CODEWITHSUNDEEP
javaencryption
Harpreet Singh
Harpreet Singh

Reputation: 21

How to write Logic for Affine Cipher Decryption in JAVA?

Hello i m working on the affine cipher in JAVA. I have successfully written the code for Encryption but now i m not getting any idea about the logic for decryption.

Below is my logic for encryption:

void encryption()
{
    char character; 
    int plainTextLength=input.length();
    int a=1,b=2,c=3,d=4,e=5,f=6,g=7,h=8,i=9,j=10,k=11,l=12,m=13,n=14,
        o=15,p=16,q=17,r=18,s=19,t=20,u=21,v=22,w=23,x=24,y=25,z=26;

    System.out.print("Cipher text is:" );

    for (int in = 0; in < plainTextLength; in++)
    {
        character = input.charAt(in);

        if (Character.isLetter(character))
        {
            character = (char)((firstKey*(character - 'a') + secondKey) % 26 + 'a');
        }
        System.out.print(character);
    }
    System.out.println();       
}

This is my Encryption logic: character = (char)((firstKey*(character - 'a') + secondKey) % 26 + 'a');

What would be the Decryption logic. I am totally confused ?

Upvotes: 2

Views: 11379

Answers (1)

Jk1
Jk1

Reputation: 11443

General affine cipher decryption formula is quite simple:

enter image description here

where a is your firstKey, b is your secondKey.

So encryption/decryption may be implemented in the following way:

private static int firstKey = 5;
private static int secondKey = 19;
private static int module = 26;

public static void main(String[] args) {
    String input = "abcdefghijklmnopqrstuvwxyz";
    String cipher = encrypt(input);
    String deciphered = decrypt(cipher);
    System.out.println("Source:    " + input);
    System.out.println("Encrypted: " + cipher);
    System.out.println("Decrypted: " + deciphered);
}

static String encrypt(String input) {
    StringBuilder builder = new StringBuilder();
    for (int in = 0; in < input.length(); in++) {
        char character = input.charAt(in);
        if (Character.isLetter(character)) {
            character = (char) ((firstKey * (character - 'a') + secondKey) % module + 'a');
        }
        builder.append(character);
    }
    return builder.toString();
}

static String decrypt(String input) {
    StringBuilder builder = new StringBuilder();
    // compute firstKey^-1 aka "modular inverse"
    BigInteger inverse = BigInteger.valueOf(firstKey).modInverse(BigInteger.valueOf(module));
    // perform actual decryption
    for (int in = 0; in < input.length(); in++) {
        char character = input.charAt(in);
        if (Character.isLetter(character)) {
            int decoded = inverse.intValue() * (character - 'a' - secondKey + module);
            character = (char) (decoded % module + 'a');
        }
        builder.append(character);
    }
    return builder.toString();
}

Output:

Source:    abcdefghijklmnopqrstuvwxyz
Encrypted: dinsxchmrwbglqvafkpuzejoty
Decrypted: abcdefghijklmnopqrstuvwxyz

Upvotes: 7

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