I tip my (red) hat to you, Netcraft. This is an absolutely excellent article that I recommend everybody to read. Great suff.
Millions of websites and billions of people rely on SSL to protect the transmission of sensitive information such as passwords, credit card details, and personal information with the expectation that encryption guarantees privacy. However, recently leaked documents appear to reveal that the NSA, the United States National Security Agency, logs very high volumes of internet traffic and retains captured encrypted communication for later cryptanalysis.
The reason that governments might consider going to great lengths to log and store high volumes of encrypted traffic is that if the SSL private key to the encrypted traffic later becomes available — perhaps through court order, social engineering, successful attack against the website, or through cryptanalysis — all of the affected site’s historical traffic may then be decrypted at once.
There is a defence against this, known as perfect forward secrecy (PFS). When PFS is used, the compromise of an SSL site’s private key does not necessarily reveal the secrets of past private communication; connections to SSL sites which use PFS have a per-session key which is not revealed if the long-term private key is compromised. The security of PFS depends on both parties discarding the shared secret after the transaction is complete (or after a reasonable period to allow for session resumption).