apk_signature.py

sb_data = b''
with open(signed_apk, 'rb') as unsigned:
    unsigned.seek(-6, 2)  # From file end, 2 byte comment length, 4 byte offset of CD
    cd_start = 0
    for i in reversed(unsigned.read(4)):
        cd_start *= (0xff + 1)
        cd_start += i
    print('cd_start:', cd_start)
    unsigned.seek(cd_start - 16 - 8, 0)  # 16 byte magic, uint64 size
    sb_size = 0
    for i in reversed(unsigned.read(8)):
        sb_size *= (0xff + 1)
        sb_size += i
    print('sb_size:', sb_size)
    sb_start = cd_start - sb_size - 8  # sb_size excludes first sb_size field
    print('sb_start:', sb_start)
    unsigned.seek(sb_start, 0)
    sb_data = unsigned.read(sb_size + 8)
    
print(sb_data)