[BitTorrent] sha-0 and md5 breaks
voodoo at gmail.com
Wed Sep 15 12:29:26 EDT 2004
This bit of writing from Bruce Schneier's CRYPTO-GRAM may interest
everyone following the SHA-1 discussion. It's a very high-level
overview, but it very much supports Bram's position of, it's not
broken yet, but we might want to move to a different hashing function
in the next release.
Cryptanalysis of MD5 and SHA
At the CRYPTO conference in Santa Barbara, CA, last month, researchers
announced several weaknesses in common hash functions. These results,
while mathematically significant, aren't cause for alarm. But even so,
it's probably time for the cryptography community to get together and
create a new hash standard.
One-way hash functions are a cryptographic construct used in many
applications. They are used in conjunction with public-key algorithms
for both encryption and digital signatures. They are used in integrity
checking. They are used in authentication. They have all sorts of
applications in a great many different protocols. Much more than
encryption algorithms, one-way hash functions are the workhorses of
In 1990, Ron Rivest invented the hash function MD4. In 1992, he
improved on MD4 and developed another hash function: MD5. In 1993, the
National Security Agency published a hash function very similar to MD5,
called SHA (Secure Hash Algorithm). Then, in 1995, citing a newly
discovered weakness that it refused to elaborate on, the NSA made a
change to SHA. The new algorithm was called SHA-1. Today, the most
popular hash function is SHA-1, with MD5 still being used in older
One-way hash functions are supposed to have two properties. One,
they're one way. This means that it is easy to take a message and
compute the hash value, but it's impossible to take a hash value and
recreate the original message. (By "impossible" I mean "can't be done
in any reasonable amount of time.") Two, they're collision free. This
means that it is impossible to find two messages that hash to the same
hash value. The cryptographic reasoning behind these two properties is
subtle, and I invite curious readers to learn more in my book "Applied
Breaking a hash function means showing that either -- or both -- of
those properties are not true. Cryptanalysis of the MD4 family of hash
functions has proceeded in fits and starts over the last decade or so,
with results against simplified versions of the algorithms and partial
results against the whole algorithms. This year, Eli Biham and Rafi
Chen, and separately Antoine Joux, announced some pretty impressive
cryptographic results against MD5 and SHA. Collisions have been
demonstrated in SHA. And there are rumors, unconfirmed at this
writing, of results against SHA-1.
The magnitude of these results depends on who you are. If you're a
cryptographer, this is a huge deal. While not revolutionary, these
results are substantial advances in the field. The techniques
described by the researchers are likely to have other applications, and
we'll be better able to design secure systems as a result. This is how
the science of cryptography advances: we learn how to design new
algorithms by breaking other algorithms. Additionally, algorithms from
the NSA are considered a sort of alien technology: they come from a
superior race with no explanations. Any successful cryptanalysis
against an NSA algorithm is an interesting data point in the eternal
question of how good they really are in there.
To a user of cryptographic systems -- as I assume most readers are --
this news is important, but not particularly worrisome. MD5 and SHA
aren't suddenly insecure. No one is going to be breaking digital
signatures or reading encrypted messages anytime soon with these
techniques. The electronic world is no less secure after these
announcements than it was before.
But there's an old saying inside the NSA: "Attacks always get better;
they never get worse." These techniques will continue to improve, and
probably someday there will be practical attacks based on these techniques.
It's time for us all to migrate away from SHA-1.
Luckily, there are alternatives. The National Institute of Standards
and Technology already has standards for longer -- and harder to break
-- hash functions: SHA-224, SHA-256, SHA-384, and SHA-512. They're
already government standards, and can already be used. This is a good
stopgap, but I'd like to see more.
I'd like to see NIST orchestrate a worldwide competition for a new hash
function, like they did for the new encryption algorithm, AES, to
replace DES. NIST should issue a call for algorithms, and conduct a
series of analysis rounds, where the community analyzes the various
proposals with the intent of establishing a new standard.
Most of the hash functions we have, and all the ones in widespread use,
are based on the general principles of MD4. Clearly we've learned a
lot about hash functions in the past decade, and I think we can start
applying that knowledge to create something even more secure.
Better to do it now, when there's no reason to panic, than years from
now, when there might be.
3655.html> or <http://makeashorterlink.com/?Z3F612849>
004-09.ps.gz> or <http://makeashorterlink.com/?O11735849>
4.ps.gz> or <http://makeashorterlink.com/?T23731849>
NIST's SHA site:
This essay originally appeared in Computerworld:
On Tue, 7 Sep 2004 07:32:48 -0700 (PDT), Bram Cohen
<bram at bitconjurer.org> wrote:
> BitTorrent currently uses sha-1, a different algorithm from the recently
> broken sha-0 and md5. While those attacks raise some concern, there aren't
> critical right now, since -
> * The attacks don't (yet) work on sha-1.
> * The attacks are birthday attacks, not pre-image attacks, which are what
> would be what would cause a security concern for BitTorrent.
> That said, I'm going to hold off on deciding on what secure hash algorithm
> to use for bt2 until the dust settles. My current best guess is it will be
> In other bt2 news, I'm fairly certain at this point that the one-the-wire
> piece size will be 16k, as a hard-coded magic number. The larger piece
> size will probably also be a magic number, but I'm less sure of that
> value, probably somewhere around a quarter meg. Yay magic numbers.
> Also, for the merkle hash trees the very last piece will always be padded
> out with zeros rather than be shorter than everything else, because I'm
> sick of all the stupid edge cases that was causing. The padding will of
> course not be actually sent down the wire.
> Some of you may have noticed that work on ipv6 support has stalled. That's
> because ipv6 addresses are bigger than ipv4 addresses, and the bandwidth
> taken up by sending those around can hurt trackers, which are already the
> bottleneck on the whole system. Since it's mostly something I want to
> support just on principle with no immediate benefit, and it appears to be
> a real problem, it's been put off until later.
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