March 31, 2014

NSA caught again -- deliberate weakening of TLS revealed!?

In a scandal that is now entertaining that legal term of art "slam-dunk" there is news of a new weakness introduced into the TLS suite by the NSA:

We also discovered evidence of the implementation in the RSA BSAFE products of a non-standard TLS extension called "Extended Random." This extension, co-written at the request of the National Security Agency, allows a client to request longer TLS random nonces from the server, a feature that, if it enabled, would speed up the Dual EC attack by a factor of up to 65,000. In addition, the use of this extension allows for for attacks on Dual EC instances configured with P-384 and P-521 elliptic curves, something that is not apparently possible in standard TLS.

This extension to TLS was introduced 3 distinct times through an open IETF Internet Draft process, twice by an NSA employee and a well known TLS specialist, and once by another. The way the extension works is that it increases the quantity of random numbers fed into the cleartext negotiation phase of the protocol. If the attacker has a heads up to those random numbers, that makes his task of divining the state of the PRNG a lot easier. Indeed, the extension definition states more or less that:

4.1. Threats to TLS

When this extension is in use it increases the amount of data that an attacker can inject into the PRF. This potentially would allow an attacker who had partially compromised the PRF greater scope for influencing the output.

The use of Dual_EC, the previously fingered dodgy standard, makes this possible. Which gives us 2 compromises of the standards process that when combined magically work together.

Our analysis strongly suggests that, from an attacker's perspective, backdooring a PRNG should be combined not merely with influencing implementations to use the PRNG but also with influencing other details that secretly improve the exploitability of the PRNG.

Red faces all round.

Posted by iang at March 31, 2014 06:12 PM | TrackBack
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