Separating Crypto Negotiation and Communication
draft-kuehlewind-taps-crypto-sep-00

Network Working Group                                      M. Kuehlewind
Internet-Draft                                                ETH Zurich
Intended status: Informational                                  T. Pauly
Expires: January 4, 2018                                         C. Wood
                                                              Apple Inc.
                                                           July 03, 2017

            Separating Crypto Negotiation and Communication
                  draft-kuehlewind-taps-crypto-sep-00

Abstract

   Due to the latency involved in connection setup and security
   handshakes, there is an increasing deployment of cryptographic
   session resumption mechanisms.  While cryptographic context and
   endpoint capabilities need to be be known before encrypted
   application data can be sent, there is otherwise no technical
   constraint that the crypto handshake must be performed on the same
   transport connection.  This document recommends a logical separation
   between the mechanism(s) used to negotiate capabilities and set up
   encryption context (handshake protocol), the application of
   encryption and authentication state to data (record protocol), and
   the associated transport connection(s).

Status of This Memo

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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Protocol Interfaces . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Handshake-Transport Interface . . . . . . . . . . . . . .   5
     3.2.  Handshake-Record Interface  . . . . . . . . . . . . . . .   6
     3.3.  Transport-Record Interface  . . . . . . . . . . . . . . .   6
   4.  Existing Mappings . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Benefits of Separation  . . . . . . . . . . . . . . . . . . .   8
     5.1.  Reducing Connection Latency . . . . . . . . . . . . . . .   9
     5.2.  Protocol Flexibility  . . . . . . . . . . . . . . . . . .   9
     5.3.  Protocol Capability Negotiation . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Secure transport protocols are generally composed of three pieces:

   1.  A transport protocol to control the transfer of data.

   2.  A record protocol to frame, encrypt and/or authenticate data

   3.  A handshake protocol to negotiate cryptographic secrets.

   For ease of deployment and standardization, among other reasons,
   these constituents are often tightly coupled.  For example, in TLS
   [RFC5246], the handshake protocol depends on the record protocol, and
   vice versa.  However, more recent transport protocols such as QUIC
   [I-D.ietf-quic-tls] keep these pieces separate.  QUIC uses TLS to
   negotiate secrets, and _exports_ those secrets to encrypt packets
   directly.

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   Separating these pieces is important, as new secure transport
   protocols increasingly rely on session resumption mechanisms where
   cryptographic context can be resumed to transmit application data
   with the first packet without delay for connection setup and
   negotiation.  In the case where there is no cryptographic context
   available when an application expresses the need to transmit data to
   a certain endpoint, it must first run the handshake protocol on a
   transport connection before being able to transmit application data.
   If the handshake protocol can be separated from the other components,
   then it can use another transport connection to establish secrets
   without blocking the application's main transport connection.  This
   also opens up the possibility to run the handshake protocol well in
   advance of the need to send application data, to avoid unnecessary
   delays.  For example, a client system could maintain a database of
   endpoints it is likely to communicate with, and establish keying
   material with a handshake protocol at periodic intervals to ensure
   fresh keys for new transport connections.

   [I-D.moskowitz-sse] proposes a similar approach.  However while
   [I-D.moskowitz-sse] proposes a new protocol to negotiate and maintain
   long-term cryptographic sessions, this document relies on the use of
   existing protocols and only discusses requirements for the evolution
   of these protocols and exchange of information within one endpoint
   locally.

2.  Terminology

   o  Transport Protocol: A protocol that can transport messages between
      two endpoints.  This may represent the service offered to
      applications to allow them to send and receive data before
      encryption; and also represent the protocol that can transmit
      handshake data and encrypted records.

   o  Handshake Protocol: A protocol that can validate and authenticate
      endpoints, encrypt and authenticate its negotiation, and
      ultimately generate keying material.

   o  Record Protocol: A protocol that can use keying material to
      transform messages.  A record will generally add a frame around
      application data, and authenticate and/or encrypt the data.

   o  Keying Material: One or more pre-shared keys that can be used to
      encrypt and authenticate data, generated by a handshake protocol
      and used by a record protocol.

