Re: Basic File Encyption

From: Brian Gladman (brg_at_nowhere.at.all)
Date: 12/15/03 

Date: Mon, 15 Dec 2003 16:38:02 -0000

"No One" <no-one-no-spam@home.com> wrote in message
news:Y3VydHdpbGw=.c7591f94d9610e972714d73216a1aeb4@1071499059.cotse.net...
> Joe wrote:
> [snip]
> > program, how does the program know if you have entered the right
> > passphrase? [snip]
>
> As the other posters have pointed out there are two (that I know of, may
be
> more) ways this is done. One is the MAC, the other is to store a MD5 or
SHA-
> 1 hash of the passphrase with the cyphertext.
>
> Sorry I can't give a better answer, but I have a question. By providing an
> MD5 or SHA-1 hash, am I not aiding the brute-force attacker? With the hash
> he calulates the hash for the phrase he is trying; if it doesn't match he
> moves on to the next. Without the hash he has to inspect the resultant
> plaintext to determine if it is gibberish, right?

If a simple hash is used as the one-way function, it does seem likely that a
attack via this hash will be faster than one via decryption of the
ciphertext. If, however, a more complex one way function is used (for
example, an iterated hash) it might then be faster to attack the ciphertext.

However, quite a few issues are involved here so it is not possible to give
a definite answer to your question without considering more specific details
of any such schemes under consideration.

   Brian Gladman

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