rsa_example.rs

rsa_example.rs

use num_bigint::{BigInt, BigUint, Sign};
use num_traits::{One, Zero};
use std::str::FromStr; // For parsing BigUint from strings (e.g., hex)

// --- RSA Core Functions ---

/// Performs modular exponentiation: base^exp % modulus
fn modpow(base: &BigUint, exp: &BigUint, modulus: &BigUint) -> BigUint {
    // This is the core of RSA. `num-bigint` provides an optimized `modpow` method.
    base.modpow(exp, modulus)
}

// --- RSA Key Structure (Hardcoded for simplicity) ---

// In a real application, you'd generate these based on large random primes.
// Using small primes (p=61, q=53) for a tiny example.
// p = 61
// q = 53
// n = p * q = 61 * 53 = 3233
// phi = (p-1)*(q-1) = 60 * 52 = 3120
// e = 17 (a common choice, coprime with 3120)
// d = 17^-1 mod 3120 = 413 (modular multiplicative inverse)

const N_STR: &str = "3233"; // n = p * q
const E_STR: &str = "17";   // e = public exponent
const D_STR: &str = "413";  // d = private exponent

// --- Main Application Logic ---
fn main() -> Result<(), Box<dyn std::error::Error>> {
    let args: Vec<String> = std::env::args().collect();
#[cfg(not(debug_assertions))]
    if args.len() < 2 {
        eprintln!("Usage: {} <encrypt|decrypt> <message_or_ciphertext_decimal>", args[0]);
        eprintln!("  <message_or_ciphertext_decimal>: The number to encrypt/decrypt, in decimal.");
        eprintln!("NOTE: This is a minimal, INSECURE RSA implementation for demonstration ONLY.");
        eprintln!("      It does NOT use padding and should NOT be used for real cryptography.");
        return Ok(());
    }
#[cfg(not(debug_assertions))]
    let command = &args[1];
    let command = &"encrypt";
#[cfg(not(debug_assertions))]
    let input_decimal_str = &args[2];
    let input_decimal_str = &"333";
    // Parse the pre-defined RSA parameters
    let n = BigUint::from_str(N_STR).expect("Invalid N constant");
    let e = BigUint::from_str(E_STR).expect("Invalid E constant");
    let d = BigUint::from_str(D_STR).expect("Invalid D constant");

    // Parse the input message/ciphertext
    let input_num = BigUint::from_str(input_decimal_str)
        .map_err(|_| "Input must be a valid decimal number")?;

    match command.as_ref() {
        "encrypt" => {
            // Check if message is smaller than n (required for raw RSA)
            if input_num >= n {
                eprintln!("Error: Message ({}) must be smaller than the modulus N ({}).", input_num, n);
                return Err("Message too large for raw RSA".into());
            }
            // C = M^e mod n
            let ciphertext = modpow(&input_num, &e, &n);
            println!("Ciphertext (decimal): {}", ciphertext);
        }
        "decrypt" => {
            // M = C^d mod n
            let decrypted_message = modpow(&input_num, &d, &n);
            println!("Decrypted Message (decimal): {}", decrypted_message);
        }
        _ => {
            eprintln!("Unknown command: {}", command);
            eprintln!("Use 'encrypt' or 'decrypt'.");
        }
    }

    Ok(())
}