Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

{ 
 This two functions are used to encrypt and decrypt text. 
 Here's how to use it: 
 The four entries Key1, Key2, Key3 and Key4 are numbers 
 that can range from 1 to 120. In order to decrypt a text, 
 you must use the same numbers you used to encrypt the text. 
 No one that doesn't know what values were used on Key1, Key2, Key3 and Key4 
 will be able to decrypt your text! 
 Note that Key1*Key4 MUST be different than Key2*Key3. 
 If any Key is zero, or Key1*Key4 is equal to Key2*Key3, 
 the function will return ''. 
 In Brief: 
      Key1, Key2, Key3, Key4 : integer from range[1..120] 
      Key1*Key4  Key2*Key3 
}
 function Encrypt(Text : string; Key1, Key2, Key3, Key4 : Integer) : string;
 var
  BufS, Hexa, Hexa1, Hexa2 : string;
  BufI, BufI2, Sc, Sl, Num1, Num2, Num3, Num4, Res1, Res2, Res3, Res4 : Integer;
 begin
  Sl := Length(Text);
  Sc := 0;
  BufS := '';
  if (Key1 in [1 .. 120]) and (Key2 in [1 .. 120]) and (Key3 in [1 .. 120]) and (Key4 in [1 .. 120]) then
  begin
   BufI := Key1 * Key4;
   BufI2 := Key3 * Key2;
   BufI := BufI - BufI2;
   if BufI = 0 then
   begin
    Result := '';
    Exit;
   end;
  end
  else
  begin
   Result := '';
   Exit;
  end;
  repeat
   Inc(Sc);
   if Sc > Sl then Num1 := 0 else Num1 := Ord(Text[Sc]);
   Inc(Sc);
   if Sc > Sl then Num2 := 0 else Num2 := Ord(Text[Sc]);
   Inc(Sc);
   if Sc > Sl then Num3 := 0 else Num3 := Ord(Text[Sc]);
   Inc(sc);
   if Sc > Sl then Num4 := 0 else Num4 := Ord(Text[Sc]);
   Res1 := Num1 * Key1;
   BufI := Num2 * Key3;
   Res1 := Res1 + BufI;
   Res2 := Num1 * Key2;
   BufI := Num2 * Key4;
   Res2 := Res2 + BufI;
   Res3 := Num3 * Key1;
   BufI := Num4 * Key3;
   Res3 := Res3 + BufI;
   Res4 := Num3 * Key2;
   BufI := Num4 * Key4;
   Res4 := Res4 + BufI;
   for BufI := 1 to 4 do
   begin
    case BufI of
     1 : Hexa := IntToHex(Res1, 4);
     2 : Hexa := IntToHex(Res2, 4);
     3 : Hexa := IntToHex(Res3, 4);
     4 : Hexa := IntToHex(Res4, 4);
    end;
    Hexa1 := '$' + Hexa[1] + Hexa[2];
    Hexa2 := '$' + Hexa[3] + Hexa[4];
    if (Hexa1 = '$00') and (Hexa2 = '$00') then
    begin
     Hexa1 := '$FF';
     Hexa2 := '$FF';
    end;
    if Hexa1 = '$00' then Hexa1 := '$FE';
    if Hexa2 = '$00' then
    begin
     Hexa2 := Hexa1;
     Hexa1 := '$FD';
    end;
    BufS := BufS + Chr(StrToInt(Hexa1)) + Chr(StrToInt(Hexa2));
   end;
   until Sc >= Sl;
  Result := BufS;
 end;
 function Decrypt(Text : string; Key1, Key2, Key3, Key4 : Integer) : string;
 var
  BufS, Hexa1, Hexa2 : string;
  BufI, BufI2, Divzr, Sc, Sl, Num1, Num2, Num3, Num4, Res1, Res2, Res3, Res4 : Integer;
 begin
  Sl := Length(Text);
  Sc := 0;
  BufS := '';
  if (Key1 in [1 .. 120]) and (Key2 in [1 .. 120]) and (Key3 in [1 .. 120]) and (Key4 in [1 .. 120]) then
  begin
   Divzr := Key1 * Key4;
   BufI2 := Key3 * Key2;
   Divzr := Divzr - BufI2;
   if Divzr = 0 then
   begin
    Result := '';
    Exit;
   end;
  end
  else
  begin
   Result := '';
   Exit;
  end;
  repeat
   for BufI := 1 to 4 do
   begin
    Inc(Sc);
    Hexa1 := IntToHex(Ord(Text[Sc]), 2);
    Inc(Sc);
    Hexa2 := IntToHex(Ord(Text[Sc]), 2);
    if Hexa1 = 'FF' then
    begin
     Hexa1 := '00';
     Hexa2 := '00';
    end;
    if Hexa1 = 'FE' then Hexa1 := '00';
    if Hexa1 = 'FD' then
    begin
     Hexa1 := Hexa2;
     Hexa2 := '00';
    end;
    case BufI of
     1 : Res1 := StrToInt('$' + Hexa1 + Hexa2);
     2 : Res2 := StrToInt('$' + Hexa1 + Hexa2);
     3 : Res3 := StrToInt('$' + Hexa1 + Hexa2);
     4 : Res4 := StrToInt('$' + Hexa1 + Hexa2);
    end;
   end;
   BufI := Res1 * Key4;
   BufI2 := Res2 * Key3;
   Num1 := BufI - BufI2;
   Num1 := Num1 div Divzr;
   BufI := Res2 * Key1;
   BufI2 := Res1 * Key2;
   Num2 := BufI - BufI2;
   Num2 := Num2 div Divzr;
   BufI := Res3 * Key4;
   BufI2 := Res4 * Key3;
   Num3 := BufI - BufI2;
   Num3 := Num3 div Divzr;
   BufI := Res4 * Key1;
   BufI2 := Res3 * Key2;
   Num4 := BufI - BufI2;
   Num4 := Num4 div Divzr;
   BufS := BufS + Chr(Num1) + Chr(Num2) + Chr(Num3) + Chr(Num4);
   until Sc >= Sl;
  Result := BufS;
 end;

Источник: delphiworld.narod.ru