Network Working Group                                         M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Standards Track                             R. Hamilton
Expires: April 28, 2017                                           Google
                                                        October 25, 2016

          Using Transport Layer Security (TLS) to Secure QUIC


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

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 April 28, 2017.

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   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Handshake Overview  . . . . . . . . . . . . . . . . . . .   4
   3.  TLS in Stream 1 . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Handshake and Setup Sequence  . . . . . . . . . . . . . .   6
   4.  QUIC Record Protection  . . . . . . . . . . . . . . . . . . .   8
     4.1.  Key Phases  . . . . . . . . . . . . . . . . . . . . . . .   8
       4.1.1.  Retransmission of TLS Handshake Messages  . . . . . .   9
       4.1.2.  Key Update  . . . . . . . . . . . . . . . . . . . . .  10
     4.2.  QUIC Key Expansion  . . . . . . . . . . . . . . . . . . .  11
     4.3.  QUIC AEAD application . . . . . . . . . . . . . . . . . .  12
     4.4.  Sequence Number Reconstruction  . . . . . . . . . . . . .  12
   5.  Pre-handshake QUIC Messages . . . . . . . . . . . . . . . . .  13
     5.1.  Unprotected Frames Prior to Handshake Completion  . . . .  14
       5.1.1.  STREAM Frames . . . . . . . . . . . . . . . . . . . .  14
       5.1.2.  ACK Frames  . . . . . . . . . . . . . . . . . . . . .  15
       5.1.3.  WINDOW_UPDATE Frames  . . . . . . . . . . . . . . . .  15
       5.1.4.  Denial of Service with Unprotected Packets  . . . . .  15
     5.2.  Use of 0-RTT Keys . . . . . . . . . . . . . . . . . . . .  16
     5.3.  Protected Frames Prior to Handshake Completion  . . . . .  17
   6.  QUIC-Specific Additions to the TLS Handshake  . . . . . . . .  18
     6.1.  Protocol and Version Negotiation  . . . . . . . . . . . .  18
     6.2.  QUIC Extension  . . . . . . . . . . . . . . . . . . . . .  18
     6.3.  Source Address Validation . . . . . . . . . . . . . . . .  19
     6.4.  Priming 0-RTT . . . . . . . . . . . . . . . . . . . . . .  19
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
     7.1.  Packet Reflection Attack Mitigation . . . . . . . . . . .  20
     7.2.  Peer Denial of Service  . . . . . . . . . . . . . . . . .  20
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  21
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   QUIC [I-D.hamilton-quic-transport-protocol] provides a multiplexed
   transport for HTTP [RFC7230] semantics that provides several key
   advantages over HTTP/1.1 [RFC7230] or HTTP/2 [RFC7540] over TCP

   This document describes how QUIC can be 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

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   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, zero round trip setup.

   This document describes how the standardized TLS 1.3 can act a
   security component of QUIC.  The same design could work for TLS 1.2,
   though few of the benefits QUIC provides would be realized due to the
   handshake latency in versions of TLS prior to 1.3.

1.1.  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].

2.  Protocol Overview

   QUIC [I-D.hamilton-quic-transport-protocol] can be separated into
   several modules:

   1.  The basic frame envelope describes the common packet layout.
       This layer includes connection identification, version
       negotiation, and includes markers that allow the framing and
       public reset to be identified.

   2.  The public reset is an unprotected packet that allows an
       intermediary (an entity that is not part of the security context)
       to request the termination of a QUIC connection.

   3.  Version negotiation frames are used to agree on a common version
       of QUIC to use.

   4.  Framing comprises most of the QUIC protocol.  Framing provides a
       number of different types of frame, each with a specific purpose.
       Framing supports frames for both congestion management and stream
       multiplexing.  Framing additionally provides a liveness testing
       capability (the PING frame).

   5.  Encryption provides confidentiality and integrity protection for
       frames.  All frames are protected based on keying material
       derived from the TLS connection running on stream 1.  Prior to
       this, data is protected with the 0-RTT keys.

   6.  Multiplexed streams are the primary payload of QUIC.  These
       provide reliable, in-order delivery of data and are used to carry
       the encryption handshake and transport parameters (stream 1),
       HTTP header fields (stream 3), and HTTP requests and responses.

