QUIC                                                     M. Thomson, Ed.
Internet-Draft                                                   Mozilla
Intended status: Standards Track                          S. Turner, Ed.
Expires: December 15, 2017                                         sn3rd
                                                           June 13, 2017

          Using Transport Layer Security (TLS) to Secure QUIC


   This document describes how Transport Layer Security (TLS) is used to
   secure QUIC.

Note to Readers

   Discussion of this draft takes place on the QUIC working group
   mailing list (quic@ietf.org), which is archived at

   Working Group information can be found at https://github.com/quicwg;
   source code and issues list for this draft can be found at

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on December 15, 2017.

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
   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
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   4
   3.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  TLS Overview  . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  TLS Handshake . . . . . . . . . . . . . . . . . . . . . .   6
   4.  TLS Usage . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Handshake and Setup Sequence  . . . . . . . . . . . . . .   7
     4.2.  Interface to TLS  . . . . . . . . . . . . . . . . . . . .   9
       4.2.1.  Handshake Interface . . . . . . . . . . . . . . . . .   9
       4.2.2.  Source Address Validation . . . . . . . . . . . . . .  10
       4.2.3.  Key Ready Events  . . . . . . . . . . . . . . . . . .  11
       4.2.4.  Secret Export . . . . . . . . . . . . . . . . . . . .  12
       4.2.5.  TLS Interface Summary . . . . . . . . . . . . . . . .  12
     4.3.  TLS Version . . . . . . . . . . . . . . . . . . . . . . .  13
     4.4.  ClientHello Size  . . . . . . . . . . . . . . . . . . . .  13
     4.5.  Peer Authentication . . . . . . . . . . . . . . . . . . .  13
     4.6.  TLS Errors  . . . . . . . . . . . . . . . . . . . . . . .  14
   5.  QUIC Packet Protection  . . . . . . . . . . . . . . . . . . .  14
     5.1.  Installing New Keys . . . . . . . . . . . . . . . . . . .  14
     5.2.  QUIC Key Expansion  . . . . . . . . . . . . . . . . . . .  15
       5.2.1.  0-RTT Secret  . . . . . . . . . . . . . . . . . . . .  15
       5.2.2.  1-RTT Secrets . . . . . . . . . . . . . . . . . . . .  15
       5.2.3.  Packet Protection Key and IV  . . . . . . . . . . . .  17
     5.3.  QUIC AEAD Usage . . . . . . . . . . . . . . . . . . . . .  17
     5.4.  Packet Numbers  . . . . . . . . . . . . . . . . . . . . .  18
     5.5.  Receiving Protected Packets . . . . . . . . . . . . . . .  19
     5.6.  Packet Number Gaps  . . . . . . . . . . . . . . . . . . .  19
   6.  Unprotected Packets . . . . . . . . . . . . . . . . . . . . .  19
     6.1.  Integrity Check Processing  . . . . . . . . . . . . . . .  19
     6.2.  The 64-bit FNV-1a Algorithm . . . . . . . . . . . . . . .  20
   7.  Key Phases  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     7.1.  Packet Protection for the TLS Handshake . . . . . . . . .  21
       7.1.1.  Initial Key Transitions . . . . . . . . . . . . . . .  21
       7.1.2.  Retransmission and Acknowledgment of Unprotected
               Packets . . . . . . . . . . . . . . . . . . . . . . .  22
     7.2.  Key Update  . . . . . . . . . . . . . . . . . . . . . . .  23

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   8.  Client Address Validation . . . . . . . . . . . . . . . . . .  24
     8.1.  HelloRetryRequest Address Validation  . . . . . . . . . .  24
       8.1.1.  Stateless Address Validation  . . . . . . . . . . . .  25
       8.1.2.  Sending HelloRetryRequest . . . . . . . . . . . . . .  26
     8.2.  NewSessionTicket Address Validation . . . . . . . . . . .  26
     8.3.  Address Validation Token Integrity  . . . . . . . . . . .  27
   9.  Pre-handshake QUIC Messages . . . . . . . . . . . . . . . . .  27
     9.1.  Unprotected Packets Prior to Handshake Completion . . . .  28
       9.1.1.  STREAM Frames . . . . . . . . . . . . . . . . . . . .  28
       9.1.2.  ACK Frames  . . . . . . . . . . . . . . . . . . . . .  28
       9.1.3.  Updates to Data and Stream Limits . . . . . . . . . .  29
       9.1.4.  Denial of Service with Unprotected Packets  . . . . .  29
     9.2.  Use of 0-RTT Keys . . . . . . . . . . . . . . . . . . . .  30
     9.3.  Receiving Out-of-Order Protected Frames . . . . . . . . .  30
   10. QUIC-Specific Additions to the TLS Handshake  . . . . . . . .  31
     10.1.  Protocol and Version Negotiation . . . . . . . . . . . .  31
     10.2.  QUIC Transport Parameters Extension  . . . . . . . . . .  31
     10.3.  Priming 0-RTT  . . . . . . . . . . . . . . . . . . . . .  32
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  32
     11.1.  Packet Reflection Attack Mitigation  . . . . . . . . . .  33
     11.2.  Peer Denial of Service . . . . . . . . . . . . . . . . .  33
   12. Error codes . . . . . . . . . . . . . . . . . . . . . . . . .  33
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  34
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  34
     14.2.  Informative References . . . . . . . . . . . . . . . . .  35
   Appendix A.  Contributors . . . . . . . . . . . . . . . . . . . .  36
   Appendix B.  Acknowledgments  . . . . . . . . . . . . . . . . . .  36
   Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . .  36
     C.1.  Since draft-ietf-quic-tls-02  . . . . . . . . . . . . . .  36
     C.2.  Since draft-ietf-quic-tls-01  . . . . . . . . . . . . . .  36
     C.3.  Since draft-ietf-quic-tls-00  . . . . . . . . . . . . . .  37
     C.4.  Since draft-thomson-quic-tls-01 . . . . . . . . . . . . .  37
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

1.  Introduction

   This document describes how QUIC [QUIC-TRANSPORT] is secured using
   Transport Layer Security (TLS) version 1.3 [I-D.ietf-tls-tls13].  TLS
   1.3 provides critical latency improvements for connection
   establishment over previous versions.  Absent packet loss, most new
   connections can be established and secured within a single round
   trip; on subsequent connections between the same client and server,
   the client can often send application data immediately, that is,
   using a zero round trip setup.

   This document describes how the standardized TLS 1.3 acts a security
   component of QUIC.  The same design could work for TLS 1.2, though

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   few of the benefits QUIC provides would be realized due to the
   handshake latency in versions of TLS prior to 1.3.

2.  Notational Conventions

   The words "MUST", "MUST NOT", "SHOULD", and "MAY" are used in this
   document.  It's not shouting; when they are capitalized, they have
   the special meaning defined in [RFC2119].

   This document uses the terminology established in [QUIC-TRANSPORT].

   For brevity, the acronym TLS is used to refer to TLS 1.3.

   TLS terminology is used when referring to parts of TLS.  Though TLS
   assumes a continuous stream of octets, it divides that stream into
   _records_. Most relevant to QUIC are the records that contain TLS
   _handshake messages_, which are discrete messages that are used for
   key agreement, authentication and parameter negotiation.  Ordinarily,
   TLS records can also contain _application data_, though in the QUIC
   usage there is no use of TLS application data.

3.  Protocol Overview

   QUIC [QUIC-TRANSPORT] assumes responsibility for the confidentiality
   and integrity protection of packets.  For this it uses keys derived
   from a TLS 1.3 connection [I-D.ietf-tls-tls13]; QUIC also relies on
   TLS 1.3 for authentication and negotiation of parameters that are
   critical to security and performance.

   Rather than a strict layering, these two protocols are co-dependent:
   QUIC uses the TLS handshake; TLS uses the reliability and ordered
   delivery provided by QUIC streams.

   This document defines how QUIC interacts with TLS.  This includes a
   description of how TLS is used, how keying material is derived from
   TLS, and the application of that keying material to protect QUIC
   packets.  Figure 1 shows the basic interactions between TLS and QUIC,
   with the QUIC packet protection being called out specially.

