Re: DES CBC Mode

"Ocean" <oceansoftware@rogers.com> wrote in message
news:1128351319.060217.160750@g43g2000cwa.googlegroups.com...
> If they're not always using the same IV, they have to add it (the IV)
> to the data that they are sending me, not XOR it to the data.

There seems to be a misunderstanding here. Ocean seems to have meant "they
have to include the IV along with the data" while Peter heard "they are
using addition to combine the IV into the data for CBC mode." So to answer
the original questions:

1) [Are they] always using the same IV (let's say 0)[?]
Probably not. The IV is often prepended to the data, and is usually
(pseudo)randomly chosen. Because Cryptool appears to be a learning aid it is
possible that it always uses the same IV.

2) do they HAVE to [include it in] the encrypted data[?]
It is necessary to transfer it in some way (otherwise the first block is
lost), the most common way of doing this is to prepend it.

3) do I have to change my algorithm to always use the same IV?
This question doesn't make sense, but I think it's based on a yes to #1, but
phrased poorly. The IV is not a part of the encryption algorithm, it is a
part of the chaining mode. While we all know that the chaining mode is an
algorithm we generally refer to it as the mode in order to avoid the same
type of confusion that Peter experienced, and that I experienced when I read
your first post.
                Joe
Received on Sat Oct 15 04:38:26 2005