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3.  Protocol Interfaces

   In traditional models in which the protocols are not separated out
   into the three elements of handshake, record, and transport
   protocols, there are two basic approaches to the interactions:

   1.  The transport protocol provides data to the security protocol and
       gets back an encrypted version of the data to be sent (handshake
       and record protocols are combined)

   2.  The security protocol provides keying material to the transport
       protocol, and the transport protocol is responsible for
       encrypting data (transport and record protocols are combined)

   By teasing apart all three portions as separate protocols, there end
   up being six interface points:

   Application Data
        |    ^
        |    |
   +----V----+-----+      (1)       +---------------+
   |               +---------------->               |
   |   Transport   |                |   Handshake   |
   |               <----------------+               |
   +-+-----^-------+      (2)       +-----+-----^---+
     |     |                              |     |
     |     |(6)                        (3)|     |
     |     |                              |     |(4)
     |     |        +---------------+     |     |
     |     +--------+               <-----+     |
     |(5)           |    Record     |           |
     +-------------->               +-----------+
                    +---------------+

      Figure 1: Secure Transport Protocol Components and Interactions

   1.  A transport protocol depends upon a handshake protocol to
       establish keying material to protect application data being sent
       through the transport.  The main interface it relies upon is
       starting the handshake, or ensuring that the material is ready.

   2.  A handshake protocol depends upon a transport protocol in order
       to send and receive negotiation messages with the remote peer.

   3.  A handshake protocol sends its keying material and cryptographic
       context to the record protocol to use

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   4.  A record protocol may signal state expiration events to a
       handshake protocol

   5.  A transport protocol uses a record protocol to send and receive
       application data

   6.  A record protocol uses a transport protocol to send and receive
       encrypted data

3.1.  Handshake-Transport Interface

   Note that for the purposes of this interface description, it is
   assumed that the application is primarily interacting with the
   transport protocol, and thus the handshake protocol interacts with
   the application primarily through the abstraction of the transport
   protocol.

   o  Start negotiation: The interface MUST provide an indication to
      start the protocol handshake for key negotiation, and have a way
      to be notified when the handshake is complete.

   o  Identity constraints: The interface MUST allow the application to
      constrain the identities that it will accept a connection to, such
      as the hostname it expects to be provided in certificate SAN.

   o  Local identities: The interface MUST allow the local identity to
      be set via a raw private key or interface to one to perform
      cryptographic operations such as signing and decryption.

   o  State changes: The interface SHOULD provide a way for the
      transport to be notified of important state changes during the
      protocol execution and session lifetime, e.g., when the handshake
      begins, ends, or when a key update occurs.

   o  Validation: The interface MUST provide a way for the application
      to participate in the endpoint authentication and validation,
      which can either be specified as parameters to define how the
      peer's authentication can be validated, or when the protocol
      provides the authentication information for the application to
      inspect directly.

   o  Caching domain and lifetime: The application SHOULD be able to
      specify the instances of the protocol that can share cached keys,
      as well as the lifetime of cached resources.

   o  The protocol SHOULD allow applications to negotiate application
      protocols and related information.

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   o  The protocol SHOULD allow applications to specify negotiable
      cryptographic algorithm suites.

   o  The protocol SHOULD expose the peer's identity information.

3.2.  Handshake-Record Interface

   o  Key export: The interface MUST provide a way to export keying
      material from a handshake protocol to a record protocol with well-
      defined cryptographic properties, e.g., "forward-secure" or
      "perfectly forward secure"

   o  Key lifetime and rotation: The interface MUST provide a way for
      the handshake protocol to define key lifetime bounds in terms of
      _time_ or _bytes encrypted_ and, additionally, provide a way to
      forcefully update cryptographic session keys at will.  The record
      protocol MUST be able to signal back to the handshake protocol
      that a lifetime has been reached and that rotation is required.
      These values SHOULD be configurable by the application.

