If something has been compressed, it will have a near flat distribution of values(otherwise you could compress it again and achieve better compression).
If something were compressed before encryption then it would maintain that flat distribution.
If you are confident something is either compressed or encrypted but not both or neither than you just have to look at how the values are distributed. Nonrandom data will have some values more common than others. Compressed data will look random.
The big problem is probably that encryption is going to use some huge number of bits, so there could be a huge number of possible values.
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Last edited by Wizlem; 11-06-2012 at 10:49 AM.