Playfair Cipher with Examples - GeeksforGeeks

24 Dec, 2021
The Playfair cipher was the first practical digraph substitution cipher. The scheme was invented in 1854 by Charles Wheatstone but was named after Lord Playfair who promoted the use of the cipher. In playfair cipher unlike traditional cipher we encrypt a pair of alphabets(digraphs) instead of a single alphabet.It was used for tactical purposes by British forces in the Second Boer War and in World War I and for the same purpose by the Australians during World War II. This was because Playfair is reasonably fast to use and requires no special equipment. 
For the encryption process let us consider the following example: 
The Playfair Cipher Encryption Algorithm: The Algorithm consists of 2 steps:  
Generate the key Square(5×5): The key square is a 5×5 grid of alphabets that acts as the key for encrypting the plaintext. Each of the 25 alphabets must be unique and one letter of the alphabet (usually J) is omitted from the table (as the table can hold only 25 alphabets). If the plaintext contains J, then it is replaced by I.  The initial alphabets in the key square are the unique alphabets of the key in the order in which they appear followed by the remaining letters of the alphabet in order.  Algorithm to encrypt the plain text: The plaintext is split into pairs of two letters (digraphs). If there is an odd number of letters, a Z is added to the last letter. For example:  
Generate the key Square(5×5): The key square is a 5×5 grid of alphabets that acts as the key for encrypting the plaintext. Each of the 25 alphabets must be unique and one letter of the alphabet (usually J) is omitted from the table (as the table can hold only 25 alphabets). If the plaintext contains J, then it is replaced by I.  The initial alphabets in the key square are the unique alphabets of the key in the order in which they appear followed by the remaining letters of the alphabet in order.  
The key square is a 5×5 grid of alphabets that acts as the key for encrypting the plaintext. Each of the 25 alphabets must be unique and one letter of the alphabet (usually J) is omitted from the table (as the table can hold only 25 alphabets). If the plaintext contains J, then it is replaced by I.  
The initial alphabets in the key square are the unique alphabets of the key in the order in which they appear followed by the remaining letters of the alphabet in order.  
Algorithm to encrypt the plain text: The plaintext is split into pairs of two letters (digraphs). If there is an odd number of letters, a Z is added to the last letter. For example:  
PlainText: "instruments" 
After Split: 'in' 'st' 'ru' 'me' 'nt' 'sz'
1. Pair cannot be made with same letter. Break the letter in single and add a bogus letter to the previous letter.
Plain Text: “hello”
After Split: ‘he’ ‘lx’ ‘lo’
Here ‘x’ is the bogus letter.
2. If the letter is standing alone in the process of pairing, then add an extra bogus letter with the alone letter
Plain Text: “helloe”
AfterSplit: ‘he’ ‘lx’ ‘lo’ ‘ez’
Here ‘z’  is the bogus letter.
Rules for Encryption:  
If both the letters are in the same column: Take the letter below each one (going back to the top if at the bottom).For example:  
Diagraph: "me"
Encrypted Text: cl
Encryption: 
  m -> c
  e -> l
 
 
If both the letters are in the same row: Take the letter to the right of each one (going back to the leftmost if at the rightmost position).For example:  
Diagraph: "st"
Encrypted Text: tl
Encryption: 
  s -> t
  t -> l
 
 
If neither of the above rules is true: Form a rectangle with the two letters and take the letters on the horizontal opposite corner of the rectangle.For example:  
Diagraph: "nt"
Encrypted Text: rq
Encryption: 
  n -> r
  t -> q
 
 
For example:  
Plain Text: "instrumentsz"
Encrypted Text: gatlmzclrqtx
Encryption: 
  i -> g
  n -> a
  s -> t
  t -> l
  r -> m
  u -> z
  m -> c
  e -> l
  n -> r
  t -> q
  s -> t
  z -> x
 
Below is an implementation of Playfair Cipher in C: 
C
// C program to implement Playfair Cipher #include <stdio.h>#include <stdlib.h>#include <string.h> #define SIZE 30 // Function to convert the string to lowercasevoid toLowerCase(char plain[], int ps){    int i;    for (i = 0; i < ps; i++) {        if (plain[i] > 64 && plain[i] < 91)            plain[i] += 32;    }} // Function to remove all spaces in a stringint removeSpaces(char* plain, int ps){    int i, count = 0;    for (i = 0; i < ps; i++)        if (plain[i] != ' ')            plain[count++] = plain[i];    plain[count] = '\0';    return count;} // Function to generate the 5x5 key squarevoid generateKeyTable(char key[], int ks, char keyT[5][5]){    int i, j, k, flag = 0, *dicty;     // a 26 character hashmap    // to store count of the alphabet    dicty = (int*)calloc(26, sizeof(int));    for (i = 0; i < ks; i++) {        if (key[i] != 'j')            dicty[key[i] - 97] = 2;    }     dicty['j' - 97] = 1;     i = 0;    j = 0;     for (k = 0; k < ks; k++) {        if (dicty[key[k] - 97] == 2) {            dicty[key[k] - 97] -= 1;            keyT[i][j] = key[k];            j++;            if (j == 5) {                i++;                j = 0;            }        }    }     for (k = 0; k < 26; k++) {        if (dicty[k] == 0) {            keyT[i][j] = (char)(k + 97);            j++;            if (j == 5) {                i++;                j = 0;            }        }    }} // Function to search for the characters of a digraph// in the key square and return their positionvoid search(char keyT[5][5], char a, char b, int arr[]){    int i, j;     if (a == 'j')        a = 'i';    else if (b == 'j')        b = 'i';     for (i = 0; i < 5; i++) {         for (j = 0; j < 5; j++) {             if (keyT[i][j] == a) {                arr[0] = i;                arr[1] = j;            }            else if (keyT[i][j] == b) {                arr[2] = i;                arr[3] = j;            }        }    }} // Function to find the modulus with 5int mod5(int a) { return (a % 5); } // Function to make the plain text length to be evenint prepare(char str[], int ptrs){    if (ptrs % 2 != 0) {        str[ptrs++] = 'z';        str[ptrs] = '\0';    }    return ptrs;} // Function for performing the encryptionvoid encrypt(char str[], char keyT[5][5], int ps){    int i, a[4];     for (i = 0; i < ps; i += 2) {         search(keyT, str[i], str[i + 1], a);         if (a[0] == a[2]) {            str[i] = keyT[a[0]][mod5(a[1] + 1)];            str[i + 1] = keyT[a[0]][mod5(a[3] + 1)];        }        else if (a[1] == a[3]) {            str[i] = keyT[mod5(a[0] + 1)][a[1]];            str[i + 1] = keyT[mod5(a[2] + 1)][a[1]];        }        else {            str[i] = keyT[a[0]][a[3]];            str[i + 1] = keyT[a[2]][a[1]];        }    }} // Function to encrypt using Playfair Ciphervoid encryptByPlayfairCipher(char str[], char key[]){    char ps, ks, keyT[5][5];     // Key    ks = strlen(key);    ks = removeSpaces(key, ks);    toLowerCase(key, ks);     // Plaintext    ps = strlen(str);    toLowerCase(str, ps);    ps = removeSpaces(str, ps);     ps = prepare(str, ps);     generateKeyTable(key, ks, keyT);     encrypt(str, keyT, ps);} // Driver codeint main(){    char str[SIZE], key[SIZE];     // Key to be encrypted    strcpy(key, "Monarchy");    printf("Key text: %s\n", key);     // Plaintext to be encrypted    strcpy(str, "instruments");    printf("Plain text: %s\n", str);     // encrypt using Playfair Cipher    encryptByPlayfairCipher(str, key);     printf("Cipher text: %s\n", str);     return 0;} // This code is contributed by AbhayBhat
Key text: Monarchy
Plain text: instruments
Cipher text: gatlmzclrqtx
Decrypting the Playfair cipher is as simple as doing the same process in reverse. The receiver has the same key and can create the same key table, and then decrypt any messages made using that key. 
The Playfair Cipher Decryption Algorithm: The Algorithm consistes of 2 steps:  
Generate the key Square(5×5) at the receiver’s end: The key square is a 5×5 grid of alphabets that acts as the key for encrypting the plaintext. Each of the 25 alphabets must be unique and one letter of the alphabet (usually J) is omitted from the table (as the table can hold only 25 alphabets). If the plaintext contains J, then it is replaced by I.  The initial alphabets in the key square are the unique alphabets of the key in the order in which they appear followed by the remaining letters of the alphabet in order.  Algorithm to decrypt the ciphertext: The ciphertext is split into pairs of two letters (digraphs).  
Generate the key Square(5×5) at the receiver’s end: The key square is a 5×5 grid of alphabets that acts as the key for encrypting the plaintext. Each of the 25 alphabets must be unique and one letter of the alphabet (usually J) is omitted from the table (as the table can hold only 25 alphabets). If the plaintext contains J, then it is replaced by I.  The initial alphabets in the key square are the unique alphabets of the key in the order in which they appear followed by the remaining letters of the alphabet in order.  
The key square is a 5×5 grid of alphabets that acts as the key for encrypting the plaintext. Each of the 25 alphabets must be unique and one letter of the alphabet (usually J) is omitted from the table (as the table can hold only 25 alphabets). If the plaintext contains J, then it is replaced by I.  
The initial alphabets in the key square are the unique alphabets of the key in the order in which they appear followed by the remaining letters of the alphabet in order.  
Algorithm to decrypt the ciphertext: The ciphertext is split into pairs of two letters (digraphs).  
Note: The ciphertext always have even number of characters.
For example:  
For example:  
CipherText: "gatlmzclrqtx" 
After Split: 'ga' 'tl' 'mz' 'cl' 'rq' 'tx'
Rules for Decryption: If both the letters are in the same column: Take the letter above each one (going back to the bottom if at the top).For example:  
Rules for Decryption: If both the letters are in the same column: Take the letter above each one (going back to the bottom if at the top).For example:  
If both the letters are in the same column: Take the letter above each one (going back to the bottom if at the top).For example:  
Diagraph: "cl" 
Decrypted Text: me
Decryption: 
  c -> m
  l -> e
 
