Forward secrecy

In cryptography, forward secrecy (FS), also known as perfect forward secrecy (PFS), is a property of secure communication protocols in which compromises of long-term keys do 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, for example via a man-in-the-middle attack.

The following is a hypothetical example of a simple instant messaging protocol that employs forward secrecy:

1. 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.
2. 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.
3. Alice sends Bob a message, encrypting it with a symmetric cipher using the session key negotiated in step 2.
4. Bob decrypts Alice's message using the key negotiated in step 2.
5. 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 (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), 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.^[8]

OpenSSL supports forward secrecy using elliptic curve Diffie–Hellman since version 1.0,^[9] with a computational overhead of approximately 15%.^[10]

The Signal Protocol uses the Double Ratchet Algorithm to provide forward secrecy.^[11]

On the other hand, among popular protocols currently in use, WPA doesn't 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.^[9] Since November 2013, Twitter provided forward secrecy with TLS to its users.^[12] Wikis hosted by the Wikimedia Foundation have all provided forward secrecy to users since July 2014.^[13]

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.^[14] As of June 2016, 51.9% of TLS-enabled websites are configured to use cipher suites that provide forward secrecy to modern web browsers.^[15]

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.^[16]ATS became mandatory for apps on Jan 1st, 2017.^[17]

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