C++ Program to Decode a Message Encoded Using Playfair Cipher

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This C++ program decodes any message encoded using the technique of traditional playfair cipher. The Playfair cipher or Playfair square is a manual symmetric encryption technique and was the first literal digraph substitution cipher. Input is not case sensitive and works only for characters from ‘a’ to ‘z’ and ‘A’ to ‘Z’.

This C++ program is successfully compiled and tested on our system. The program output is given below.

  1. /*
  2.  * C++ Program to Decode a Message Encoded Using Playfair Cipher
  3.  */
  4. #include<iostream>
  5. #include<vector>
  6. using namespace std;
  7.  
  8. const char encoder[5][5] = {{'A','B','C','D','E'},
  9.                       {'F','G','H','I','K'},
  10.                       {'L','M','N','O','P'},
  11.                       {'Q','R','S','T','U'},
  12.                       {'V','W','X','Y','Z'}};
  13.  
  14. void input_string(vector<char>& a)
  15. {
  16.     char c;
  17.     while (1)
  18.     {
  19.         c=getchar();
  20.         if (c >= 97 && c <= 122)
  21.         c -= 32;
  22.         if (c == '\n')
  23.             break;
  24.         else if (c==' ')
  25.             continue;
  26.         else if (c == 'J')
  27.         a.push_back('I');
  28.         a.push_back(c);
  29.     }
  30.     return;
  31. }
  32.  
  33.  
  34.  
  35. void get_pos(char p, int& r, int& c)
  36. {
  37.     if (p < 'J')
  38.     {
  39.         r = (p - 65) / 5;
  40.         c = (p - 65) % 5;
  41.     }
  42.     else if (p > 'J')
  43.     {
  44.         r = (p - 66) / 5;
  45.         c = (p - 66) % 5;
  46.     }
  47.     return;
  48. }
  49.  
  50. void same_row(int r, vector<char>& code, int c1, int c2)
  51. {
  52.     code.push_back(encoder[r][(c1 + 4) % 5]);
  53.     code.push_back(encoder[r][(c2 + 4) % 5]);
  54.     return;
  55. }
  56.  
  57. void same_column(int c, vector<char>& code, int r1, int r2)
  58. {
  59.     code.push_back(encoder[(r1 + 4) % 5][c]);
  60.     code.push_back(encoder[(r2 + 4) % 5][c]);
  61.     return;
  62. }
  63.  
  64. void diff_col_row(int r1, int c1, vector<char>& code, int r2, int c2)
  65. {
  66.     code.push_back(encoder[r1][c2]);
  67.     code.push_back(encoder[r2][c1]);
  68.     return;
  69. }
  70.  
  71.  
  72. void encode(vector<char> msgx, int len)
  73. {
  74.     vector<char> code;
  75.     int i = 0, j = 0;
  76.     int r1, c1, r2, c2;
  77.     while (i < len)
  78.     {
  79.         get_pos(msgx[i], r1, c1);
  80.         i++;
  81.         get_pos(msgx[i], r2, c2);
  82.         if (r1 == r2)
  83.         {
  84.             same_row(r1, code, c1, c2);
  85.         }
  86.         else if (c1 == c2)
  87.         {
  88.             same_column(c1, code, r1, r2);
  89.         }
  90.         else
  91.         {
  92.             diff_col_row(r1, c1, code, r2, c2);
  93.         }
  94.         i++;
  95.     }
  96.     cout<<"\nCODE: ";
  97.     for (j = 0;j < code.size();j++)
  98.     {
  99.         if (code[j] == 'X')
  100.         continue;
  101.         cout<<code[j];
  102.     }
  103.     return;
  104. }
  105.  
  106.  
  107. int main()
  108. {
  109.     vector<char> msg;
  110.     std::cout<<"Enter the Encrypted Message:";
  111.     input_string(msg);
  112.     int len=msg.size();
  113.     encode(msg,len);
  114.     return 0;
  115. }

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Output:
 
Enter the Encrypted Message:CBNVMPPO
CODE: BALLOON

Sanfoundry Global Education & Learning Series – 1000 C++ Programs.

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If you wish to look at all C++ Programming examples, go to C++ Programs.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn