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Hi Russell,
The CFB mode according to your quote of Schneier "can be
encrypted in units smaller
than the block size". Even more I'm using
that with CFB mode with CryptoPP and that
proved to work fine :)
Thank
you for concern,
Voronkov Konstantin
----- Original Message -----
Sent: Wednesday, June 02, 2004 12:30
PM
Subject: Re[2]: SymmetricCipher Decoder
is not working if created dynamically.
Hi Konstantin,
Voronkov> I wanted to ask you question
about padding. IFAIK the CFB encryption
Voronkov> mode produces the same
output buffer size and
Voronkov> input, and in the
sample:
Voronkov> http://www.eskimo.com/~weidai/cgi-bin/fom-serve/cache/79.html
Voronkov>
there is the same size for output buffer as for
Voronkov> input. This is
the CFB mode
Voronkov> design to produce the same length buffer, am I
wrong?
According to Bruce Schneier in Applied Cryptography (p
200):
"Block ciphers
can also be implemented as
a
self-synchronizing
stream cipher; this is
called
cipher-feedback (CFB) mode.....In CFB mode, data can
be
encrypted in
units smaller than the block size."
Apart from the IV needing to be
transmitted with the ciphertext, I
would take this to mean that the output
buffer can indeed be the same
size as the input buffer.
However, if
I don't something for sure, I make a conservative decision
when
implementing. I use CFB mode almost exclusively, and I've never
assumed the
output will be the same size as the input. Plus, the
addition of the
IV to send to the decryptor means what I transmit
is always going to be
bigger.
That's the really nice thing about Wei's filter classes.
They work
with variable input and output, so you never really have to work
with
fixed length buffers.
In the general case for block ciphers,
however, I would assume that
the output buffer must be a multiple of the
block size.
For example, if the block size is 8 bytes, and your
plaintext is 100
bytes, then your output buffer would have to be 104 bytes
(13 * 8 =
104) on encryption.
I'm no expert in this field, so if the
above conclusion is incorrect,
could someone correct it, *PLEASE*, for the
benefit of all?
--
Russell Robinson (mailto:[EMAIL PROTECTED])
Author
of Tectite (CRM and Licensing for Software Developers)
Download your free
CRM from: http://www.tectite.com/
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