3.3.  Transport-Record Interface

   o  Transform data: The interface MUST provide a way to send raw
      application data from the transport protocol to a record protocol
      to transform it based on the keying material.  This data is then
      sent out by the transport protocol.  The same applies for inbound
      data, in which inbound transport data is transformed by the record
      protocol into raw application data.

   o  Reliability: The transport MUST specify if messages are
      transmitted reliable and in order.

   o  Maximum message size (optional): The transport may specify a
      maximum message size for the encrypted data if e.g. a datagram
      transport is used

4.  Existing Mappings

   In this section we document existing mappings between common
   transport security protocols and the three components described in
   Section I.

   o  TLS/DTLS: TLS [RFC5246] and DTLS [RFC6347] is a combination of a
      handshake and record protocol, with a dependency on some
      underlying transport.

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                 Application (configure and I/O)
                   |     ^
                   |     |
         +---------V-----+--------+
         |        Connection      |
         +----+----^--------------+
   +----------|----|------------------------------------+
   |          |    |       --TLS--                      |
   |     +----V----+-----+         +---------------+    |
   |     |               +--------->               |    |
   |     |   Handshake   |         |     Record    |    |
   |     |               <---------+               |    |
   |     +---------------+         +----+------^---+    |
   |                                    |      |        |
   +------------------------------------|------|--------+
                                        |      |
                                   +----V------+----+
                                   |    Transport   |
                                   +----------------+

   o  QUIC + TLS: The emerging QUIC standard is decomposed into the
      three pieces outlined in Section I [I-D.ietf-quic-tls].  TLS is
      used as the handshake protocol running on a dedicated QUIC stream,
      a QUIC-specific record protocol encrypts and encapsulates stream
      frames, and the main QUIC component handles the transport of these
      frames.

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       Application (configure and I/O)
         |     ^
   +-----|-----|------------------------------------+
   |     |     |      --QUIC--                      |
   |     |     |                                    |
   |  +--V-----+---+             +--------------+   |
   |  |    QUIC    |------------>|      TLS     |   |
   |  | (transport)|             |  (handshake) |   |
   |  |            <-------------+              |   |
   |  ++---^--+--^-+             +--^-------+---+   |
   |   |   |  |  |                  |       |       |
   |   |   |  |  |                  |       |       |
   |   |   |  |  |  +V---------+-+  |       |       |
   |   |   |  |  +-->   Packet   +--+       |       |
   |   |   |  |     | Protection |          |       |
   |   |   |  +-----+  (record)  <----------+       |
   |   |   |        +------------+                  |
   |   |   |                                        |
   +---|---|----------+-----------------------------+
       |   |
   +---V---+--------+
   |    Transport   |
   +----------------+

   o  IKEv2 + ESP: IKEv2 [RFC7296] is a handshake protocol commonly used
      to establish keys for use in IPsec (often VPN) deployments.  It is
      already a distinct protocol from its commonly paired record
      protocol, which is ESP [RFC4303].  ESP encrypts and authenticates
      IP datagrams, and sends them as datagrams over a transport
      mechanism such, e.g., IP or UDP.

           Application (configure)    Application (I/O)
             |    ^                     |    ^
        +----V----+-----+         +-----V----+----+
        |               +--------->               |
        |     IKEv2     |         |     Record    |
        |               <---------+               |
        +----+------^---+         +----+------^---+
             |      |                    |      |
        +----V------+------------------V------+----+
        |            (Unreliable) Transport        |
        +------------------------------------------+

5.  Benefits of Separation

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5.1.  Reducing Connection Latency

   One of the clearest benefits of separating the handshake protocol
   from the record protocol is that the handshake can be performed out-
   of-band from the application's data transfer.  This should
   essentially reduce the number of RTTs required before being able to
   send data by the full length of the handshake (which is commonly 1 or
   2 RTTs in the best cases for TLS 1.2 and IKEv2, potentially more if
   cookie challenges or extended authentication are required).

