A keyword cipher is a form of monoalphabetic substitution. A keyword is used as the key, and it determines the letter matchings of the cipher alphabet to the plain alphabet. Repeats of letters in the word are removed, then the cipher alphabet is generated with the keyword matching to A, B, C etc. until the keyword is used up, whereupon the rest of the ciphertext letters are used in alphabetical order, excluding those already used in the key.
Encryption
First line of input contains keyword which you wish to enter. Second line of input contains the string which you have to encrypt.
Plaintext : A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Encrypted : K R Y P T O S A B C D E F G H I J L M N Q U V W X Z
With KRYPTOS as the keyword, all As become Ks, all Bs become Rs and so on. Encrypting the message “knowledge is power” using the keyword “kryptos”:
Encrypting the message: Knowledge is Power
Encoded message: IlmWjbaEb gq NmWbp
Examples:
Input : Keyword : secret Message : Zombie Here Output : Ciphered String : ZLJEFT DTOT Take the first example, we used "secret" keyword there. Plain Text : A B C D E F G H I J K L M N O P Q R S T U V W X Y Z When "secret" keyword is used, the new encypting text becomes : Encrypting : S E C R T A B D F G H I J K L M N O P Q U V W X Y Z This means 'A' means 'S', 'B' means 'E' and 'C' means 'C' and so on. Lets encode the given message "Zombie Here" ZOMBIE HERE becomes ZLJEFT DTOT Input : Keyword : Star War Message : Attack at dawn Output : Ciphered String : SPPSAG SP RSVJ
Few points to be noted in this method:
- All the messages are encoded in uppercase.
- Whitespace, special character and numbers does not takes into consideration in keyword although you can put it in there.
- While encrypting the message, whitespace, special characters and numbers remain unaffected.
C++
// CPP program for encoding the string// using classical cipher #include<bits/stdc++.h>using namespace std; // Function generates the encoded textstring encoder(string key){ string encoded = ""; // This array represents the // 26 letters of alphabets bool arr[26] = {0}; // This loop inserts the keyword // at the start of the encoded string for (int i=0; i<key.size(); i++) { if(key[i] >= 'A' && key[i] <= 'Z') { // To check whether the character is inserted // earlier in the encoded string or not if (arr[key[i]-65] == 0) { encoded += key[i]; arr[key[i]-65] = 1; } } else if (key[i] >= 'a' && key[i] <= 'z') { if (arr[key[i]-97] == 0) { encoded += key[i] - 32; arr[key[i]-97] = 1; } } } // This loop inserts the remaining // characters in the encoded string. for (int i=0; i<26; i++) { if(arr[i] == 0) { arr[i]=1; encoded += char(i + 65); } } return encoded;} // Function that generates encodes(cipher) the messagestring cipheredIt(string msg, string encoded){ string cipher=""; // This loop ciphered the message. // Spaces, special characters and numbers remain same. for (int i=0; i<msg.size(); i++) { if (msg[i] >='a' && msg[i] <='z') { int pos = msg[i] - 97; cipher += encoded[pos]; } else if (msg[i] >='A' && msg[i] <='Z') { int pos = msg[i] - 65; cipher += encoded[pos]; } else { cipher += msg[i]; } } return cipher;} // Driver codeint main(){ // Hold the Keyword string key; key = "Computer"; cout << "Keyword : " <<key << endl; // Function call to generate encoded text string encoded = encoder(key); // Message that need to encode string message = "GeeksforGeeks"; cout << "Message before Ciphering : " << message << endl; // Function call to print ciphered text cout << "Ciphered Text : " << cipheredIt(message,encoded) << endl; return 0;} |
Java
// Java program for encoding the string// using classical cipher class GFG { // Function generates the encoded text static String encoder(char[] key) { String encoded = ""; // This array represents the // 26 letters of alphabets boolean[] arr = new boolean[26]; // This loop inserts the keyword // at the start of the encoded string for (int i = 0; i < key.length; i++) { if (key[i] >= 'A' && key[i] <= 'Z') { // To check whether the character is inserted // earlier in the encoded string or not if (arr[key[i] - 65] == false) { encoded += (char) key[i]; arr[key[i] - 65] = true; } } else if (key[i] >= 'a' && key[i] <= 'z') { if (arr[key[i] - 97] == false) { encoded += (char) (key[i] - 32); arr[key[i] - 97] = true; } } } // This loop inserts the remaining // characters in the encoded string. for (int i = 0; i < 26; i++) { if (arr[i] == false) { arr[i] = true; encoded += (char) (i + 65); } } return encoded; } // Function that generates encodes(cipher) the message static String cipheredIt(String msg, String encoded) { String cipher = ""; // This loop ciphered the message. // Spaces, special characters and numbers remain same. for (int i = 0; i < msg.length(); i++) { if (msg.charAt(i) >= 'a' && msg.charAt(i) <= 'z') { int pos = msg.charAt(i) - 97; cipher += encoded.charAt(pos); } else if (msg.charAt(i) >= 'A' && msg.charAt(i) <= 'Z') { int pos = msg.charAt(i) - 65; cipher += encoded.charAt(pos); } else { cipher += msg.charAt(i); } } return cipher; } // Driver code public static void main(String[] args) { // Hold the Keyword String key; key = "Computer"; System.out.println("Keyword : " + key); // Function call to generate encoded text String encoded = encoder(key.toCharArray()); // Message that need to encode String message = "GeeksforGeeks"; System.out.println("Message before Ciphering : " + message); // Function call to print ciphered text System.out.println("Ciphered Text : " + cipheredIt(message, encoded)); }} // This code is contributed by 29AjayKumar |
