Implementation of Vigenere Cipher
``````
// CSci 4509 Vigenere Cipher implementation
// Elena Machkasova, 1/31/05

public class Vigenere {
public static void main(String[] args) {

if (args.length < 3) {
System.out.println("Usage: java Vigenere mode key message");
System.out.println("mode: e for encrypt. d for decrypt");
System.out.println("key: a sequence of letters, no spaces, etc.");
System.out.println("message: the message to encrypt or decrypt");
System.exit(0);
}

if (args[0].equals("e") || args[0].equals("E")) {
// encrypting
System.out.println(encrypt(args[2], args[1]));
} else if (args[0].equals("d") || args[0].equals("D")) {
// decrypting
System.out.println(decrypt(args[2], args[1]));
} else {
System.out.println("the first argument must be e or d");
System.exit(0);
}

}

// computes n mod m defined as the smallest
// non-negative integer r such that n = k * m + r,
// where k is an integer
// Assume that m > 0
public static int mod(int n, int m) {
int r = n % m;
if (r < 0) r = r + m;
return r;
}

// converts an upper or lower case letter
// into an integer 0 <= n < = 25 which
// corresponds to the position of the letter in
// English alphabet
public static int letterToNum(char letter) {
int position = 0;
if (Character.isUpperCase(letter)) {
position = letter - 'A';
}
else if (Character.isLowerCase(letter)) {
position = letter - 'a';
} else {
System.out.println("Illegal character " + letter);
System.exit(0);
}
return position;
}

public static String encrypt(String plaintext, String key) {
int messageLength = plaintext.length();
int keyLength = key.length();
StringBuffer ciphertext = new StringBuffer();
int j = 0;
for (int i = 0; i < messageLength; ++i) {
// convert the letter to a number 0 to 25
char letter = plaintext.charAt(i);
int position = letterToNum(letter);

// convert the next key letter to a number 0 to 25
char keyLetter = key.charAt(j);
int shift = letterToNum(keyLetter);

// encrypt the letter
ciphertext.append((char) ('A' + mod(position + shift,26)));

// index of the next key letter (wraps around)
j = mod(j + 1, keyLength);
}
return ciphertext.toString();
}

public static String decrypt(String ciphertext, String key) {
int messageLength = ciphertext.length();
StringBuffer plaintext = new StringBuffer();

int keyLength = key.length();
int j = 0;

for (int i = 0; i < messageLength; ++i) {
// convert the letter to a number 0 to 25
char letter = ciphertext.charAt(i);
int position = letterToNum(letter);

// convert the next key letter to a number 0 to 25
char keyLetter = key.charAt(j);
int shift = letterToNum(keyLetter);

// encrypt the letter
plaintext.append((char) ('A' + mod(position - shift,26)));

// index of the next key letter (wraps around)
j = mod(j + 1, keyLength);
}

return plaintext.toString();
}
}
``````

Try encrypting this
idonotlikegreeneggsandhamidonotlikethemsamiam
using the key KEY and the key GREEN
The course main page.

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