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       Frames for managing multiplexing include those for creating and
       destroying streams as well as flow control and priority frames.

   7.  Congestion management includes packet acknowledgment and other
       signal required to ensure effective use of available link

   8.  A complete TLS connection is run on stream 1.  This includes the
       entire TLS record layer.  As the TLS connection reaches certain
       states, keying material is provided to the QUIC encryption layer
       for protecting the remainder of the QUIC traffic.

   9.  HTTP mapping provides an adaptation to HTTP that is based on

   The relative relationship of these components are pictorally
   represented in Figure 1.

      | TLS | HTTP |
      |  Streams   | Congestion |
      |         Frames          +--------+---------+
      +   +---------------------+ Public | Version |
      |   |     Encryption      | Reset  |  Nego.  |
      |                   Envelope                 |
      |                     UDP                    |

                         Figure 1: QUIC Structure

   This document defines the cryptographic parts of QUIC.  This includes
   the handshake messages that are exchanged on stream 1, plus the
   record protection that is used to encrypt and authenticate all other

2.1.  Handshake Overview

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

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

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   o  A 0-RTT handshake in which the client uses information about the
      server to send immediately.  This 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)
      (end_of_early_data)        -------->
                                <--------      [Application Data]
      {Finished}                -------->

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

                    Figure 2: TLS Handshake with 0-RTT

   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 Section 6.3).

   o  A pre-shared key mode can be used for subsequent handshakes to
      avoid 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.

3.  TLS in Stream 1

   QUIC completes its cryptographic handshake on stream 1, which means
   that the negotiation of keying material happens after the QUIC
   protocol has started.  This simplifies the use of TLS since QUIC is

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   able to ensure that the TLS handshake packets are delivered reliably
   and in order.

   QUIC Stream 1 carries a complete TLS connection.  This includes the
   TLS record layer in its entirety.  QUIC provides for reliable and in-
   order delivery of the TLS handshake messages on this stream.

   Prior to the completion of the TLS handshake, QUIC frames can be
   exchanged.  However, these frames are not authenticated or
   confidentiality protected.  Section 5 covers some of the implications
   of this design and limitations on QUIC operation during this phase.

   Once complete, QUIC frames are protected using QUIC record
   protection, see Section 4.

3.1.  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 1 carry the TLS
   handshake.  QUIC is responsible for ensuring that the handshake
   packets are re-sent in case of loss and that they can be ordered

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       Client                                             Server

   @A QUIC STREAM Frame(s) <1>:
          + QUIC Setup Parameters
                            0-RTT Key -> @B

   @B QUIC STREAM Frame(s) <1>:
      Replayable QUIC Frames <any stream>

                                         QUIC STREAM Frame <1>: @B/A
                                         {Handshake Messages}
                           1-RTT Key -> @C

                                                    QUIC Frames @C
   @B QUIC STREAM Frame(s) <1>:
        (end_of_early_data <1>)

   @C QUIC Frames              <------->            QUIC Frames @C

                     Figure 3: QUIC over TLS Handshake

   In Figure 3, symbols mean:

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

   o  "@" indicates the key phase that is currently used for protecting
      QUIC packets.

   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 possible, then the client does not send frames
   protected by the 0-RTT key (@B).  The only key transition on the
   client is from cleartext (@A) to 1-RTT protection (@C).

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   If 0-RTT data is not accepted by the server, then the server sends
   its handshake messages without protection (@A).  The client still
   transitions from @A to @B, but it can stop sending 0-RTT data and
   progress immediately to 1-RTT data when it receives a cleartext

4.  QUIC Record Protection

   QUIC provides a record protection layer that is responsible for
   authenticated encryption of packets.  The record protection layer
   uses keys provided by the TLS connection and authenticated encryption
   to provide confidentiality and integrity protection for the content
   of packets.

   Different keys are used for QUIC and TLS record protection.  Having
   separate QUIC and TLS record protection means that TLS records can be
   protected by two different keys.  This redundancy is maintained for
   the sake of simplicity.

