A := [ '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', ',' , '.','!','?', ' '];
 


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; 
  


AffinLinear := function(M, key,alphabet)  

   return(NumbersToText( List(  Apply(List(key[1]*M + key[2]), i-> Element(Integers(),i) mod Length(alphabet))   ), alphabet));

end;








E := function(key, M)    
 
   local l,n,alphabet;

      alphabet := A;
      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(Transpose(Matrix(Z,1,n,TextToNumbers(i,alphabet))), key, alphabet));
      return(Concatenation(l));

end;



D := function(key, C)     
 
   local l,n,k,M,b,alphabet;

      alphabet := A;
      n := Length(List(key[2]));
      l := ListSplit(C,n);
      k := FiniteField(31,1);
      M := Matrix(k, n,n, Apply(List(key[1]), i-> Element(k,i)));
      b := Apply(List(key[2]), i-> Element(k,i));
      l := Apply(l, i-> AffinLinear(Transpose(Matrix(k,1,n,TextToNumbers(i,alphabet))), [M^(-1),Transpose(Matrix(k,1,n,-b))] , alphabet));      


      return(Concatenation(l));

end;


########### Beisipiele: 

N    := Matrix(Z, 4,4, Apply([1..16], i-> 0));
I4   := Identity(Parent(N));
b    := Transpose(Matrix(Z, 1,4, [1,2,3,4]));
key  := [ I4, b];
M    := "DIESER TEXT SOLL VERSCHLUESSELT WERDEN";