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Re^2: Crypt::CipherSaber replacement... Crypt::CBC?

by hardburn (Abbot)
on Apr 30, 2010 at 16:46 UTC ( #837820=note: print w/replies, xml ) Need Help??

in reply to Re: Crypt::CipherSaber replacement... Crypt::CBC?
in thread Crypt::CipherSaber replacement... Crypt::CBC?

After thinking about it some more, I can almost guarantee your system is insecure.

The way stream ciphers like RC4 work is to take a key as a seed value to a pseudo-random number generation function. Each bit of output in the PRNG is XOR'd with the data (much like an OTP).

The problem with this is that if you use the same encryption key twice (which I assume your password database would be, and is also where Microsoft screwed up with NT3), an attacker can easily recover the key by comparing the two encrypted values. This can also be a problem when encrypting large amounts of data with the same key (which is where WEP screwed up).

The solution is to stick with block ciphers like AES.

"There is no shame in being self-taught, only in not trying to learn in the first place." -- Atrus, Myst: The Book of D'ni.

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Re^3: Crypt::CipherSaber replacement... Crypt::CBC?
by Anonymous Monk on Jan 17, 2013 at 04:03 UTC

    I can actually nearly guarantee his system is secure...

    You are correct that using the same key twice with the RC4 algorithm alone would indeed create two streams that could be used to derive the keystream. However, this is what the initialization vector is for. It's appended to the key before encryption and sent in the clear to be used for decryption. It's not part of the key (it's no secret), but it ensures that two identical messages sent with identical keys will always result in different keystreams (and ciphertext). Two examples of ciphersaber-created ciphertext cannot simply be XOR'd together to glean anything useful.

    The way these IV's are chosen and they way they express themselves in the ciphertext can be determined looking at a rather large number of messages with the same key. THIS is why WEP is screwed up. If you create the key array more than once (ciphersaber-2 does this), or don't use the key many times, it's secure.

    I know this is very old, but I can't leave a page that's relatively prevalent on Google searches with significant errors on it. RC4 with properly chosen keys is not broken by any means.

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