Warning! New cryptographic modes!

[Jerry Leichter]

> To handle smaller inserts or deletes, you need to ensure that the  
> underlying blocks "get back into sync".  The gzip technique I  
> mentioned earlier works.  Keep a running cryptographically secure  
> checksum over the last blocksize bytes.  When some condition on the  
> checksum is met - equals 0 mod M - insert filler to the beginning of  
> the next block before encrypting; discard to the beginning of the  
> next block when decrypting.  Logically, this is dividing the file up  
> into segments whose ends occur at runs of blocksize bytes that give  
> a checksum obeying the condition.  A change within a segment can at  
> most destroy that segment and the following one; any other segments  
> eventually match up.
Oh, feh.  I'm typing without thinking.  In the worst case, an  
insertion (deletion) of K bytes could create (delete) K/M new  
(existing) segments.  In practice, this is unlikely except in an  
adversarial situation, and all it can do is force extra data to be  
transferred.
                                                         -- Jerry

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