(**
 * Attacks affine cipher text string via analysis of char-frequency
 *
 **) 
function attackAffineCipher(cipherText: string): string;

  var
    i, j, a, b, c, d, cipherCharDeci, mostFreqCipherChar, secondMostFreqCipherChar, 
    tempTopCount, modInv: 
      integer;
    mostFreqGerChars: 
      array of integer;
    cipherCharFrequency: 
      array[95] of integer (* only ascii chars from 32 to 127 are allowed in plaintext: 95 chars in alphabet! *)
  
  begin
    mostFreqGerChars := ( ord(' ')-32, ord('e')-32, ord('n')-32, ord('i')-32, ord('s')-32); 
    
    (* create array with number of frequence of each ciphertext char *)
    for i:=0 to length(cipherText)-1 do
      cipherCharDeci := ord(cipherText[i])-32;
      cipherCharFrequency[cipherCharDeci] := cipherCharFrequency[cipherCharDeci]+1;
    end;
    
    (* compute 1st most frequently ciphertext char *)
    for i:=0 to length(cipherCharFrequency)-1 do
      if cipherCharFrequency[i] > tempTopCount then
        tempTopCount := cipherCharFrequency[i];
        mostFreqCipherChar := i;
      end;
    end;
    
    (* compute 2nd most frequently ciphertext char *)
    tempTopCount := -1; (* reset temporary counter *)
    for i:=0 to length(cipherCharFrequency)-1 do
      if i <> mostFreqCipherChar then
        if cipherCharFrequency[i] > tempTopCount then
          tempTopCount := cipherCharFrequency[i];
          secondMostFreqCipherChar := i;
        end;
      end;
    end;
    
    writeln();
    writeln("1st most frequently ciphertext char: ", concat(''',chr(mostFreqCipherChar+32),'''), " .. and 2nd most: ", concat(''',chr(secondMostFreqCipherChar+32),'''));
    writeln()
    writeln("starting decription:");
    
    (* iterate all permutations of given most frequently german chars (extendable) *)
    for i:=0 to length(mostFreqGerChars)-1 do
      for j:=0 to length(mostFreqGerChars)-1 do
        modInv := mod_inverse(mostFreqGerChars[i]-mostFreqGerChars[j],95);
        (* skip loop if mod-inverse is not given (is also needed to eliminate coefficient of 'a' in upcoming equation to compute 'a') *)
        if modInv = 0 then
          continue;
        end;
        writeln("__________________________________________");
        writeln("using y1 = ",mostFreqGerChars[i]," (",chr(mostFreqGerChars[i]+32),") and y2 = ",mostFreqGerChars[j]," (",chr(mostFreqGerChars[j]+32),")");
        a := ((mostFreqCipherChar - secondMostFreqCipherChar) * modInv) mod 95;
        b := (secondMostFreqCipherChar - (mostFreqGerChars[j] * a)) mod 95;
        
        (* compare value of b with value of dedicated second equation to make paranoia sure *)
        if b = (mostFreqCipherChar - (mostFreqGerChars[i] * a)) mod 95 then
          writeln("using a: ",a," and b: ",b);
          (* compute coefficients 'c' and 'd' for upcoming decryption procedure *)
          c := mod_inverse(a,95);
          d := mod_inverse(a,95) * b mod 95;
          writeln("decrypted plain text: ", decrypt(cipherText,c,d));
        end;
      end;
    end;
    
  end;


(**
 * Encrypts plain text (ASCII 32 to 127) into affine chiffre
 *
 **)  
function encrypt(plainText: string): string;

  var
    i, a, b, y: integer;
    encryptedText: string;
    
  begin
    encryptedText := "";
       
    a := random(95);
    while gcd(a,95) <> 1 do 
      a := random(95); 
    end;
    
    b := random(95);
    
    writeln("encypted plain text '", plainText, "' with a: ",a," and b: ",b);
    for i:=0 to length(plainText)-1 do
      y := ord(plainText[i]) - 32; 
      y := (a * y + b) mod 95; 
      y := y + 32;
      encryptedText := concat(encryptedText,chr(y));        
    end;
    
    return encryptedText;

  end;


(**
 * Decrypts affine cipher chiffre into plain text via factor c and sub d
 *
 **)  
function decrypt(cipherText: string; c,d: integer): string;

  var 
    decryptedText: string;
  	i,x: integer;
  	ch: char;
  	
  begin
    decryptedText := "";
    x := 0;
  	for i:=0 to length(cipherText)-1 do
      x := ord(cipherText[i]) - 32;
      x := (c * x - d) mod 95;
      x := x + 32; 
      decryptedText := concat(decryptedText,chr(x));
    end;
    
    return decryptedText;
    
  end;