Rust’s const fn feature opens up exciting possibilities for compile-time cryptography. I’ve been exploring this advanced technique, and it’s a game-changer for creating ultra-efficient cryptographic routines.

The idea is simple yet powerful: perform key expansion for symmetric encryption algorithms at compile-time. This means the round keys are pre-computed and baked into the binary, resulting in zero runtime overhead for key scheduling. It’s perfect for embedded systems, high-performance computing, and anywhere else where every CPU cycle matters.

Let’s dive into how we can leverage const fn for this purpose. First, we need to understand that const fn has some limitations. Not all operations are allowed in a const context, so we need to be creative in our implementations.

Here’s a basic example of a const fn that performs a simple key expansion:

const fn expand_key(key: &[u8; 16]) -> [u32; 44] {
    let mut expanded_key = [0u32; 44];
    let mut i = 0;
    while i < 4 {
        expanded_key[i] = u32::from_be_bytes([key[4*i], key[4*i+1], key[4*i+2], key[4*i+3]]);
        i += 1;
    }
    // More key expansion logic here...
    expanded_key
}

This function takes a 16-byte key and expands it into 44 32-bit words. The real power comes when we use this in our encryption function:

const EXPANDED_KEY: [u32; 44] = expand_key(b"mysecretkey12345");

fn encrypt(data: &mut [u8]) {
    // Use EXPANDED_KEY for encryption
}

Now, EXPANDED_KEY is computed at compile-time, and our encrypt function can use it without any runtime key expansion overhead.

But let’s get more ambitious. What about implementing a full AES key expansion at compile-time? This is where things get tricky. AES key expansion involves operations like rotations and S-box lookups, which aren’t typically allowed in const fn.

To work around this, we can implement our own const-compatible versions of these operations. For example, here’s how we might implement a const S-box lookup:

const SBOX: [u8; 256] = [
    0x63, 0x7c, 0x77, 0x7b, /* ... rest of S-box ... */
];

const fn s_box_lookup(byte: u8) -> u8 {
    SBOX[byte as usize]
}

With this in place, we can start building our AES key expansion:

const fn aes_key_expansion(key: &[u8; 16]) -> [u32; 44] {
    let mut w = [0u32; 44];
    let mut i = 0;
    while i < 4 {
        w[i] = u32::from_be_bytes([key[4*i], key[4*i+1], key[4*i+2], key[4*i+3]]);
        i += 1;
    }

    i = 4;
    while i < 44 {
        let mut temp = w[i-1];
        if i % 4 == 0 {
            temp = sub_word(rot_word(temp)) ^ (RCON[i/4 - 1] as u32) << 24;
        }
        w[i] = w[i-4] ^ temp;
        i += 1;
    }
    w
}

const fn sub_word(word: u32) -> u32 {
    let b0 = s_box_lookup((word >> 24) as u8);
    let b1 = s_box_lookup((word >> 16) as u8);
    let b2 = s_box_lookup((word >> 8) as u8);
    let b3 = s_box_lookup(word as u8);
    (b0 as u32) << 24 | (b1 as u32) << 16 | (b2 as u32) << 8 | (b3 as u32)
}

const fn rot_word(word: u32) -> u32 {
    (word << 8) | (word >> 24)
}

This implementation performs the full AES key expansion at compile-time. It’s a bit verbose due to the limitations of const fn, but the result is worth it: a fully expanded AES key with zero runtime overhead.

But we’re not limited to just AES. We can apply this technique to other algorithms like ChaCha20. ChaCha20’s key setup is simpler, making it a great candidate for const fn:

const fn chacha20_key_setup(key: &[u8; 32], nonce: &[u8; 12]) -> [u32; 16] {
    let mut state = [0u32; 16];
    state[0] = 0x61707865;
    state[1] = 0x3320646e;
    state[2] = 0x79622d32;
    state[3] = 0x6b206574;

    let mut i = 0;
    while i < 8 {
        state[4 + i] = u32::from_le_bytes([key[4*i], key[4*i+1], key[4*i+2], key[4*i+3]]);
        i += 1;
    }

    state[12] = 0;
    state[13] = u32::from_le_bytes([nonce[0], nonce[1], nonce[2], nonce[3]]);
    state[14] = u32::from_le_bytes([nonce[4], nonce[5], nonce[6], nonce[7]]);
    state[15] = u32::from_le_bytes([nonce[8], nonce[9], nonce[10], nonce[11]]);

    state
}

This setup can be used to create a const initial state for ChaCha20, which can then be used in the encryption process with minimal runtime overhead.

One challenge you might face when implementing these const cryptographic functions is handling large lookup tables. While we can define const arrays, accessing them in complex ways within a const fn can be tricky. One approach is to break down complex lookups into simpler operations that are allowed in const contexts.

For example, if you need to perform a complex permutation, you might break it down into a series of simpler swaps:

const fn permute(mut value: u64) -> u64 {
    let mut i = 0;
    while i < 32 {
        let j = PERMUTATION[i];
        let bit_i = (value >> i) & 1;
        let bit_j = (value >> j) & 1;
        value ^= (bit_i ^ bit_j) << i;
        value ^= (bit_i ^ bit_j) << j;
        i += 1;
    }
    value
}

This approach, while potentially less efficient than a direct lookup, allows us to perform complex permutations in a const context.

Another area where const fn shines is in implementing complex bit manipulations. Many cryptographic algorithms rely heavily on bitwise operations, and these are generally well-suited to const fn. Here’s an example of implementing a const bit rotation:

const fn rotate_left(value: u32, shift: u32) -> u32 {
    (value << shift) | (value >> (32 - shift))
}

This function can be used as a building block for more complex operations, all computed at compile-time.

It’s important to note that while these techniques can lead to very efficient implementations, they should be used judiciously. Compile-time computation can increase compile times and binary sizes. For small keys or simple algorithms, the tradeoff is usually worth it. For more complex scenarios, you’ll need to weigh the benefits against the costs.

Security is another crucial consideration. When implementing cryptographic algorithms, especially in novel ways like this, it’s essential to be aware of potential side-channel attacks. Const fn can help here by eliminating certain types of timing attacks, as the key expansion happens at compile-time. However, you still need to be cautious about how the expanded keys are used in your runtime code.

In terms of practical applications, these techniques are particularly valuable in resource-constrained environments. Imagine you’re developing firmware for a small IoT device. By using const fn for key expansion, you can include robust encryption in your firmware without significantly impacting the device’s limited processing power or memory.

Similarly, in high-performance computing scenarios where every cycle counts, eliminating the runtime overhead of key expansion can lead to noticeable performance improvements, especially when dealing with large volumes of data.

As Rust and const fn continue to evolve, we can expect even more powerful compile-time cryptography techniques to emerge. The ability to perform complex computations at compile-time opens up new possibilities for optimizing critical code paths and improving overall system security.

I’m excited about the potential of these techniques, and I encourage you to experiment with them in your own projects. Remember, the key to mastering this approach is understanding the limitations of const fn and finding creative ways to work within those constraints. With practice, you’ll be able to craft incredibly efficient cryptographic routines that push the boundaries of what’s possible in systems programming.

As we continue to explore the intersection of cryptography and systems programming, techniques like these will play an increasingly important role in building secure, efficient systems. Whether you’re working on embedded devices, high-performance servers, or anything in between, mastering compile-time cryptography with Rust’s const fn is a valuable skill that can set your implementations apart.