[OpenVPN-NL] Deployment advisory update: use of OpenVPN-NL within virtualised environments

[Adriaan de Jong]

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Feedback from the security community has triggered an investigation
into the quality of PolarSSL's random number generation within
virtualised environments. PolarSSL performs the random number
generation and cryptographic functions required by OpenVPN-NL.

To ensure that security is maintained while this investigation
proceeds, the NLNCSA has added an extra restriction to its deployment
advisory:

 - OpenVPN-NL should not be used in virtualised environments.

This restriction might be lifted in the future, depending on the
results of the investigation.

We would like to thank Jacob Appelbaum, Marsh Ray, and Oscar Koeroo
for starting the discussion on this matter.

Details
- -------

PolarSSL versions prior to v1.1 use only the HAVEGE random number
generation algorithm. At its heart, this uses timing information based
on the processor's high resolution timer (the RDTSC instruction). This
instruction can be virtualised, and some virtual machine hosts have
chosen to disable this instruction, returning 0s or predictable results.

Currently, the problem appears to be limited to commercial cloud
server providers.

Solution
- --------

A new version of PolarSSL, v1.1, will be released within a few days.
This contains a new random number generator based on the CTR_DRBG
algorithm specified in NIST SP-800-90. The entropy used by this
algorithm is accumulated from multiple sources, including the existing
Havege RNG, platform-specific entropy sources and timing sources. This
increases the quality of the entropy pool, and makes it less
vulnerable to problems within a single source.

Once this new RNG has been succesfully evaluated, it will be included
in future releases of OpenVPN-NL.

Further research is also being done to determine the overall quality
of random number generation within virtual machines.


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