4.1.  Key Phases

   The transition to use of a new QUIC key occurs immediately after
   sending the TLS handshake messages that produced the key transition.
   Every time that a new set of keys is used for protecting outbound
   messages, the KEY_PHASE bit in the public flags is toggled.  The
   KEY_PHASE bit on unencrypted messages is 0.

   The KEY_PHASE bit on the public flags is the most significant bit

   The KEY_PHASE bit allows a recipient to detect a change in keying
   material without needing to receive the message that triggers the
   change.  This avoids head-of-line blocking around transitions between
   keys without relying on trial decryption.

   The following transitions are defined:

   o  The client transitions to using 0-RTT keys after sending the
      ClientHello.  This causes the KEY_PHASE bit on packets sent by the
      client to be set to 1.

   o  The server transitions to using 0-RTT keys before sending the
      ServerHello, but only if the early data from the client is
      accepted.  This transition causes the KEY_PHASE bit on packets
      sent by the server to be set to 1.  If the server rejects 0-RTT
      data, the server's handshake messages are sent without QUIC-level
      record protection with a KEY_PHASE of 0.  TLS handshake messages

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      will still be protected by TLS record protection based on the TLS
      handshake traffic keys.

   o  The server transitions to using 1-RTT keys after sending its
      Finished message.  This causes the KEY_PHASE bit to be set to 0 if
      early data was accepted, and 1 if the server rejected early data.

   o  The client transitions to 1-RTT keys after sending its Finished
      message.  Subsequent messages from the client will then have a
      KEY_PHASE of 0 if 0-RTT data was sent, and 1 otherwise.

   o  Both peers start sending messages protected by a new key
      immediately after sending a TLS KeyUpdate message.  The value of
      the KEY_PHASE bit is changed each time.

   At each point, both keying material (see Section 4.2) and the AEAD
   function used by TLS is interchanged with the values that are
   currently in use for protecting outbound packets.  Once a change of
   keys has been made, packets with higher sequence numbers MUST use the
   new keying material until a newer set of keys (and AEAD) are used.
   The exception to this is that retransmissions of TLS handshake
   packets MUST use the keys that they were originally protected with.

   Once a packet protected by a new key has been received, a recipient
   SHOULD retain the previous keys for a short period.  Retaining old
   keys allows the recipient to decode reordered packets around a change
   in keys.  Keys SHOULD be discarded when an endpoints has received all
   packets with sequence numbers lower than the lowest sequence number
   used for the new key, or when it determines that reordering of those
   packets is unlikely.  0-RTT keys SHOULD be retained until the
   handshake is complete.

   The KEY_PHASE bit does not directly indicate which keys are in use.
   Depending on whether 0-RTT data was sent and accepted, packets
   protected with keys derived from the same secret might be marked with
   different KEY_PHASE values.

4.1.1.  Retransmission of TLS Handshake Messages

   TLS handshake messages need to be retransmitted with the same level
   of cryptographic protection that was originally used to protect them.
   Newer keys cannot be used to protect QUIC packets that carry TLS

   A client would be unable to decrypt retransmissions of a server's
   handshake messages that are protected using the 1-RTT keys, since the
   calculation of the application data keys depends on the contents of
   the handshake messages.

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   This restriction means the creation of an exception to the
   requirement to always use new keys for sending once they are
   available.  A server MUST mark the retransmitted handshake messages
   with the same KEY_PHASE as the original messages to allow a recipient
   to distinguish the messages.

4.1.2.  Key Update

   Once the TLS handshake is complete, the KEY_PHASE bit allows for the
   processing of messages without having to receive the TLS KeyUpdate
   message that triggers the key update.  This allows endpoints to start
   using updated keys immediately without the concern that a lost
   KeyUpdate will cause their messages to be indecipherable to their

   An endpoint MUST NOT initiate more than one key update at a time.  A
   new key update cannot be sent until the endpoint has received a
   matching KeyUpdate message from its peer; or, if the endpoint did not
   initiate the original key update, it has received an acknowledgment
   of its own KeyUpdate.

   This ensures that there are at most two keys to distinguish between
   at any one time, for which the KEY_PHASE bit is sufficient.

