Playfair cipher

Question

I want to write a source code of playfair cipher by C# but I dont't know how to. Please help me!

meteor pt · Answer

Thanks Alan so much!

Alan · Answer

The basis of the Playfair cipher is explained in this Wikipedia article: http://crypto.dsplabs.com.au/classical/playfair-cipher-cryptanalysis.php I also found another article which includes some C code for the algorithm: http://crypto.dsplabs.com.au/classical/playfair-cipher-cryptanalysis.php but surprisingly I couldn't find a C# implementation at all! I've therefore translated the C code into C# and tested it on the example string it uses as well as the Wikipedia example string. When the second of a pair of letters is the same as the first you're supposed to insert an 'x' and I've allowed for this in my version even though the C code didn't: using System; using System.Text; class Test { static void Main() { string originalText = Defend the east wall of the castle.; Console.WriteLine(originalText); string plainText = Playfair.Prepare(originalText); Console.WriteLine(plainText); string key = cdefghiklmnopqrstuvwxyzab; string cipherText = Playfair.Encipher(key plainText); Console.WriteLine(cipherText); plainText = Playfair.Decipher(key cipherText); Console.WriteLine(plainText); Console.WriteLine(); originalText = Hide the gold in the tree stump.; Console.WriteLine(originalText); plainText = Playfair.Prepare(originalText); Console.WriteLine(plainText); key = playfirexmbcdghjknostuvwz; cipherText = Playfair.Encipher(key plainText); Console.WriteLine(cipherText); plainText = Playfair.Decipher(key cipherText); Console.WriteLine(plainText); Console.ReadLine(); } } public class Playfair { /* 'Prepare' removes all characters that are not letters i.e. all numbers punctuation spaces etc. are removed (uppercase is also converted to lowercase). If the seond letter of a pair is the same as the first letter an 'x' is inserted. Also if the length of the string is odd an 'x' is appended to make it an even length as Playfair can only encrypt even length strings. If you want numbers punctuation etc. you must spell it out e.g. 'stop' for period 'one' 'two' etc. */ public static string Prepare(string originalText) { int length = originalText.Length; originalText = originalText.ToLower(); StringBuilder sb = new StringBuilder(); for(int i = 0; i { char c = originalText[i]; if (c >= 97 && c { // If the second letter of a pair is the same as the first insert an 'x' if (sb.Length % 2 == 1 && sb[sb.Length - 1] == c) { sb.Append('x'); } sb.Append(c); } } // If the string is an odd length append an 'x' if (sb.Length % 2 == 1) { sb.Append('x'); } return sb.ToString(); } /* 'Encipher' uses the Playfair cipher to encipher some text. The key is a string containing all 26 letters in the alphabet except one'. */ public static string Encipher(string key string plainText) { int length = plainText.Length; char ab; int a_ind b_ind a_row b_row a_col b_col; StringBuilder sb = new StringBuilder(); for(int i = 0; i { a = plainText[i]; b = plainText[i+1]; a_ind = key.IndexOf(a); b_ind = key.IndexOf(b); a_row = a_ind / 5; b_row = b_ind / 5; a_col = a_ind % 5; b_col = b_ind % 5; if(a_row == b_row) { if(a_col == 4) { sb.Append(key[a_ind - 4]); sb.Append(key[b_ind + 1]); } else if(b_col == 4) { sb.Append(key[a_ind + 1]); sb.Append(key[b_ind - 4]); } else { sb.Append(key[a_ind + 1]); sb.Append(key[b_ind + 1]); } } else if(a_col == b_col) { if(a_row == 4) { sb.Append(key[a_ind - 20]); sb.Append(key[b_ind + 5]); } else if(b_row == 4) { sb.Append(key[a_ind + 5]); sb.Append(key[b_ind - 20]); } else { sb.Append(key[a_ind + 5]); sb.Append(key[b_ind + 5]); } } else { sb.Append(key[5*a_row + b_col]); sb.Append(key[5*b_row + a_col]); } } return sb.ToString(); } /* 'Decipher' uses the Playfair cipher to decipher some text. The key is a string containing all 26 letters of the alphabet except one. */ public static string Decipher(string key string cipherText) { int length = cipherText.Length; char ab; int a_ind b_ind a_row b_row a_col b_col; StringBuilder sb = new StringBuilder(); for(int i = 0; i { a = cipherText[i]; b = cipherText[i+1]; a_ind = key.IndexOf(a); b_ind = key.IndexOf(b); a_row = a_ind / 5; b_row = b_ind / 5; a_col = a_ind % 5; b_col = b_ind % 5; if(a_row == b_row) { if(a_col == 0) { sb.Append(key[a_ind + 4]); sb.Append(key[b_ind - 1]); } else if(b_col == 0) { sb.Append(key[a_ind - 1]); sb.Append(key[b_ind + 4]); } else { sb.Append(key[a_ind - 1]); sb.Append(key[b_ind - 1]); } } else if(a_col == b_col) { if(a_row == 0) { sb.Append(key[a_ind + 20]); sb.Append(key[b_ind - 5]); } else if(b_row == 0) { sb.Append(key[a_ind - 5]); sb.Append(key[b_ind + 20]); } else { sb.Append(key[a_ind - 5]); sb.Append(key[b_ind - 5]); } } else { sb.Append(key[5*a_row + b_col]); sb.Append(key[5*b_row + a_col]); } } return sb.ToString(); } }