On the desktop we have many different executable compactors, compressors and encryptors. These are used to protect and/or obfuscate binary files. These can be employed by software authors and malicious code authors to protect their code from reverse engineering (though, typically in vain). A while back, we saw a surge of malicious code authors using these tools to obfuscate their code against signatures. It became a case of:
10 Download executable compactor
20 Pass existing malicious code through it
30 Release on Internet
40 Wait for signature to be added to antivirus
50 GOTO 10
This got a bit boring for antivirus vendors like Symantec, so we introduced executable decompression support to our AV engines (as discussed in the Internet Security Threat...