Skip to content

Instantly share code, notes, and snippets.

@castleberrysam
Created July 29, 2018 19:56
  • Star 2 You must be signed in to star a gist
  • Fork 1 You must be signed in to fork a gist
Star You must be signed in to star a gist
Save castleberrysam/2510f3e87b3d313a27320f165ef5e1cc to your computer and use it in GitHub Desktop.
Image extractor/decrypter for Beiying K8xx firmware updater utilities
#!/usr/bin/env python3
import sys
from struct import *
def decrypt8(int0, int1, key):
garbage = 0xc6ed3720
for i in range(32):
edi = (key[3] + (int0 >> 5)) & 0xffffffff
ebx = (key[2] + (int0 << 4)) & 0xffffffff
edi ^= ebx
edi ^= (garbage + int0) & 0xffffffff
int1 -= edi
if int1 < 0:
int1 += 0x100000000
int1 &= 0xffffffff
edi = (key[0] + (int1 << 4)) & 0xffffffff
ebx = (key[1] + (int1 >> 5)) & 0xffffffff
edi ^= ebx
edi ^= (garbage + int1) & 0xffffffff
int0 -= edi
if int0 < 0:
int0 += 0x100000000
int0 &= 0xffffffff
garbage = (garbage + 0x61c89647) & 0xffffffff
return int0,int1
def decrypt(data, key):
buf = []
for i in range(len(data)//4):
buf.append(unpack_from(">I", data, i*4)[0])
for i in range(len(data)//8):
int0 = buf[(i*2)+0]
int1 = buf[(i*2)+1]
int0,int1 = decrypt8(int0, int1, key)
buf[(i*2)+0] = int0
buf[(i*2)+1] = int1
return b"".join([pack(">I", val) for val in buf])
fw_magic = b"\x5a\xa5" + "SINO".encode()
def main():
if len(sys.argv) < 2:
print("Usage: {} <fw installer>".format(sys.argv[0]))
exit(1)
with open(sys.argv[1], "rb") as fw_installer:
i = 0
while i < len(fw_magic):
byte = fw_installer.read(1)
if len(byte) < 1:
print("Couldn't find a fw header in the file.")
exit(1)
elif byte[0] != fw_magic[i]:
i = 0
continue
i += 1
print("Found a fw header.")
var_98c0,var_98c4,fw_len = unpack(">HHH", fw_installer.read(6))
fw_data = fw_installer.read(fw_len);
print("Successfully read encrypted fw data from file.")
key = fw_installer.read(4)
print("Decryption key: {:08x}".format(unpack("<I", key)[0]))
print("Image size: {} bytes.".format(fw_len))
with open("fw_encrypted.bin", "wb") as fw_encrypted:
fw_encrypted.write(fw_data)
print("Wrote fw_encrypted.bin.")
fw_data = decrypt(fw_data, key)
print("Successfully decrypted fw data.")
with open("fw.bin", "wb") as fw:
fw.write(fw_data)
print("Wrote fw.bin.")
if __name__ == "__main__":
main()
@kobin23
Copy link

kobin23 commented Sep 30, 2018

Hello! Thank you for this unpacker! But can you do opposite program, that will crypt and pack firmware into exe?

@castleberrysam
Copy link
Author

@Olim98 Sorry about the response time. I don't have a repacker for the firmware data, but I believe that the algorithm should be similar since the encryption is XOR-based. It might interest you to know that I've identified the BYK870 as a rebranded version of the Sinowealth SH79F6489, an 8051 chip. This part is well documented in the relevant datasheet. I made some progress in reverse engineering the existing firmware a few months back but it became quite tedious. If you plan on creating a custom firmware it might be a better idea to start from scratch. I have another gist posted which documents how the keyswitches and LEDs are connected to the GPIO pins of the microcontroller, although I have a tenkeyless layout, so some of the connections for the numpad, etc. are likely missing.

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment