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RFC 7918

Informational
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Transport Layer Security (TLS) False Start

 


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Internet Engineering Task Force (IETF)                        A. Langley
Request for Comments: 7918                                   N. Modadugu
Category: Informational                                       B. Moeller
ISSN: 2070-1721                                                   Google
                                                             August 2016


               Transport Layer Security (TLS) False Start

Abstract

   This document specifies an optional behavior of Transport Layer
   Security (TLS) client implementations, dubbed "False Start".  It
   affects only protocol timing, not on-the-wire protocol data, and can
   be implemented unilaterally.  A TLS False Start reduces handshake
   latency to one round trip.

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 7841.

   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/rfc7918.

Copyright Notice

   Copyright (c) 2016 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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   4
   3.  False Start Compatibility . . . . . . . . . . . . . . . . . .   4
   4.  Client-Side False Start . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
     5.1.  Symmetric Cipher  . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Protocol Version  . . . . . . . . . . . . . . . . . . . .   7
     5.3.  Key Exchange and Client Certificate Type  . . . . . . . .   7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Implementation Notes . . . . . . . . . . . . . . . .  10
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   A full handshake in TLS protocol versions up to TLS 1.2 [RFC5246]
   requires two full protocol rounds (four flights) before the handshake
   is complete and the protocol parties may begin to send application
   data.  Thus, using TLS can add a latency penalty of two network
   round-trip times for application protocols in which the client sends
   data first, such as HTTP [RFC7230].  Figure 1 (copied from [RFC5246])
   shows the message flow for a full handshake.

      Client                                               Server

      ClientHello                  -------->
                                                      ServerHello
                                                     Certificate*
                                               ServerKeyExchange*
                                              CertificateRequest*
                                   <--------      ServerHelloDone
      Certificate*
      ClientKeyExchange
      CertificateVerify*
      [ChangeCipherSpec]
      Finished                     -------->
                                               [ChangeCipherSpec]
                                   <--------             Finished
      Application Data             <------->     Application Data

                Figure 1: Message Flow for a Full Handshake

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   This document describes a technique that alleviates the latency
   burden imposed by TLS: the client-side TLS False Start.  If certain
   conditions are met, the client can start to send application data
   when the full handshake is only partially complete, namely, when the
   client has sent its own ChangeCipherSpec and Finished messages (thus
   having updated its TLS Record Protocol write state as negotiated in
   the handshake) but has yet to receive the server's ChangeCipherSpec
   and Finished messages.  (Per Section 7.4.9 of [RFC5246], after a full
   handshake, the client would have to delay sending application data
   until it has received and validated the server's Finished message.)
   Accordingly, the latency penalty for using TLS with HTTP can be kept
   at one round-trip time.

   Note that in practice, the TCP three-way handshake [RFC0793]
   typically adds one round-trip time before the client can even send
   the ClientHello.  See [RFC7413] for a latency improvement at that
   level.

   When an earlier TLS session is resumed, TLS uses an abbreviated
   handshake with only three protocol flights.  For application
   protocols in which the client sends data first, this abbreviated
   handshake adds just one round-trip time to begin with, so there is no
   need for a client-side False Start.  However, if the server sends
   application data first, the abbreviated handshake adds two round-trip
   times, and this could be reduced to just one added round-trip time by
   doing a server-side False Start.  There is little need for this in
   practice, so this document does not consider server-side False Starts
   further.

   Note also that TLS versions 1.3 [TLS13] and beyond are out of scope
   for this document.  False Start will not be needed with these newer
   versions since protocol flows minimizing the number of round trips
   have become a first-order design goal.

   In a False Start, when the client sends application data before it
   has received and verified the server's Finished message, there are
   two possible outcomes:

   o  The handshake completes successfully: The handshake is
      retroactively validated when both Finished messages have been
      received and verified.  This retroactively validates the
      handshake.  In this case, the transcript of protocol data carried
      over the transport underlying TLS will look as usual, apart from
      the different timing.

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   o  The handshake fails: If a party does not receive the other side's
      Finished message or if the Finished message's contents are not
      correct, the handshake never gets validated.  This means that an
      attacker may have removed, changed, or injected handshake
      messages.  In this case, data has been sent over the underlying
      transport that would not have been sent without the False Start.

   The latter scenario makes it necessary to restrict when a False Start
   is allowed, as described in this document.  Section 3 considers basic
   requirements for using False Start.  Section 4 specifies the behavior
   for clients, referring to important security considerations in
   Section 5.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT","RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  False Start Compatibility

   TLS False Start as described in detail in the subsequent sections, if
   implemented, is an optional feature.

   A TLS server implementation is defined to be "False Start compatible"
   if it tolerates receiving TLS records on the transport connection
   early, before the protocol has reached the state to process them.
   For successful use of client-side False Start in a TLS connection,
   the server has to be False Start compatible.  Out-of-band knowledge
   that the server is False Start compatible may be available, e.g., if
   this is mandated by specific application profile standards.  As
   discussed in Appendix A, the requirement for False Start
   compatibility generally does not pose a hindrance in practice.

4.  Client-Side False Start

   This section specifies a change to the behavior of TLS client
   implementations in full TLS handshakes.

   When the client has sent its ChangeCipherSpec and Finished messages,
   its default behavior per [RFC5246] is to not send application data
   until it has received the server's ChangeCipherSpec and Finished
   messages, which completes the handshake.  With the False Start
   protocol modification, the client MAY send application data earlier
   (under the new Cipher Spec) if each of the following conditions is
   satisfied:

   o  The application layer has requested the TLS False Start option.

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   o  The symmetric cipher defined by the cipher suite negotiated in
      this handshake has been whitelisted for use with False Start
      according to the Security Considerations in Section 5.1.

   o  The protocol version chosen by ServerHello.server_version has been
      whitelisted for use with False Start according to the Security
      Considerations in Section 5.2.

   o  The key exchange method defined by the cipher suite negotiated in
      this handshake and, if applicable, its parameters have been
      whitelisted for use with False Start according to the Security
      Considerations in Section 5.3.

   o  In the case of a handshake with client authentication, the client
      certificate type has been whitelisted for use with False Start
      according to the Security Considerations in Section 5.3.

   The rules for receiving data from the server remain unchanged.

   Note that the TLS client cannot infer the presence of an
   authenticated server until all handshake messages have been received.
   With False Start, unlike with the default handshake behavior,
   applications are able to send data before this point has been
   reached: from an application point of view, being able to send data
   does not imply that an authenticated peer is present.  Accordingly,
   it is recommended that TLS implementations allow the application
   layer to query whether the handshake has completed.

5.  Security Considerations

   In a TLS handshake, the Finished messages serve to validate the
   entire handshake.  These messages are based on a hash of the
   handshake so far processed by a Pseudorandom Function (PRF) keyed
   with the new master secret (serving as a Message Authentication Code
   (MAC)) and are also sent under the new Cipher Spec with its keyed
   MAC, where the MAC key again is derived from the master secret.  The
   protocol design relies on the assumption that any server and/or
   client authentication done during the handshake carries over to this.
   While an attacker could, for example, have changed the cipher suite
   list sent by the client to the server and thus influenced cipher
   suite selection (presumably towards a less secure choice) or could
   have made other modifications to handshake messages in transmission,
   the attacker would not be able to round off the modified handshake
   with a valid Finished message: every TLS cipher suite is presumed to
   key the PRF appropriately to ensure unforgeability.  Verifying the
   Finished messages validates the handshake and confirms that the
   handshake has not been tampered with; thus, secure encryption is
   bootstrapped from secure authentication.

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   Using False Start interferes with this approach of bootstrapping
   secure encryption from secure authentication, as application data may
   have already been sent before Finished validation confirms that the
   handshake has not been tampered with -- so there is generally no way
   to be sure that communication with the expected peer is indeed taking
   place during the False Start.  Instead, the security goal is to
   ensure that if anyone at all can decrypt the application data sent in
   a False Start, it must be the legitimate peer.  While an attacker
   could be influencing the handshake (restricting cipher suite
   selection, modifying key exchange messages, etc.), the attacker
   should not be able to benefit from this.  The TLS protocol already
   relies on such a security property for authentication; with False
   Start, the same is needed for encryption.  This motivates the rules
   put forth in the following subsections.

   It is prudent for applications to be even more restrictive.  If
   heuristically a small list of cipher suites and a single protocol
   version is found to be sufficient for the majority of TLS handshakes
   in practice, it could make sense to forego False Start for any
   handshake that does not match this expected pattern, even if there is
   no concrete reason to assume a cryptographic weakness.  Similarly, if
   handshakes almost always use ephemeral Elliptic Curve Diffie-Hellman
   (ECDH) over one of a few named curves, it could make sense to
   disallow False Start with any other supported curve.

5.1.  Symmetric Cipher

   Clients MUST NOT use the False Start protocol modification in a
   handshake unless the cipher suite uses a symmetric cipher that is
   considered cryptographically strong.

   Implementations may have their own classification of ciphers (and may
   additionally allow the application layer to provide a
   classification), but generally only symmetric ciphers with an
   effective key length of 128 bits or more can be considered strong.
   Also, various ciphers specified for use with TLS are known to have
   cryptographic weaknesses regardless of key length (none of the
   ciphers specified in [RFC4492] and [RFC5246] can be recommended for
   use with False Start).  The AES_128_GCM_SHA256 or AES_256_GCM_SHA384
   ciphers specified in [RFC5288] and [RFC5289] can be considered
   sufficiently strong for most uses.  Implementations that support
   additional cipher suites have to be careful to whitelist only
   suitable symmetric ciphers; if in doubt, False Start should not be
   used with a given symmetric cipher.

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   While an attacker can change handshake messages to force a downgrade
   to a less secure symmetric cipher than otherwise would have been
   chosen, this rule ensures that in such a downgrade attack, no
   application data will be sent under an insecure symmetric cipher.

5.2.  Protocol Version

   Clients MUST NOT use the False Start protocol modification in a
   handshake unless the protocol version chosen by
   ServerHello.server_version has been whitelisted for this use.

