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
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
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
participants (3)
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Brad Huntting -
Eric Hughes -
Jason Zions