LogJam encryption flaw fix will
block some websites
· Technology
Web-browser makers are preparing a fix for a flaw in an encryption algorithm that makes it possible to spy on supposedly secure communications.
However, the updates
will mean a minority of websites will be blocked by the new software.
The "LogJam attack" was
discovered by researchers at
Microsoft and a number of US and French universities.
They believe about 8% of the top one million
HTTPS security-protected sites are made vulnerable by the flaw.
Users would therefore be
given false reassurance by the padlock icon that such sites display in a
browser's address bar.
Some email servers and
services that use the Transport Layer Security (TLS) cryptographic protocol are
also at risk of being hacked until their operators update their systems.
Export controls
The LogJam attack
vulnerability is a legacy of the US 1990s-era export restrictions on
cryptographic tools.
These limited the
complexity of the secret encryption codes that could be generated by
"international versions" of US-made software, including Netscape's
web browser.
The export rules were
later relaxed, but the researchers say an unintended consequence is that a
commonly used process, called a Diffie-Hellman key exchange, can be compromised
by a "man-in-the-middle" attack.
A Diffie-Hellman key exchange was one of the first techniques
developed to allow two or more parties to create and share an encryption key by
exchanging parts of the key in public.
What the researchers
discovered was that by intercepting the communications, a hacker could ensure a
512-bit key was used rather than a more complicated one.
In this context, 512-bit
means there are two to the power of 512 possible combinations - representing a
huge number.
Nevertheless, the
researchers said it was still possible for computers to crack such codes in
"minutes".
Even more complicated
types of encryption were susceptible to cyber-spies using supercomputers at the
National Security Agency, they added.
"In the 1024-bit
case, we estimate that such computations are plausible given nation-state resources,
and a close reading of published NSA leaks shows that the agency's attacks on
VPNs [virtual private networks] are consistent with having achieved such a
break," they wrote.
Nevertheless, the fix
that web-browser makers have agreed on is that their software should block
512-bit or weaker encryption keys.
"The solution is relatively simple - you disable this
legacy function on your system," said Prof Alan Woodward, a cybersecurity
expert at the University of Surrey.
"Unfortunately,
some older web servers might then be prevented from starting a secure conversation
with the updated web browsers as they would support only that older, shorter,
weaker key lengths.
"But do you really
want this backward compatibility if it means others could be forced to use this
weaker form of encryption?
"Browsers can be
updated and servers can be reconfigured easily, and it really is no bad thing
to force this to happen bearing in mind the alternative is that a 'secure
connection" could potentially be broken by an eavesdropper."
According to a report by
the Wall Street Journal, it has been estimated that 20,00
websites could become blocked as a consequence.
'Storm in a tea cup'
Mozilla - which is
responsible for the Firefox browser - said its new software should be released
in the "next few days".
"Most of the coordination in this case was done thanks to
the researchers who found the bug. They provided valuable measurement data to
the various browser vendors, which allowed us all to calibrate our
response," said Richard Barnes, the organisation's cryptographic
engineering manager.
Another security expert
said that internet users should not be worried about being unprotected in the
meantime.
"The fact that
LogJam can only be exploited when hackers and targets are on the same network,
as well as patches being imminent, means that hype around it is likely to be a
bit of a storm in a tea cup," said Ross Brewer, from security research
company LogRhythm.
"Organisations
should, however, use flaws like this as an excuse to give themselves a security
health-check.
"With flaws like
LogJam being identified with increasing frequency, the only real way to know
you're safe is to know you can stop an attack in its tracks as soon as it gets
going."
