In cryptography, forward secrecy (FS; also known as perfect forward secrecy) is a property of secure communication protocols in which compromise of long-term keys does not compromise past session keys. Forward secrecy protects past sessions against future compromises of secret keys or passwords. If forward secrecy is used, encrypted communications and sessions recorded in the past cannot be retrieved and decrypted should long-term secret keys or passwords be compromised in the future, even if the adversary actively interfered.
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History
The term "perfect forward secrecy" was coined by C. G. Günther in 1990 and further discussed by Whitfield Diffie, Paul van Oorschot, and Michael James Wiener in 1992 where it was used to describe a property of the Station-to-Station protocol.
Forward secrecy has also been used to describe the analogous property of password-authenticated key agreement protocols where the long-term secret is a (shared) password.
Annex D.5.1 of IEEE 1363-2000 discusses the related one-party and two-party forward secrecy properties of various standard key agreement schemes (for two-party forward secrecy properties compare below 2WIPFS: "2-Way-Instant-Perfect-Forward-Secrecy").
Forward secrecy
A public-key system has the property of forward secrecy if it generates one random secret key per session to complete a key agreement, without using a deterministic algorithm. This means that the compromise of one message cannot compromise others as well, and there is no one secret value whose acquisition would compromise multiple messages. This is not to be confused with the perfect secrecy demonstrated by one-time pads: when it is used properly, the one-time pad involves multiple parties agreeing on a set of disposable keys by communicating it fully in private—without a formalized key agreement system—and then using each key for one message only.
Attacks
Forward secrecy is designed to prevent the compromise of a long-term secret key from affecting the confidentiality of past conversations. However, forward secrecy cannot defend against a successful cryptanalysis of the underlying ciphers being used, since a cryptanalysis consists of finding a way to decrypt an encrypted message without the key, and forward secrecy only protects keys, not the ciphers themselves. A patient attacker can capture a conversation whose confidentiality is protected through the use of public-key cryptography and wait until the underlying cipher is broken (e.g. large quantum computers could be created which allow the discrete logarithm problem to be computed quickly). This would allow the recovery of old plaintexts even in a system employing forward secrecy.
Weak perfect forward secrecy
Weak perfect forward secrecy (wPFS) is the weaker property whereby when agents' long-term keys are compromised, the secrecy of previously established session-keys is guaranteed, but only for sessions in which the adversary did not actively interfere. This new notion, and the distinction between this and forward secrecy was introduced by Hugo Krawczyk in 2005. This weaker definition implicitly requires that full (perfect) forward secrecy maintains the secrecy of previously established session keys even in sessions where the adversary did actively interfere, or attempted to act as a man in the middle.
Protocols
Forward secrecy is present in several major protocol implementations, such as SSH and as an optional feature in IPsec (RFC 2412). Off-the-Record Messaging, a cryptography protocol and library for many instant messaging clients, provides forward secrecy as well as deniable encryption.
In Transport Layer Security (TLS), Diffie–Hellman key exchange-based PFSs (DHE-RSA, DHE-DSA) and elliptic curve Diffie–Hellman-based PFSs (ECDHE-RSA, ECDHE-ECDSA) are available. In theory, TLS can choose appropriate ciphers since SSLv3, but in everyday practice many implementations have refused to offer forward secrecy or only provide it with very low encryption grade.
OpenSSL supports forward secrecy using elliptic curve Diffie–Hellman since version 1.0, with a computational overhead of approximately 15%.
The Signal Protocol uses the Double Ratchet Algorithm to provide forward secrecy. The protocol was developed by Open Whisper Systems in 2013 and was first introduced in the Signal app in February 2014. It has since been implemented into WhatsApp, Facebook Messenger, and Google Allo, encrypting the conversations of "more than a billion people worldwide".
On the other hand, among popular protocols currently in use, WPA doesn't support forward secrecy.
Use
Forward secrecy is seen as an important security feature by several large Internet information providers. Since late 2011, Google provided forward secrecy with TLS by default to users of its Gmail service, Google Docs service, and encrypted search services. Since November 2013, Twitter provided forward secrecy with TLS to its users. Wikis hosted by the Wikimedia Foundation have all provided forward secrecy to users since July 2014.
Facebook reported as part of an investigation into email encryption that, as of May 2014, 74% of hosts that support STARTTLS also provide Forward Secrecy. As of June 2016, 51.9% of TLS-enabled websites are configured to use cipher suites that provide forward secrecy to modern web browsers.
At WWDC 2016, Apple announced that all iOS apps would need to use "ATS" (App Transport Security), a feature which enforces the use of HTTPS transmission. Specifically, ATS requires the use of an encryption cipher that provides forward secrecy. ATS became mandatory for apps on Jan 1st, 2017.