[e-lang] Weaknesses in crypto-has functions

Mark Miller markm at caplet.com
Wed Sep 15 19:30:41 EDT 2004


>From Bruce Schneier's Cryptogram.


         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 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.


<http://news.com.com/Crypto+researchers+abuzz+over+flaws/2100-1002_3-531 3655.html> or <http://makeashorterlink.com/?Z3F612849>
<http://www.freedom-to-tinker.com/archives/000661.html>
<http://www.mail-archive.com/cryptography%40metzdowd.com/msg02554.html> or <http://makeashorterlink.com/?Q20743849>

Technical information:
<http://www.cs.technion.ac.il/users/wwwb/cgi-bin/tr-get.cgi/2004/CS/CS-2 004-09.ps.gz> or <http://makeashorterlink.com/?O11735849>
<http://www.cs.technion.ac.il/~biham/Reports/Slides/invited-talk-sac-200 4.ps.gz> or <http://makeashorterlink.com/?T23731849>

NIST's SHA site:
<http://csrc.nist.gov/CryptoToolkit/tkhash.html>

This essay originally appeared in Computerworld:
<http://www.computerworld.com/printthis/2004/0,4814,95343,00.html>




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