[rc5] Suggestion for protocol

Bob Krzaczek rskpci at cis.rit.edu
Tue Jun 24 19:43:45 EDT 1997

On Tue, 24 Jun 1997, Remi Guyomarch wrote:
> Fedor Kouranov wrote:
> > Ahem... what's iv?
> Inversion Value ? Well, I don't know ;-)

IV is the ``initialization vector'' for some modes you can run a block
cipher in.  For example, the RSA contests use the RC5 algorithm in Cipher
Block Chaining mode, or CBC for short.

CBC is really simple: every block of plaintext to be encrypted is first
XOR'd with the ciphertext from the last block. 

The IV is the ``fake'' ciphertext used in the XOR when the very first
block of plaintext is to be encrypted.  When you're encrypting that first
block, there is no ``previous block's ciphertext'' to draw on.  So, the IV
is used in its place.

In short, it's just a random word chosen at encryption time. 

// bob

// Bob Krzaczek                              <http://www.cis.rit.edu/~rskpci/>
// Center for Imaging Science, RIT                        <rskpci at cis.rit.edu>

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