cipher_crypt/
caesar.rs

1//! The Caesar cipher is named after Julius Caesar, who used it (allegedly) with a shift of three
2//! to protect messages of military significance.
3//!
4//! As with all single-alphabet substitution ciphers, the Caesar cipher is easily broken
5//! and in modern practice offers essentially no communication security.
6//!
7use crate::common::alphabet::Alphabet;
8use crate::common::cipher::Cipher;
9use crate::common::{alphabet, substitute};
10
11/// A Caesar cipher.
12///
13/// This struct is created by the `new()` method. See its documentation for more.
14pub struct Caesar {
15    shift: usize,
16}
17
18impl Cipher for Caesar {
19    type Key = usize;
20    type Algorithm = Caesar;
21
22    /// Initialise a Caesar cipher given a specific shift value.
23    ///
24    /// # Panics
25    /// * `shift` is not in the inclusive range `1 - 26`.
26    ///
27    fn new(shift: usize) -> Caesar {
28        if shift < 1 || shift > 26 {
29            panic!("The shift factor must be within the range 1 <= n <= 26.");
30        }
31
32        Caesar { shift }
33    }
34
35    /// Encrypt a message using a Caesar cipher.
36    ///
37    /// # Examples
38    /// Basic usage:
39    ///
40    /// ```
41    /// use cipher_crypt::{Cipher, Caesar};
42    ///
43    /// let c = Caesar::new(3);
44    /// assert_eq!("Dwwdfn dw gdzq!", c.encrypt("Attack at dawn!").unwrap());
45    /// ```
46    ///
47    fn encrypt(&self, message: &str) -> Result<String, &'static str> {
48        // Encryption of a letter:
49        //         E(x) = (x + n) mod 26
50        // Where;  x = position of letter in alphabet
51        //         n = shift factor (or key)
52
53        Ok(substitute::shift_substitution(message, |idx| {
54            alphabet::STANDARD.modulo((idx + self.shift) as isize)
55        }))
56    }
57
58    /// Decrypt a message using a Caesar cipher.
59    ///
60    /// # Examples
61    /// Basic usage:
62    ///
63    /// ```
64    /// use cipher_crypt::{Cipher, Caesar};
65    ///
66    /// let c = Caesar::new(3);
67    /// assert_eq!("Attack at dawn!", c.decrypt("Dwwdfn dw gdzq!").unwrap());
68    /// ```
69    ///
70    fn decrypt(&self, ciphertext: &str) -> Result<String, &'static str> {
71        // Decryption of a letter:
72        //         D(x) = (x - n) mod 26
73        // Where;  x = position of letter in alphabet
74        //         n = shift factor (or key)
75
76        Ok(substitute::shift_substitution(ciphertext, |idx| {
77            alphabet::STANDARD.modulo(idx as isize - self.shift as isize)
78        }))
79    }
80}
81
82#[cfg(test)]
83mod tests {
84    use super::*;
85
86    #[test]
87    fn encrypt_message() {
88        let c = Caesar::new(2);
89        assert_eq!("Cvvcem cv fcyp!", c.encrypt("Attack at dawn!").unwrap());
90    }
91
92    #[test]
93    fn decrypt_message() {
94        let c = Caesar::new(2);
95        assert_eq!("Attack at dawn!", c.decrypt("Cvvcem cv fcyp!").unwrap());
96    }
97
98    #[test]
99    fn with_utf8() {
100        let c = Caesar::new(3);
101        let message = "Peace, Freedom and Liberty! 🗡️";
102        let encrypted = c.encrypt(message).unwrap();
103        let decrypted = c.decrypt(&encrypted).unwrap();
104
105        assert_eq!(decrypted, message);
106    }
107
108    #[test]
109    fn exhaustive_encrypt() {
110        //Test with every possible shift combination
111        let message = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
112
113        for i in 1..27 {
114            let c = Caesar::new(i);
115            let encrypted = c.encrypt(message).unwrap();
116            let decrypted = c.decrypt(&encrypted).unwrap();
117            assert_eq!(decrypted, message);
118        }
119    }
120
121    #[test]
122    #[should_panic]
123    fn key_to_small() {
124        Caesar::new(0);
125    }
126
127    #[test]
128    #[should_panic]
129    fn key_to_big() {
130        Caesar::new(27);
131    }
132}
cipher_crypt/caesar.rs

cipher_crypt/
caesar.rs
