how to tell if decryption was successfull?

[Steven M. Bellovin]

In message <41FFD8A7.6090200 at terra.com.br>, Andreas writes:
>[newbie here]
>
>I was wondering how can one tell if some data was successfully 
>decrypted. Isn't there an assumption going on about what the cleartext 
>data should be? Text? Image? ZIP file? Ziped jpeg? Another cyphertext? 
>rot-13?

There are a lot of ways to tell, but you generally have to have some 
idea what you're looking for.  For two examples of how to do it, see
http://www1.cs.columbia.edu/~smb/papers/probtxt.ps (or .pdf) and
http://www1.cs.columbia.edu/~smb/papers/recog.ps (or .pdf)

		--Prof. Steven M. Bellovin, http://www.cs.columbia.edu/~smb



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