####################   Einige vorgegebene Dinge  ###########################

A1 := [ '1','2','3','4','5','6','7','8','9', '0','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',')' , '(' , ',' , '.', '=','!','?','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', ' '];
A2 := [ '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'];
A3 := ['0','1'];

    
Z  := Integers(); 




NumbersToText := function(L,alphabet)
   
      return(Apply(L, i-> alphabet[i+1]));
end;



TextToNumbers := function(text,alphabet)
   
      return(Apply(List(text), i-> (Position(alphabet,i)-1)));
end; 


    
TextAnalysis := function(text,alphabet)
   
       return(Apply(alphabet, i-> [i,Length(Filtered(List(text), j-> j=i))]));

end;








AffinLinear := function(block, key,alphabet)           # key = [A,b]

   local m,n,A,b,M,C;
      
      m := Length(alphabet);       
      n := Length(block);


      A := key[1];
      b := key[2]; 

      if not( Type(A) = elt-alg^mat/ord^rat) or not( Type(b) =  elt-mdl^mat/ord^rat) then
         Print(" Error in AffinLinear(Block, key, alphabet: key[1], key[2] have to be matrices or vectors...\n");
      fi;
     
      M := Transpose(Matrix(Z,1,n,TextToNumbers(block,alphabet)));            

      C := Apply(List(A*M + b), i-> i mod Length(alphabet));


      return(NumbersToText( List(C), alphabet));

   end;












########### So koennten zum Beispiel die Verschluesselung und Entschluesselung aussehen:


E := function(key, M,alphabet)     # key=Schluessel, n=Blocklaenge, M=Klartext
 
   local l,n;

      n := Length(List(key[2]));
      l := ListSplit(M,n);
      l := Apply(l, i-> Append(i, Apply([Length(i)+1..n], j-> alphabet[1])));
      l := Apply(l, i-> AffinLinear(i, key, alphabet));
      return(Concatenation(l));

end;


D := function(key, C,alphabet)     # key=Schluessel, n=Blocklaenge, C=Chiffretext
 
   local l,a,n;

      n := Length(List(key[2]));
      l := ListSplit(C,n);
      a := Apply(l, i-> Apply(List(key[1]^(-1)*(Transpose(Matrix(Z, 1,n,TextToNumbers(i,alphabet)))-key[2])), j-> j mod Length(alphabet))); 

      return( Concatenation(Apply(a, i-> NumbersToText(i, alphabet))));

end;



A    := Matrix( Z, 2,2, [ 1,0,0, 1]); 
b    := Transpose(Matrix( Z, 2, [1,2]));
key  := [A,b];


text := "DIESE NACHRICHT IST GEHEIM";

C := E(key, text, A1);
M := D(key, C   , A1);