      Initiating Peer                    Responding Peer

   @M KeyUpdate
                       New Keys -> @N
   @N QUIC Frames
                                               KeyUpdate @N
     -- Initiating Peer can initiate another KeyUpdate --
    @N Acknowledgment
     -- Responding Peer can initiate another KeyUpdate --

                           Figure 4: Key Update

   As shown in Figure 3 and Figure 4, there is never a situation where
   there are more than two different sets of keying material that might
   be received by a peer.

   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 it has

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   received an acknowledgment that its Finished message has been

   Note:  This models the key changes in the handshake as a key update
      initiated by the server, with the Finished message in the place of

4.2.  QUIC Key Expansion

   The following table shows QUIC keys, when they are generated and the
   TLS secret from which they are derived:

       | Key   | TLS Secret           | Phase                      |
       | 0-RTT | early_traffic_secret | "QUIC 0-RTT key expansion" |
       |       |                      |                            |
       | 1-RTT | traffic_secret_N     | "QUIC 1-RTT key expansion" |

   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 5.2.  0-RTT keys are used after sending or receiving a

   1-RTT keys are used after the TLS handshake completes.  There are
   potentially multiple sets of 1-RTT keys; new 1-RTT keys are created
   by sending a TLS KeyUpdate message.  1-RTT keys are used after
   sending a Finished or KeyUpdate message.

   The complete key expansion uses the same process for key expansion as
   defined in Section 7.3 of [I-D.ietf-tls-tls13].  For example, the
   Client Write Key for the data sent immediately after sending the TLS
   Finished message is:

      label = "QUIC 1-RTT key expansion, client write key"
      client_write = HKDF-Expand-Label(traffic_secret_0, label,
                                       "", key_length)

   This results in a label input to HKDF that includes a two-octet
   length field, the string "TLS 1.3, QUIC 1-RTT key expansion, client
   write key" and a zero octet.

   The QUIC record protection initially starts without keying material.
   When the TLS state machine produces the corresponding secret, new
   keys are generated from the TLS connection and used to protect the
   QUIC record protection.

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

4.3.  QUIC AEAD application

   Regular QUIC packets are protected by an AEAD [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, for the AEAD is either the Client Write Key or the Server
   Write Key, derived as defined in Section 4.2.

   The nonce, N, for the AEAD is formed by combining either the Client
   Write IV or Server Write IV with the sequence numbers.  The 48 bits
   of the reconstructed QUIC sequence number (see Section 4.4) in
   network byte order is left-padded with zeros to the N_MAX parameter
   of the AEAD (see Section 4 of [RFC5116]).  The exclusive OR of the
   padded sequence number and the IV forms the AEAD nonce.

   The associated data, A, for the AEAD is an empty sequence.

   The input plaintext, P, for the AEAD is the contents of the QUIC
   frame following the packet number, as described in

   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.

   Note:  QUIC defined a null-encryption that had an additional, hash-
      based checksum for cleartext packets.  This might be added here,
      but it is more complex.

4.4.  Sequence Number Reconstruction

   Each peer maintains a 48-bit sequence number that is incremented with
   every packet that is sent, including retransmissions.  The least
   significant 8-, 16-, 32-, or 48-bits of this number is encoded in the
   QUIC sequence number field in every packet.

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   A receiver maintains the same values, but recovers values based on
   the packets it receives.  This is based on the sequence number of
   packets that it has received.  A simple scheme predicts the receive
   sequence number of an incoming packet by incrementing the sequence
   number of the most recent packet to be successfully decrypted by one
   and expecting the sequence number to be within a range centered on
   that value.

   A more sophisticated algorithm can almost double the search space by
   checking backwards from the most recent sequence for a received (or
   abandoned) packet.  If a packet was received, then the packet
   contains a sequence number that is greater than the most recent
   sequence number.  If no such packet was found, the number is assumed
   to be in the smaller window centered on the next sequence number, as
   in the simpler scheme.

   Note:  QUIC has a single, contiguous sequence 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 discarded).  QUIC does not assume a reliable
      transport and is therefore required to handle attacks where
      packets are dropped in other ways.  TLS maintains a separate
      sequence number that is used for record protection on the
      connection that is hosted on stream 1.  This sequence number is
      reset according to the rules in the TLS protocol.

