Re: [Libaes-devel] Proposed API for libaes

[Sandy H. <sa...@st...>]

Nigel Metheringham wrote:

> In general aes.h is getting less complex, and is losing its block size
> dependancy.  libaes is going to support the 3 main block sizes - 128,
> 192 and 256 bits.   Key sizes supported will be the same

Those are all the sizes in the original Rijndael spec. That allows key and
block sizes to vary independently over 128, 192, 256.

However, AES as standardised is /not/ identical to that. See below.

> - I am not
> currently intending to do anything to support weird intermediate key
> sizes (OK thats pejorative language, but are there any reasonable
> requirements for intermediate keysizes especially as it really hits
> efficiency at present).

Why even consider it? Neither the original Rijndael nor the AES spec
have those, and you aren't doing research on cipher variations. You're
either implementing the cipher or providing a new interface for Brian
Gladwin's code. Either way, you have no business adding things.
 
> There will be 4 sets of function entries:-
>   aes_...       - any block size handling code
>   aes128_...    - 128 bit block size code
>   aes192_...    - 192 bit block size code
>   aes256_...    - 256 bit block size code
> 
> Ideally all of the aes<nnn>_ entries will use a highly optimised version
> of the code - assembler if we have it.

I think even this complexity is unnecessary.

The AES contest specified that all entries must use a 128-bit block
size. All the evaluation of Rijndael (security and speed) during the
AES process used that size, so much less is known about whether the
other sizes might have security weaknesses. The AES standard is for
a cipher with 128-bit block size, Rijndael minus the variable block
size feature. Last time I looked, that was all Brian Gladwin's code
supported as well.

I think you should do only the one block size, unless someone can
turn up a compelling reason that the others are quite useful.

If you really want to do them all, then I think the names should be:

   aes		128-bit block
#ifdef REST_OF_RIJNDAEL
   rijndael		any block size
   rijndael_128blk	128-bit block
   rijndael_192blk	192-bit block
   rijndael_256blk	256-bit block
#endif

You cannot call it AES if it uses any block size other than 128.

You need rijndeal_128blk, not just rijndael_128, to make it clear