Affine Cipher in C#

 

The affine cipher is a type of monoalphabetic substitution cipher, wherein each letter in an alphabet is mapped to its numeric equivalent, encrypted using a simple mathematical function, and converted back to a letter. The formula used means that each letter encrypts to one other letter, and back again, meaning the cipher is essentially a standard substitution cipher with a rule governing which letter goes to which. As such, it has the weaknesses of all substitution ciphers. Each letter is enciphered with the function , where is the magnitude of the shift.

For more information on affine cipher visit here

Following code is written in c# which has the affine cipher algorithm

1.  
2. public static string AffineEncrypt(string plainText, int a, int b)
3. {
4.     string cipherText = "";
5.  
6.     // Put Plain Text (all capitals) into Character Array
7.     char[] chars = plainText.ToUpper().ToCharArray();
8.  
9.     // Compute e(x) = (ax + b)(mod m) for every character in the Plain Text
10.     foreach (char c in chars)
11.     {
12.         int x = Convert.ToInt32(c - 65);
13.         cipherText += Convert.ToChar(((a * x + b) % 26) + 65);
14.     }
15.  
16.     return cipherText;
17. }
18.  
19.  
20. ///
21. /// This function takes cipher text and decrypts it using the Affine Cipher
22. /// d(x) = aInverse * (e(x)  b)(mod m).
23. ///
24. public static string AffineDecrypt(string cipherText, int a, int b)
25. {
26.     string plainText = "";
27.  
28.     // Get Multiplicative Inverse of a
29.     int aInverse = MultiplicativeInverse(a);
30.  
31.     // Put Cipher Text (all capitals) into Character Array
32.     char[] chars = cipherText.ToUpper().ToCharArray();
33.  
34.     // Computer d(x) = aInverse * (e(x)  b)(mod m)
35.     foreach (char c in chars)
36.     {
37.         int x = Convert.ToInt32(c - 65);
38.         if (x - b < 0) x = Convert.ToInt32(x) + 26;
39.         plainText += Convert.ToChar(((aInverse * (x - b)) % 26) + 65);
40.     }
41.  
42.     return plainText;
43. }
44.  
45. ///
46. /// This functions returns the multiplicative inverse of integer a mod 26.
47. ///
48. public static int MultiplicativeInverse(int a)
49. {
50.     for (int x = 1; x < 27; x++)
51.     {
52.         if ((a * x) % 26 == 1)
53.             return x;
54.     }
55.  
56.     throw new Exception("No multiplicative inverse found!");
57. }

Happy Coding !!!!