On Thu, 30 May 1996, Marshall Clow wrote:
At 12:47 AM -0700 5/30/96, E. ALLEN SMITH wrote:
From: IN%"mclow@owl.csusm.edu" "Marshall Clow" 29-MAY-1996 19:18:31.60
* Why encrypt before compression? If the encryption is any good, then the encrypted data won't compress much at all. However, compression before encryption has its own problems.
What problems does compression before encryption have? It at least seems to work for PGP.
Most compression schemes put a header/index on the front of the compressed data. This makes recognizing the correct decryption very simple.
Call it a limited "known plaintext" situation.
PGP, and I'm sure other encryption programs, strip this header off as there is no need for it. Compression actually makes encryption much stronger because it eliminates a lot of the patterns found in plaintext and makes cryptanalysis much harder. -- Mark =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= markm@voicenet.com | finger -l for PGP key 0xe3bf2169 http://www.voicenet.com/~markm/ | d61734f2800486ae6f79bfeb70f95348 ((2b) || !(2b)) | Old key now used only for signatures "The concept of normalcy is just a conspiracy of the majority" -me