SANS: Decoding malware
When ISC handler Bojan Zdrnja mentioned a "pretty interesting piece of malware" he had found, those of us who like to analyze and reverse engineer such critters immediately jumped onto it.
The malware was talking to a handful of servers over HTTP to fetch additional content, and only by faking user agent headers to look exactly like the malware was setting them was Bojan able to retrieve the additional files. The files he got were "big strings" of ASCII character sequences which Bojan quickly figured out how to decode/translate from the Ceasarean substitution cipher into URLs. But when requesting one of these URLs, all he got was another messy big string, and one whose coding method was different...
At first, we were convinced that the long string we were looking at was just another collection of URLs. But all the pattern matching we could cook up did not turn up anything looking like an encoded URL. So it was time to try a different approach - statistical analysis. Counting characters and character sequences can frequently tell something about the code or cipher used. SANS - Internet Storm Center - Cooperative Cyber Threat Monitor And Alert System
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