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   +------------+                        +------------+
   |            |------ Handshake ------>|            |
   |            |<-- Validate Address ---|            |
   |            |-- OK/Error/Validate -->|            |
   |            |<----- Handshake -------|            |
   |   QUIC     |------ Validate ------->|    TLS     |
   |            |                        |            |
   |            |<------ 0-RTT OK -------|            |
   |            |<------ 1-RTT OK -------|            |
   |            |<--- Handshake Done ----|            |
   +------------+                        +------------+
    |         ^                               ^ |
    | Protect | Protected                     | |
    v         | Packet                        | |
   +------------+                             / /
   |   QUIC     |                            / /
   |  Packet    |-------- Get Secret -------' /
   | Protection |<-------- Secret -----------'

                    Figure 1: QUIC and TLS Interactions

   The initial state of a QUIC connection has packets exchanged without
   any form of protection.  In this state, QUIC is limited to using
   stream 0 and associated packets.  Stream 0 is reserved for a TLS
   connection.  This is a complete TLS connection as it would appear
   when layered over TCP; the only difference is that QUIC provides the
   reliability and ordering that would otherwise be provided by TCP.

   At certain points during the TLS handshake, keying material is
   exported from the TLS connection for use by QUIC.  This keying
   material is used to derive packet protection keys.  Details on how
   and when keys are derived and used are included in Section 5.

3.1.  TLS Overview

   TLS provides two endpoints with a way to establish a means of
   communication over an untrusted medium (that is, the Internet) that
   ensures that messages they exchange cannot be observed, modified, or

   TLS features can be separated into two basic functions: an
   authenticated key exchange and record protection.  QUIC primarily
   uses the authenticated key exchange provided by TLS but provides its
   own packet protection.

   The TLS authenticated key exchange occurs between two entities:
   client and server.  The client initiates the exchange and the server

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   responds.  If the key exchange completes successfully, both client
   and server will agree on a secret.  TLS supports both pre-shared key
   (PSK) and Diffie-Hellman (DH) key exchanges.  PSK is the basis for
   0-RTT; the latter provides perfect forward secrecy (PFS) when the DH
   keys are destroyed.

   After completing the TLS handshake, the client will have learned and
   authenticated an identity for the server and the server is optionally
   able to learn and authenticate an identity for the client.  TLS
   supports X.509 [RFC5280] certificate-based authentication for both
   server and client.

   The TLS key exchange is resistent to tampering by attackers and it
   produces shared secrets that cannot be controlled by either
   participating peer.

3.2.  TLS Handshake

   TLS 1.3 provides two basic handshake modes of interest to QUIC:

   o  A full 1-RTT handshake in which the client is able to send
      application data after one round trip and the server immediately
      after receiving the first handshake message from the client.

   o  A 0-RTT handshake in which the client uses information it has
      previously learned about the server to send application data
      immediately.  This application data can be replayed by an attacker
      so it MUST NOT carry a self-contained trigger for any non-
      idempotent action.

   A simplified TLS 1.3 handshake with 0-RTT application data is shown
   in Figure 2, see [I-D.ietf-tls-tls13] for more options and details.

       Client                                             Server

      (0-RTT Application Data)  -------->
                                <--------      [Application Data]
      {Finished}                -------->

      [Application Data]        <------->      [Application Data]

                    Figure 2: TLS Handshake with 0-RTT

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   This 0-RTT handshake is only possible if the client and server have
   previously communicated.  In the 1-RTT handshake, the client is
   unable to send protected application data until it has received all
   of the handshake messages sent by the server.

   Two additional variations on this basic handshake exchange are
   relevant to this document:

   o  The server can respond to a ClientHello with a HelloRetryRequest,
      which adds an additional round trip prior to the basic exchange.
      This is needed if the server wishes to request a different key
      exchange key from the client.  HelloRetryRequest is also used to
      verify that the client is correctly able to receive packets on the
      address it claims to have (see [QUIC-TRANSPORT]).

   o  A pre-shared key mode can be used for subsequent handshakes to
      reduce the number of public key operations.  This is the basis for
      0-RTT data, even if the remainder of the connection is protected
      by a new Diffie-Hellman exchange.

4.  TLS Usage

   QUIC reserves stream 0 for a TLS connection.  Stream 0 contains a
   complete TLS connection, which includes the TLS record layer.  Other
   than the definition of a QUIC-specific extension (see Section 10.2),
   TLS is unmodified for this use.  This means that TLS will apply
   confidentiality and integrity protection to its records.  In
   particular, TLS record protection is what provides confidentiality
   protection for the TLS handshake messages sent by the server.

   QUIC permits a client to send frames on streams starting from the
   first packet.  The initial packet from a client contains a stream
   frame for stream 0 that contains the first TLS handshake messages
   from the client.  This allows the TLS handshake to start with the
   first packet that a client sends.

   QUIC packets are protected using a scheme that is specific to QUIC,
   see Section 5.  Keys are exported from the TLS connection when they
   become available using a TLS exporter (see Section 7.5 of
   [I-D.ietf-tls-tls13] and Section 5.2).  After keys are exported from
   TLS, QUIC manages its own key schedule.

4.1.  Handshake and Setup Sequence

   The integration of QUIC with a TLS handshake is shown in more detail
   in Figure 3.  QUIC "STREAM" frames on stream 0 carry the TLS
   handshake.  QUIC performs loss recovery [QUIC-RECOVERY] for this

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   stream and ensures that TLS handshake messages are delivered in the
   correct order.

       Client                                             Server

   @C QUIC STREAM Frame(s) <0>:
          + QUIC Extension
                           0-RTT Key => @0

   @0 QUIC STREAM Frame(s) <any stream>:
      Replayable QUIC Frames

                                         QUIC STREAM Frame <0>: @C
                                     {TLS Handshake Messages}
                           1-RTT Key => @1

                                              QUIC Frames <any> @1
   @C QUIC STREAM Frame(s) <0>:

   @1 QUIC Frames <any>        <------->      QUIC Frames <any> @1

                     Figure 3: QUIC over TLS Handshake

   In Figure 3, symbols mean:

   o  "<" and ">" enclose stream numbers.

   o  "@" indicates the keys that are used for protecting the QUIC
      packet (C = cleartext, with integrity only; 0 = 0-RTT keys; 1 =
      1-RTT keys).

   o  "(" and ")" enclose messages that are protected with TLS 0-RTT
      handshake or application keys.

   o  "{" and "}" enclose messages that are protected by the TLS
      Handshake keys.

   If 0-RTT is not attempted, then the client does not send packets
   protected by the 0-RTT key (@0).  In that case, the only key

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   transition on the client is from cleartext packets (@C) to 1-RTT
   protection (@1), which happens after it sends its final set of TLS
   handshake messages.

   Note: the client uses two different types of cleartext packet during
   the handshake.  The Client Initial packet carries a TLS ClientHello
   message; the remainder of the TLS handshake is carried in Client
   Cleartext packets.

   The server sends TLS handshake messages without protection (@C).  The
   server transitions from no protection (@C) to full 1-RTT protection
   (@1) after it sends the last of its handshake messages.

   Some TLS handshake messages are protected by the TLS handshake record
   protection.  These keys are not exported from the TLS connection for
   use in QUIC.  QUIC packets from the server are sent in the clear
   until the final transition to 1-RTT keys.

   The client transitions from cleartext (@C) to 0-RTT keys (@0) when
   sending 0-RTT data, and subsequently to to 1-RTT keys (@1) after its
   second flight of TLS handshake messages.  This creates the potential
   for unprotected packets to be received by a server in close proximity
   to packets that are protected with 1-RTT keys.

   More information on key transitions is included in Section 7.1.

4.2.  Interface to TLS

   As shown in Figure 1, the interface from QUIC to TLS consists of four
   primary functions: Handshake, Source Address Validation, Key Ready
   Events, and Secret Export.

   Additional functions might be needed to configure TLS.

