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//! The Railfence Cipher is a transposition cipher. It has a very low keyspace and is therefore
//! incredibly insecure.
//!
//! This implementation currently transposes all input characters including whitespace and
//! punctuation.
/// A Railfence cipher.
///
/// This struct is created by the `new()` method. See its documentation for more.
use crate::common::cipher::Cipher;
pub struct Railfence {
rails: usize,
}
impl Cipher for Railfence {
type Key = usize;
type Algorithm = Railfence;
/// Initialise a Railfence cipher given a specific key (number of rails).
///
/// # Panics
/// * The `key` is 0.
///
fn new(key: usize) -> Railfence {
if key == 0 {
panic!("The key is 0.");
}
Railfence { rails: key }
}
/// Encrypt a message using a Railfence cipher.
///
/// # Examples
/// Basic usage:
///
/// ```
/// use cipher_crypt::{Cipher, Railfence};
///
/// let r = Railfence::new(3);
/// assert_eq!("Src s!ue-ertmsaepseeg", r.encrypt("Super-secret message!").unwrap());
/// ```
///
fn encrypt(&self, message: &str) -> Result<String, &'static str> {
// Encryption process:
// First a table is created with a height given by the key and a length
// given by the message length.
// e.g.
// For a key of 3 and the message "Hello, World!" of length 13:
// .............
// .............
// .............
// The message can then be written onto the grid in a zigzag going right:
// H...o...o...!
// .e.l.,.W.r.d.
// ..l... ...l..
// The encrypted message is then read line by line:
// Hoo!el,Wrdl l
// We simply return the message as the 'encrypted' message when there is one rail.
// This is because the message is transposed along a single rail without being altered.
if self.rails == 1 {
return Ok(message.to_string());
}
// Initialise the fence (a simple table)
// The form of an entry is (bool, char) => (is_msg_element, msg_element)
let mut table = vec![vec![(false, '.'); message.len()]; self.rails];
//Transpose the message along the fence
for (col, element) in message.chars().enumerate() {
//Given the column (ith element of the message), determine which row to place the
//character on
let rail = Railfence::calc_current_rail(col, self.rails);
table[rail][col] = (true, element);
}
Ok(table
.iter()
.flatten()
.filter(|(is_element, _)| *is_element)
.map(|(_, element)| element)
.collect::<String>())
}
/// Decrypt a message using a Railfence cipher.
///
/// # Examples
/// Basic usage:
///
/// ```
/// use cipher_crypt::{Cipher, Railfence};
///
/// let r = Railfence::new(3);
/// assert_eq!("Super-secret message!", r.decrypt("Src s!ue-ertmsaepseeg").unwrap());
/// ```
///
fn decrypt(&self, ciphertext: &str) -> Result<String, &'static str> {
// Decryption process:
// First a table is created with a height given by the key and a length
// given by the ciphertext length.
// e.g.
// For a key of 3 and the ciphertext "Hoo!el,Wrdl l" of length 13:
// .............
// .............
// .............
// The positions in the table that would be used to encrypt a message are identified
// x...x...x...x
// .x.x.x.x.x.x.
// ..x...x...x..
// The ciphertext is then written onto the identified positions, line by line
// H...o...o...!
// .e.l.,.W.r.d.
// ..l... ...l..
// The decrypted message is then read in a zigzag:
// Hello, World!
// As mentioned previously, a single rail means that the original message has not been
// altered
if self.rails == 1 {
return Ok(ciphertext.to_string());
}
let mut table = vec![vec![(false, '.'); ciphertext.len()]; self.rails];
// Traverse the table and mark the elements that will be filled by the cipher text
for col in 0..ciphertext.len() {
let rail = Railfence::calc_current_rail(col, self.rails);
table[rail][col].0 = true;
}
// Fill the identified positions in the table with the ciphertext, line by line
let mut ct_chars = ciphertext.chars();
'outer: for row in &mut table {
// For each element in the row, determine if a char should be placed there
for element in row.iter_mut() {
if element.0 {
if let Some(c) = ct_chars.next() {
*element = (element.0, c);
} else {
// We have transposed all chars of the cipher text
break 'outer;
}
}
}
}
// From the transposed cipher text construct the original message
let mut message = String::new();
for col in 0..ciphertext.len() {
// For this column, determine which row we should read from to get the next char
// of the message
let rail = Railfence::calc_current_rail(col, self.rails);
message.push(table[rail][col].1);
}
Ok(message)
}
}
impl Railfence {
/// For a given column and the total number of 'rails' (rows), determine the current rail
/// that should be referenced.
///
fn calc_current_rail(col: usize, total_rails: usize) -> usize {
// In the Railfence cipher the letters are placed diagonally in a zigzag,
// so, with a key of 4 say, the row numbers will go
// 0, 1, 2, 3, 2, 1, 0, 1, 2, 3, 2, 1, 0, ...
// This repeats with a cycle (or period) given by (2*key - 2)
// [0, 1, 2, 3, 2, 1], [0, 1, 2, 3, 2, 1], 0, ...
// This cycle is always even.
let cycle = 2 * total_rails - 2;
// For the first half of a cycle, the row is given by the index,
// but for the second half it decreases and is therefore given by the reverse index,
// the distance from the end of the cycle.
if col % cycle <= cycle / 2 {
col % cycle
} else {
cycle - col % cycle
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn encrypt_test() {
let message = "attackatdawn";
let r = Railfence::new(6);
assert_eq!("awtantdatcak", r.encrypt(message).unwrap());
}
#[test]
fn encrypt_mixed_case() {
let message = "Hello, World!";
let r = Railfence::new(3);
assert_eq!("Hoo!el,Wrdl l", r.encrypt(message).unwrap());
}
#[test]
fn encrypt_short_key() {
let message = "attackatdawn";
let r = Railfence::new(1);
assert_eq!("attackatdawn", r.encrypt(message).unwrap());
}
#[test]
fn encrypt_long_key() {
let message = "attackatdawn";
let r = Railfence::new(20);
assert_eq!("attackatdawn", r.encrypt(message).unwrap());
}
#[test]
fn decrypt_test() {
let message = "awtantdatcak";
let r = Railfence::new(6);
assert_eq!("attackatdawn", r.decrypt(message).unwrap());
}
#[test]
fn decrypt_short_key() {
let message = "attackatdawn";
let r = Railfence::new(1);
assert_eq!("attackatdawn", r.decrypt(message).unwrap());
}
#[test]
fn decrypt_mixed_case() {
let message = "Hoo!el,Wrdl l";
let r = Railfence::new(3);
assert_eq!("Hello, World!", r.decrypt(message).unwrap());
}
#[test]
fn decrypt_long_key() {
let message = "attackatdawn";
let r = Railfence::new(20);
assert_eq!("attackatdawn", r.decrypt(message).unwrap());
}
#[test]
#[should_panic]
fn incorrect_key_test() {
Railfence::new(0);
}
#[test]
fn unicode_test() {
let r = Railfence::new(3);
let message = "ÂƮƮäƈķ ɑƬ Ðawŋ ✓";
assert_eq!("ÂƈƬwƮäķɑ aŋ✓Ʈ Ð ", r.encrypt(message).unwrap());
}
}