[Cryptography] Summary: compression before encryption

[Jerry Leichter]

On Jan 13, 2015, at 11:22 PM, Phillip Hallam-Baker <phill at hallambaker.com> wrote:
> ...Yes, it is not possible to compress encrypted data if the encryption is any good and this is actually a good check to see if you have goofed by using ECB or whatever. But don't be too sure that random data can't compress. Sometimes it can - we get down to 3-4 bits per revoked cert.
That's because you aren't actually compressing the certs - you're compressing deltas on pre-communicated sets of certs - and you can make assumptions about the nature of the deltas.  Very different problem.  The technique is *very* nice, but it amounts to recognizing and working with the exact kind of non-randomness inherent in the data you're compressing.
                                                        -- Jerry


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