C#
// C# program for encoding the string// using classical cipherusing System; class GFG { // Function generates the encoded text static String encoder(char[] key) { String encoded = ""; // This array represents the // 26 letters of alphabets Boolean[] arr = new Boolean[26]; // This loop inserts the keyword // at the start of the encoded string for (int i = 0; i < key.Length; i++) { if (key[i] >= 'A' && key[i] <= 'Z') { // To check whether the character is inserted // earlier in the encoded string or not if (arr[key[i] - 65] == false) { encoded += (char) key[i]; arr[key[i] - 65] = true; } } else if (key[i] >= 'a' && key[i] <= 'z') { if (arr[key[i] - 97] == false) { encoded += (char) (key[i] - 32); arr[key[i] - 97] = true; } } } // This loop inserts the remaining // characters in the encoded string. for (int i = 0; i < 26; i++) { if (arr[i] == false) { arr[i] = true; encoded += (char) (i + 65); } } return encoded; } // Function that generates encodes(cipher) the message static String cipheredIt(String msg, String encoded) { String cipher = ""; // This loop ciphered the message. // Spaces, special characters and numbers remain same. for (int i = 0; i < msg.Length; i++) { if (msg[i] >= 'a' && msg[i] <= 'z') { int pos = msg[i] - 97; cipher += encoded[pos]; } else if (msg[i] >= 'A' && msg[i] <= 'Z') { int pos = msg[i] - 65; cipher += encoded[pos]; } else { cipher += msg[i]; } } return cipher; } // Driver code public static void Main(String[] args) { // Hold the Keyword String key; key = "Computer"; Console.WriteLine("Keyword : " + key); // Function call to generate encoded text String encoded = encoder(key.ToCharArray()); // Message that need to encode String message = "GeeksforGeeks"; Console.WriteLine("Message before Ciphering : " + message); // Function call to print ciphered text Console.WriteLine("Ciphered Text : " + cipheredIt(message, encoded)); }} /* This code contributed by PrinciRaj1992 */ |
Output:
Keyword : Computer Message before Ciphering : GeeksforGeeks Ciphered Text : EUUDNTILEUUDN
Decryption
To decode the message you check the position of given message in encrypting text with the plain text.
Plaintext : A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Encrypted : K R Y P T O S A B C D E F G H I J L M N Q U V W X Z
Message : PTYBIATLEP
Deciphered Text : DECIPHERED
Now, how we generate the deciphered string?
We search for ‘P’ in Encrypted Text and compare its position with plain text letter and generate that letter. So ‘P’ becomes ‘D’, ‘T’ becomes ‘E’, ‘Y’ becomes ‘C’ and so on.
Examples:
Input : Keyword : secret Message : zljeft dtOT Output : Deciphered String : ZOMBIE HERE Input : Keyword : joker0O7hack123 Message : QjTijl Output : Deciphered String : BATMAN
// CPP program for decoding the string// which generate using classical cipher #include<bits/stdc++.h>using namespace std; // Original Set of lettersstring plaintext = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"; // Function generates the encoded textstring encoder(string key){ string encoded = ""; bool arr[26] = {0}; // This loop inserts the keyword // at the start of the encoded string for (int i=0; i<key.size(); i++) { if(key[i] >= 'A' && key[i] <= 'Z') { // To check whether the character is inserted // earlier in the encoded string or not if (arr[key[i]-65] == 0) { encoded += key[i]; arr[key[i]-65] = 1; } } else if (key[i] >= 'a' && key[i] <= 'z') { if (arr[key[i]-97] == 0) { encoded += key[i] - 32; arr[key[i]-97] = 1; } } } // This loop inserts the remaining // characters in the encoded string. for (int i=0; i<26; i++) { if(arr[i] == 0) { arr[i]=1; encoded += char(i + 65); } } return encoded;} // This function will decode the messagestring decipheredIt(string msg, string encoded){ // Hold the position of every character (A-Z) // from encoded string map <char,int> enc; for(int i=0; i<encoded.size(); i++) { enc[encoded[i]]=i; } string decipher=""; // This loop deciphered the message. // Spaces, special characters and numbers remain same. for (int i=0; i<msg.size(); i++) { if (msg[i] >='a' && msg[i] <='z') { int pos = enc[msg[i]-32]; decipher += plaintext[pos]; } else if(msg[i] >='A' && msg[i] <='Z') { int pos = enc[msg[i]]; decipher += plaintext[pos]; } else { decipher += msg[i]; } } return decipher;} // Driver codeint main(){ // Hold the Keyword string key; key = "Computer"; cout << "Keyword : "<< key << endl; // Function call to generate encoded text string encoded = encoder(key); // Message that need to decode string message = "EUUDN TIL EUUDN"; cout << "Message before Deciphering : " << message << endl; // Function call to print deciphered text cout << "Deciphered Text : " << decipheredIt(message,encoded) << endl; return 0;} |
Output:
Keyword : Computer Message before Deciphering : EUUDN TIL EUUDN Deciphered Text : GEEKS FOR GEEKS
You can improve this Classical Cipher : Keyword also. Here we are only taking A-Z in plain text. You can take uppercase,lowercase and numbers also into consideration.
Ways to attack a keyword cipher : The best ways to attack a keyword cipher without knowing
the keyword are through known – plaintext attack, frequency analysis and discovery of the keyword (often a cryptanalyst will combine all three techniques). Keyword discovery allows immediate decryption since the table can be made immediately.
This article is contributed by Sachin Bisht. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.
