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Sydney Morning Herald
Time for new hash standard
By Bruce Schneier
Comment
September 20, 2004 - 9:09AM
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 modern
cryptography.
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 applications.
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 Cryptography."
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.
Bruce Schneier is a world-renowned security technologist. His latest book
is Beyond Fear: Thinking Sensibly About Security in an Uncertain World. He
can be reached at www.schneier.com. This article first appeared in his
monthly newsletter Crypto-Gram and is reproduced with permission. Copyright
rests with the author.
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R. A. Hettinga