That code:

  PDWord(longword(@OutData)+ 0)^:= a;

Complains:

../lazarus/components/DCP/Ciphers/dcpserpent.pas(275,10) Error:
Typecast has different size (4 -> 8) in assignment

I search for definition of Oudata

    procedure DecryptECB(const Indata; var Outdata); virtual;
      { Decrypt a block of data using the ECB method of decryption }

Which type is outdata? Im confused. :(

Any idea where to start?

TIA