Network Working Group V. Dukhovni
Internet-Draft Two Sigma
Intended status: Informational July 7, 2014
Expires: January 8, 2015

Opportunistic Security: some protection most of the time


This memo defines the term "opportunistic security". In contrast to the established approach of delivering strong protection some of the time, opportunistic security strives to deliver at least some protection most of the time. The primary goal is therefore broad interoperability, with security policy tailored to the capabilities of peer systems.

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Table of Contents

1. Introduction

Historically, Internet security protocols have prioritized strong protection for peers capable and motivated to absorb the associated costs. Since strong protection is not universally applicable, while communications traffic was sometimes strongly secured, more typically it was not protected at all. The fact that most traffic is unprotected facilitates nation-state pervasive monitoring (PM [RFC7258]) by making it cost-effective (or at least not cost-prohibitive). Indiscriminate collection of communications traffic would be substantially less attractive if security protocols were designed to operate at a range of protection levels with encrypted transmission accessible to most if not all peers, and stronger security still available where required by policy or opportunistically negotiated.

Encryption is easy, but key management is difficult. Key management at Internet scale remains an incompletely solved problem. The PKIX ([RFC5280]) key management model introduces costs that not all peers are willing to bear and is also not sufficient to secure communications when the peer reference identity is obtained indirectly over an insecure channel or communicating parties don't agree on a mutually trusted certification authority (CA). DNSSEC is not at this time sufficiently widely adopted to make DANE a viable alternative at scale. Trust on first use (TOFU) key management models (as with saved SSH fingerprints and various certificate pinning approaches) don't protect initial contact and require user intervention when key continuity fails.

Without Internet-scale key management, authentication is often not possible. When protocols only offer the options of strongly-authenticated secure channels or else no security, most traffic gets no security protection. Therefore, in order to make encryption more ubiquitous, authentication needs to be optional. When strongly authenticated communication is not possible, unauthenticated encryption is still substantially stronger than cleartext. Opportunistic security encourages peers to employ as much security as possible, without falling back to unnecessarily weak options. In particular, opportunistic security encourages unauthenticated encryption when authentication is not an option.

2. Opportunistic Security Design Philosophy

Interoperate to maximize deployment:
The primary goal of designs that feature opportunistic security is to be able to communicate with any reasonably configured peer. If many peers are only capable of cleartext, then it is acceptable to fall back to cleartext when encryption is not possible. If authentication is only possible for some peers, then it is acceptable to authenticate only those peers and not the rest. Interoperability must be possible without bilateral coordination. Applications employing opportunistic security need to be deployable at Internet scale, with each peer independently configured to meet its own security needs (within the practical bounds of the application protocol). Opportunistic security must not get in the way of the peers communicating if neither end is misconfigured.
Maximize security peer by peer:
Subject to the above large-scale interoperability goal, opportunistic security strives to maximize security based on the capabilities of the peer (or peers). For some opportunistic security protocols the maximal protection possible may be just unauthenticated encryption. For others, greater security may be an option, and opportunistic security may at times (in partial conflict with the interoperability goal) refuse to continue with peers where higher security is expected, but for some reason not achieved. The conditions under which connections fail should generally be limited to operational errors at one or the other peer or an active attack, so that well-maintained systems rarely encounter problems in normal use of opportunistic security.
Encrypt by default:
An opportunistic security protocol MUST interoperably achieve at least unauthenticated encryption between peer systems that don't explicitly disable this capability. Over time, as peer software is updated to support opportunistic security, only legacy systems or a minority of systems where encryption is disabled should be communicating in cleartext. Whenever possible, opportunistic security should employ Perfect Forward Secrecy (PFS) to make recovery of previously sent keys and plaintext computationally expensive even after disclosure of long-term keys.
No misrepresentation of security:
Unauthenticated communication or use of authentication that is vulnerable to MiTM attacks is not represented as strong security. Where strong security is required, opportunistic security is not a substitute, though the underlying mechanisms may in some cases be very similar.

In summary, opportunistic security is an umbrella term that encompasses protocol designs that remove barriers to the widespread use of encryption in the Internet. The actual protection provided by opportunistic security depends on the capabilities of the communicating peers; opportunistic security MUST attempt to at least encrypt network traffic, while allowing fallback to cleartext with peers that do not appear to be encryption capable.

It is important to note that opportunistic security is not limited to unauthenticated encryption. When possible, opportunistic security SHOULD provide stronger security on a peer-by-peer basis. For example, some peers may be authenticated via DANE, TOFU or other means. Though authentication failure MAY be a reason to abort a connection to a peer that is expected to be authenticated, it MUST NOT instead lead to communication in cleartext when encryption is an option. Some sending MTAs employing STARTTLS have been observed to abort TLS transmission when the receiving MTA fails authentication, only to immediately deliver the same message over a cleartext connection. This design blunder MUST be avoided.

3. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

The following definitions are derived from the Internet Security Glossary [RFC4949], where applicable.

Perfect Forward Secrecy (PFS):
For a key management protocol, the property that compromise of long-term keys does not compromise session/traffic/content keys that are derived from or distributed using the long-term keys.
Man-in-the-Middle attack (MiTM):
A form of active wiretapping attack in which an attacker intercepts and may selectively modify communicated data to masquerade as one of the entities involved in a communication. Masquerading enables the MiTM to violate the confidentiality and/or the integrity of communicated data passing through it.
Trust on First Use (TOFU):
In a protocol, TOFU typically consists of accepting an asserted identity, without authenticating that assertion, and caching a key or credential associated with the identity. Subsequent communication using the cached key/credential is secure against a MiTM attack, if such an attack did not succeed during the (vulnerable) initial communication or if the MiTM is not present for all subsequent communications. The SSH protocol makes use of TOFU. The phrase "leap of faith" (LoF) is sometimes used as a synonym.
Unauthenticated Encryption:
Encryption using a key management technique that enables unauthenticated communication between parties. The communication may be 1-way or 2-way unauthenticated. If 1-way, the initiator (client) or the target (server) may be anonymous.

4. Security Considerations

Though opportunistic security potentially supports transmission in cleartext, unauthenticated encryption, or other protection levels short of the strongest potentially applicable, the effective security for users is increased, not reduced. Provided strong security is not required by policy or securely negotiated, nothing is lost by allowing weaker protection levels, indeed opportunistic security is strictly stronger than the alternative of providing no security services when maximal security is not applicable.

5. Acknowledgements

I would like to thank Steve Kent. Some of the text in this document is based on his earlier draft.

6. References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC 4949, August 2007.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R. and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, May 2014.

Author's Address

Viktor Dukhovni Two Sigma EMail: