secure hash modes for rijndael
sao19677 at terra.com.br
sao19677 at terra.com.br
Fri Mar 30 06:19:12 EST 2001
Why not using tandem or abreast Davies-Meyer, as
it is done with IDEA? These modes are designed for
block ciphers whose key length is twice the block
length -- certainly the case for AES-256 -- and
generate hashes with twice the block length.
I'm resisting the temptation to say that they were
also more thoroughly analyzed (this should be the
case because they are long known by now, but I'm not
aware of any such analysis).
I have asked NIST's Jim Foti about this issue some
time ago. Maybe it's a good idea to submit a public
comment for NIST's modes of operation process, just
Bram Cohen wrote:
> sha-256 is ridiculously slow, so I've done some thinking about hash modes
> for rijndael.
> To begin with, there's the issue of padding - this can be done by
> appending a 1 and then padding with zeros to the next multiple of a block
> size. If the data to be hashed is already a multiple of a block size and
> doesn't end in a 1 to begin with, no padding is necessary. This gets rid
> of a lot of unnecessary work for hashing small files.
> Making a hash requires two fixed keys, the logical values for them are all
> 0 and all 1 bits.
> First, compute CBC MACs using the two fixed keys (a CBC MAC is where
> encrypt the first block, xor with the second, encrypt again, xor with the
> third, etc.) Call the two MACs A and B. Now encrypt A using B and B with
> it's last byte xored with FF using A (the xor is in case the data is only
> a single block length, making A and B both be the original
> file). Concatenate those two values together and that's the hash.
> I believe this algorithm is quite secure. It produces an output twice the
> length of the block size, which should be as resistant to birthday attacks
> as the block cipher is to regular attacks. It has a hash rate of 1/2,
> which makes sense since it's output is twice as large. Even at that rate,
> it's still much faster than sha.
> It would be nice if there was an algorithm which used rijndael with 256
> bit blocks to produce a hash of 256 bits and had a hash rate of 1, but I
> haven't been able to come up with one.
> One neat trick - if you have a short file (about 20 bytes) and want a 160
> bit hash of it, you can encrypt the file using itself as the key.
> I think releasing a sha256 standard at all was a bad idea - sha1 will last
> a while yet, and a standard hashing mode for rijndael will work much
> better. This is a very serious issue with regards to blob identification,
> since you can't just switch your metadata to refer to the same files but
> using a different hash function. I strongly urge everyone who indexes
> blobs to continue using sha1 until a decent hashing algorithm has been
> picked as the next standard.
> -Bram Cohen
> Soko! puzzle game - http://ch.havenco.com:4201/
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