Der Dialog der Schwestern

main.rs

// Der Dialog der Schwestern
// Quick 'n dirty solution in Rust
// see https://www.heise.de/ct/artikel/Der-Dialog-der-Schwestern-4274601.html

#[derive(Debug)]
struct Encryption {
    n: u32,         // encryption modulus
    e: u32,         // encryption exponent
    n2: u32,        // decryption modulus
    e2: u32,        // decryption exponent
}

impl Encryption {
    fn new(p: u32, q: u32, e: u32) -> Self {
        let n = p * q;
        let n2 = (p-1) * (q-1);
        let e2 = (n2 + 1) / e;
        Self { n, e, n2, e2 }
    }

    fn decrypt(&self, ciphertext: &[u32]) -> Vec<u32> {
        ciphertext.iter().map(|ch| {
            (((ch % self.n) as u64).pow(self.e2) % self.n as u64) as u32
        }).collect()
    }
}

fn print(text: &[u32]) {
    const ALPHABET: [char; 26] = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'];
    for ch in text {
        if *ch >= 1 && *ch <= ALPHABET.len() as u32 {
            print!("{}", ALPHABET[*ch as usize - 1]);
        } else {
            print!("?");
        }
    }
    println!();
}

fn main() {
    let key = Encryption::new(3, 227, 151);
    println!("key: {:?}", key);
    let ciphertext = [172, 1734, 315, 641, 372, 3491, 360, 387, 586, 602, 2358];
    println!("ciphertext: {:?}", ciphertext);
    let plaintext = key.decrypt(&ciphertext);
    println!("plaintext: {:?}", plaintext);
    print(&plaintext);
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn smoke() {
        let key = Encryption::new(3, 17, 3);
        assert_eq!(key.n, 51);
        assert_eq!(key.e, 3);
        assert_eq!(key.n2, 32);
        assert_eq!(key.e2, 11);
        assert_eq!(
            key.decrypt(&[1647, 741, 665, 1487, 780, 2897, 620, 525, 2500, 2336, 486, 1174]),
            [9, 3, 8, 2, 9, 14, 2, 9, 1, 14, 3, 1]      // ICHBINBIANCA
        );
    }
}