decryptor – Malware University

This is a script used to unpack a hidden area of the bootsector after a TDL4 infection. This is classic code but still useful is anyone needs it. Written by your humble admin many years ago.

import idaapi

# Simulate C-style for() loop.
def cfor(first, test, update):
    while test(first):
        yield first
        first = update(first)


# Bit-shifting operations.
#def ROR(x, n, bits = 32):
#    mask = (2L**n) - 1
#    mask_bits = x & mask
#    return (x >> n) | (mask_bits << (bits - n))

#def ROL(x, n, bits = 32):
#    return ROR(x, bits - n, bits)
def ROR(byte, count):
    while count > 0:
        byte = (byte >> 1 | byte << 7) & 0xFF
        count -= 1
    return byte


if __name__ == "__main__":
    print "TDL4 Decrypt"
    print "A script to unpack the contents hidden on the bootsector of a TDL4 infected host"
    print ""
    
    encryptedBytes = [None] * 311
    
    rorCount = 311  # 0x137 (CX)
    count = 0
    # target starts at 0x62A in memory
    for i in cfor(int('0x2A', 16), lambda i : i < (311 + i), lambda i : i + 1):
        cryptedByte = idaapi.get_byte(i)
        print "Byte[" + hex(i) + "]:    " + hex(cryptedByte)
        idaapi.patch_byte(i, ROR(cryptedByte, rorCount - count))
        count += 1
        if count == 311:
            break

Tag: decryptor

TDL4 IDAPython Decryptor