4.2.1.  Handshake Interface

   In order to drive the handshake, TLS depends on being able to send
   and receive handshake messages on stream 0.  There are two basic
   functions on this interface: one where QUIC requests handshake
   messages and one where QUIC provides handshake packets.

   Before starting the handshake QUIC provides TLS with the transport
   parameters (see Section 10.2) that it wishes to carry.

   A QUIC client starts TLS by requesting TLS handshake octets from TLS.
   The client acquires handshake octets before sending its first packet.

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   A QUIC server starts the process by providing TLS with stream 0

   Each time that an endpoint receives data on stream 0, it delivers the
   octets to TLS if it is able.  Each time that TLS is provided with new
   data, new handshake octets are requested from TLS.  TLS might not
   provide any octets if the handshake messages it has received are
   incomplete or it has no data to send.

   Once the TLS handshake is complete, this is indicated to QUIC along
   with any final handshake octets that TLS needs to send.  TLS also
   provides QUIC with the transport parameters that the peer advertised
   during the handshake.

   Once the handshake is complete, TLS becomes passive.  TLS can still
   receive data from its peer and respond in kind, but it will not need
   to send more data unless specifically requested - either by an
   application or QUIC.  One reason to send data is that the server
   might wish to provide additional or updated session tickets to a

   When the handshake is complete, QUIC only needs to provide TLS with
   any data that arrives on stream 0.  In the same way that is done
   during the handshake, new data is requested from TLS after providing
   received data.

   Important:  Until the handshake is reported as complete, the
      connection and key exchange are not properly authenticated at the
      server.  Even though 1-RTT keys are available to a server after
      receiving the first handshake messages from a client, the server
      cannot consider the client to be authenticated until it receives
      and validates the client's Finished message.

      The requirement for the server to wait for the client Finished
      message creates a dependency on that message being delivered.  A
      client can avoid the potential for head-of-line blocking that this
      implies by sending a copy of the STREAM frame that carries the
      Finished message in multiple packets.  This enables immediate
      server processing for those packets.

4.2.2.  Source Address Validation

   During the processing of the TLS ClientHello, TLS requests that the
   transport make a decision about whether to request source address
   validation from the client.

   An initial TLS ClientHello that resumes a session includes an address
   validation token in the session ticket; this includes all attempts at

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   0-RTT.  If the client does not attempt session resumption, no token
   will be present.  While processing the initial ClientHello, TLS
   provides QUIC with any token that is present.  In response, QUIC
   provides one of three responses:

   o  proceed with the connection,

   o  ask for client address validation, or

   o  abort the connection.

   If QUIC requests source address validation, it also provides a new
   address validation token.  TLS includes that along with any
   information it requires in the cookie extension of a TLS
   HelloRetryRequest message.  In the other cases, the connection either
   proceeds or terminates with a handshake error.

   The client echoes the cookie extension in a second ClientHello.  A
   ClientHello that contains a valid cookie extension will always be in
   response to a HelloRetryRequest.  If address validation was requested
   by QUIC, then this will include an address validation token.  TLS
   makes a second address validation request of QUIC, including the
   value extracted from the cookie extension.  In response to this
   request, QUIC cannot ask for client address validation, it can only
   abort or permit the connection attempt to proceed.

   QUIC can provide a new address validation token for use in session
   resumption at any time after the handshake is complete.  Each time a
   new token is provided TLS generates a NewSessionTicket message, with
   the token included in the ticket.

   See Section 8 for more details on client address validation.

4.2.3.  Key Ready Events

   TLS provides QUIC with signals when 0-RTT and 1-RTT keys are ready
   for use.  These events are not asynchronous, they always occur
   immediately after TLS is provided with new handshake octets, or after
   TLS produces handshake octets.

   When TLS completed its handshake, 1-RTT keys can be provided to QUIC.
   On both client and server, this occurs after sending the TLS Finished

   This ordering means that there could be frames that carry TLS
   handshake messages ready to send at the same time that application
   data is available.  An implementation MUST ensure that TLS handshake

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   messages are always sent in cleartext packets.  Separate packets are
   required for data that needs protection from 1-RTT keys.

   If 0-RTT is possible, it is ready after the client sends a TLS
   ClientHello message or the server receives that message.  After
   providing a QUIC client with the first handshake octets, the TLS
   stack might signal that 0-RTT keys are ready.  On the server, after
   receiving handshake octets that contain a ClientHello message, a TLS
   server might signal that 0-RTT keys are available.

   1-RTT keys are used for packets in both directions.  0-RTT keys are
   only used to protect packets sent by the client.

4.2.4.  Secret Export

   Details how secrets are exported from TLS are included in
   Section 5.2.

4.2.5.  TLS Interface Summary

   Figure 4 summarizes the exchange between QUIC and TLS for both client
   and server.

   Client                                                    Server

   Get Handshake
   0-RTT Key Ready
                         --- send/receive --->
                                                 Handshake Received
                                                    0-RTT Key Ready
                                                      Get Handshake
                                                   1-RTT Keys Ready
                        <--- send/receive ---
   Handshake Received
   Get Handshake
   Handshake Complete
   1-RTT Keys Ready
                         --- send/receive --->
                                                 Handshake Received
                                                      Get Handshake
                                                 Handshake Complete
                        <--- send/receive ---
   Handshake Received
   Get Handshake

            Figure 4: Interaction Summary between QUIC and TLS

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4.3.  TLS Version

   This document describes how TLS 1.3 [I-D.ietf-tls-tls13] is used with

   In practice, the TLS handshake will negotiate a version of TLS to
   use.  This could result in a newer version of TLS than 1.3 being
   negotiated if both endpoints support that version.  This is
   acceptable provided that the features of TLS 1.3 that are used by
   QUIC are supported by the newer version.

   A badly configured TLS implementation could negotiate TLS 1.2 or
   another older version of TLS.  An endpoint MUST terminate the
   connection if a version of TLS older than 1.3 is negotiated.

4.4.  ClientHello Size

   QUIC requires that the initial handshake packet from a client fit
   within the payload of a single packet.  The size limits on QUIC
   packets mean that a record containing a ClientHello needs to fit
   within 1197 octets.

   A TLS ClientHello can fit within this limit with ample space
   remaining.  However, there are several variables that could cause
   this limit to be exceeded.  Implementations are reminded that large
   session tickets or HelloRetryRequest cookies, multiple or large key
   shares, and long lists of supported ciphers, signature algorithms,
   versions, QUIC transport parameters, and other negotiable parameters
   and extensions could cause this message to grow.

   For servers, the size of the session tickets and HelloRetryRequest
   cookie extension can have an effect on a client's ability to connect.
   Choosing a small value increases the probability that these values
   can be successfully used by a client.

   The TLS implementation does not need to ensure that the ClientHello
   is sufficiently large.  QUIC PADDING frames are added to increase the
   size of the packet as necessary.

4.5.  Peer Authentication

   The requirements for authentication depend on the application
   protocol that is in use.  TLS provides server authentication and
   permits the server to request client authentication.

   A client MUST authenticate the identity of the server.  This
   typically involves verification that the identity of the server is

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   included in a certificate and that the certificate is issued by a
   trusted entity (see for example [RFC2818]).

   A server MAY request that the client authenticate during the
   handshake.  A server MAY refuse a connection if the client is unable
   to authenticate when requested.  The requirements for client
   authentication vary based on application protocol and deployment.

   A server MUST NOT use post-handshake client authentication (see
   Section 4.6.2 of [I-D.ietf-tls-tls13]).

4.6.  TLS Errors

   Errors in the TLS connection SHOULD be signaled using TLS alerts on
   stream 0.  A failure in the handshake MUST be treated as a QUIC
   connection error of type TLS_HANDSHAKE_FAILED.  Once the handshake is
   complete, an error in the TLS connection that causes a TLS alert to
   be sent or received MUST be treated as a QUIC connection error of

5.  QUIC Packet Protection

   QUIC packet protection provides authenticated encryption of packets.
   This provides confidentiality and integrity protection for the
   content of packets (see Section 5.3).  Packet protection uses keys
   that are exported from the TLS connection (see Section 5.2).

   Different keys are used for QUIC packet protection and TLS record
   protection.  TLS handshake messages are protected solely with TLS
   record protection, but post-handshake messages are redundantly
   proteted with both both the QUIC packet protection and the TLS record
   protection.  These messages are limited in number, and so the
   additional overhead is small.

5.1.  Installing New Keys

   As TLS reports the availability of keying material, the packet
   protection keys and initialization vectors (IVs) are updated (see
   Section 5.2).  The selection of AEAD function is also updated to
   match the AEAD negotiated by TLS.

   For packets other than any unprotected handshake packets (see
   Section 7.1), once a change of keys has been made, packets with
   higher packet numbers MUST be sent with the new keying material.  The
   KEY_PHASE bit on these packets is inverted each time new keys are
   installed to signal the use of the new keys to the recipient (see
   Section 7 for details).

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   An endpoint retransmits stream data in a new packet.  New packets
   have new packet numbers and use the latest packet protection keys.
   This simplifies key management when there are key updates (see
   Section 7.2).

5.2.  QUIC Key Expansion

   QUIC uses a system of packet protection secrets, keys and IVs that
   are modelled on the system used in TLS [I-D.ietf-tls-tls13].  The
   secrets that QUIC uses as the basis of its key schedule are obtained
   using TLS exporters (see Section 7.5 of [I-D.ietf-tls-tls13]).

   QUIC uses HKDF with the same hash function negotiated by TLS for key
   derivation.  For example, if TLS is using the TLS_AES_128_GCM_SHA256,
   the SHA-256 hash function is used.

5.2.1.  0-RTT Secret

   0-RTT keys are those keys that are used in resumed connections prior
   to the completion of the TLS handshake.  Data sent using 0-RTT keys
   might be replayed and so has some restrictions on its use, see
   Section 9.2.  0-RTT keys are used after sending or receiving a

   The secret is exported from TLS using the exporter label "EXPORTER-
   QUIC 0-RTT Secret" and an empty context.  The size of the secret MUST
   be the size of the hash output for the PRF hash function negotiated
   by TLS.  This uses the TLS early_exporter_secret.  The QUIC 0-RTT
   secret is only used for protection of packets sent by the client.

          = TLS-Exporter("EXPORTER-QUIC 0-RTT Secret"
                         "", Hash.length)

5.2.2.  1-RTT Secrets

   1-RTT keys are used by both client and server after the TLS handshake
   completes.  There are two secrets used at any time: one is used to
   derive packet protection keys for packets sent by the client, the
   other for packet protection keys on packets sent by the server.

   The initial client packet protection secret is exported from TLS
   using the exporter label "EXPORTER-QUIC client 1-RTT Secret"; the
   initial server packet protection secret uses the exporter label
   "EXPORTER-QUIC server 1-RTT Secret".  Both exporters use an empty
   context.  The size of the secret MUST be the size of the hash output
   for the PRF hash function negotiated by TLS.

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          = TLS-Exporter("EXPORTER-QUIC client 1-RTT Secret"
                         "", Hash.length)
          = TLS-Exporter("EXPORTER-QUIC server 1-RTT Secret"
                         "", Hash.length)

   These secrets are used to derive the initial client and server packet
   protection keys.

   After a key update (see Section 7.2), these secrets are updated using
   the HKDF-Expand-Label function defined in Section 7.1 of
   [I-D.ietf-tls-tls13].  HKDF-Expand-Label uses the PRF hash function
   negotiated by TLS.  The replacement secret is derived using the
   existing Secret, a Label of "QUIC client 1-RTT Secret" for the client
   and "QUIC server 1-RTT Secret" for the server, an empty HashValue,
   and the same output Length as the hash function selected by TLS for
   its PRF.

          = HKDF-Expand-Label(client_pp_secret_<N>,
                              "QUIC client 1-RTT Secret",
                              "", Hash.length)
          = HKDF-Expand-Label(server_pp_secret_<N>,
                              "QUIC server 1-RTT Secret",
                              "", Hash.length)

   This allows for a succession of new secrets to be created as needed.

   HKDF-Expand-Label uses HKDF-Expand [RFC5869] with a specially
   formatted info parameter, as shown:

       HKDF-Expand-Label(Secret, Label, HashValue, Length) =
            HKDF-Expand(Secret, HkdfLabel, Length)

       Where HkdfLabel is specified as:

       struct {
           uint16 length = Length;
           opaque label<10..255> = "TLS 1.3, " + Label;
           uint8 hashLength;     // Always 0
       } HkdfLabel;

   For example, the client packet protection secret uses an info
   parameter of:

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      info = (HashLen / 256) || (HashLen % 256) || 0x21 ||
             "TLS 1.3, QUIC client 1-RTT secret" || 0x00

5.2.3.  Packet Protection Key and IV

   The complete key expansion uses an identical process for key
   expansion as defined in Section 7.3 of [I-D.ietf-tls-tls13], using
   different values for the input secret.  QUIC uses the AEAD function
   negotiated by TLS.

   The packet protection key and IV used to protect the 0-RTT packets
   sent by a client are derived from the QUIC 0-RTT secret.  The packet
   protection keys and IVs for 1-RTT packets sent by the client and
   server are derived from the current generation of client_pp_secret
   and server_pp_secret respectively.  The length of the output is
   determined by the requirements of the AEAD function selected by TLS.
   The key length is the AEAD key size.  As defined in Section 5.3 of
   [I-D.ietf-tls-tls13], the IV length is the larger of 8 or N_MIN (see
   Section 4 of [RFC5116]).  For any secret S, the corresponding key and
   IV are derived as shown below:

      key = HKDF-Expand-Label(S, "key", "", key_length)
      iv  = HKDF-Expand-Label(S, "iv", "", iv_length)

   The QUIC record protection initially starts without keying material.
   When the TLS state machine reports that the ClientHello has been
   sent, the 0-RTT keys can be generated and installed for writing.
   When the TLS state machine reports completion of the handshake, the
   1-RTT keys can be generated and installed for writing.

5.3.  QUIC AEAD Usage

   The Authentication Encryption with Associated Data (AEAD) [RFC5116]
   function used for QUIC packet protection is AEAD that is negotiated
   for use with the TLS connection.  For example, if TLS is using the
   TLS_AES_128_GCM_SHA256, the AEAD_AES_128_GCM function is used.

   Regular QUIC packets are protected by an AEAD algorithm [RFC5116].
   Version negotiation and public reset packets are not protected.

   Once TLS has provided a key, the contents of regular QUIC packets
   immediately after any TLS messages have been sent are protected by
   the AEAD selected by TLS.

   The key, K, is either the client packet protection key
   (client_pp_key_n) or the server packet protection key
   (server_pp_key_n), derived as defined in Section 5.2.

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   The nonce, N, is formed by combining the packet protection IV (either
   client_pp_iv_n or server_pp_iv_n) with the packet number.  The 64
   bits of the reconstructed QUIC packet number in network byte order is
   left-padded with zeros to the size of the IV.  The exclusive OR of
   the padded packet number and the IV forms the AEAD nonce.

   The associated data, A, for the AEAD is the contents of the QUIC
   header, starting from the flags octet in the common header.

   The input plaintext, P, for the AEAD is the contents of the QUIC
   frame following the packet number, as described in [QUIC-TRANSPORT].

   The output ciphertext, C, of the AEAD is transmitted in place of P.

   Prior to TLS providing keys, no record protection is performed and
   the plaintext, P, is transmitted unmodified.

5.4.  Packet Numbers

   QUIC has a single, contiguous packet number space.  In comparison,
   TLS restarts its sequence number each time that record protection
   keys are changed.  The sequence number restart in TLS ensures that a
   compromise of the current traffic keys does not allow an attacker to
   truncate the data that is sent after a key update by sending
   additional packets under the old key (causing new packets to be

   QUIC does not assume a reliable transport and is required to handle
   attacks where packets are dropped in other ways.  QUIC is therefore
   not affected by this form of truncation.

   The QUIC packet number is not reset and it is not permitted to go
   higher than its maximum value of 2^64-1.  This establishes a hard
   limit on the number of packets that can be sent.

