NODE 575c32eare: fast des
Jason Zions <jazz@hal.com>Thu, 17 Jun 93 14:13:23 PDT
>>Usually the limiting factor is examining the <ostensibly> decrypted data
>>for statistically significant patterns indicating that you have the
>>correct key.
>
>If you know that your plaintext is 7-bit ASCII, then you can reject if you
>see too many 8th bits set. [ ... ] Hint for implementors: don't allow such
>easy bit correlations in your plaintext.
Run your plaintext through compress first; remove the compress header; then
encrypt. Compression will screw up character frequencies (and use all eight
bits) enough to make automated detection of a successfully-broken encryption
really darn hard. Especially if you keep changing compression technology
each message.
Jazz
NODE 3cb8cc7cRe: fast des
Brad Huntting <huntting@glarp.com>Fri, 18 Jun 93 09:30:04 PDT
> Run your plaintext through compress first; remove the compress
> header; then encrypt. Compression will screw up character frequencies
> (and use all eight bits) enough to make automated detection of a
> successfully-broken encryption really darn hard. Especially if you
> keep changing compression technology each message.
Most encryption scheams use cypher block chaining or some other
mechanism where a change in one block will affect every block to
come after it, no?
Given this, would inserting a block of random data at the begining
of the datastream help?
brad
NODE 6a70b040fast des
Eric Hughes <hughes@soda.berkeley.edu>Thu, 17 Jun 93 18:06:27 PDT
> Compression will screw up character frequencies [...] enough to make
>automated detection of a successfully-broken encryption really darn
>hard.
The question is just how hard is "really darn hard"?
Compressed English text has characteristic patterns just as plain
English does. The salient difference is that these patterns take
longer to emerge at the same confidence level. The compressibility
limit is a limit not usually reached; the difference between that
limit and the actual compressed text will be non-zero. This
difference manifests itself in patterns in the compressed text.
Some estimates of this size are necessary in order that the designer
have an assurance that automatic recognition of decrypted text is
difficult.
These concerns are largely obviated by using ciphers with longer key
lengths, of course.
Eric