[sllug-members]: Encrypted backup tapes

[Jason Holt]

On Thu, 11 Jan 2007, Corey Edwards wrote:

> On Thu, 2007-01-11 at 12:24 -0700, M. Alton jensen wrote:
>> I'm not sure if this is distro specific, but the defaults for gnupg on
>> my desktop(SuSE) include compressing the item being encrypted.  I ran a
>> simple test with a text file to confirm this and here are the results.
>>
>>> du -sk test.txt*
>> 328     test.txt
>> 40      test.txt.gpg
>> 40      test.txt.gz
>
> A lot of encryption schemes, and I believe PGP is one of them, implement
> a compression step before the encryption. The idea is to increase the
> entropy. Plain text really isn't that random, whereas compression by

Schneier (even acknowledges that) he did us a disservice by talking about data 
compression as a way to strengthen a cipher in Applied Cryptography, since any 
good cipher doesn't need compression to be strong.  In fact, if you were using 
a weak cipher, compression could work against you, since compression data 
structures put certain bits in predictable places.

So, I consider it bad to tell people that you're compressing to strengthen the 
cipher, since that gives people the impression that good ciphers still need 
help.  But compressing to strengthen the cipher even though you know it 
shouldn't be necessary, or because you know that it won't do any good to 
compress after encrypting, are okay.  I am reminded of The Power Cycle Koan:

    A novice was trying to fix a broken lisp machine by turning the
    power off and on.  Knight, seeing what the student was doing spoke sternly,
    "You cannot fix a machine by just power-cycling it with no understanding of
    what is going wrong."  Knight turned the machine off and on.  The machine
    worked.

 							-J