A transposition cipher is a method of encryption where the positions of characters are shifted according to a certain system, without changing the actual characters themselves. It rearranges the characters in a message to create ciphertext, and the same method in reverse restores the original message.
This post walks you through how a transposition cipher works using a Java program. This particular example arranges characters into a matrix and reads them column-wise (for encryption) or row-wise (for decryption).
How the Transposition Cipher Works
- Encryption:
- Fill a matrix column-wise with characters of the message.
- Read the characters row-wise to get the encrypted message.
- The operation is repeated twice for added confusion.
- Decryption:
- Reverse the above process by filling the matrix row-wise.
- Read it column-wise to retrieve the original message.
Java Implementation of Transposition Cipher
package an_tans;
import java.io.*;
public class An_tans {
// Method for encryption
public static String enc(String ip, int m_row, int m_col) {
char[][] op = new char[100][100];
int len = ip.length();
String op2 = "";
int i1, i2, i;
// Fill matrix column-wise with input characters
for (i = 0, i1 = 0, i2 = 0; i < len; i++) {
op[i2][i1] = ip.charAt(i);
i2++;
if (i2 == m_row) {
i2 = 0;
i1++;
}
}
// Display the matrix
System.out.println("Cipher matrix:");
for (i1 = 0; i1 < m_row; i1++) {
for (i2 = 0; i2 < m_col; i2++) {
System.out.print(op[i1][i2] + " ");
}
System.out.println();
}
// Read the matrix row-wise to form the encrypted message
for (i1 = 0; i1 < m_row; i1++) {
for (i2 = 0; i2 < m_col; i2++) {
op2 = op2 + op[i1][i2];
}
}
return (op2);
}
// Method for decryption
public static String dec(String ip, int m_row, int m_col) {
char[][] op = new char[100][100];
int len = ip.length();
String op2 = "";
int i1, i2, i;
// Fill matrix row-wise with input characters
for (i = 0, i1 = 0, i2 = 0; i < len; i++) {
op[i1][i2] = ip.charAt(i);
i2++;
if (i2 == m_col) {
i2 = 0;
i1++;
}
}
// Display the matrix
System.out.println("Cipher matrix");
for (i1 = 0; i1 < m_row; i1++) {
for (i2 = 0; i2 < m_col; i2++) {
System.out.print(op[i1][i2] + " ");
}
System.out.println();
}
// Read the matrix column-wise to recover the original message
for (i1 = 0; i1 < m_col; i1++) {
for (i2 = 0; i2 < m_row; i2++) {
op2 = op2 + op[i2][i1];
}
}
return (op2);
}
public static void main(String[] args) throws IOException {
BufferedReader obj = new BufferedReader(new InputStreamReader(System.in));
int m_col;
int m_row;
String ip;
int len;
String op2;
System.out.println("Enter input:");
ip = obj.readLine();
len = ip.length();
System.out.println("Enter number of rows:");
m_row = Integer.parseInt(obj.readLine());
// Calculate number of columns based on input length
m_col = (int) Math.ceil((float) len / m_row);
System.out.println("What do you want to perform:\n1.Encryption\n2.Decryption");
int ch;
ch = Integer.parseInt(obj.readLine());
if (ch == 1) {
op2 = enc(ip, m_row, m_col);
op2 = enc(op2, m_row, m_col);
System.out.println("Cipher text:" + op2);
} else if (ch == 2) {
op2 = dec(ip, m_row, m_col);
op2 = dec(op2, m_row, m_col);
System.out.println("Plain text:" + op2);
} else {
System.out.println("Invalid Choice");
}
}
}
Sample Output and Explanation
Enter input:
MEETMEATPARK
Enter number of rows:
2
What do you want to perform:
1.Encryption
2.Decryption
1
Cipher matrix:
M E M A P R
E T E T A K
Cipher matrix:
M M P E E A
E A R T T K
Cipher text:MMPEEAEARTTK
Enter input:
MMPEEAEARTTK
Enter number of rows:
2
What do you want to perform:
1.Encryption
2.Decryption
2
Cipher matrix
M M P E E A
E A R T T K
Cipher matrix
M E M A P R
E T E T A K
Plain text:MEETMEATPARKStep-by-Step Explanation of Output
- Input Message:
MEETMEATPARK - Number of Rows:
2 - Length:
12→ so columns = 12 / 2 = 6
First Encryption:
- Fill the matrix column-wise:
- Column 1: M E
- Column 2: E T
- Column 3: M E
- Column 4: A T
- Column 5: P A
- Column 6: R K
- Read the matrix row-wise:
- Row 1: M E M A P R
- Row 2: E T E T A K → Encrypted once:
MEMAPRETETAK
Second Encryption:
- Fill matrix again column-wise with the above result
- Columns: M E, M A, P R, E T, E T, A K
- Read row-wise:
- Row 1: M M P E E A
- Row 2: E A R T T K → Final Cipher Text:
MMPEEAEARTTK
Decryption:
- Fill the matrix row-wise with the cipher text (MMPEEAEARTTK):
- Row 1: M M P E E A
- Row 2: E A R T T K
- Read column-wise:
- M E M A P R
- E T E T A K → After one decryption:
MEMAPRETETAK
- Repeat decryption to recover original message:
- Rows: M E M A P R and E T E T A K
- Read column-wise again:
MEETMEATPARK