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 at Sophos, for his thoughts on Twitter's latest security move. Pointing to when Google began using HTTPS by default, he told us it often takes a bit of time for all the big players to come around and set the new standard for security and privacy online.
"Perfect forward secrecy is another one of these moments. It is great news to see high-profile organizations like Twitter adopt changes that will improve the privacy and security of its users. Will it make a difference? Probably not, but it does help establish this as a new baseline that all Web sites should comply with," Wisniewski said.
PFS largely protects intercepted communications against future encryption key disclosure. For example, the NSA intercepts communications between you and Twitter. Before PFS it could go to court and persuade Twitter to disclose its keys. If Twitter were to comply, Wisniewski said, the NSA could read the intercepted communications. PFS prevents that from working. Even if the private keys are intercepted or handed over, he continued, the messages remain protected.
"The reason it might not matter? Who is sending private messages over Twitter? It is a platform designed for blasting out messages, publicly, to anyone willing to listen," he concluded. "It would seem that this move is largely symbolic more than about preventing interception."