5.  Pre-handshake QUIC Messages

   Implementations MUST NOT exchange data on any stream other than
   stream 1 prior to the completion of the TLS handshake.  However, QUIC
   requires the use of several types of frame for managing loss
   detection and recovery.  In addition, it might be useful to use the
   data acquired during the exchange of unauthenticated messages for
   congestion management.

   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.

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   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.

   o  Transport parameters and options are made usable and authenticated
      as part of the TLS handshake (see Section 6.2).

   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 5.1).

   o  Protected packets can either be discarded or saved and later used
      (see Section 5.3).

5.1.  Unprotected Frames 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.

5.1.1.  STREAM Frames

   "STREAM" frames for stream 1 are permitted.  These carry the TLS
   handshake messages.

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

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5.1.2.  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
   protected data.  Such an acknowledgement can only serve as a denial
   of service, since an endpoint that can read protected data is always
   permitted to send protected data.

   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.

5.1.3.  WINDOW_UPDATE Frames

   "WINDOW_UPDATE" frames MUST NOT be sent unprotected.

   Though data is exchanged on stream 1, the initial flow control window
   is 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 1 is exempt from the connection-level flow control window.

5.1.4.  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.  The handshake is complete when the server
   receives a client's Finished message and when a client receives an
   acknowledgement that their Finished message was received.  From that
   point onward, unprotected messages can be safely dropped.  Note that

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   the client could retransmit its Finished message to the server, so
   the server cannot reject such a message.

   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.

   ISSUE:  This would not be an issue if QUIC had a randomized starting
      sequence number.  If we choose to randomize, we fix this problem
      and reduce the denial of service exposure to on-path attackers.
      The only possible problem is in authenticating the initial value,
      so that peers can be sure that they haven't missed an initial

   In addition to denying endpoints messages, an attacker to generate
   packets that cause no state change in a recipient.  See Section 7.2
   for a discussion of these risks.

   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 - other than a single end_of_early_data at the
   appropriate time - that is received prior to the end of the handshake
   MUST be treated as a fatal error.

5.2.  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.  In
   addition to a ServerHello without an early_data extension, an
   unprotected handshake message with a KEY_PHASE bit set to 0 indicates
   that 0-RTT data has been rejected.

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   A client SHOULD send its end_of_early_data alert only after it has
   received all of the server's handshake messages.  Alternatively
   phrased, a client is encouraged to use 0-RTT keys until 1-RTT keys
   become available.  This prevents stalling of the connection and
   allows the client to send continuously.

   A server MUST NOT use 0-RTT keys to protect anything other than TLS
   handshake messages.  Servers therefore treat packets protected with
   0-RTT keys as equivalent to unprotected packets in determining what
   is permissible to send.  A server protects handshake messages using
   the 0-RTT key if it decides to accept a 0-RTT key.  A server MUST
   still include the early_data extension in its ServerHello message.

   This restriction prevents a server from responding to a request using
   frames protected by the 0-RTT keys.  This ensures that all
   application data from the server are always protected with keys that
   have forward secrecy.  However, this results in head-of-line blocking
   at the client because server responses cannot be decrypted until all
   the server's handshake messages are received by the client.

5.3.  Protected Frames Prior to Handshake Completion

   Due to reordering and loss, protected packets might be received by an
   endpoint before the final handshake messages are received.  If these
   can be decrypted successfully, such packets MAY be stored and used
   once the handshake is complete.

   Unless expressly permitted below, encrypted packets MUST NOT be used
   prior to completing the TLS handshake, in particular the receipt of a
   valid Finished message and any authentication of the peer.  If
   packets are processed prior to completion of the handshake, an
   attacker might use the willingness of an implementation to use these
   packets to mount attacks.

   TLS handshake messages are covered by record protection during the
   handshake, once key agreement has completed.  This means that
   protected messages need to be decrypted to determine if they are TLS
   handshake messages or not.  Similarly, "ACK" and "WINDOW_UPDATE"
   frames might be needed to successfully complete the TLS handshake.

