[42500] in Cypherpunks
Re: MD4-derived hash functions
daemon@ATHENA.MIT.EDU (Childers James)
Sun Oct 29 22:51:43 1995
Date: Sun, 29 Oct 1995 21:43:01 -0600 (CST)
From: Childers James <ic58@jove.acs.unt.edu>
To: Mark <mark@lochard.com.au>
Cc: David A Wagner <daw@quito.CS.Berkeley.EDU>, cypherpunks@toad.com
In-Reply-To: <199510300353.AA55657@junkers.lochard.com.au>
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.
"Freedom is meaningless unless | ic58@jove.acs.unt.edu - James Childers
you can give to those with whom| No man's freedom is safe
you disagree." - Jefferson | while Congress is in session
EA 73 53 12 4E 08 27 6C 21 64 28 51 92 0E 7C F7