 
If both the letters are in the same row: Take the letter to the left of each one (going back to the rightmost if at the leftmost position).For example:  
Diagraph: "tl" 
Decrypted Text: st 
Decryption: 
  t -> s
  l -> t
 
 
If neither of the above rules is true: Form a rectangle with the two letters and take the letters on the horizontal opposite corner of the rectangle.For example:  
Diagraph: "rq" 
Decrypted Text: nt 
Decryption: 
  r -> n
  q -> t
 
 
For example:  
Plain Text: "gatlmzclrqtx"
Decrypted Text: instrumentsz
Decryption: 
(red)-> (green)
  ga -> in
  tl -> st
  mz -> ru
  cl -> me
  rq -> nt
  tx -> sz
 
Below is an implementation of Playfair Cipher Decryption in C:  
C
#include <stdio.h>#include <stdlib.h>#include <string.h>#define SIZE 30 // Convert all the characters// of a string to lowercasevoid toLowerCase(char plain[], int ps){    int i;    for (i = 0; i < ps; i++) {        if (plain[i] > 64 && plain[i] < 91)            plain[i] += 32;    }} // Remove all spaces in a string// can be extended to remove punctuationint removeSpaces(char* plain, int ps){    int i, count = 0;    for (i = 0; i < ps; i++)        if (plain[i] != ' ')            plain[count++] = plain[i];    plain[count] = '\0';    return count;} // generates the 5x5 key squarevoid generateKeyTable(char key[], int ks, char keyT[5][5]){    int i, j, k, flag = 0, *dicty;     // a 26 character hashmap    // to store count of the alphabet    dicty = (int*)calloc(26, sizeof(int));     for (i = 0; i < ks; i++) {        if (key[i] != 'j')            dicty[key[i] - 97] = 2;    }    dicty['j' - 97] = 1;     i = 0;    j = 0;    for (k = 0; k < ks; k++) {        if (dicty[key[k] - 97] == 2) {            dicty[key[k] - 97] -= 1;            keyT[i][j] = key[k];            j++;            if (j == 5) {                i++;                j = 0;            }        }    }    for (k = 0; k < 26; k++) {        if (dicty[k] == 0) {            keyT[i][j] = (char)(k + 97);            j++;            if (j == 5) {                i++;                j = 0;            }        }    }} // Search for the characters of a digraph// in the key square and return their positionvoid search(char keyT[5][5], char a, char b, int arr[]){    int i, j;     if (a == 'j')        a = 'i';    else if (b == 'j')        b = 'i';     for (i = 0; i < 5; i++) {        for (j = 0; j < 5; j++) {            if (keyT[i][j] == a) {                arr[0] = i;                arr[1] = j;            }            else if (keyT[i][j] == b) {                arr[2] = i;                arr[3] = j;            }        }    }} // Function to find the modulus with 5int mod5(int a){    if (a < 0)        a += 5;    return (a % 5);} // Function to decryptvoid decrypt(char str[], char keyT[5][5], int ps){    int i, a[4];    for (i = 0; i < ps; i += 2) {        search(keyT, str[i], str[i + 1], a);        if (a[0] == a[2]) {            str[i] = keyT[a[0]][mod5(a[1] - 1)];            str[i + 1] = keyT[a[0]][mod5(a[3] - 1)];        }        else if (a[1] == a[3]) {            str[i] = keyT[mod5(a[0] - 1)][a[1]];            str[i + 1] = keyT[mod5(a[2] - 1)][a[1]];        }        else {            str[i] = keyT[a[0]][a[3]];            str[i + 1] = keyT[a[2]][a[1]];        }    }} // Function to call decryptvoid