How to decrypt manufacturer's firmware
From S1MP3 Wiki
Actions has implemented a simple encryption for it's firmware files. And some firmware files comes on it's encypted state. Before a firmware can be modified or disassembled, it has to be decrypted first.
- Download S1fwx from the download tools page 
- Put the firmware you want to decrypt into a folder of your choice along with S1FWX
- Open command prompt and go to the S1FWX directory and type:
- C:\S1FWX>s1fwx l the_firmware_to_be_decrypted.bin
this will decrypt the firmware and list its contents, the decrypted firmware will be called: the_firmware_to_be_decrypted.bi$
- feel free to disasemble/edit it now
For decrypting the newer format
The new format is identified by the letters "3DU" appearing near the beginning of the file. When viewed in a hex editor, the first 16 bytes are 11 22 33 44 55 66 77 88 99 aa bb cc dd ee ff 75.
- RockBox ATJBootTool can decrypt this: http://git.rockbox.org/?p=rockbox.git;a=tree;f=utils/atj2137/atjboottool;hb=HEAD
- Use AmyK's decryptor program : http://s1mp3.pastebin.com/f45e2a83d (fwdc.c) ; http://pastebin.com/UBg4tFXL (enc_data.h)