Choosing RC5 is an irrelevant implementation detail; they probably used whatever was in hand at the time. The only requirement is that the function makes things

Choosing RC5 is an irrelevant implementation detail; they probably used whatever was in hand at the time. The only requirement is that the function makes things random-looking, really. I agree with alister in that the reason---if there was any reasoning applied here---was likely code size / simplicity. Look at Flame's encryption functions [1, §4]. This is the malware that burned a chosen-prefix MD5 collision elsewhere, but still went ahead and used the simplest mangling functions possible. You also find plenty of other 'state-sponsored' malware using similarly stupid/weak/outdated encryption. [1] http://www.crysys.hu/skywiper/skywiper.pdf http://www.crysys.hu/skywiper/skywiper.pdf