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Re: MD4-derived hash functions
On Mon, 30 Oct 1995, Mark wrote:
> >The conclusion to take away from this is simple: double encryption
> >doesn't give you much extra security over single encryption. Don't
> >use double encryption.
> That doesnt make sense. If one accepts that double encryption is securer than
> single encryption, wether marginally or twice as secure, why not use it?
Ah yes, but the vagarities of crypto don't lend themselves to real-world
analogies so easily. With crypto schemes, if you use double-encryption,
you effectively halve the amount of time needed to crack them. This is
because of the "man in the middle attack." Schneier talks about it in
Applied Crypto, and I am sure others on this list know the technical
details better than I.
What Schneier says has been proven to be secure is, instead, a triple
encryption scheme. Using two different keys, it goes something like this
(if memory serves):
Cipertext = P1xorEK1 -> C1xorDK1 -> C2xorEK1
Where P1 is the plaintext, EK1 is encrypt key 1, and DK1 is decrypt key 1.
That doesn't look right the longer I consider it, but the basic idea is
there. Encrypt, decrypt, then encrypt again.
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