Tool for decrypting DSi TADs (not the ones listed on dsibrew, that's wrong, the installable ones that are closer to WADs), py2/3

decrypt_tad.py

from io import BytesIO
from struct import unpack
from binascii import hexlify, unhexlify
from Crypto.Cipher import AES #pip install pycryptodome
import sys

dsi_common_key = unhexlify(b"%032X" % 0xAF1BF516A807D21AEA45984F04742861)      # DSi common key
debugger_common_key = unhexlify(b"%032X" % 0xA2FDDDF2E423574AE7ED8657B5AB19D3) # DSi debugger common key (used for `maketad.updater` TADs)
wii_debug_key = unhexlify(b"%032X" % 0xA1604A6A7123B529AE8BEC32C816FCAA)       # Wii debug key

def align(val): #Tads have 64-byte alignment between sections
    return val + (64 - (val % 64))

with open(sys.argv[1], "rb") as f:
    header = unpack(">I2sH6I", f.read(0x20))
    if header[1] != b"Is": #Installable? same-ish as Wii WADs
        raise BaseException("Invalid TAD file")
    if header[0] != 0x20: #header size
        raise BaseException("unknown header size %d" % header[0])
    if header[2] != 0: #WAD version according to wiibrew
        raise BaseException("unknown TAD version %d" % header[2])
    
    print("Input Tad file: %s" % sys.argv[1])
    print("Header: %08X cert %08X ticket %08X tmd %08X content %08X footer" %\
          (header[3], header[5], header[6], header[7], header[8]))
    
    with open("title.cert", "wb") as o:
        o.write(f.read(header[3]))
        f.seek(align(f.tell()))
        
    ticket = BytesIO(f.read(header[5])) #store to parse a bit
    f.seek(align(f.tell()))
    with open("title.tik", "wb") as o:
        o.write(ticket.read())
        ticket.seek(0)

    tmd = BytesIO(f.read(header[6])) #store to parse a bit
    f.seek(align(f.tell()))
    with open("title.tmd", "wb") as o:
        o.write(tmd.read())
        tmd.seek(0)

    tmd.seek(0x18C)
    title_id = tmd.read(8)
    tmd.seek(0x1DE)
    content_count, boot_index = unpack(">2H", tmd.read(4))
    print("Title ID: %016X %s" % (unpack(">Q", title_id)[0], title_id[4:].decode("UTF-8")))
    
    ticket.seek(0x1BF)
    enc_title_key = ticket.read(0x10)
    print("Encrypted Title Key: %032X" % (int(hexlify(enc_title_key), 16)))
    if content_count > 1:
        raise BaseException("Multiple contents not supported, very easy to add")

    #if you encounter multi-content files, handle tmd table here, and then assume
    #content is in a row in the content blob in the TAD

    content = BytesIO(f.read(header[7]))
    f.seek(align(f.tell()))
    with open("%08X" % boot_index, "wb") as o: #store the encrypted content
        o.write(content.read())
        content.seek(0)

    if header[8] != 0: #there is a footer
        with open("footer.bin", "wb") as o:
            o.write(f.read(header[8]))
        f.seek(align(f.tell()))

    title_id += b"\x00" * 8 #pad to 16 bytes for IV

    obj = AES.new(dsi_common_key, AES.MODE_CBC, title_id)
    dsi_dec_title_key = obj.decrypt(enc_title_key)
    obj = AES.new(debugger_common_key, AES.MODE_CBC, title_id)
    debugger_dec_title_key = obj.decrypt(enc_title_key)
    obj = AES.new(wii_debug_key, AES.MODE_CBC, title_id)
    wii_dec_title_key = obj.decrypt(enc_title_key)

    #try both keys and check the srl's "reserved" bytes, we decrypt the entire file
    #because otherwise the CBC blocks get off and the start of the srl gets garbled
    obj = AES.new(dsi_dec_title_key, AES.MODE_CBC, b"\x00" * 16)
    decrypted_content = BytesIO(obj.decrypt(content.read()))
    content.seek(0)
    decrypted_content.seek(0x15) #reserved bytes
    if decrypted_content.read(7) == b"\x00" * 7: #this is an srl
        decrypted_content.seek(0xC)
        print("DSi Common Key Used")
        game_code = decrypted_content.read(6).decode("UTF-8") #for export filename
        print("Game Code: %s" % game_code)
        decrypted_content.seek(0)
        print("Output file name: %s.srl" % game_code)
        with open("%s.srl" % game_code, "wb") as o:
            o.write(decrypted_content.read())
        sys.exit(1)

    obj = AES.new(debugger_dec_title_key, AES.MODE_CBC, b"\x00" * 16)
    decrypted_content = BytesIO(obj.decrypt(content.read()))
    content.seek(0)
    decrypted_content.seek(0x15) #reserved bytes
    if decrypted_content.read(7) == b"\x00" * 7: #this is an srl
        decrypted_content.seek(0xC)
        print("Debugger Common Key Used")
        game_code = decrypted_content.read(6).decode("UTF-8") #for export filename
        print("Game Code: %s" % game_code)
        decrypted_content.seek(0)
        print("Output file name: %s.srl" % game_code)
        with open("%s.srl" % game_code, "wb") as o:
            o.write(decrypted_content.read())
        sys.exit(1)

    obj = AES.new(wii_dec_title_key, AES.MODE_CBC, b"\x00" * 16)
    decrypted_content = BytesIO(obj.decrypt(content.read()))
    content.seek(0)
    decrypted_content.seek(0x15) #reserved bytes
    if decrypted_content.read(7) == b"\x00" * 7: #this is an srl
        decrypted_content.seek(0xC)
        print("Wii Debug Key Used")
        game_code = decrypted_content.read(6).decode("UTF-8") #for export filename
        print("Game Code: %s" % game_code)
        decrypted_content.seek(0)
        print("Output file name: %s.srl" % game_code)
        with open("%s.srl" % game_code, "wb") as o:
            o.write(decrypted_content.read())
        sys.exit(1)

    raise BaseException("Was not able to decrypt the content, oops")