Saturday, March 24, 2012

Simple Substitution Cipher Algorithms in C

A substitution cipher is a method of encryption by which units of the original alphabet (or plain text) are replaced with units of a coded alphabet (or cipher text) according to a regular system. The units may be single letters, two letters or triplets or letters, etc. A simple substitution cipher uses operates with single letter units.

To encrypt a message using a substitution cipher one needs the plain text alphabet (original alphabet) and the cipher text alphabet (the coded alphabet) which shall serve as a private key.
The formula for the substitution is:
  • Ei represents the i-th character from the encoded string
  • Cj represents the j-th character from the cipher text alphabet, where j represents the position of the Ei character in the original alphabet.
In order to implement such algorithm we shall define the following enumeration:
typedef enum
Below you can see the C implementation for the algorithm:
OPERATION_STATUS SimpleSubstitutionCipher_Code(char* originalAlphabet,
                                               char* codedAlphabet,
                                               char* originalMessage,
                                               char* encodedMessage)
 unsigned short alphabetLength = strlen(originalAlphabet);
 unsigned int messageLength    = strlen(originalMessage);
 unsigned int i = 0U;
 char* pointer = NULL;
 unsigned int position = 0U;
 if( alphabetLength != strlen(codedAlphabet) )
  //The lengths of the alphabets do not match
 for(i=0; i<messageLength; i++)
  pointer  = strchr(originalAlphabet, originalMessage[i]);
   //A character in the message was not found in the
   //original alphabet
   position = pointer - originalAlphabet;
   encodedMessage[i] = codedAlphabet[position];
 encodedMessage[messageLength] = '\0';
The function receives 4 parameters:
  • originalAlphabet - a pointer to a string containing the plain text alphabet (who has all the letters that appear in the originalMessage). All symbols in the originalAlphabet should appear only once.
  • codedAlphabet - a pointer to a string containing the cipher text alphabet (who has the same length as the originalAlphabet string). All symbols in the codedAlphabet should appear only once.
  • originalMessage - a pointer to a string containing the message who will be encoded
  • codedMessage - a pointer to a pre-allocated empty string who will contain the encoded message.
The function returns OPERATIONS_SUCCES if all preconditions have been respected, respectively OPERATON_FAILED if the lengths of the alphabets are different or if the function found a symbol in the original message who has not been defined in the originalAlphabet.

A simple encoding/decoding example
 char originalAlphabet[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ ";
 char codedAlphabet[]    = "QWERTYUIOPASDFGHJKLZXCVBNM#";
 char originalMessage[]  = "SUBSTITUTION CIPHER";
 char encodedMessage[20];
 char decodedMessage[20];
                               originalMessage, encodedMessage);
                               encodedMessage, decodedMessage);
As you probably already observed, the same function is used for both encryption and decryption. This is because the same operations are performed in both cases.

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