   Generally, to avoid potential protocol downgrade attacks,
   implementations should whitelist only their latest (highest-valued)
   supported TLS protocol version (and, if applicable, any earlier
   protocol versions that they would use in fallback retries without
   TLS_FALLBACK_SCSV [RFC7507]).

   The details of nominally identical cipher suites can differ between
   protocol versions, so this reinforces Section 5.1.

5.3.  Key Exchange and Client Certificate Type

   Clients MUST NOT use the False Start protocol modification in a
   handshake unless the cipher suite uses a key exchange method that has
   been whitelisted for this use.  Also, clients MUST NOT use the False
   Start protocol modification unless any parameters to the key exchange
   methods (such as ServerDHParams or ServerECDHParams) have been
   whitelisted for this use.  Furthermore, when using client
   authentication, clients MUST NOT use the False Start protocol
   modification unless the client certificate type has been whitelisted
   for this use.

   Implementations may have their own whitelists of key exchange
   methods, parameters, and client certificate types (and may
   additionally allow the application layer to specify whitelists).
   Generally, out of the options from [RFC5246] and [RFC4492], the
   following whitelists are recommended:

   o  Key exchange methods: DHE_RSA, ECDHE_RSA, DHE_DSS, ECDHE_ECDSA

   o  Parameters: well-known DH groups (at least 3,072 bits), named
      curves (at least 256 bits)

   o  Client certificate types: none

   However, if an implementation that supports only key exchange methods
   from [RFC5246] and [RFC4492] does not support any of the above key
   exchange methods, all of its supported key exchange methods can be

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   whitelisted for False Start use.  Care is required with any
   additional key exchange methods, as these may not have similar
   properties.

   The recommended whitelists are such that if cryptographic algorithms
   suitable for forward secrecy would possibly be negotiated, no False
   Start will take place if the current handshake fails to provide
   forward secrecy.  (Forward secrecy can be achieved using ephemeral
   Diffie-Hellman or ephemeral Elliptic Curve Diffie-Hellman; there is
   no forward secrecy when using a key exchange method of RSA, RSA_PSK,
   DH_DSS, DH_RSA, ECDH_ECDSA, or ECDH_RSA, or a client certificate type
   of rsa_fixed_dh, dss_fixed_dh, rsa_fixed_ecdh, or ecdsa_fixed_ecdh.)
   As usual, the benefits of forward secrecy may need to be balanced
   against efficiency, and accordingly, even implementations that
   support the above key exchange methods might whitelist further key
   exchange methods and client certificate types.

   Client certificate types rsa_sign, dss_sign, and ecdsa_sign do allow
   forward security, but using False Start with any of these means
   sending application data tied to the client's signature before the
   server's authenticity (and thus the CertificateRequest message) has
   been completely verified, so these too are not generally suitable for
   the client certificate type whitelist.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492,
              DOI 10.17487/RFC4492, May 2006,
              <http://www.rfc-editor.org/info/rfc4492>.

   [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>.

   [RFC5288]  Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
              Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
              DOI 10.17487/RFC5288, August 2008,
              <http://www.rfc-editor.org/info/rfc5288>.

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   [RFC5289]  Rescorla, E., "TLS Elliptic Curve Cipher Suites with
              SHA-256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
              DOI 10.17487/RFC5289, August 2008,
              <http://www.rfc-editor.org/info/rfc5289>.

6.2.  Informative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <http://www.rfc-editor.org/info/rfc793>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <http://www.rfc-editor.org/info/rfc7413>.

   [RFC7507]  Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
              Suite Value (SCSV) for Preventing Protocol Downgrade
              Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015,
              <http://www.rfc-editor.org/info/rfc7507>.

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", Work in Progress, draft-ietf-tls-tls13-14,
              July 2016.

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Appendix A.  Implementation Notes

   TLS False Start is a modification to the TLS protocol, and some
   implementations that conform to [RFC5246] may have problems
   interacting with implementations that use the False Start
   modification.  If the peer uses a False Start, application data
   records may be received directly following the peer's Finished
   message, before the TLS implementation has sent its own Finished
   message.  False Start compatibility as defined in Section 3 ensures
   that these records with application data will simply remain buffered
   for later processing.

   A False Start compatible TLS implementation does not have to be aware
   of the False Start concept and is certainly not expected to detect
   whether a False Start handshake is currently taking place: thanks to
   transport layer buffering, typical implementations will be False
   Start compatible without having been designed for it.

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Acknowledgments

   The authors wish to thank Wan-Teh Chang, Ben Laurie, Martin Thomson,
   Eric Rescorla, and Brian Smith for their input.

Authors' Addresses

   Adam Langley
   Google Inc.
   345 Spear St
   San Francisco, CA  94105
   United States of America

   Email: agl@google.com


   Nagendra Modadugu
   Google Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   United States of America

   Email: nagendra@cs.stanford.edu


   Bodo Moeller
   Google Switzerland GmbH
   Brandschenkestrasse 110
   Zurich  8002
   Switzerland

   Email: bmoeller@acm.org