How is NSA breaking into VPNs?

[Peter Fairbrother]

On 16/10/15 17:29, Brian Gladman wrote:
> An interesting paper on the weaknesses of DH key exchange protocols:
>
> https://weakdh.org/imperfect-forward-secrecy-ccs15.pdf


The paper describes the Logjam attack, and the state-level attacks on 
shared DH primes.

Logjam, like FREAK, degrades crypto suite choices, in this case TLS DH 
to 512-bit "export grade" crypto. It's a bit more sophisticated than 
FREAK, but not much.

7th principle: Holes for "good guys" are holes for bad guys too.

8th principle: In code, nothing ever really goes away.

(big nod to Jerry Leichter)




More controversial, and quite possibly more damaging, is this:

"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.

[...]

A close reading of published NSA leaks shows that the agency's attacks 
on VPNs are consistent with having achieved a break [of the single, most 
common 1024-bit prime]."




I wonder whether the "state level threat" of breaking common 1024-bit DH 
primes is the "major breakthrough" which NSA told Congress about a few 
years ago, for which they got all that lovely extra money.

If so, the people who in 2013 were supporting the idea of replacing 
2048-bit RSA with ubiquitous 1024-bit DH in order to provide FS look a 
bit silly ..

[ the major browsers supported only 1024-bit DH, but supported 2048-bit 
RSA, perhaps due to people mistakenly thinking that DH keys needed to be 
half the size of RSA keys - it might be interesting to see where that 
rumour came from.

To quote Peter Gutmann, posting on the cryptography at metzdowd list:

"It's a debate between two groups, the security practitioners, "we'd 
like a PFS solution as soon as we can, and given currently-deployed 
infrastructure DH-1024 seems to be the best bet", and the theoreticians, 
"only a theoretically perfect solution is acceptable, even if it takes 
us forever to get it"."


By the way, this was just after Snowden, when Google and the like were 
moving to ubiquitous 2048-bit RSA, and supposedly crypto people were 
running around like headless chickens saying "we must do something".

NSA must have been laughing all the way to the bank. ]

-- Peter Fairbrother