OpenVPN, NSA, & Diffie-Hellman key exchange

chef

Read an interesting article in the Hacker News on how the NSA was probably able to crack web and vpn encryption https://thehackernews.com/2015/10/nsa-crack-encryption.html. The article provides a link https://weakdh.org/imperfect-forward-secrecy-ccs15.pdf to the paper that describes how a vulnerability in the Diffee-Hillman key exchange could be exploited. The site where the paper originates from https://weakdh.org provides configurations to thwart this exploit.

2 Questions:
1) Is OpenVPN susceptible to this attack?
2) What should one include in their configuration files to eliminate this exploit and other similar ones?

thanks
-cheflc

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chef wroteIs OpenVPN susceptible to this attack?

To which attack do you refer, the Logjam attack ?

chef wroteWhat should one include in their configuration files to eliminate this exploit and other similar ones?

Create your PKI with a minimum 2048bit DH key and use --tls-auth for that extra layer of security.

https://weakdh.org:

Good News! Your browser is safe against the Logjam attack.

:mrgreen:

chef

not logjam.

from the weakdh.org site:

"Threats from state-level adversaries. Millions of HTTPS, SSH, and VPN servers all use the same prime numbers for Diffie-Hellman key exchange. Practitioners believed this was safe as long as new key exchange messages were generated for every connection. However, the first step in the number field sieve—the most efficient algorithm for breaking a Diffie-Hellman connection—is dependent only on this prime. After this first step, an attacker can quickly break individual connections.

We carried out this computation against the most common 512-bit prime used for TLS and demonstrate that the Logjam attack can be used to downgrade connections to 80% of TLS servers supporting DHE_EXPORT. We further estimate that an academic team can break a 768-bit prime and that a nation-state can break a 1024-bit prime. Breaking the single, most common 1024-bit prime used by web servers would allow passive eavesdropping on connections to 18% of the Top 1 Million HTTPS domains. A second prime would allow passive decryption of connections to 66% of VPN servers and 26% of SSH servers. A close reading of published NSA leaks shows that the agency's attacks on VPNs are consistent with having achieved such a break."

from what I can gather it is the exchange method for the DH keys themselves, which I know nothing about. check out the hackernews link I posted.

not sure if "--tls-auth" would help in this case

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chef wroteThreats from state-level adversaries. Millions of HTTPS, SSH, and VPN servers all use the same prime numbers for Diffie-Hellman key exchange.

If you built your own PKI from scratch then you have a unique DH.key .. which rules this threat out. The state can, of course, throw billions of dollars at your VPN and break it eventually, if they want, but I think you would notice they were doing so when they could no longer afford to pick up your garbage as they wasted all your tax on the wrong department.

chef wrotenot sure if "--tls-auth" would help in this case

would not hurt ..