problem with reference AES implementation (CBC) ?
Eli Bendersky
I downloaded the AES reference implementation (the one sent by the
authors to the AES commitee) in C, compiled it and tried using it in
different modes. While generally it works OK and the encryption of
blocks is correct, I think there is a problem in its implementation of
the CBC mode. Namely, it does not actually XOR C(i-1) with P(i) prior
to feeding it to the block encryptor.
The Perl implementation Crypt::Rijndael that is based on this reference
implementation suffers from exactly the same problem.
Has anyone else run into this, or am I just missing something ?
Thanks in advance
Kristian Gjųsteen
>Namely, it does not actually XOR C(i-1) with P(i) prior
>to feeding it to the block encryptor.
I just looked at reference version 2.2, and it seems to do this.
It does however mention a bug in CBC mode in a previous version,
so perhaps you have an old version?
--
Kristian Gjøsteen
Eli Bendersky
implementation, and it was fixed in version 2.2
I hope not too many people are happily implementing their AES-CBC MACs
using the older reference version, which has a very nasty bug that
_significantly_ weakens the algorithm.
David Wagner
>I hope not too many people are happily implementing their AES-CBC MACs
>using the older reference version, which has a very nasty bug that
>_significantly_ weakens the algorithm.
Out of curiousity, would you mind saying what was the bug?
Eli Bendersky
The bug was that instead of XORing with the result of the previous
block, it was always XORing with IV (which is zero). So in effect,
instead of implementing CBC, it was implementing ECB :-)
David Wagner
Ahh, thanks. Cute. So in essence the MAC only depended on the last
block of the message -- and an attacker could freely change any of the
other blocks without affecting the MAC digest. That's a nasty one.
I'm reminded of a bug in the standard GOST-MAC implementation that was
floating around the net for a while. It used the 64-bit GOST cipher
in CBC-MAC mode, but it only did a 32-bit xor instead of a 64-bit xor.
The 64-bit internal state was held in two 32-bit ints, call them "l"
and "r", and the code looked something like this:
l ^= x;
r = y; // Bug: Should have been r ^= y
The consequence is that only 32 bits of the internal state were used,
and the other 32 bits of were thrown away. Normally, CBC-MAC with a
64-bit cipher is secure for up to around 2^32 messages (after which you
encounter a birthday attack). With this implementation error, the GOST
CBC-MAC implementation would only be secure up to around 2^16 messages
(after which a birthday attack breaks it). I don't recall whether this
was ever fixed.
Implementation errors in crypto code are nasty, because they can be
hard to detect (how do you know when you got the wrong answer, when the
right answer looks like random bits?) and can degrade the security
far below what one would expect. I've been wanted for years to someday
write a survey paper on some of the more amusing implementation errors
I've seen, but I've never had time. Oh well, maybe next decade.
Douglas A. Gwyn
encryption algorithm, where an attempt to improve the salting
ended up reducing the range of possible salt values.
David Wagner
I don't know if I've seen that one. Do you know of anywhere I could
learn more? What was the bug?
Dennis Ritchie
"David Wagner" <d...@taverner.cs.berkeley.edu> wrote in message news:dt8b5i$192u$1...@agate.berkeley.edu...
This may not be the same thing, but it was similar: in the late 70s
Ken Thompson taught for an academic year at UCB, and
one of the other CS courses assigned randomly generated passwords
to the students for their accounts. Ken noticed that the
RNG being used passed through a 16-bit gate, giving an
easily searchable 65K different possible passwords.
Dennis
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