decryptByPlayfairCipher(char str[], char key[]){    char ps, ks, keyT[5][5];     // Key    ks = strlen(key);    ks = removeSpaces(key, ks);    toLowerCase(key, ks);     // ciphertext    ps = strlen(str);    toLowerCase(str, ps);    ps = removeSpaces(str, ps);     generateKeyTable(key, ks, keyT);     decrypt(str, keyT, ps);} // Driver codeint main(){    char str[SIZE], key[SIZE];     // Key to be encrypted    strcpy(key, "Monarchy");    printf("Key text: %s\n", key);     // Ciphertext to be decrypted    strcpy(str, "gatlmzclrqtx");    printf("Plain text: %s\n", str);     // encrypt using Playfair Cipher    decryptByPlayfairCipher(str, key);     printf("Deciphered text: %s\n", str);     return 0;} // This code is contributed by AbhayBhat
Key text: Monarchy
Plain text: gatlmzclrqtx
Deciphered text: instrumentsz
Advantages: It is significantly harder to break since the frequency analysis technique used to break simple substitution ciphers is difficult but still can be used on (25*25) = 625 digraphs rather than 25 monographs which is difficult.  Frequency analysis thus requires more cipher text to crack the encryption.  
It is significantly harder to break since the frequency analysis technique used to break simple substitution ciphers is difficult but still can be used on (25*25) = 625 digraphs rather than 25 monographs which is difficult.  Frequency analysis thus requires more cipher text to crack the encryption.  
It is significantly harder to break since the frequency analysis technique used to break simple substitution ciphers is difficult but still can be used on (25*25) = 625 digraphs rather than 25 monographs which is difficult.  
Frequency analysis thus requires more cipher text to crack the encryption.  
Disadvantages: An interesting weakness is the fact that a digraph in the ciphertext (AB) and it’s reverse (BA) will have corresponding plaintexts like UR and RU (and also ciphertext UR and RU will correspond to plaintext AB and BA, i.e. the substitution is self-inverse). That can easily be exploited with the aid of frequency analysis, if the language of the plaintext is known.  Another disadvantage is that playfair cipher is a symmetric cipher thus same key is used for both encryption and decryption.  
An interesting weakness is the fact that a digraph in the ciphertext (AB) and it’s reverse (BA) will have corresponding plaintexts like UR and RU (and also ciphertext UR and RU will correspond to plaintext AB and BA, i.e. the substitution is self-inverse). That can easily be exploited with the aid of frequency analysis, if the language of the plaintext is known.  Another disadvantage is that playfair cipher is a symmetric cipher thus same key is used for both encryption and decryption.  
An interesting weakness is the fact that a digraph in the ciphertext (AB) and it’s reverse (BA) will have corresponding plaintexts like UR and RU (and also ciphertext UR and RU will correspond to plaintext AB and BA, i.e. the substitution is self-inverse). That can easily be exploited with the aid of frequency analysis, if the language of the plaintext is known.  
Another disadvantage is that playfair cipher is a symmetric cipher thus same key is used for both encryption and decryption.  
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