One way to find PE files that start at offset 0 and have a single byte xor key:
rule single_byte_xor_pe_and_mz {
meta:
author = "Wesley Shields <wxs@atarininja.org>"
description = "Look for single byte xor of a PE starting at offset 0"
strings:
$b = "PE\x00\x00" xor(0x01-0xff)
condition: