TLS                                                            R. Segers
Internet-Draft                           Federal Aviation Administration
Intended status: Informational                                 A. Kopman
Expires: 27 November 2022                                Concepts Beyond
                                                             26 May 2022

               Use Case Validation Request TLS Extension


   This document describes a civil aviation, air-to-ground communication
   use case for the Path Validation extension to Transport Layer
   Security (TLS) and Datagram Transport Layer Security (DTLS) using the
   Server-based Certificate Validation Protocol (SCVP).

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   This Internet-Draft will expire on 27 November 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terms and Definitions . . . . . . . . . . . . . . . . . . . .   3
   3.  Use Case Discussion . . . . . . . . . . . . . . . . . . . . .   4
   4.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   In a digital aviation environment, there is a need for
   interoperability and cyber resilience.  Establishing trusted
   communications presents additional challenges when it is integrated
   at a global level.

   The International Civil Aviation Organisation (ICAO) Work Group I
   (WG-I) of the Communication Panel has standardized the use of DTLS
   for end to end information security protection between the aircraft
   and the ground user system.

   The ICAO Trust Framework Study Group (TFSG) has worked to develop
   policy and guidance material for a global and interoperable
   International Aviation Trust Framework (IATF) that will enable
   trusted ground-ground, air-ground, and air-air exchange of
   information.  This trust framework is based on a Public Key
   Infrastructure (PKI) with a cross-certified Certificate Authority
   (CA) hierarchy and is designed to map commercial aviation identity
   and access requirements to a common set of operating rules.  These
   rules are governed by the trust framework Certificate Policy (CP).

   The Federal Aviation Administration (FAA) and European Organisation
   for the Safety of Air Navigation (EUROCONTROL) collaborated under
   Coordination Plan 1.8 between FAA Next Generation Air Transportation
   System (NextGen) and Single European Sky ATM Research (SESAR)
   Deployment Manager (SDM) to prototype a solution for secure ground-
   to-ground communications based on this trust framework.  This
   prototype demonstrated the viability of leveraging the SCVP for
   identity validation.  The SCVP offloads the complexity of certificate
   path discovery and validation from the client, which is establishing
   trust to a common server.  This reduces the computational load and
   complexity of the client.  It also ensures that policies are
   consistently applied across all clients by moving the policy
   validation to a common server.

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   SCVP has been used for validating ground-to-ground communications and
   can be leveraged for air-to-ground communications.  The additional
   challenges in air-to-ground communications including limited
   bandwidth and higher latency must be considered in development of a
   solution.  These limitations can be addressed by enabling validation
   without requiring the aircraft to have direct communication with the
   SCVP responder.  This paper proposes offloading the SCVP request to
   the ground-based server and providing the aircraft with the outcome
   of this interaction.  A use case presentation [path_val_use_case] of
   the proposed path validation extension to TLS as presented to the
   ICAO Internet Protocol Suite (IPS) Security Subgroup is available.

2.  Terms and Definitions

   To encourage comprehension necessary for adoption of the TLS
   Extension for Path Validation by the intended user community, the
   civil aviation community's norms are respected herein.  The terms
   listed below are from that community.

      An Air Navigation Service Provider (ANSP) is an organization that
      provides the service of managing the aircraft in flight or on the
      maneuvering area of an and which is the legitimate holder of that

      European Organisation for the Safety of Air Navigation.
      EUROCONTROL is pan-European, civil-military organisation dedicated
      to supporting European aviation.

      International Aviation Trust Framework.  ICAO effort to develop an
      organizational entity managing a resilient and secure by design
      operational framework for digital identity management and
      information exchange in support of all aspects of aviation.

      International Civil Aviation Organization.  A specialized agency
      of the United Nations that develops and harmonizes international
      standards relating to aviation.

      Next Generation Air Transportation System.  FAA program area to
      implement advanced technologies and capabilities that improve the
      operation of the National Airspace System (NAS).

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      Single European Sky ATM Research.  SESAR defines, develops and
      deploys technologies to transform air traffic management in

      The Trust Framework Study Group.  An ICAO Study Group assisting in
      the development of a globally harmonized trust framework for the
      exchange of information in a digitally connected aviation
      environment to enable trusted ground-ground, air-ground and air-
      air exchange of information.

3.  Use Case Discussion

   A significant step in establishing security in air-to-ground
   communications is for the aircraft to validate the ground server's
   identity.  Protocols, such as DTLS, use certificates to exchange
   identity information.  Validation of trust in the ground server's
   identity is done by constructing and validating a trust chain from
   the ground server's end-entity certificate to a root of trust.  This
   can be done in two ways.  Implementation of certificate path
   construction and validation can be done onboard the aircraft.  This
   approach requires loading each aircraft with the trust anchors in use
   by all ground servers communicating with the aircraft worldwide and
   any intermediated trust anchors between the ground server certificate
   and the root.  Validation of identity onboard the aircraft also
   requires regularly uploading revocation lists for validating the
   certificates.  These steps place a significant burden on the
   aircraft.  Alternatively, the responsibility of certificate path
   construction and validation can be delegated to a trusted SCVP
   responder.  In this case, the aircraft only requires awareness of a
   single trust anchor to verify the SCVP response is signed by a
   trusted SCVP responder.

   The resource limitations of air-to-ground communications necessitates
   consideration of bandwidth usage and number of round-trips.  To
   address these limitations, the overhead of the SCVP request can be
   performed by the ground-based server.  The outcome of the SCVP
   request can be provided to the aircraft.