   Some AEAD functions have limits for how many packets can be encrypted
   under the same key and IV (see for example [AEBounds]).  This might
   be lower than the packet number limit.  An endpoint MUST initiate a
   key update (Section 7.2) prior to exceeding any limit set for the
   AEAD that is in use.

   TLS maintains a separate sequence number that is used for record
   protection on the connection that is hosted on stream 0.  This
   sequence number is not visible to QUIC.

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5.5.  Receiving Protected Packets

   Once an endpoint successfully receives a packet with a given packet
   number, it MUST discard all packets with higher packet numbers if
   they cannot be successfully unprotected with either the same key, or
   - if there is a key update - the next packet protection key (see
   Section 7.2).  Similarly, a packet that appears to trigger a key
   update, but cannot be unprotected successfully MUST be discarded.

   Failure to unprotect a packet does not necessarily indicate the
   existence of a protocol error in a peer or an attack.  The truncated
   packet number encoding used in QUIC can cause packet numbers to be
   decoded incorrectly if they are delayed significantly.

5.6.  Packet Number Gaps

   [QUIC-TRANSPORT]; Section also requires a secret to compute
   packet number gaps on connection ID transitions.  That secret is
   computed as:

             = TLS-Exporter("EXPORTER-QUIC Packet Number Secret"
                            "", Hash.length)

6.  Unprotected Packets

   QUIC adds an integrity check to all unprotected packets.  Any packet
   that is not protected by the negotiated AEAD (see Section 5),
   includes an integrity check.  This check does not prevent the packet
   from being altered, it exists for added resilience against data
   corruption and to provided added assurance that the sender intends to
   use QUIC.

   Unprotected packets all use the long form of the QUIC header and so
   will include a version number.  For this version of QUIC, the
   integrity check uses the 64-bit FNV-1a hash (see Section 6.2).  The
   output of this hash is appended to the payload of the packet.

   The integrity check algorithm MAY change for other versions of the

6.1.  Integrity Check Processing

   An endpoint sending a packet that has a long header and a type that
   does not indicate that the packet will be protected (that is, 0-RTT
   Encrypted (0x05), 1-RTT Encrypted (key phase 0) (0x06), or 1-RTT
   Encrypted (key phase 1) (0x07)) first constructs the packet that it
   sends without the integrity check.

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   The sender then calculates the integrity check over the entire
   packet, starting from the type field.  The output of the hash is
   appended to the packet.

   A receiver that receives an unprotected packet first checks that the
   version is correct, then removes the trailing 8 octets.  It
   calculates the integrity check over the remainder of the packet.
   Unprotected packets that do not contain a valid integrity check MUST
   be discarded.

6.2.  The 64-bit FNV-1a Algorithm

   QUIC uses the 64-bit version of the alternative Fowler/Noll/Vo hash
   (FNV-1a) [FNV].

   FNV-1a can be expressed in pseudocode as:

   "hash := offset basis for each input octet: hash := hash XOR input
   octet hash := hash * prime "

   That is, a 64-bit unsigned integer is initialized with an offset
   basis.  Then, for each octet of the input, the exclusive binary OR of
   the value is taken, then multiplied by a prime.  Any overflow from
   multiplication is discarded.

   The offset basis for the 64-bit FNV-1a is the decimal value
   14695981039346656037 (in hex, 0xcbf29ce484222325).  The prime is
   1099511628211 (in hex, 0x100000001b3; or as an expression 2^40 + 2^8
   + 0xb3).

   Once all octets have been processed in this fashion, the final
   integer value is encoded as 8 octets in network byte order.

7.  Key Phases

   As TLS reports the availability of 0-RTT and 1-RTT keys, new keying
   material can be exported from TLS and used for QUIC packet
   protection.  At each transition during the handshake a new secret is
   exported from TLS and packet protection keys are derived from that

   Every time that a new set of keys is used for protecting outbound
   packets, the KEY_PHASE bit in the public flags is toggled.  0-RTT
   protected packets use the QUIC long header, they do not use the
   KEY_PHASE bit to select the correct keys (see Section 7.1.1).

   Once the connection is fully enabled, the KEY_PHASE bit allows a
   recipient to detect a change in keying material without necessarily

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   needing to receive the first packet that triggered the change.  An
   endpoint that notices a changed KEY_PHASE bit can update keys and
   decrypt the packet that contains the changed bit, see Section 7.2.

   The KEY_PHASE bit is included as the 0x20 bit of the QUIC short
   header, or is determined by the packet type from the long header (a
   type of 0x06 indicates a key phase of 0, 0x07 indicates key phase 1).

   Transitions between keys during the handshake are complicated by the
   need to ensure that TLS handshake messages are sent with the correct
   packet protection.

7.1.  Packet Protection for the TLS Handshake

   The initial exchange of packets are sent without protection.  These
   packets use a cleartext packet type.

   TLS handshake messages MUST NOT be protected using QUIC packet
   protection.  All TLS handshake messages up to the TLS Finished
   message sent by either endpoint use cleartext packets.

   Any TLS handshake messages that are sent after completing the TLS
   handshake do not need special packet protection rules.  Packets
   containing these messages use the packet protection keys that are
   current at the time of sending (or retransmission).

   Like the client, a server MUST send retransmissions of its
   unprotected handshake messages or acknowledgments for unprotected
   handshake messages sent by the client in cleartext packets.

7.1.1.  Initial Key Transitions

   Once the TLS handshake is complete, keying material is exported from
   TLS and QUIC packet protection commences.

   Packets protected with 1-RTT keys initially have a KEY_PHASE bit set
   to 0.  This bit inverts with each subsequent key update (see
   Section 7.2).

   If the client sends 0-RTT data, it uses the 0-RTT packet type.  The
   packet that contains the TLS EndOfEarlyData and Finished messages are
   sent in cleartext packets.

   Using distinct packet types during the handshake for handshake
   messages, 0-RTT data, and 1-RTT data ensures that the server is able
   to distinguish between the different keys used to remove packet
   protection.  All of these packets can arrive concurrently at a

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   A server might choose to retain 0-RTT packets that arrive before a
   TLS ClientHello.  The server can then use those packets once the
   ClientHello arrives.  However, the potential for denial of service
   from buffering 0-RTT packets is significant.  These packets cannot be
   authenticated and so might be employed by an attacker to exhaust
   server resources.  Limiting the number of packets that are saved
   might be necessary.

   The server transitions to using 1-RTT keys after sending its first
   flight of TLS handshake messages.  From this point, the server
   protects all packets with 1-RTT keys.  Future packets are therefore
   protected with 1-RTT keys.  Initially, these are marked with a
   KEY_PHASE of 0.

7.1.2.  Retransmission and Acknowledgment of Unprotected Packets

   TLS handshake messages from both client and server are critical to
   the key exchange.  The contents of these messages determines the keys
   used to protect later messages.  If these handshake messages are
   included in packets that are protected with these keys, they will be
   indecipherable to the recipient.

   Even though newer keys could be available when retransmitting,
   retransmissions of these handshake messages MUST be sent in cleartext
   packets.  An endpoint MUST generate ACK frames for these messages and
   send them in cleartext packets.

   A HelloRetryRequest handshake message might be used to reject an
   initial ClientHello.  A HelloRetryRequest handshake message is sent
   in a Server Stateless Retry packet; any second ClientHello that is
   sent in response uses a Client Initial packet type.  Neither packet
   is protected.  This is natural, because no new keying material will
   be available when these messages need to be sent.  Upon receipt of a
   HelloRetryRequest, a client SHOULD cease any transmission of 0-RTT
   data; 0-RTT data will only be discarded by any server that sends a

   The packet type ensures that protected packets are clearly
   distinguished from unprotected packets.  Loss or reordering might
   cause unprotected packets to arrive once 1-RTT keys are in use,
   unprotected packets are easily distinguished from 1-RTT packets using
   the packet type.

   Once 1-RTT keys are available to an endpoint, it no longer needs the
   TLS handshake messages that are carried in unprotected packets.
   However, a server might need to retransmit its TLS handshake messages
   in response to receiving an unprotected packet that contains ACK
   frames.  A server MUST process ACK frames in unprotected packets

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   until the TLS handshake is reported as complete, or it receives an
   ACK frame in a protected packet that acknowledges all of its
   handshake messages.

