Implement Hill Cipher with 2x2 Key Matrix in Ciphers Module

src/ciphers/hill.rs

@@ -0,0 +1,128 @@
+use nalgebra::Matrix2;
+
+/// Returns Some(inverse) if it exists (i.e. when gcd(a, m) == 1), else None.
+fn mod_inv(a: i32, m: i32) -> Option<i32> {
+    (1..m).find(|&x| (a * x) % m == 1)
+}
+
+fn check_key_validity(key: &Matrix2<i32>) -> Result<(), &'static str> {
+    let det = key[(0, 0)] * key[(1, 1)] - key[(0, 1)] * key[(1, 0)];
+
+    if det.rem_euclid(26) == 0 {
+        return Err("Error: key matrix is not invertible mod 26");
+    }
+    Ok(())
+}
+
+fn text_to_numbers(text: &str) -> Vec<u8> {
+    text.chars()
+        .filter_map(|c| {
+            if c.is_ascii_alphabetic() {
+                Some(c.to_ascii_uppercase() as u8 - b'A')
+            } else {
+                None
+            }
+        })
+        .collect()
+}
+
+fn numbers_to_text(nums: &[u8]) -> String {
+    nums.iter()
+        .map(|&n| ((n.rem_euclid(26)) + b'A') as char)
+        .collect()
+}
+
+fn process_2x2_chunks(text_vector: Vec<u8>, key: &Matrix2<i32>) -> Vec<u8> {
+    let mut result_data = Vec::new();
+
+    for chunk in text_vector.chunks(2) {
+        // if chunk is incomplete then padding with 0
+        let a = *chunk.first().unwrap_or(&0) as i32;
+        let b = *chunk.get(1).unwrap_or(&0) as i32;
+
+        // matrix mult
+        let x = key[(0, 0)] * a + key[(0, 1)] * b;
+        let y = key[(1, 0)] * a + key[(1, 1)] * b;
+        result_data.push(x.rem_euclid(26) as u8);
+        result_data.push(y.rem_euclid(26) as u8);
+    }
+
+    result_data
+}
+
+pub fn encode_hill(text: &str, key: Matrix2<i32>) -> Result<String, &'static str> {
+    check_key_validity(&key)?;
+
+    let text_vector = text_to_numbers(text);
+    let encrypted_data = process_2x2_chunks(text_vector, &key);
+    Ok(numbers_to_text(&encrypted_data))
+}
+
+pub fn decode_hill(text: &str, key: Matrix2<i32>) -> Result<String, &'static str> {
+    check_key_validity(&key)?;
+
+    let det = key[(0, 0)] * key[(1, 1)] - key[(0, 1)] * key[(1, 0)];
+    let det_mod = det.rem_euclid(26);
+    let det_inv = match mod_inv(det_mod, 26) {
+        Some(x) => x,
+        None => return Err("Error: key matrix is not invertible mod 26"),
+    };
+
+    // compute the inverse using 2x2 formula
+    let inv_key = Matrix2::new(
+        (key[(1, 1)] * det_inv).rem_euclid(26),
+        (-key[(0, 1)] * det_inv).rem_euclid(26),
+        (-key[(1, 0)] * det_inv).rem_euclid(26),
+        (key[(0, 0)] * det_inv).rem_euclid(26),
+    );
+
+    let text_vector = text_to_numbers(text);
+    let decrypted_data = process_2x2_chunks(text_vector, &inv_key);
+    Ok(numbers_to_text(&decrypted_data))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_key_not_invertible_should_fail() {
+        let key = Matrix2::new(2, 4, 1, 2);
+        assert_eq!(
+            encode_hill("test", key),
+            Err("Error: key matrix is not invertible mod 26")
+        );
+    }
+
+    #[test]
+    fn test_text_to_numbers_conversion() {
+        assert_eq!(text_to_numbers("123 ABC xyz!?"), vec![0, 1, 2, 23, 24, 25]);
+    }
+
+    #[test]
+    fn test_numbers_to_text_conversion() {
+        assert_eq!(numbers_to_text(&[0, 1, 2, 23, 24, 25]), "ABCXYZ");
+    }
+
+    #[test]
+    fn test_encoding_with_valid_key() {
+        let key = Matrix2::new(3, 3, 2, 5);
+        let result = encode_hill("HELP", key);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_decoding_with_valid_key() {
+        let key = Matrix2::new(3, 3, 2, 5);
+        let encoded_text = encode_hill("HELP", key).unwrap();
+        let decoded_text = decode_hill(&encoded_text, key).unwrap();
+        assert_eq!(decoded_text, "HELP");
+    }
+
+    #[test]
+    fn test_encoding_with_padding() {
+        let key = Matrix2::new(3, 3, 2, 5);
+        let result = encode_hill("ABC", key);
+        assert!(result.is_ok());
+    }
+}

src/ciphers/mod.rs

@@ -7,6 +7,7 @@ mod caesar;
 mod chacha;
 mod diffie_hellman;
 mod hashing_traits;
+mod hill;
 mod kerninghan;
 mod morse_code;
 mod polybius;
@@ -30,6 +31,7 @@ pub use self::chacha::chacha20;
 pub use self::diffie_hellman::DiffieHellman;
 pub use self::hashing_traits::Hasher;
 pub use self::hashing_traits::HMAC;
+pub use self::hill::{decode_hill, encode_hill};
 pub use self::kerninghan::kerninghan;
 pub use self::morse_code::{decode, encode};
 pub use self::polybius::{decode_ascii, encode_ascii};