Diffie–Hellman key exchange
The 2013 mass surveillance disclosures made it more widely known that IIS is particularly bad in supporting perfect forward secrecy (PFS), especially when used in conjunction with Internet Explorer. Possessing one of the long term asymmetric secret keys used to establish a HTTPS session should not make it easier to derive the short term session key to then decrypt the conversation, even at a later time. Diffie–Hellman key exchange (DHE) and elliptic curve Diffie–Hellman key exchange (ECDHE) are in 2013 the only ones known to have that property. Only 30% of Firefox, Opera, and Chromium Browser sessions use it, and nearly 0% of Apple's Safari and Microsoft Internet Explorer sessions.