   Any timestamps present in "ACK" frames MUST be ignored rather than
   causing a fatal error.  Timestamps on protected frames MAY be saved
   and used once the TLS handshake completes successfully.

   An endpoint MAY save the last protected "WINDOW_UPDATE" frame it
   receives for each stream and apply the values once the TLS handshake
   completes.  Failing to do this might result in temporary stalling of
   affected streams.

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6.  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.

6.1.  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 during the handshake, though it means that such a
   downgrade causes a handshake failure.

   Protocols that use the QUIC transport MUST use Application Layer
   Protocol Negotiation (ALPN) [RFC7301].  The ALPN identifier for the
   protocol MUST be specific to the QUIC version that it operates over.
   When constructing a ClientHello, clients MUST include a list of all
   the ALPN identifiers that they support, regardless of whether the
   QUIC version that they have currently selected supports that

   Servers SHOULD select an application protocol based solely on the
   information in the ClientHello, not using the QUIC version that the
   client has selected.  If the protocol that is selected is not
   supported with the QUIC version that is in use, the server MUST
   either send a QUIC version negotiation packet if this is possible, or
   fail the connection otherwise.

6.2.  QUIC Extension

   QUIC defines an extension for use with TLS.  That extension defines
   transport-related parameters.  This provides integrity protection for
   these values.  Including these in the TLS handshake also make the
   values that a client sets available to a server one-round trip
   earlier than parameters that are carried in QUIC frames.  This
   document does not define that extension.

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6.3.  Source Address Validation

   QUIC implementations describe a source address token.  This is an
   opaque blob that a server might provide to clients when they first
   use a given source address.  The client returns this token in
   subsequent messages as a return routeability check.  That is, the
   client returns this token to prove that it is able to receive packets
   at the source address that it claims.  This prevents the server from
   being used in packet reflection attacks (see Section 7.1).

   A source address token is opaque and consumed only by the server.
   Therefore it can be included in the TLS 1.3 pre-shared key identifier
   for 0-RTT handshakes.  Servers that use 0-RTT are advised to provide
   new pre-shared key identifiers after every handshake to avoid
   linkability of connections by passive observers.  Clients MUST use a
   new pre-shared key identifier for every connection that they
   initiate; if no pre-shared key identifier is available, then
   resumption is not possible.

   A server that is under load might include a source address token in
   the cookie extension of a HelloRetryRequest.  (Note: the current
   version of TLS 1.3 does not include the ability to include a cookie
   in HelloRetryRequest.)

6.4.  Priming 0-RTT

   QUIC uses TLS without modification.  Therefore, it is possible to use
   a pre-shared key that was obtained in a TLS connection 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, and the certificate
   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.

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7.  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.

7.1.  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.

   A client SHOULD also pad [RFC7685] its ClientHello to at least 1024
   octets (TODO: tune this value).  A server is less likely to generate
   a packet reflection attack if the data it sends is a small multiple
   of the data it receives.  A server SHOULD use a HelloRetryRequest if
   the size of the handshake messages it sends is likely to exceed the
   size of the ClientHello.

7.2.  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.

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   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.

8.  IANA Considerations

   This document has no IANA actions.  Yet.

9.  References

9.1.  Normative References

              Hamilton, R., Iyengar, J., Swett, I., and A. Wilk, "QUIC:
              A UDP-Based Multiplexed and Secure Transport", draft-
              hamilton-quic-transport-protocol-00 (work in progress),
              July 2016.

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-17 (work in progress),
              October 2016.

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

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

   [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, <>.

9.2.  Informative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [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,

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   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [RFC7685]  Langley, A., "A Transport Layer Security (TLS) ClientHello
              Padding Extension", RFC 7685, DOI 10.17487/RFC7685,
              October 2015, <>.

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

Appendix A.  Acknowledgments

   Christian Huitema's knowledge of QUIC is far better than my own.
   This would be even more inaccurate and useless if not for his
   assistance.  This document has variously benefited from a long series
   of discussions with Jana Iyengar, Adam Langley, Roberto Peon, Eric
   Rescorla, Ian Swett, and likely many others who are merely forgotten
   by a faulty meat computer.

Authors' Addresses

   Martin Thomson


   Ryan Hamilton


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