   To limit the number of key phases that could be active, an endpoint
   MUST NOT initiate a key update while there are any unacknowledged
   handshake messages, see Section 7.2.

7.2.  Key Update

   Once the TLS handshake is complete, the KEY_PHASE bit allows for
   refreshes of keying material by either peer.  Endpoints start using
   updated keys immediately without additional signaling; the change in
   the KEY_PHASE bit indicates that a new key is in use.

   An endpoint MUST NOT initiate more than one key update at a time.  A
   new key cannot be used until the endpoint has received and
   successfully decrypted a packet with a matching KEY_PHASE.  Note that
   when 0-RTT is attempted the value of the KEY_PHASE bit will be
   different on packets sent by either peer.

   A receiving endpoint detects an update when the KEY_PHASE bit doesn't
   match what it is expecting.  It creates a new secret (see
   Section 5.2) and the corresponding read key and IV.  If the packet
   can be decrypted and authenticated using these values, then the keys
   it uses for packet protection are also updated.  The next packet sent
   by the endpoint will then use the new keys.

   An endpoint doesn't need to send packets immediately when it detects
   that its peer has updated keys.  The next packet that it sends will
   simply use the new keys.  If an endpoint detects a second update
   before it has sent any packets with updated keys it indicates that
   its peer has updated keys twice without awaiting a reciprocal update.
   An endpoint MUST treat consecutive key updates as a fatal error and
   abort the connection.

   An endpoint SHOULD retain old keys for a short period to allow it to
   decrypt packets with smaller packet numbers than the packet that
   triggered the key update.  This allows an endpoint to consume packets
   that are reordered around the transition between keys.  Packets with
   higher packet numbers always use the updated keys and MUST NOT be
   decrypted with old keys.

   Keys and their corresponding secrets SHOULD be discarded when an
   endpoint has received all packets with sequence numbers lower than
   the lowest sequence number used for the new key.  An endpoint might
   discard keys if it determines that the length of the delay to
   affected packets is excessive.

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   This ensures that once the handshake is complete, packets with the
   same KEY_PHASE will have the same packet protection keys, unless
   there are multiple key updates in a short time frame succession and
   significant packet reordering.

      Initiating Peer                    Responding Peer

   @M QUIC Frames
                  New Keys -> @N
   @N QUIC Frames
                                             QUIC Frames @M
                             New Keys -> @N
                                             QUIC Frames @N

                           Figure 5: Key Update

   As shown in Figure 3 and Figure 5, there is never a situation where
   there are more than two different sets of keying material that might
   be received by a peer.  Once both sending and receiving keys have
   been updated,

   A server cannot initiate a key update until it has received the
   client's Finished message.  Otherwise, packets protected by the
   updated keys could be confused for retransmissions of handshake
   messages.  A client cannot initiate a key update until all of its
   handshake messages have been acknowledged by the server.

   A packet that triggers a key update could arrive after successfully
   processing a packet with a higher packet number.  This is only
   possible if there is a key compromise and an attack, or if the peer
   is incorrectly reverting to use of old keys.  Because the latter
   cannot be differentiated from an attack, an endpoint MUST immediately
   terminate the connection if it detects this condition.

8.  Client Address Validation

   Two tools are provided by TLS to enable validation of client source
   addresses at a server: the cookie in the HelloRetryRequest message,
   and the ticket in the NewSessionTicket message.

8.1.  HelloRetryRequest Address Validation

   The cookie extension in the TLS HelloRetryRequest message allows a
   server to perform source address validation during the handshake.

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   When QUIC requests address validation during the processing of the
   first ClientHello, the token it provides is included in the cookie
   extension of a HelloRetryRequest.  As long as the cookie cannot be
   successfully guessed by a client, the server can be assured that the
   client received the HelloRetryRequest if it includes the value in a
   second ClientHello.

   An initial ClientHello never includes a cookie extension.  Thus, if a
   server constructs a cookie that contains all the information
   necessary to reconstruct state, it can discard local state after
   sending a HelloRetryRequest.  Presence of a valid cookie in a
   ClientHello indicates that the ClientHello is a second attempt from
   the client.

   An address validation token can be extracted from a second
   ClientHello and passed to the transport for further validation.  If
   that validation fails, the server MUST fail the TLS handshake and
   send an illegal_parameter alert.

   Combining address validation with the other uses of HelloRetryRequest
   ensures that there are fewer ways in which an additional round-trip
   can be added to the handshake.  In particular, this makes it possible
   to combine a request for address validation with a request for a
   different client key share.

   If TLS needs to send a HelloRetryRequest for other reasons, it needs
   to ensure that it can correctly identify the reason that the
   HelloRetryRequest was generated.  During the processing of a second
   ClientHello, TLS does not need to consult the transport protocol
   regarding address validation if address validation was not requested
   originally.  In such cases, the cookie extension could either be
   absent or it could indicate that an address validation token is not

8.1.1.  Stateless Address Validation

   A server can use the cookie extension to store all state necessary to
   continue the connection.  This allows a server to avoid committing
   state for clients that have unvalidated source addresses.

   For instance, a server could use a statically-configured key to
   encrypt the information that it requires and include that information
   in the cookie.  In addition to address validation information, a
   server that uses encryption also needs to be able recover the hash of
   the ClientHello and its length, plus any information it needs in
   order to reconstruct the HelloRetryRequest.

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8.1.2.  Sending HelloRetryRequest

   A server does not need to maintain state for the connection when
   sending a HelloRetryRequest message.  This might be necessary to
   avoid creating a denial of service exposure for the server.  However,
   this means that information about the transport will be lost at the
   server.  This includes the stream offset of stream 0, the packet
   number that the server selects, and any opportunity to measure round
   trip time.

   A server MUST send a TLS HelloRetryRequest in a Server Stateless
   Retry packet.  Using a Server Stateless Retry packet causes the
   client to reset stream offsets.  It also avoids the need for the
   server select an initial packet number, which would need to be
   remembered so that subsequent packets could be correctly numbered.

   A HelloRetryRequest message MUST NOT be split between multiple Server
   Stateless Retry packets.  This means that HelloRetryRequest is
   subject to the same size constraints as a ClientHello (see
   Section 4.4).

8.2.  NewSessionTicket Address Validation

   The ticket in the TLS NewSessionTicket message allows a server to
   provide a client with a similar sort of token.  When a client resumes
   a TLS connection - whether or not 0-RTT is attempted - it includes
   the ticket in the handshake message.  As with the HelloRetryRequest
   cookie, the server includes the address validation token in the
   ticket.  TLS provides the token it extracts from the session ticket
   to the transport when it asks whether source address validation is

   If both a HelloRetryRequest cookie and a session ticket are present
   in the ClientHello, only the token from the cookie is passed to the
   transport.  The presence of a cookie indicates that this is a second
   ClientHello - the token from the session ticket will have been
   provided to the transport when it appeared in the first ClientHello.

   A server can send a NewSessionTicket message at any time.  This
   allows it to update the state - and the address validation token -
   that is included in the ticket.  This might be done to refresh the
   ticket or token, or it might be generated in response to changes in
   the state of the connection.  QUIC can request that a
   NewSessionTicket be sent by providing a new address validation token.

   A server that intends to support 0-RTT SHOULD provide an address
   validation token immediately after completing the TLS handshake.

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8.3.  Address Validation Token Integrity

   TLS MUST provide integrity protection for address validation token
   unless the transport guarantees integrity protection by other means.
   For a NewSessionTicket that includes confidential information - such
   as the resumption secret - including the token under authenticated
   encryption ensures that the token gains both confidentiality and
   integrity protection without duplicating the overheads of that

9.  Pre-handshake QUIC Messages

   Implementations MUST NOT exchange data on any stream other than
   stream 0 without packet protection.  QUIC requires the use of several
   types of frame for managing loss detection and recovery during this
   phase.  In addition, it might be useful to use the data acquired
   during the exchange of unauthenticated messages for congestion

   This section generally only applies to TLS handshake messages from
   both peers and acknowledgments of the packets carrying those
   messages.  In many cases, the need for servers to provide
   acknowledgments is minimal, since the messages that clients send are
   small and implicitly acknowledged by the server's responses.

