In cryptography, forward secrecy (FS), also known as perfect forward secrecy (PFS), is a feature of specific key agreement protocols that gives assurances your session keys will not be compromised even if the private key of the server is compromised. Forward secrecy protects past sessions against future compromises of secret keys or passwords. By generating a unique session key for every session a user initiates, even the compromise of a single session key will not affect any data other than that exchanged in the specific session protected by that particular key. Forward secrecy further protects data on the transport layer of a network that uses common SSL/TLS protocols, including OpenSSL, which had previously been affected by the Heartbleed exploit. 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, for example via a man-in-the-middle attack.
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.
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 session cannot compromise others as well, and there is no one secret value whose acquisition would compromise multiple sessions. 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.
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The following is a hypothetical example of a simple instant messaging protocol that employs forward secrecy:
- Alice and Bob each generate a pair of long-term, asymmetric public and private keys, then verify public-key fingerprints in person or over an already-authenticated channel. The only thing these keys will be used for is authentication, including signing messages and signing things during session key exchange. These keys will not be used for encryption of any kind.
- Alice and Bob use a key exchange algorithm such as Diffie–Hellman, to securely agree on an ephemeral session key. They use the keys from step 1 only to authenticate one another during this process.
- Alice sends Bob a message, encrypting it with a symmetric cipher using the session key negotiated in step 2.
- Bob decrypts Alice's message using the key negotiated in step 2.
- The process repeats for each new message sent, starting from step 2 (and switching Alice and Bob's roles as sender/receiver as appropriate). Step 1 is never repeated.
Forward secrecy (achieved by generating new session keys for each message) ensures that past communications cannot be decrypted if one of the keys generated in an iteration of step 2 is compromised, since such a key is only used to encrypt a single message. Forward secrecy also ensures that past communications cannot be decrypted if the long-term private keys from step 1 are compromised. However, masquerading as Alice or Bob would be possible going forward if this occurred, possibly compromising all future messages.
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.
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), cipher suites based on Diffie–Hellman key exchange (DHE-RSA, DHE-DSA) and elliptic curve Diffie–Hellman key exchange (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. TLS 1.3 leaves ephemeral Diffie–Hellman as the only key exchange mechanism to provide forward secrecy.
On the other hand, among popular protocols currently in use, WPA does not support forward secrecy.
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 and are requiring the use of forward secrecy since August 2018.
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. TLS 1.3, published in August 2018, dropped support for ciphers without forward secrecy. As of February 2019[update], 96.6% of web servers surveyed support some form of forward secrecy, and 52.1% will use forward secrecy with most browsers.
At WWDC 2016, Apple announced that all iOS apps would need to use App Transport Security (ATS), 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 January 1, 2017.
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