# monty.rs.html -- source
Source to the Rust file `/Users/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/num-bigint-0.1.43/src/monty.rs`.
Keywords: rust, rustlang, rust-lang

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    use integer::Integer;
    use traits::Zero;

    use biguint::BigUint;

    struct MontyReducer<'a> {
        n: &'a BigUint,
        n0inv: u32
    }

    // Calculate the modular inverse of `num`, using Extended GCD.
    //
    // Reference:
    // Brent & Zimmermann, Modern Computer Arithmetic, v0.5.9, Algorithm 1.20
    fn inv_mod_u32(num: u32) -> u32 {
        // num needs to be relatively prime to 2**32 -- i.e. it must be odd.
        assert!(num % 2 != 0);

        let mut a: i64 = num as i64;
        let mut b: i64 = (u32::max_value() as i64) + 1;

        // ExtendedGcd
        // Input: positive integers a and b
        // Output: integers (g, u, v) such that g = gcd(a, b) = ua + vb
        // As we don't need v for modular inverse, we don't calculate it.

        // 1: (u, w) <- (1, 0)
        let mut u = 1;
        let mut w = 0;
        // 3: while b != 0
        while b != 0 {
            // 4: (q, r) <- DivRem(a, b)
            let q = a / b;
            let r = a % b;
            // 5: (a, b) <- (b, r)
            a = b; b = r;
            // 6: (u, w) <- (w, u - qw)
            let m = u - w*q;
            u = w; w = m;
        }

        assert!(a == 1);
        // Downcasting acts like a mod 2^32 too.
        u as u32
    }

    impl<'a> MontyReducer<'a> {
        fn new(n: &'a BigUint) -> Self {
            let n0inv = inv_mod_u32(n.data[0]);
            MontyReducer { n: n, n0inv: n0inv }
        }
    }

    // Montgomery Reduction
    //
    // Reference:
    // Brent & Zimmermann, Modern Computer Arithmetic, v0.5.9, Algorithm 2.6
    fn monty_redc(a: BigUint, mr: &MontyReducer) -> BigUint {
        let mut c = a.data;
        let n = &mr.n.data;
        let n_size = n.len();

        // Allocate sufficient work space
        c.resize(2 * n_size + 2, 0);

        // β is the size of a word, in this case 32 bits. So "a mod β" is
        // equivalent to masking a to 32 bits.
        // mu <- -N^(-1) mod β
        let mu = 0u32.wrapping_sub(mr.n0inv);

        // 1: for i = 0 to (n-1)
        for i in 0..n_size {
            // 2: q_i <- mu*c_i mod β
            let q_i = c[i].wrapping_mul(mu);

            // 3: C <- C + q_i * N * β^i
            super::algorithms::mac_digit(&mut c[i..], n, q_i);
        }

        // 4: R <- C * β^(-n)
        // This is an n-word bitshift, equivalent to skipping n words.
        let ret = BigUint::new(c[n_size..].to_vec());

        // 5: if R >= β^n then return R-N else return R.
        if &ret < mr.n {
            ret
        } else {
            ret - mr.n
        }
    }

    // Montgomery Multiplication
    fn monty_mult(a: BigUint, b: &BigUint, mr: &MontyReducer) -> BigUint {
        monty_redc(a * b, mr)
    }

    // Montgomery Squaring
    fn monty_sqr(a: BigUint, mr: &MontyReducer) -> BigUint {
        // TODO: Replace with an optimised squaring function
        monty_redc(&a * &a, mr)
    }

    pub fn monty_modpow(a: &BigUint, exp: &BigUint, modulus: &BigUint) -> BigUint{
        let mr = MontyReducer::new(modulus);

        // Calculate the Montgomery parameter
        let mut v = vec![0; modulus.data.len()];
        v.push(1);
        let r = BigUint::new(v);

        // Map the base to the Montgomery domain
        let mut apri = a * &r % modulus;

        // Binary exponentiation
        let mut ans = &r % modulus;
        let mut e = exp.clone();
        while !e.is_zero() {
            if e.is_odd() {
                ans = monty_mult(ans, &apri, &mr);
            }
            apri = monty_sqr(apri, &mr);
            e = e >> 1;
        }

        // Map the result back to the residues domain
        monty_redc(ans, &mr)
    }

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