   The actions that a peer takes as a result of receiving an
   unauthenticated packet needs to be limited.  In particular, state
   established by these packets cannot be retained once record
   protection commences.

   There are several approaches possible for dealing with
   unauthenticated packets prior to handshake completion:

   o  discard and ignore them

   o  use them, but reset any state that is established once the
      handshake completes

   o  use them and authenticate them afterwards; failing the handshake
      if they can't be authenticated

   o  save them and use them when they can be properly authenticated

   o  treat them as a fatal error

   Different strategies are appropriate for different types of data.
   This document proposes that all strategies are possible depending on
   the type of message.

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   o  Transport parameters are made usable and authenticated as part of
      the TLS handshake (see Section 10.2).

   o  Most unprotected messages are treated as fatal errors when
      received except for the small number necessary to permit the
      handshake to complete (see Section 9.1).

   o  Protected packets can either be discarded or saved and later used
      (see Section 9.3).

9.1.  Unprotected Packets Prior to Handshake Completion

   This section describes the handling of messages that are sent and
   received prior to the completion of the TLS handshake.

   Sending and receiving unprotected messages is hazardous.  Unless
   expressly permitted, receipt of an unprotected message of any kind
   MUST be treated as a fatal error.

9.1.1.  STREAM Frames

   "STREAM" frames for stream 0 are permitted.  These carry the TLS
   handshake messages.  Once 1-RTT keys are available, unprotected
   "STREAM" frames on stream 0 can be ignored.

   Receiving unprotected "STREAM" frames for other streams MUST be
   treated as a fatal error.

9.1.2.  ACK Frames

   "ACK" frames are permitted prior to the handshake being complete.
   Information learned from "ACK" frames cannot be entirely relied upon,
   since an attacker is able to inject these packets.  Timing and packet
   retransmission information from "ACK" frames is critical to the
   functioning of the protocol, but these frames might be spoofed or

   Endpoints MUST NOT use an unprotected "ACK" frame to acknowledge data
   that was protected by 0-RTT or 1-RTT keys.  An endpoint MUST ignore
   an unprotected "ACK" frame if it claims to acknowledge data that was
   sent in a protected packet.  Such an acknowledgement can only serve
   as a denial of service, since an endpoint that can read protected
   data is always able to send protected data.

   ISSUE:  What about 0-RTT data?  Should we allow acknowledgment of
      0-RTT with unprotected frames?  If we don't, then 0-RTT data will
      be unacknowledged until the handshake completes.  This isn't a
      problem if the handshake completes without loss, but it could mean

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      that 0-RTT stalls when a handshake packet disappears for any

   An endpoint SHOULD use data from unprotected or 0-RTT-protected "ACK"
   frames only during the initial handshake and while they have
   insufficient information from 1-RTT-protected "ACK" frames.  Once
   sufficient information has been obtained from protected messages,
   information obtained from less reliable sources can be discarded.

9.1.3.  Updates to Data and Stream Limits

   "MAX_STREAM_ID" frames MUST NOT be sent unprotected.

   Though data is exchanged on stream 0, the initial flow control window
   on that stream is sufficiently large to allow the TLS handshake to
   complete.  This limits the maximum size of the TLS handshake and
   would prevent a server or client from using an abnormally large
   certificate chain.

   Stream 0 is exempt from the connection-level flow control window.

   Consequently, there is no need to signal being blocked on flow

   Similarly, there is no need to increase the number of allowed streams
   until the handshake completes.

9.1.4.  Denial of Service with Unprotected Packets

   Accepting unprotected - specifically unauthenticated - packets
   presents a denial of service risk to endpoints.  An attacker that is
   able to inject unprotected packets can cause a recipient to drop even
   protected packets with a matching sequence number.  The spurious
   packet shadows the genuine packet, causing the genuine packet to be
   ignored as redundant.

   Once the TLS handshake is complete, both peers MUST ignore
   unprotected packets.  From that point onward, unprotected messages
   can be safely dropped.

   Since only TLS handshake packets and acknowledgments are sent in the
   clear, an attacker is able to force implementations to rely on
   retransmission for packets that are lost or shadowed.  Thus, an
   attacker that intends to deny service to an endpoint has to drop or
   shadow protected packets in order to ensure that their victim
   continues to accept unprotected packets.  The ability to shadow
   packets means that an attacker does not need to be on path.

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   In addition to causing valid packets to be dropped, an attacker can
   generate packets with an intent of causing the recipient to expend
   processing resources.  See Section 11.2 for a discussion of these

   To avoid receiving TLS packets that contain no useful data, a TLS
   implementation MUST reject empty TLS handshake records and any record
   that is not permitted by the TLS state machine.  Any TLS application
   data or alerts that is received prior to the end of the handshake
   MUST be treated as a fatal error.

9.2.  Use of 0-RTT Keys

   If 0-RTT keys are available, the lack of replay protection means that
   restrictions on their use are necessary to avoid replay attacks on
   the protocol.

   A client MUST only use 0-RTT keys to protect data that is idempotent.
   A client MAY wish to apply additional restrictions on what data it
   sends prior to the completion of the TLS handshake.  A client
   otherwise treats 0-RTT keys as equivalent to 1-RTT keys.

   A client that receives an indication that its 0-RTT data has been
   accepted by a server can send 0-RTT data until it receives all of the
   server's handshake messages.  A client SHOULD stop sending 0-RTT data
   if it receives an indication that 0-RTT data has been rejected.

   A server MUST NOT use 0-RTT keys to protect packets.

9.3.  Receiving Out-of-Order Protected Frames

   Due to reordering and loss, protected packets might be received by an
   endpoint before the final TLS handshake messages are received.  A
   client will be unable to decrypt 1-RTT packets from the server,
   whereas a server will be able to decrypt 1-RTT packets from the

   Packets protected with 1-RTT keys MAY be stored and later decrypted
   and used once the handshake is complete.  A server MUST NOT use 1-RTT
   protected packets before verifying either the client Finished message
   or - in the case that the server has chosen to use a pre-shared key -
   the pre-shared key binder (see Section 4.2.8 of
   [I-D.ietf-tls-tls13]).  Verifying these values provides the server
   with an assurance that the ClientHello has not been modified.

   A server could receive packets protected with 0-RTT keys prior to
   receiving a TLS ClientHello.  The server MAY retain these packets for
   later decryption in anticipation of receiving a ClientHello.

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   Receiving and verifying the TLS Finished message is critical in
   ensuring the integrity of the TLS handshake.  A server MUST NOT use
   protected packets from the client prior to verifying the client
   Finished message if its response depends on client authentication.

10.  QUIC-Specific Additions to the TLS Handshake

   QUIC uses the TLS handshake for more than just negotiation of
   cryptographic parameters.  The TLS handshake validates protocol
   version selection, provides preliminary values for QUIC transport
   parameters, and allows a server to perform return routeability checks
   on clients.

10.1.  Protocol and Version Negotiation

   The QUIC version negotiation mechanism is used to negotiate the
   version of QUIC that is used prior to the completion of the
   handshake.  However, this packet is not authenticated, enabling an
   active attacker to force a version downgrade.

   To ensure that a QUIC version downgrade is not forced by an attacker,
   version information is copied into the TLS handshake, which provides
   integrity protection for the QUIC negotiation.  This does not prevent
   version downgrade prior to the completion of the handshake, though it
   means that a downgrade causes a handshake failure.

   TLS uses Application Layer Protocol Negotiation (ALPN) [RFC7301] to
   select an application protocol.  The application-layer protocol MAY
   restrict the QUIC versions that it can operate over.  Servers MUST
   select an application protocol compatible with the QUIC version that
   the client has selected.

   If the server cannot select a compatible combination of application
   protocol and QUIC version, it MUST abort the connection.  A client
   MUST abort a connection if the server picks an incompatible
   combination of QUIC version and ALPN identifier.

