = 0;
    while i < 8 {
        result[i * 4] = (state[i] >> 24) as u8;
        result[i * 4 + 1] = (state[i] >> 16) as u8;
        result[i * 4 + 2] = (state[i] >> 8) as u8;
        result[i * 4 + 3] = state[i] as u8;
        i += 1;
    }

    result
}

const HASH: [u8; 32] = sha256(b"Hello, World!");

This implementation of SHA-256 runs entirely at compile-time, allowing us to bake cryptographic hashes directly into our binary. It’s a powerful tool for verifying the integrity of embedded data or creating compile-time unique identifiers.

The applications of compile-time cryptography extend far beyond these examples. We can use it to generate encryption keys, create secure boot loaders, or even implement parts of asymmetric cryptography algorithms.

One particularly interesting use case is in embedded systems and IoT devices. By moving cryptographic operations to compile-time, we can significantly reduce the runtime overhead and memory usage of these resource-constrained devices. Imagine a smart lock that has its encryption keys and algorithms baked in at compile-time, leaving more resources for other critical operations.

However, it’s important to note that compile-time cryptography isn’t a silver bullet. While it offers significant performance benefits and can enhance security in certain scenarios, it also comes with limitations. The deterministic nature of compile-time operations means that true randomness can’t be achieved, which is crucial for many cryptographic applications. Additionally, hardcoding cryptographic data into binaries can make it more difficult to update or rotate keys.

As we continue to explore the possibilities of compile-time cryptography in Rust, we’re likely to see more advanced techniques emerge. For instance, future versions of Rust might allow for more complex const operations, enabling us to implement even more sophisticated cryptographic algorithms at compile-time.

One area that’s particularly exciting is the potential for compile-time implementation of post-quantum cryptography algorithms. As quantum computers threaten traditional cryptographic methods, having efficient, compile-time implementations of quantum-resistant algorithms could be a game-changer for secure systems.

Another frontier is the use of compile-time cryptography in creating zero-knowledge proofs. While the full implementation of zk-SNARKs at compile-time is currently beyond reach, we might see parts of these complex systems moved to compile-time in the future, potentially revolutionizing blockchain and privacy-preserving technologies.

The intersection of compile-time cryptography and formal verification is another area ripe for exploration. By implementing cryptographic algorithms as const functions, we open up new possibilities for proving their correctness at compile-time. This could lead to cryptographic implementations with unprecedented levels of assurance.

As we push the boundaries of what’s possible with const evaluation, we’re also likely to see improvements in the tooling and ecosystem around compile-time cryptography. Better static analysis tools, linters, and IDE support will make it easier for developers to work with these advanced techniques.

In conclusion, Rust’s const evaluation system offers a powerful toolset for implementing cryptographic operations at compile-time. From simple encryption algorithms to complex hash functions, we can bake security directly into our binaries, enhancing both performance and safety. As the Rust ecosystem continues to evolve, I’m excited to see how developers and researchers will push the boundaries of compile-time cryptography, creating ever more secure and efficient systems.