There have been rumors for years that the NSA can decrypt a significant fraction of encrypted Internet traffic. Edward Snowden documents show that NSA has built extensive infrastructure to intercept and decrypt VPN traffic and suggest that the agency can decrypt at least some HTTPS and SSH connections on demand. The key is, somewhat ironically, Diffie-Hellman key exchange, an algorithm that we and many others have advocated as a defense against mass surveillance. The payoff, in terms of connections they could decrypt, would be enormous.”]
Source: https://freedom-to-tinker.com/2015/10/14/how-is-nsa-breaking-so-much-crypto/