10.2.  QUIC Transport Parameters Extension

   QUIC transport parameters are carried in a TLS extension.  Different
   versions of QUIC might define a different format for this struct.

   Including transport parameters in the TLS handshake provides
   integrity protection for these values.

      enum {
         quic_transport_parameters(26), (65535)
      } ExtensionType;

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   The "extension_data" field of the quic_transport_parameters extension
   contains a value that is defined by the version of QUIC that is in
   use.  The quic_transport_parameters extension carries a
   TransportParameters when the version of QUIC defined in
   [QUIC-TRANSPORT] is used.

   The quic_transport_parameters extension is carried in the ClientHello
   and the EncryptedExtensions messages during the handshake.  The
   extension MAY be included in a NewSessionTicket message.

10.3.  Priming 0-RTT

   QUIC uses TLS without modification.  Therefore, it is possible to use
   a pre-shared key that was established in a TLS handshake over TCP to
   enable 0-RTT in QUIC.  Similarly, QUIC can provide a pre-shared key
   that can be used to enable 0-RTT in TCP.

   All the restrictions on the use of 0-RTT apply, with the exception of
   the ALPN label, which MUST only change to a label that is explicitly
   designated as being compatible.  The client indicates which ALPN
   label it has chosen by placing that ALPN label first in the ALPN

   The certificate that the server uses MUST be considered valid for
   both connections, which will use different protocol stacks and could
   use different port numbers.  For instance, HTTP/1.1 and HTTP/2
   operate over TLS and TCP, whereas QUIC operates over UDP.

   Source address validation is not completely portable between
   different protocol stacks.  Even if the source IP address remains
   constant, the port number is likely to be different.  Packet
   reflection attacks are still possible in this situation, though the
   set of hosts that can initiate these attacks is greatly reduced.  A
   server might choose to avoid source address validation for such a
   connection, or allow an increase to the amount of data that it sends
   toward the client without source validation.

11.  Security Considerations

   There are likely to be some real clangers here eventually, but the
   current set of issues is well captured in the relevant sections of
   the main text.

   Never assume that because it isn't in the security considerations
   section it doesn't affect security.  Most of this document does.

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11.1.  Packet Reflection Attack Mitigation

   A small ClientHello that results in a large block of handshake
   messages from a server can be used in packet reflection attacks to
   amplify the traffic generated by an attacker.

   Certificate caching [RFC7924] can reduce the size of the server's
   handshake messages significantly.

   QUIC requires that the packet containing a ClientHello be padded to a
   minimum size.  A server is less likely to generate a packet
   reflection attack if the data it sends is a small multiple of this
   size.  A server SHOULD use a HelloRetryRequest if the size of the
   handshake messages it sends is likely to significantly exceed the
   size of the packet containing the ClientHello.

11.2.  Peer Denial of Service

   QUIC, TLS and HTTP/2 all contain a messages that have legitimate uses
   in some contexts, but that can be abused to cause a peer to expend
   processing resources without having any observable impact on the
   state of the connection.  If processing is disproportionately large
   in comparison to the observable effects on bandwidth or state, then
   this could allow a malicious peer to exhaust processing capacity
   without consequence.

   QUIC prohibits the sending of empty "STREAM" frames unless they are
   marked with the FIN bit.  This prevents "STREAM" frames from being
   sent that only waste effort.

   TLS records SHOULD always contain at least one octet of a handshake
   messages or alert.  Records containing only padding are permitted
   during the handshake, but an excessive number might be used to
   generate unnecessary work.  Once the TLS handshake is complete,
   endpoints SHOULD NOT send TLS application data records unless it is
   to hide the length of QUIC records.  QUIC packet protection does not
   include any allowance for padding; padded TLS application data
   records can be used to mask the length of QUIC frames.

   While there are legitimate uses for some redundant packets,
   implementations SHOULD track redundant packets and treat excessive
   volumes of any non-productive packets as indicative of an attack.

12.  Error codes

   The portion of the QUIC error code space allocated for the crypto
   handshake is 0xC0000000-0xFFFFFFFF.  The following error codes are
   defined when TLS is used for the crypto handshake:

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   TLS_HANDSHAKE_FAILED (0xC000001C):  The TLS handshake failed.

   TLS_FATAL_ALERT_GENERATED (0xC000001D):  A TLS fatal alert was sent,
      causing the TLS connection to end prematurely.

   TLS_FATAL_ALERT_RECEIVED (0xC000001E):  A TLS fatal alert was
      received, causing the TLS connection to end prematurely.

13.  IANA Considerations

   This document does not create any new IANA registries, but it does
   utilize the following registries:

   o  QUIC Transport Parameter Registry - IANA is to register the three
      values found in Section 12.

   o  TLS ExtensionsType Registry - IANA is to register the
      quic_transport_parameters extension found in Section 10.2.
      Assigning 26 to the extension would be greatly appreciated.  The
      Recommended column is to be marked Yes.

   o  TLS Exporter Label Registry - IANA is requested to register
      "EXPORTER-QUIC 0-RTT Secret" from Section 5.2.1; "EXPORTER-QUIC
      client 1-RTT Secret" and "EXPORTER-QUIC server 1-RTT Secret" from
      Section 5.2.2; "EXPORTER-QUIC Packet Number Secret" Section 5.6.
      The DTLS column is to be marked No.  The Recommended column is to
      be marked Yes.

14.  References

14.1.  Normative References

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-20 (work in progress),
              April 2017.

              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", draft-ietf-quic-
              transport (work in progress), June 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,

   [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>.

14.2.  Informative References

              Luykx, A. and K. Paterson, "Limits on Authenticated
              Encryption Use in TLS", March 2016,

   [FNV]      Fowler, G., Noll, L., Vo, K., Eastlake, D., and T. Hansen,
              "The FNV Non-Cryptographic Hash Algorithm", draft-
              eastlake-fnv-13 (work in progress), June 2017.

              Bishop, M., Ed., "Hypertext Transfer Protocol (HTTP) over
              QUIC", draft-ietf-quic-http (work in progress), June 2017.

              Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
              and Congestion Control", draft-ietf-quic-recovery (work in
              progress), June 2017.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

   [RFC7924]  Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", RFC 7924,
              DOI 10.17487/RFC7924, July 2016,

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Appendix A.  Contributors

   Ryan Hamilton was originally an author of this specification.

Appendix B.  Acknowledgments

   This document has benefited from input from Dragana Damjanovic,
   Christian Huitema, Jana Iyengar, Adam Langley, Roberto Peon, Eric
   Rescorla, Ian Swett, and many others.

Appendix C.  Change Log

      *RFC Editor's Note:* Please remove this section prior to
      publication of a final version of this document.

   Issue and pull request numbers are listed with a leading octothorp.

C.1.  Since draft-ietf-quic-tls-02

   o  Updates to match changes in transport draft

C.2.  Since draft-ietf-quic-tls-01

   o  Use TLS alerts to signal TLS errors (#272, #374)

   o  Require ClientHello to fit in a single packet (#338)

   o  The second client handshake flight is now sent in the clear (#262,

   o  The QUIC header is included as AEAD Associated Data (#226, #243,

   o  Add interface necessary for client address validation (#275)

   o  Define peer authentication (#140)

   o  Require at least TLS 1.3 (#138)

   o  Define transport parameters as a TLS extension (#122)

   o  Define handling for protected packets before the handshake
      completes (#39)

   o  Decouple QUIC version and ALPN (#12)

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C.3.  Since draft-ietf-quic-tls-00

   o  Changed bit used to signal key phase

   o  Updated key phase markings during the handshake

   o  Added TLS interface requirements section

   o  Moved to use of TLS exporters for key derivation

   o  Moved TLS error code definitions into this document

C.4.  Since draft-thomson-quic-tls-01

   o  Adopted as base for draft-ietf-quic-tls

   o  Updated authors/editors list

   o  Added status note

Authors' Addresses

   Martin Thomson (editor)

   Email: martin.thomson@gmail.com

   Sean Turner (editor)

   Email: sean@sn3rd.com

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