#!/usr/bin/python3

import sys

def getvaluefrombytes(tbytes):
    va = 0
    for j in range(len(tbytes)):
        va += tbytes[j] * 256 ** j
    return va


def decodebytesfromnum(num, lens=4):
    tbytes = bytearray()
    for i in range(lens):
        tbytes.append((num & (0xff << 8 * i)) >> 8 * i)
    return tbytes


def set_cid(readbytes, channelname):
    
    # find End of central directory
    for i in range(len(readbytes) - 22):
        if (readbytes[-22 - i] == 0x50 and readbytes[-21 - i] == 0x4b and readbytes[-20 - i] == 0x05 and readbytes[
                -19 - i] == 0x06):
            break
            

    start_central_directory_value_pos = -6 - i
    start_central_directory = readbytes[start_central_directory_value_pos + 3] * 256 ** 3  \
        + readbytes[start_central_directory_value_pos + 2] * 256 ** 2                      \
        + readbytes[start_central_directory_value_pos + 1] * 256                           \
        + readbytes[start_central_directory_value_pos]

    if readbytes[start_central_directory - 16:start_central_directory] == 'APK Sig Block 42'.encode("utf-8"):
        print("check v2 true")
    else:
        print("check v2 fail")
        return


    sign_block_size = 0
    sgin_block_value_pos = start_central_directory - 16 - 8
    for i in range(8):
        sign_block_size += 256 ** i * readbytes[sgin_block_value_pos + i]

    sign_block_start_pos = start_central_directory - sign_block_size - 8

    k = 0;
    keybytes = {}
    tempbytes = []
        
    strkey = 0
    k = 0
    
    #walleid 0x71777777
    walleIDbytes = bytearray()
    walleIDbytes.append(0x77)
    walleIDbytes.append(0x77)
    walleIDbytes.append(0x77)
    walleIDbytes.append(0x71)

    pairlen = 0
    for i in range(sign_block_start_pos + 8, sgin_block_value_pos):
        tempbytes.append(readbytes[i])
        if k == 7:
            pairlen = getvaluefrombytes(tempbytes)
            tempbytes.clear()
        if k == 11:
            strkey = getvaluefrombytes(tempbytes)
            tempbytes.clear()
        if k == pairlen + 7:
            keybytes[strkey] = bytes(tempbytes)
            tempbytes.clear()
            k = -1
        k += 1


    channel = "{\"channel\":\"%s\"}" % channelname
    channelbytes = bytearray(channel.encode("utf-8"))
    if len(channelbytes) > 252:
        print(" channel str len < 252")
        return
        
    insertbytes = bytearray()
       
    channelbytes_len = len(channelbytes) + 4
    insertbytes.append(channelbytes_len)
    for i in range(7):
        insertbytes.append(0)

    for i in walleIDbytes:
        insertbytes.append(i)
    for i in channelbytes:
        insertbytes.append(i)


    start_central_directory += len(insertbytes)
    dictoffsetbytes = decodebytesfromnum(start_central_directory)
    for index in range(4):
        readbytes[start_central_directory_value_pos + index] = dictoffsetbytes[index]
    sign_block_size += len(insertbytes)
    signsizebytes = decodebytesfromnum(sign_block_size, 8)
    for index in range(8):
        readbytes[sign_block_start_pos + index] = signsizebytes[index]
        readbytes[sgin_block_value_pos + index] = signsizebytes[index]
    for b in insertbytes[::-1]:
        readbytes.insert(sgin_block_value_pos, b)

    outfilepath = "channel_%s.apk" % channelname
    with open(outfilepath, "wb") as fout:
        fout.write(readbytes)







def main():
    if len(sys.argv) != 3:
        print("i need apk_path and channel_id.")
        exit(-1)

    filepath = sys.argv[1]
    cid      = sys.argv[2]

    readbytes = bytearray()
    with open(filepath, "rb") as f:
        readbytes = bytearray(f.read())

    set_cid(readbytes, cid)


if __name__ == "__main__":
    main()
    
            

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