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Versions: 00                                                            
Network Working Group                                     R. Atkinson,
Internet Draft                                            Editor
                                                          2 June 1997


                   Guidelines for Writing RFC Text on
                        Security Considerations


Status of this Memo

   This is an Internet Draft.  Internet Drafts are publications of the
   IETF, its working groups, and other non-IETF groups.  This document
   was created as output from the IAB Security Workshop held in early
   1997.  It is intended that the IESG or IAB would publish a revised
   version of this document as an RFC in future.

   Distribution of this memo is unlimited.


1. INTRODUCTION


           This memo provides guidelines on how to write a useful "Security
   Considerations" section for an RFC.  It is intended to provide information
   for RFC Authors and Editors in the hope of making their job easier.

           Participants at the recent IAB Security Workshop came to consensus
   that any new protocol must not worsen the overall security of The Internet.
   The act of writing the "Security Considerations" section for a new protocol
   or technology should cause the editor of that document to reflect on the
   security issues and clearly document those issues.

           A future version of this draft might be published as a Best Current
   Practice (BCP) RFC by the IESG or Informational RFC by the IAB.


2. OBJECTIVES


           The "Security Considerations" section of an RFC is a
   mechanism via which the authors/editors of an RFC communicate to the
   reader the security issues (including threats) of the RFC topic and
   discuss mechanisms for mitigating or eliminating those threats.  Each
   risk reduction mechanism not documented directly in that RFC should
   cite another RFC or document which describes the risk reduction
   mechanism.




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           The "Security Considerations" should be readable and focused
   on the matters discussed in that particular RFC.

           After reading the "Security Considerations" of an RFC, the
   reader should have a clear understanding of the threats, methods for
   mitigating those threats, and the residual risks of deploying or
   using the procedures, technology, or protocol described in that RFC.
   The reader should be able to use the citations to further investigate
   the potential risk reduction mechanisms.

           [outline for the Security Section needed here]

   Cyfi

3. DEFINITIONS



   Access Control
           The security service that protects against unauthorised
           use of system resources.

   Active Attack
           An attack that is not a "passive attack".

   Applicability Statement
           A formal statement of the operational environment in which
           a particular procedure, protocol, or technology is
           reasonable to use.  This statement should also clearly
           indicate which operational environments are unreasonable
           or inappropriate for that procedure, protocol, or
           technology to be used.

   Attack
           A threat action that results from an intelligent threat.
           An attack is an intentional act involving a means or
           method of exploiting a vulnerability.  Attacks might be
           either passive or active.

   Attacker
           A malicious or hostile adversary with the motivation and
           means to carry out an attack.

   Authentication
           The security service that verifies an identity claimed
           by an entity.  Note that in some situations, a particular
           mechanism that provides authentication might also provide
           integrity as an intrinsic by-product.  Despite this,



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           authentication and integrity are conceptually separate
           services.

   Availability
           The service ensuring that the network is accessible and
           usable upon demand by an authorised entity.

   Confidentiality
           The security service that protects data from disclosure
           to unauthorised entities.

   Countermeasure
           Something that reduces a threat or vulnerability by
           eliminating or preventing it, by minimising the harm
           it can cause, or by discovering and reporting it so
           that corrective action can be taken. For example,
           the vulnerability of a Cyclic Redundancy Check (CRC)
           can be reduced by suitable cryptographic techniques.

   Critical
           A communications service is critical if its denial would
           jeopardize its user's ability to perform a primary mission
           function.

   Denial of Service
           A kind of attack where the adversary seeks to deny
           a legitimate user access to some resource or service.
           For example, disrupting routing could cause packets
           to be lost or misdelivered, thus denying use of the
           network to legitimate users.

   Gateway
           Usually a synonym for "router".

   Hosts
           Computers that run full Internet protocol stacks and
           support application protocols.  Hosts range from small
           personal computers to large supercomputers.  In some
           communities these are considered 'end systems'.

   Infrastructure
           The Internet infrastructure includes networks, relays,
           routers, and any necessary support hosts (e.g. those
           hosts that provide DNS service).

   Integrity
           The security service that protects data from unauthorised
           alteration or destruction.



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   Non-repudiation
           The service that protects against false denial of
           a communication.  For example, this service would prevent
           the sender of a signed email message from being able to
           falsely deny sending that message.

   Passive Attack
           An attack that only observes the operation of network
           elements to learn about them or observes data and
           data traffic characteristics to learn the data's
           semantic content.

   Principle of Least Privilege
           TBD

   Risk
           An expectation of loss expressed as the probability that
           a particular threat will exploit a particular vulnerability
           with a particular harmful result.

   Risk Analysis
           A systemic identification of valuable resources, threats,
           and countermeasures along with quantification of the loss
           exposures based on estimated frequencies and costs.
           [NBS79, HR91]  The analysis then lists risks
           in order of cost and criticality, thereby determining
           where countermeasures should be applied first.

   Sensitive
           Data is sensitive if its disclosure, alteration, or destruction
           would adversely affect the interests or business of its owner
           or user.  There can be different degrees of sensitivity.
           For example, compromise of some sensitive data might cause
           a death while compromise of other sensitive data might merely
           cause a brief disruption to normal operations.

   Threat
           A potential violation of security.  A threat exists when
           there is a circumstance, capability, or event that could
           breach security and cause harm.  Threats might be either
           accidental or intentional.  CERT Advisories document threats,
           though not all threats are documented in CERT Advisories.

   Vulnerability
           A flaw or weakness in a system's security.  A characteristic
           that could be exploited to cause harm by disclosing, modifying,
           or destroying functions or resources, or by denying service
           to authorised users.  Existence of a vulnerability creates



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           a threat.



