> >
> > Eh? Isn't the IV length the same as the block length?
>
> Nope. Rijndael's biggest IV is 16 bytes, but the biggest block is 32
> bytes. I haven't checked why, so if someone wants to enlighten me...
>
I too was under the assumption that the IV in use needed to be
the same size as the block length. Since Rijndael encrypts at
128, 192, and 256 bits, the IV should be 16, 24, or 32 bytes
in size.
Not being a cryptography expert myself I'm afraid I provide
little help to Ben's request. However, I DID notice that in the
code downloaded from NIST that in the header file they
specify a MAX_SIZE of 32 for the IV for cipher structure
definition. However, the code in use is hard-coded 16 byte
memcpy and pointer operations. Thus it appears that, no
matter the size of block or key, the IV remains at 16 bytes
for 128, 192, 256 bits keys. I think this has much to do
with the following statement taken directly from the AES
proposal, and I quote:
4. Specification
Rijndael is an iterated block cipher with a variable block length and a
variable key length. The
block length and the key length can be independently specified to 128, 192
or 256 bits.
Note: this section is intended to explain the cipher structure and not as an
implementation
guideline. For implementation aspects, we refer to Section 5.
4.1 The State, the Cipher Key and the number of rounds
The different transformations operate on the intermediate result, called the
State:
Definition: the intermediate cipher result is called the State.
The State can be pictured as a rectangular array of bytes. This array has
four rows, the
number of columns is denoted by Nb and is equal to the block length divided
by 32.
The Cipher Key is similarly pictured as a rectangular array with four rows.
The number of
columns of the Cipher Key is denoted by Nk and is equal to the key length
divided by 32.
I'd be curious to know what would happen to the IV
code if the key size was increased beyond 256 bits as
the proposal implies...
For anyone wishing to see the code and spec for this cipher
use the following link: http://csrc.nist.gov/encryption/aes/
(as if you guys don't know where it is! ;-)
Hope this helps!
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