Assuming you have a block cipher that (when given a 128-bit key) yields an instance of a true pseudorandom permutation... how can you use that block cipher within the above constraints to achieve practically perfect secrecy?
closed as unclear what you're asking by SteveS, Xander, Adnan, GdD, Terry Chia Jan 22 at 14:36
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"Secrecy" depends on the context. Secrecy of what, where, when, against who ?
If you have an ideal block cipher, then you can use it in a mode of operation which turns it into an encryption system for data messages of arbitrary length. If the block cipher is ideal (there is no way to distinguish the block cipher from a randomly selected permutation that is easier than trying all possible keys), and the blocks are large enough, then encryption will ensure confidentiality up to the exhaustive search effort.
Note the subtle points, though: