Twitter Beefs Up Encryption Against NSA Snooping

Following in the footsteps of Google and Facebook, Twitter is moving to implement "perfect forward secrecy," a cryptographic technique to keep data safe from prying eyes. Twitter announced it has enabled forward secrecy for traffic on Twitter.com, api.twitter.com, and mobile.twitter.com.

On top of the usual confidentiality and integrity properties of HTTPS, perfect forward secrecy adds a new property, according to Twitter's Jacob Hoffman-Andrews. If an adversary is currently recording all Twitter users' encrypted traffic, and they later crack or steal Twitter's private keys, they should not be able to use those keys to decrypt the recorded traffic, he said.

The New Normal

"Under traditional HTTPS, the client chooses a random session key, encrypts it using the server's public key, and sends it over the network," Hoffman-Andrews wrote in a blog post. "Someone in possession of the server's private key and some recorded traffic can decrypt the session key and use that to decrypt the entire session. In order to support forward secrecy, we've enabled the EC Diffie-Hellman cipher suites."

Under those cipher suites, the client and server manage to come up with a shared, random session key without ever sending the key across the network, he explained, even under encryption. Hoffman-Andrews said the details are explained at Wikipedia's article on Diffie-Hellman key exchange. The server's private key is only used to sign the key exchange, preventing man-in-the-middle attacks.

"At the end of the day, we are writing this not just to discuss an interesting piece of technology, but to present what we believe should be the new normal for Web service owners," said Hoffman-Andrews. "A year and a half ago, Twitter was first served completely over HTTPS. Since then, it has become clearer and clearer how important that step was to protecting our users' privacy."

Largely Symbolic Move

We asked Chester Wisniewski, a senior security adviser...

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