4. MINIMAL THREAT ENVIRONMENT


           [This section seems mis-organised]

           This section describes the minimal threat environment
   applicable to every RFC.  Alternately put, any RFC written should
   have a Security Considerations section that assumes the following
   threats (at a minimum) exist.

           Any class of attack described in a CERT Advisory or
   equivalent is considered to be a legitimate potential threat.  CERT
   Advisories are quite predictable given knowledge of the threat.
   Hence, CERT Advisories are considered an existence proof of the
   threat, but do not constitute a threat analysis.

           Other known kinds of attacks (e.g. from published magazine
   articles, from conference papers) are also considered to be
   legitimate potential threats.  For example, many of the recently seen
   attacks on the Internet use techniques and exploit vulnerabilities
   described in the literature some years ago. [Bellovin89]


5. GUIDELINES


           There should be a clear description of the kinds of threats
   on the described protocol or technology.  This should be approached
   as an effort to perform "due diligence" in describing all known or
   foreseeable risks and threats to potential implementers and users.

           The methods via which those some or all of those threats are
   mitigated or eliminated (e.g. firewalls, packet filtering,
   encryption, cryptographic authentication) should be described along
   with an indication of the extent to which the particular method
   mitigates the risk.  If external mechanisms (e.g. IPsec) are
   identified for risk reduction, the relevant RFCs or other documents
   should be cited so the reader knows where to obtain more information.

           The threat environment addressed by the Security
   Considerations section MUST at a minimum include deployment across
   the global Internet across multiple administrative boundaries without
   assuming that firewalls are in place.  It is not acceptable to only
   discuss threats applicable to LANs and ignore the broader threat



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   environment.  All IETF standards-track protocols are considered
   likely to have deployment in the global Internet.  In some cases,
   there might be an Applicability Statement discouraging use of a
   technology or protocol in a particular environment.  Nonetheless, the
   security issues of broader deployment should be discussed in the
   document.

           There should be a clear description of the residual risk to
   the user or operator of that protocol after threat mitigation has
   been deployed.  Such risks might arise from compromise in a related
   protocol (e.g. IPsec is useless if key management has been
   compromised), from incorrect implementation, compromise of the
   security technology used for risk reduction (e.g. 40-bit DES), or
   might be risks that are not addressed by the protocol specification
   (e.g. denial of service attacks on an underlying link protocol).

           There should also be some discussion of potential security
   risks arising from obvious potential misapplications of the protocol
   or technology described in the RFC.  This might be coupled with an
   Applicability Statement for that RFC.


6. EXAMPLES


     An RFC discussing TCP should mention the SYN flooding denial of
   service attacks and possible implementation strategies for reducing
   that risk.  [CERT96]

     An RFC discussing HTTP should discuss the potential for
   eavesdroppers to obtain credit card or other personal data when
   security techniques are not in use.  Such an RFC should also
   recommend use of appropriate security techniques (e.g. SET, SSL,
   SHTTP) to mitigate that threat.  [SHTTP,SSL,SET]

     An RFC discussing a security protocol might discuss common
   implementation flaws so that implementers know how to avoid those.
   [Atk95]


7. DESIGN SUGGESTIONS

           When a protocol uses cryptography to provide some security
   service, the protocol should be designed in a manner independent of
   any particular cryptographic algorithm.  This permits future
   substitution of a new cryptographic algorithm for the originally
   specified cryptographic algorithm if the original is broken.  This
   property is often referred to as "algorithm-independence".



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           Also, when a protocol relies on the randomness of some
   number, it should clearly indicate what level of randomness is
   required.  If cryptographic randomness is required, it would be
   reasonable to help implementers by citing a reference or two (e.g.
   ECS94) on how to obtain such randomness.

REFERENCES


   [Bellovin89] Stephen M. Bellovin, "Security Problems in the TCP/IP Protocol
           Suite", ACM Computer Communications Review, Vol. 19, No. 2, ACM,
           New York, NY, March 1989.

   [CERT96] US DoD Computer Emergency Response Team, "TCP SYN Flooding Attacks",
           CERT Advisory CA-96.21, 19 September 1996.  ftp://info.cert.org/pub/
           cert_advisories/CA-96.21.tcp_syn_flooding

   [NBS79] US National Bureau of Standards, "Guideline for Automatic Data
           Processing and Risk Analysis", Federal Information Processing
           Standard (FIPS) 65, National Bureau of Standards, Gaithersburg,
           MD, USA, 1 August 1979.

   [SSL]
   [SHTTP]
   [SET]

   [SK]    Robert W. Shirey & Stephen T. Kent, "Security Principles for
           Internet Architecture".

   [HR91]  P. Holbrook, J. Reynolds, "Site Security Handbook",
           RFC-1244, 23 July 1991.

   [ECS94] D. Eastlake, S. Crocker, J. Schiller, "Randomness
           Recommendations for Security", RFC-1750, 29 December 1994.

   [Atk95] R. Atkinson, "IP Security Architecture", RFC-1825,
           July 1995.














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ACKNOWLEDGEMENTS


           This note was written after the IAB Security Workshop held in
   early 1997.  Everyone at that workshop has contributed to this
   document, either via email or in the discussions at that workshop.
   Some of the specific text above is taken from an email message
   written by Fred Baker.  Virtually all of the definitions in Section 3
   are excerpted with permission from a document "Security Principles
   for Internet Architecture" by Robert W. Shirey and Stephen T. Kent
   [SK].

           Any errors are the responsibility of the editor.

Editor's Addresses:

   Randall Atkinson        <rja@home.net>

   @Home Network
   385 Ravendale Drive
   Mountain View, CA 94043

   Voice: +1 (415) 944-7200




























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