Crypto attack against SSL outlined

[InfoSec News <isn () c4i org>]

http://www.theregister.co.uk/content/55/29423.html

By John Leyden
Posted: 21/02/2003 

Swiss security researchers have discovered an attack against
implementations of the ubiquitous SSL protocol that could potentially
compromise email passwords, though not ecommerce transactions.

The protocol itself has not been compromised and the weakness only
applies to versions of OpenSSL prior to version 0.9.6i and 0.9.7a,
according to early analysis.

Users of earlier versions of OpenSSL are strongly advised to upgrade.

At this point its unclear whether alternative implementations of SSL
are at risk.

Credit card transaction secured using even earlier versions of OpenSSL
are not at risk because of the mechanism of the attack.

Secure Sockets Layer (SSL), which is supported by all major Web
browsers, is one of the most common security protocols in use on the
Net. SSL, and its successor Transport Layer Security (TLS), manage the
security of message transmission: which can be anything from the
details of credit cards made during a ecommerce transactions to an
Outlook client logging onto an email server.

In a paper researchers at the Security and Cryptography Laboratory of
Swiss University (Lasec) EPFL demonstrate a timing-based attack on CBC
cipher suites in SSL and TLS.

The attack assumes that multiple SSL or TLS connections involve a
common fixed plaintext block, such as a password. Since credit cards
numbers are normally sent to a secure server only once this particular
attack has little or no chance of success.

When checking emails, using for example an Outlook Express 6.x client,
using a secure connection passwords are sent periodically as email is
checked. This leaves the door open for an attack.

The researchers at Lasec have demonstrated a form a man in the middle
attack (using DNS spoofing) can be used to discover email passwords.

Essentially an attacker would substitute specifically made-up cipher
text blocks in a legitimate communication and monitor the error
messages an email server generates.

In this way, through cryptanalysis of the error messages, it is
possible to glean clues on the make up of a legitimate password.  
Dictionary or brute force attacks may be used, as explained in greater
detail in the researcher's paper.

The flaw with earlier versions of OpenSSL lies in the way error
messages are constructed, a problem that doesn't apply to OpenSSL
versions 0.9.6i and 0.9.7a. This point is explained by the OpenSSL
project in much greater depth in an advisory published earlier this
week.



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