A script to decrypt uboot/kernel/mcu update images used by Allwinner T8-based car radio headunits

dcupdate.py

#!/usr/bin/env python3
import hashlib
import sys
import tqdm # Used to report progress during decryption

if len(sys.argv) not in (2,3):
    print("Usage: dcupdate.py enciphered_file [deciphered_file]")
    print("Deciphers images used by Allwinner T8-based car radio headunits")
    sys.exit(0)

print("Deciphering update '{}'".format(sys.argv[1]))

# Extracted from UBoot Firmware
unkA = bytes([0xC1, 0x81, 0x56, 0xC2, 0x44, 0xBD, 0x11, 0xE1, 0x94, 0x38, 0x00, 0x0C, 0x29, 0xBA, 0x27, 0xC0])
unkB = bytes([0x81, 0x6B, 0xAE, 0x89, 0x3F, 0x95, 0xE6, 0xDB, 0x96, 0xA3, 0xB9, 0x90, 0x57, 0x17, 0x29, 0xAF])
unk_xored = bytes([a ^ b for a,b in zip(unkA, unkB)])

print("Magic bytes:")
print("  A = {}".format(unkA.hex()))
print("  B = {}".format(unkB.hex()))
print("  A ^ B = {}".format(unk_xored.hex()))

def obfuscate_block(key_block, in_block):
    inner_digest = hashlib.md5(key_block + in_block + unk_xored).digest()
    outer_digest = hashlib.md5(inner_digest + unkB).digest()
    return outer_digest

fn = sys.argv[1]
try:
    with open(fn, 'rb') as f:
        buffer = f.read()
        key_block = buffer[-0x20:-0x10]
        print("Key Block: {}".format(key_block.hex()))
        
        # Verify checksum
        file_md5 = hashlib.md5(buffer[:-0x10]).digest()
        obfuscated_md5 = obfuscate_block(key_block, file_md5)
        exp_obfuscated_md5 = buffer[-0x10:]
        print("Obfuscated Checksum: {}".format(obfuscated_md5.hex()))
        print("Expected Checksum:   {}".format(exp_obfuscated_md5.hex()))
        
        assert obfuscated_md5 == exp_obfuscated_md5, "Checksums do not match"
        if len(sys.argv) == 2:
            print("Checksums match! No output file specified, exiting...")
            sys.exit(0)

        # Dump deciphered file
        print("Deciphering file...")
        out_fn = sys.argv[2]
        xor_block = bytes([0] * 16)
        with open(out_fn, 'wb') as fout:
            for i in tqdm.trange(0, len(buffer) - 0x20, 0x10):
                xor_block = obfuscate_block(key_block, xor_block)
                block = buffer[i:i+min(0x10, len(buffer) - 0x20)]
                dec_block = bytes([a ^ b for a,b in zip(block, xor_block)])
                fout.write(dec_block)
        
except IOError as e:
    print("I/O Error ({0}): {1}".format(e.errno, e.strerror))