[Cryptography] Summary: compression before encryption

Tom Mitchell mitch at niftyegg.com
Thu Jan 15 23:41:43 EST 2015

On Thu, Jan 15, 2015 at 12:11 PM, Ray Dillinger <bear at sonic.net> wrote:

> In general, compression algorithms ought to be beneficial in
> cryptography.  Greater information density means less redundant
> structure of the sort that results in "cribs" for the analysts
> to use.  The problem with most compression programs is that
> they have very distinctive headers that instead provide cribs
> for the analysts in the first few hundred bytes of the file.

 Well chosen, yes...
A compressed file has more "known" structure than just the header.
A large pile of encrypted files with a known structure and a common key
could be all that modern tools in Utah need to attack the key.

If compression after encryption is a net gain that would be more secure.
If compression was first than knowing how the initial data was compressed
may a loss of security as the hint is "it was compressed with ___"

A good encryption and no knowledge that the content was compressed first
would be fine.   Some compression tools have a notion of a compression
preset level which might be used when the same file is sent multiple times
with the same or different keys.


  T o m    M i t c h e l l
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