3 expanded posts ยท every known parent and child
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. -Allen
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. -- Marshall Marshall Clow Aladdin Systems <mailto:mclow@mailhost2.csusm.edu> "We don't have to take it; never have, never will. Gonna shake it, gonna break it; let's forget it: better still" --The Who, "Tommy"
--Boundary..3941.1071713581.multipart/signed Content-Type: text/plain Content-Transfer-Encoding: 7bit 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 --Boundary..3941.1071713581.multipart/signed Content-Type: application/octet-stream; name="pgp00006.pgp" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="pgp00006.pgp" Content-Description: "PGP signature" LS0tLS1CRUdJTiBQR1AgTUVTU0FHRS0tLS0tClZlcnNpb246IDIuNi4zCgpp UUNWQXdVQk1hNEFhYlpjK3N2NXNpdWxBUUhmRlFQK081aFJFbmF1dWFqMFhP MkpRODZsWlpqQlljWTAwR1kzCmJTeC82QVVYa0lvZ0FpbHBrZ0JBaGRqOE9y UjJhYktmU1BIV2JCS09iV1ZBOVJWN2Z5SERaNGlkcVVRV1dlNUMKeFNGbEF5 dTRiMFB1UFRTT0laSlBlclo5ZzdWVXRYR3p6MERpeG9ic0d4dWlhamx5eWtx ZkhOL1FMZTJwZVpWRgpObTE0Nm50M2hjND0KPS90d3cKLS0tLS1FTkQgUEdQ IE1FU1NBR0UtLS0tLQo= --Boundary..3941.1071713581.multipart/signed--