By Anne Broache, and Declan McCullagh
Published on ZDNet News
November 1, 2005
GAITHERSBURG, Md. -- A team of Chinese scientists shocked the data
security world this year by announcing a flaw in a widely used
technique used to create and verify digital signatures in e-mail and
on the Web.
Now the U.S. government is trying to figure out what to do about it.
The decade-old algorithm, called the Secure Hash Algorithm, or SHA-1,
is an official federal standard and is embedded in every modern Web
browser and operating system. Any change will be expensive and
time-consuming--and a poor choice by the government would mean that
the successor standard may not survive another 10 years.
"We're going to have to make a decision fairly soon about where to
push people," said John Kelsey of the National Institute of Standards
and Technology (NIST), which convened a workshop here on the topic
Monday. Even though NIST is only technically responsible for
government standards-setting, Kelsey noted, "we're likely to get a lot
of other people to head in that direction as well."
The findings by the researchers at China's Shangdong University, which
they described in an interview with CNET News.com in March, are still
of more theoretical than practical interest. But as computing speed
accelerates, their discovery eventually will make it easier for
intruders to insert undetectable back doors into computer code or to
forge an electronic signature--unless a different, more secure "hash"
algorithm is adopted.
NIST is weighing two broad options: selecting a newer variant of SHA-1
believed to be more secure, or undertaking the much longer process of
soliciting public suggestions for an entirely new algorithm that can
be used for digital signatures. (The agency followed the second path
before deciding on the Rijndael algorithm, used for data encryption
rather than signatures.)
Complicating the decision-making process is a belief among computer
scientists that even the newer algorithms related to SHA-1 may suffer
from similar flaws.
Variants on SHA-1--originally devised by the National Security
Agency--exist and are growing in popularity. NIST has announced a set
of algorithms known generally as SHA-2 (sometimes called SHA-256,
SHA-384, or SHA-512), but they haven't been subject to as much public
scrutiny as SHA-1, which makes some researchers nervous. Orr
Dunkelman, a doctoral student at Technion University in Israel, said
"I have a strong suspicion that in the next five years, SHA-256 might
be considered broken."
Last year, flaws also were reported in MD5, a similar algorithm widely
used on the Internet. SHA-1 yields a 160-bit output, which is longer
than MD5's 128-bit output and is considered more secure.
To computer scientists, the SHA and MD5 algorithms are known as hash
functions. They take all kinds of input, from an e-mail message to an
operating-system kernel, and generate what's supposed to be a unique
fingerprint. Changing even one letter in the input file should result
in a completely different fingerprint.
Security applications rely on these fingerprints being unique. But if
a malicious attacker could generate the same fingerprint with a
different input stream, the cloned fingerprint--known as a hash
collision--would certify that software with a back door is safe to
download and execute.
That would help a crook who wanted to falsely sign an e-mail
instructing that someone's bank account be emptied. Or a digitally
signed contract could, in theory, be altered but appear valid.
There's no need to panic, said Steven Bellovin, a professor of
computer science at Columbia University, who described the flaws in
SHA-1 as still theoretical. But "even if we decide that SHA-1 is good
enough for today, someday we are going to have to deploy new hash
functions," Bellovin said.
Complicating that deployment is the dizzying scope of the upgrade
project. Hundreds of protocols including TLS/SSL (used by Web
browsers), SSH (used for remote logins) and IPsec (used in virtual
private networks, or VPNs) eventually would have to be reworked to
support the new standard. Then Internet users would have to be
convinced to upgrade.
"You cannot deploy a new algorithm of any sort all over the place all
at once," Bellovin said. "The Internet is far too large." He said that
newer applications based on NIST's successor algorithm should be able
to "switch-hit" and support the older algorithms when talking to older
Although the U.S. government and most companies may gradually switch
from SHA-1--including PGP Corp., which sells desktop encryption
software--it won't be practical to abandon it anytime soon, said Niels
Ferguson, a cryptographer who works for Microsoft. "You have to be
able to read old files and talk to people who haven't updated their
PCs in seven years," he said.
NIST has announced plans to ditch SHA-1 by 2010. But it is still far
from making a decision. "We really have no strong preconceptions at
this point about what we want to do," said Bill Burr, manager of
NIST's computer security division.
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