function TElgamalCipher.CheckNumbers(p, x, k: Integer): Integer;
begin
   if not IsPrime(P) or (P < 256) then
      Result := 1
   else
      if (x >= p - 1) or (x <= 1) then
         Result := 2
      else
         if (GetGCD(k, p - 1) <> 1) or (k >= p - 1) or (k <= 1) then
            Result := 3
         else
            Result := 0;
end;

procedure TElgamalCipher.EncryptFile(const InputFileName, OutputFileName: string;
   p, g, y, k: Integer);
var
   InputBuff: ByteBuff;
   OutputBuff: IntBuff;
   InputFile, OutputFile: TFileStream;
   i, ByteCounter: Integer;
   ACipher, BCipher: Int64;
   NotInformationShown: Boolean;
begin
   InputFile := TFileStream.Create(InputFilename, fmOpenRead);
   OutputFile := TFileStream.Create(OutputFileName, fmCreate);
   ByteCounter := 0;
   NotInformationShown := True;
   ACipher := FastModPower(g, k, p);
   repeat
      ByteCounter := InputFile.Read(InputBuff, BytesToUpdate);
      for i := 0 to ByteCounter - 1 do
      begin
         BCipher := FastModPower(y, k, p);
         BCipher := (BCipher * InputBuff[i]) mod p;
         OutputBuff[i * 2] := ACipher;
         OutputBuff[i * 2 + 1] := BCipher;
      end;
      OutputFile.Write(OutputBuff, ByteCounter * 8);
      if NotInformationShown then
      begin
         MainForm.FillMemo(OutputBuff, ByteCounter * 2);
         NotInformationShown := False;
      end;
   until ByteCounter < BytesToUpdate;
   InputFile.Free;
   OutputFile.Free;
end;

procedure TElgamalCipher.DecryptFile(const InputFileName, OutputFileName: string;
   x, p: Int64);
var
   InputBuff: IntBuff;
   OutputBuff: ByteBuff;
   InputFile, OutputFile: TFileStream;
   i, ByteCounter: Integer;
   Decipher: Integer;
   NotInformationShown: Boolean;
   TempVal: Int64;
begin
   InputFile := TFileStream.Create(InputFilename, fmOpenRead);
   OutputFile := TFileStream.Create(OutputFileName, fmCreate);
   ByteCounter := 0;
   NotInformationShown := True;
   repeat
      ByteCounter := InputFile.Read(InputBuff, BytesToUpdate * 2 * 4);
      for i := 0 to (ByteCounter div 8) - 1 do
      begin
         TempVal := InputBuff[i * 2 + 1];
         TempVal := (FastModPower(InputBuff[i * 2], (p - 2) * x, p) * TempVal) mod p;
         OutputBuff[i] := TempVal;
      end;
      OutputFile.Write(OutputBuff, ByteCounter div 8);
      if NotInformationShown then
      begin
         MainForm.FillMemo(OutputBuff, ByteCounter div 8);
         NotInformationShown := False;
      end;
   until ByteCounter < BytesToUpdate;
   InputFile.Free;
   OutputFile.Free;
end;