On 8/6/19 8:38 AM, Phil Smith III wrote: > Ron Hawkins wrote: > >> One area where PE encryption, as implemented on z is where it is used >> together with compression. > > >> The horse must go in front of the cart, meaning compression must happen >> before encryption, because it will be ineffective if you do it after. > > > Not true with format-preserving data protection. Since the protected data has > the same format as the original, it should compress just as well. With other > forms of encryption, sure. > > > > And yes, this is a well-integrated feature of PE encryption! > Unless by format-preserving data protection you mean an encryption technique that preserves repeated characters (like blanks) and repeated combinations of characters, then NO, it will not compress well after encryption. An encryption technique that preserves repeated patterns is basically a substitution cipher, which is highly insecure. The meaning of format-preserving is that it preserves the length and the text/numeric attributes of a field, not that it preserves patterns of repetition within or among fields.
Compression techniques work by recognizing repeated sequences within a file and substituting a shorter value for a frequently occurring longer sequences. A good encryption algorithm of necessity must obfuscate repeated sequences. Compression adds control fields, so if applied to a file that has no or very few repeated sequences can even result in a larger data file rather than a smaller one (try zipping a zipped file). So, the compression step must be an integral front-end to the encryption process, or it must be separately performed prior to encryption. Joel C Ewing -- Joel C. Ewing ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN
