Opportunistic Security: Some Protection Most of the Time
RFC 7435

Document Type RFC - Informational (December 2014; No errata)
Was draft-dukhovni-opportunistic-security (individual in sec area)
Last updated 2015-10-14
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Internet Engineering Task Force (IETF)                       V. Dukhovni
Request for Comments: 7435                                     Two Sigma
Category: Informational                                    December 2014
ISSN: 2070-1721

        Opportunistic Security: Some Protection Most of the Time

Abstract

   This document defines the concept "Opportunistic Security" in the
   context of communications protocols.  Protocol designs based on
   Opportunistic Security use encryption even when authentication is not
   available, and use authentication when possible, thereby removing
   barriers to the widespread use of encryption on the Internet.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7435.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Dukhovni                      Informational                     [Page 1]
RFC 7435                 Opportunistic Security            December 2014

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   2
     1.2.  A New Perspective . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Opportunistic Security Design Principles  . . . . . . . . . .   5
   4.  Example: Opportunistic TLS in SMTP  . . . . . . . . . . . . .   8
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

1.1.  Background

   Historically, Internet security protocols have emphasized
   comprehensive "all or nothing" cryptographic protection against both
   passive and active attacks.  With each peer, such a protocol achieves
   either full protection or else total failure to communicate (hard
   fail).  As a result, operators often disable these security protocols
   when users have difficulty connecting, thereby degrading all
   communications to cleartext transmission.

   Protection against active attacks requires authentication.  The
   ability to authenticate any potential peer on the Internet requires
   an authentication mechanism that encompasses all such peers.  No IETF
   standard for authentication scales as needed and has been deployed
   widely enough to meet this requirement.

   The Public Key Infrastructure (PKI) model employed by browsers to
   authenticate web servers (often called the "Web PKI") imposes cost
   and management burdens that have limited its use.  With so many
   Certification Authorities (CAs), not all of which everyone is willing
   to trust, the communicating parties don't always agree on a mutually
   trusted CA.  Without a mutually trusted CA, authentication fails,
   leading to communications failure in protocols that mandate
   authentication.  These issues are compounded by operational
   difficulties.  For example, a common problem is for site operators to
   forget to perform timely renewal of expiring certificates.  In Web
   PKI interactive applications, security warnings are all too frequent,
   and end users learn to actively ignore security problems, or site
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