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//! An autokey cipher (also known as the autoclave cipher) is a cipher which incorporates the
//! message (the plaintext) into the key.
//!
//! For example, say the message was `ATTACK AT DAWN` and the key was `CRYPT` then the calculated
//! keystream would be `CRYPTA TT ACKA`. It was invented by Blaise de Vigenère in 1586, and is
//! generally more secure than the Vigenere cipher.
use common::alphabet::Alphabet;
use common::cipher::Cipher;
use common::{alphabet, substitute};
/// An Autokey cipher.
///
/// This struct is created by the `new()` method. See its documentation for more.
pub struct Autokey {
key: String,
}
impl Cipher for Autokey {
type Key = String;
type Algorithm = Autokey;
/// Initialise an Autokey cipher given a specific key.
///
/// Will return `Err` if the key contains non-alphabetic symbols.
fn new(key: String) -> Result<Autokey, &'static str> {
if key.len() < 1 {
return Err("Invalid key. It must have at least one character.");
} else if !alphabet::STANDARD.is_valid(&key) {
return Err("Invalid key. Autokey keys cannot contain non-alphabetic symbols.");
}
Ok(Autokey { key: key })
}
/// Encrypt a message using an Autokey cipher.
///
/// # Examples
/// Basic usage:
///
/// ```
/// use cipher_crypt::{Cipher, Autokey};
///
/// let a = Autokey::new(String::from("fort")).unwrap();
/// assert_eq!("Fhktcd 🗡 mhg otzx aade", a.encrypt("Attack 🗡 the east wall").unwrap());
/// ```
fn encrypt(&self, message: &str) -> Result<String, &'static str> {
// Encryption of a letter in a message:
// Ci = Ek(Mi) = (Mi + Ki) mod 26
// Where; Mi = position within the alphabet of ith char in message
// Ki = position within the alphabet of ith char in key
substitute::key_substitution(message, &mut self.encrypt_keystream(message), |mi, ki| {
alphabet::STANDARD.modulo((mi + ki) as isize)
})
}
/// Decrypt a message using an Autokey cipher.
///
/// # Examples
/// Basic usage:
///
/// ```
/// use cipher_crypt::{Cipher, Autokey};
///
/// let a = Autokey::new(String::from("fort")).unwrap();
/// assert_eq!("Attack 🗡 the east wall", a.decrypt("Fhktcd 🗡 mhg otzx aade").unwrap());
/// ```
fn decrypt(&self, ciphertext: &str) -> Result<String, &'static str> {
// Decryption of a letter in a message:
// Mi = Dk(Ci) = (Ci - Ki) mod 26
// Where; Ci = position within the alphabet of ith char in cipher text
// Ki = position within the alphabet of ith char in key
//
// Please note that the decrypt keystream is generated 'on the fly' whilst the ciphertext
// is being decrypted.
self.autokey_decrypt(ciphertext)
}
}
impl Autokey {
fn autokey_decrypt(&self, ciphertext: &str) -> Result<String, &'static str> {
//As each character of the ciphertext is decrypted, the un-encrypted char is appended
//to the base key 'keystream', so that it may be used to decrypt the latter part
//of the ciphertext
let mut plaintext = String::new();
//We start the stream with the base key
let mut keystream: Vec<char> = self.key.clone().chars().collect();
for cc in ciphertext.chars() {
//Find the index of the ciphertext character in the alphabet (if it exists in there)
let pos = alphabet::STANDARD.find_position(cc);
match pos {
Some(ci) => {
//Get the next key character in the stream (we always read from position 0)
if keystream.len() < 1 {
return Err(
"Keystream is not large enough for full substitution of message",
);
}
let kc = keystream[0];
if let Some(ki) = alphabet::STANDARD.find_position(kc) {
//Calculate the index and retrieve the letter to substitute
let si = alphabet::STANDARD.modulo(ci as isize - ki as isize);
//We can safely unwrap as we know the index will be within the alphabet
let s = alphabet::STANDARD
.get_letter(si, cc.is_uppercase())
.unwrap();
//Push to the decrypted text AND the keystream
plaintext.push(s);
keystream.push(s);
keystream.remove(0); //We have consumed the keystream chartacter
} else {
return Err("Keystream contains a non-alphabetic symbol.");
}
}
None => plaintext.push(cc), //Push non-alphabetic chars 'as-is'
}
}
Ok(plaintext)
}
/// Generate an encrypt keystream by concatonating the key and message itself.
///
/// Will simply return a copy of the key if its length is already larger than the message.
fn encrypt_keystream(&self, message: &str) -> Vec<char> {
//The key will only be used to encrypt the portion of the message that is alphabetic
let scrubbed_msg = alphabet::STANDARD.scrub(message);
//The key is large enough for the message already
if self.key.len() >= scrubbed_msg.len() {
return self.key[0..scrubbed_msg.len()].chars().collect();
}
//The keystream is simply a concatonation of the base key + the scrubbed message
let mut keystream = self.key.clone();
keystream.push_str(&scrubbed_msg);
keystream[0..scrubbed_msg.len()].chars().collect()
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn with_utf8() {
let m = "Attack 🗡️ the east wall";
let a = Autokey::new(String::from("fort")).unwrap();
assert_eq!(m, a.decrypt(&a.encrypt(m).unwrap()).unwrap());
}
#[test]
fn simple_encrypt_decrypt_test() {
let message = "defend the east wall of the castle";
let v = Autokey::new(String::from("fortification")).unwrap();
let c_text = v.encrypt(message).unwrap();
let p_text = v.decrypt(&c_text).unwrap();
assert_eq!(message, p_text);
}
#[test]
fn decrypt_test() {
let ciphertext = "lxfopktmdcgn";
let v = Autokey::new(String::from("lemon")).unwrap();
assert_eq!("attackatdawn", v.decrypt(ciphertext).unwrap());
}
#[test]
fn larger_base_key() {
let message = "Hello";
let v = Autokey::new(String::from("fortification")).unwrap();
assert_eq!(vec!['f', 'o', 'r', 't', 'i'], v.encrypt_keystream(message));
}
#[test]
fn smaller_base_key() {
let message = "We are under seige";
let v = Autokey::new(String::from("lemon")).unwrap();
assert_eq!(
vec![
'l', 'e', 'm', 'o', 'n', 'W', 'e', 'a', 'r', 'e', 'u', 'n', 'd', 'e', 'r'
],
v.encrypt_keystream(message)
);
}
#[test]
fn valid_key() {
assert!(Autokey::new(String::from("LeMon")).is_ok());
}
#[test]
fn key_with_symbols() {
assert!(Autokey::new(String::from("!em@n")).is_err());
}
#[test]
fn key_with_whitespace() {
assert!(Autokey::new(String::from("wow this key is a real lemon")).is_err());
}
}