   To avoid long-lived transport connections that wouldn't be actively
   used, and thus would be vulnerable to timeouts on NATs or firewalls,
   an obvious approach to separating the handshake and record protocols
   is to use different transport connections for the early handshake and
   the data transfer.  However, this approach of using separate
   connections will not always save RTTs if the handshake and data
   transfer are back-to-back.  Each connection may require its own
   transport protocol handshake, and if the data transfer must wait for
   two transport protocols to establish and the cryptographic handshake
   to be finished before sending, then it may experience higher latency.
   Implementations SHOULD avoid this by either allowing the handshake
   and record protocols to share a single transport connection or open
   two connections in parallel when the handshake protocol has not pre-
   fetched keys.  Latency benefits, however, can even be achieved when
   ensuring that this scenario does not occur by always having the
   handshake protocol refresh the keys whenever old ones are near
   expiry.

5.2.  Protocol Flexibility

   Separation of the handshake, record, and transport protocols also
   allows for more flexible composition of protocols with one another.
   If a deployment uses a handshake protocol like TLS, which requires a
   stream-based transport protocol like TCP, separation of protocols
   will allow it to use the resulting keys for record protocols that run
   on datagram transport protocols like UDP.

   This flexibility may be useful for implementations that are
   optimizing for packet size by choosing minimal/lightweight record
   protocols, while being able to use commonly supported handshake
   protocols like TLS.  One example here is the approach of a VPN tunnel
   that uses ESP or Diet-ESP [I-D.mglt-ipsecme-diet-esp] to encrypt
   datagrams, but uses TLS for establishing keys.

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5.3.  Protocol Capability Negotiation

   Enabling the use of a different transport protocol for the actual
   data transmission than for the cryptographic handshakes opens also
   the possibility to negotiate protocol capabilities for the data
   transmission.  For TLS, usually TCP is the appropriate transport
   protocol to use, as it is also widely supported by endpoints.
   Allowing an endpoint to indicate the support of other, new transport
   protocols within the TCP connection that is used for the handshake,
   provides a dynamic transition path to enable easy deployment of new
   protocols.

6.  IANA Considerations

   This document has on request to IANA.

7.  Security Considerations

   (editor's note: this section will be added later.  However, this
   document discusses the use of cryptographic context for transport
   connections and as such it has security relevant consideration within
   the whole document.)

8.  Acknowledgments

   This work is partially supported by the European Commission under
   Horizon 2020 grant agreement no. 688421 Measurement and Architecture
   for a Middleboxed Internet (MAMI), and by the Swiss State Secretariat
   for Education, Research, and Innovation under contract no. 15.0268.
   This support does not imply endorsement.

9.  Informative References

   [I-D.ietf-quic-tls]
              Thomson, M. and S. Turner, "Using Transport Layer Security
              (TLS) to Secure QUIC", draft-ietf-quic-tls-04 (work in
              progress), June 2017.

   [I-D.mglt-ipsecme-diet-esp]
              Migault, D., Guggemos, T., and C. Bormann, "ESP Header
              Compression and Diet-ESP", draft-mglt-ipsecme-diet-esp-04
              (work in progress), June 2017.

   [I-D.moskowitz-sse]
              Moskowitz, R., Faynberg, I., Lu, H., Hares, S., and P.
              Giacomin, "Session Security Envelope", draft-moskowitz-
              sse-05 (work in progress), June 2017.

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   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <http://www.rfc-editor.org/info/rfc4303>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <http://www.rfc-editor.org/info/rfc7296>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <http://www.rfc-editor.org/info/rfc7301>.

Authors' Addresses

   Mirja Kuehlewind
   ETH Zurich
   Gloriastrasse 35
   8092 Zurich
   Switzerland

   Email: mirja.kuehlewind@tik.ee.ethz.ch

   Tommy Pauly
   Apple Inc.
   1 Infinite Loop
   Cupertino, California 95014
   United States of America

   Email: tpauly@apple.com

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   Christopher A. Wood
   Apple Inc.
   1 Infinite Loop
   Cupertino, California 95014
   United States of America

   Email: cawood@apple.com

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