   The TLS and DLTS extension framework defines an extension for clients
   to request the revocation status of the server's certificate for
   Section 3 of TLS 1.2 [RFC6066] and TLS 1.3 [RFC8446].  This status
   request extension offloads the gathering of certificate revocation
   information from a TLS/DTLS client to a TLS/DTLS server.  This
   technique is widely used to provide Online Certificate Status
   Protocol (OCSP) responses to the client, but it has some limitations.
   OCSP responses and certificates are needed for each step in the trust

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   chain.  This can result in a very large data exchange.  OCSP only
   provides revocation information, trust must still be determined by
   the client.

   SCVP, in contrast, can provide a single response for the full path
   building and validation.  Additionally, the SCVP response does not
   require the full details of the path in the response.  Therefore,
   SCVP can offload more processing from the client and provide the
   outcome in a much smaller response than OCSP.  A new TLS/DTLS
   extension should be defined for SCVP validation request which can
   leverage the concepts behind the OCSP status request extension.

   In air-to-ground communications, the aircraft initiates a DTLS
   connection with a ground-based server.  The aircraft will optionally
   include a SCVP validation request extension in the Client Hello
   message.  The extension data will contain an PathValidationRequest
   consisting of an optional list of SCVP Responder URIs, an optional
   list of trust anchors, and an optional list of SCVP settings used to
   convey client preferences.

   On receipt of a Client Hello with a validation request extension the
   ground server will process the request.  If the ground server has a
   cached response matching the aircraft's settings it will use the
   cached response.  If the ground server does not have an appropriate
   response cached, it will process the PathValidationRequest and create
   an SCVP Validation Request [RFC5055] for validation of the ground
   server's TLS certificate.  If the aircraft specified SCVP responder
   URI(s) in its request, the ground server will send the SCVP request
   to the first reachable SCVP responder in the list.  If the aircraft
   specified a trust anchor, the ground server will include the trust
   anchor in the SCVP request Validation Policy and send the request to
   its pre-configured SCVP responder.  In both cases, the ground server
   sends the aircraft a PathValdiationResponse containing the signed
   SCVP response following the DTLS Certificate handshake message.  The
   aircraft validates that the included SCVP response has been signed by
   a trusted SCVP responder.

   In international aviation there is the potential to stand up many CAs
   and many SCVP responders.  These SCVP responders can be at different
   levels in the trust framework hierarchy.  For example, the responders
   can be at an airline, an Air Navigation Service Provider (ANSP), a
   region, or at an international (IATF) level.  The aircraft can
   include a list of responders that it trusts, but at a minimum should
   include the IATF SCVP responder.  By setting the IATF SCVP responder
   as a neutral fall-back, which will be reachable by any member of the
   trust framework, it is possible to establish trust from anywhere in
   the world.

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   The use of short-lived certificates has been discussed within the
   international aviation community to ease the burden of certificate
   revocation checking onboard the aircraft.  The intent is to use
   short-lived end-entity certificates by ground-based servers so that
   Certificate Revocation Lists (CRLs) are not needed onboard the
   aircraft.  However, this does not address establishing trust.  A
   certificate path from the ground server's end-entity certificate to
   an aircraft trust anchor must be constructed and validated.
   Maintaining or dynamically retrieving all of the intermediate
   certificates and their revocation information for global aviation
   communications onboard an aircraft is impractical.  Additionally,
   path validation not only checks revocation status, but also ensures
   that the certificate is fit for purpose and conforms to profile
   policies.  Whether SCVP is utilized to validate long-lived or short-
   lived certificates, the security posture and maintenance strategy are
   the same from the aircraft perspective.  This is because the SCVP
   responder performs the revocation checking as one part of the
   certificate validation process.

4.  Conclusion

   SCVP should be used to enable validation of certificates in a complex
   international aviation ecosystem.  The viability of this approach has
   been demonstrated in a ground-to-ground Proof-Of-Concept.  Short-
   lived certificates are not a viable alternative to SCVP, as even they
   need chain validation.

   The SCVP validation request extension is proposed as a mechanism to
   bring the benefits of SCVP into air-to-ground communications.  It
   will reduce the overhead for the aircraft by offloading the complex
   path validation to a server and eliminate the need for CRL downloads
   by the aircraft.  Also, by including the SCVP response with the
   ground-based server's certificate the overhead of the SCVP request is
   performed by the ground system.

5.  IANA Considerations

   IANA considerations for the Path Validation TLS extensions are
   covered in draft extension

6.  Security Considerations

   Security considerations for the Path Validation TLS extensions are
   covered in draft extension

7.  References

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7.1.  Normative References

              Segers, R. and A. Kopman, "Transport Layer Security (TLS)
              Extension: Validation Request", May 2022,

   [RFC5055]  Freeman, T., Housley, R., Malpani, A., Cooper, D., and W.
              Polk, "Server-Based Certificate Validation Protocol
              (SCVP)", RFC 5055, DOI 10.17487/RFC5055, December 2007,

7.2.  Informative References

              Segers, R. and A. Kopman, "SCVP Validation with TLS/DTLS
              in an air to ground communication", May 2022,

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

Authors' Addresses

   Robert Segers
   Federal Aviation Administration
   800 Independence Ave. SW
   Washington, DC 20591
   United States of America

   Ashley Kopman
   Concepts Beyond
   1155 F St NW
   Washington, DC 20004
   United States of America

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