Using TLS to Secure QUIC
draft-ietf-quic-tls-17

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QUIC                                                     M. Thomson, Ed.
Internet-Draft                                                   Mozilla
Intended status: Standards Track                          S. Turner, Ed.
Expires: June 21, 2019                                             sn3rd
                                                       December 18, 2018

                        Using TLS to Secure QUIC
                         draft-ietf-quic-tls-17

Abstract

   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
   https://mailarchive.ietf.org/arch/search/?email_list=quic [1].

   Working Group information can be found at https://github.com/quicwg
   [2]; source code and issues list for this draft can be found at
   https://github.com/quicwg/base-drafts/labels/-tls [3].

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 21, 2019.

Copyright Notice

   Copyright (c) 2018 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   4
     2.1.  TLS Overview  . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Carrying TLS Messages . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Interface to TLS  . . . . . . . . . . . . . . . . . . . .   9
       4.1.1.  Sending and Receiving Handshake Messages  . . . . . .   9
       4.1.2.  Encryption Level Changes  . . . . . . . . . . . . . .  11
       4.1.3.  TLS Interface Summary . . . . . . . . . . . . . . . .  12
     4.2.  TLS Version . . . . . . . . . . . . . . . . . . . . . . .  13
     4.3.  ClientHello Size  . . . . . . . . . . . . . . . . . . . .  14
     4.4.  Peer Authentication . . . . . . . . . . . . . . . . . . .  14
     4.5.  Enabling 0-RTT  . . . . . . . . . . . . . . . . . . . . .  15
     4.6.  Rejecting 0-RTT . . . . . . . . . . . . . . . . . . . . .  15
     4.7.  HelloRetryRequest . . . . . . . . . . . . . . . . . . . .  15
     4.8.  TLS Errors  . . . . . . . . . . . . . . . . . . . . . . .  16
     4.9.  Discarding Unused Keys  . . . . . . . . . . . . . . . . .  16
     4.10. Discarding Initial Keys . . . . . . . . . . . . . . . . .  17
   5.  Packet Protection . . . . . . . . . . . . . . . . . . . . . .  18
     5.1.  Packet Protection Keys  . . . . . . . . . . . . . . . . .  18
     5.2.  Initial Secrets . . . . . . . . . . . . . . . . . . . . .  18
     5.3.  AEAD Usage  . . . . . . . . . . . . . . . . . . . . . . .  19
     5.4.  Header Protection . . . . . . . . . . . . . . . . . . . .  20
       5.4.1.  Header Protection Application . . . . . . . . . . . .  21
       5.4.2.  Header Protection Sample  . . . . . . . . . . . . . .  22
       5.4.3.  AES-Based Header Protection . . . . . . . . . . . . .  23
       5.4.4.  ChaCha20-Based Header Protection  . . . . . . . . . .  24
     5.5.  Receiving Protected Packets . . . . . . . . . . . . . . .  24
     5.6.  Use of 0-RTT Keys . . . . . . . . . . . . . . . . . . . .  24
     5.7.  Receiving Out-of-Order Protected Frames . . . . . . . . .  25
   6.  Key Update  . . . . . . . . . . . . . . . . . . . . . . . . .  25
   7.  Security of Initial Messages  . . . . . . . . . . . . . . . .  27
   8.  QUIC-Specific Additions to the TLS Handshake  . . . . . . . .  28
     8.1.  Protocol and Version Negotiation  . . . . . . . . . . . .  28
     8.2.  QUIC Transport Parameters Extension . . . . . . . . . . .  28
     8.3.  Removing the EndOfEarlyData Message . . . . . . . . . . .  29

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   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  29
     9.1.  Packet Reflection Attack Mitigation . . . . . . . . . . .  29
     9.2.  Peer Denial of Service  . . . . . . . . . . . . . . . . .  30
     9.3.  Header Protection Analysis  . . . . . . . . . . . . . . .  30
     9.4.  Key Diversity . . . . . . . . . . . . . . . . . . . . . .  31
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  32
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     11.2.  Informative References . . . . . . . . . . . . . . . . .  33
     11.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  34
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  34
     A.1.  Since draft-ietf-quic-tls-14  . . . . . . . . . . . . . .  34
     A.2.  Since draft-ietf-quic-tls-13  . . . . . . . . . . . . . .  34
     A.3.  Since draft-ietf-quic-tls-12  . . . . . . . . . . . . . .  34
     A.4.  Since draft-ietf-quic-tls-11  . . . . . . . . . . . . . .  35
     A.5.  Since draft-ietf-quic-tls-10  . . . . . . . . . . . . . .  35
     A.6.  Since draft-ietf-quic-tls-09  . . . . . . . . . . . . . .  35
     A.7.  Since draft-ietf-quic-tls-08  . . . . . . . . . . . . . .  35
     A.8.  Since draft-ietf-quic-tls-07  . . . . . . . . . . . . . .  35
     A.9.  Since draft-ietf-quic-tls-05  . . . . . . . . . . . . . .  35
     A.10. Since draft-ietf-quic-tls-04  . . . . . . . . . . . . . .  35
     A.11. Since draft-ietf-quic-tls-03  . . . . . . . . . . . . . .  36
     A.12. Since draft-ietf-quic-tls-02  . . . . . . . . . . . . . .  36
     A.13. Since draft-ietf-quic-tls-01  . . . . . . . . . . . . . .  36
     A.14. Since draft-ietf-quic-tls-00  . . . . . . . . . . . . . .  36
     A.15. Since draft-thomson-quic-tls-01 . . . . . . . . . . . . .  37
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  37
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  37
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

1.  Introduction

   This document describes how QUIC [QUIC-TRANSPORT] is secured using
   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 TLS acts as a security component of QUIC.

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2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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

   For brevity, the acronym TLS is used to refer to TLS 1.3, though a
   newer version could be used (see Section 4.2).

2.1.  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
   forged.

   Internally, TLS is a layered protocol, with the structure shown
   below:

   +--------------+--------------+--------------+
   |  Handshake   |    Alerts    |  Application |
   |    Layer     |              |     Data     |
   |              |              |              |
   +--------------+--------------+--------------+
   |                                            |
   |               Record Layer                 |
   |                                            |
   +--------------------------------------------+

   Each upper layer (handshake, alerts, and application data) is carried
   as a series of typed TLS records.  Records are individually
   cryptographically protected and then transmitted over a reliable
   transport (typically TCP) which provides sequencing and guaranteed
   delivery.

   Change Cipher Spec records cannot be sent in QUIC.

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

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   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 resistant to tampering by attackers and it
   produces shared secrets that cannot be controlled by either
   participating peer.

   TLS 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
      responds 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 handshake with 0-RTT application data is shown in
   Figure 1.  Note that this omits the EndOfEarlyData message, which is
   not used in QUIC (see Section 8.3).

       Client                                             Server

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

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

       () Indicates messages protected by early data (0-RTT) keys
       {} Indicates messages protected using handshake keys
       [] Indicates messages protected using application data
          (1-RTT) keys

                    Figure 1: TLS Handshake with 0-RTT

   Data is protected using a number of encryption levels:

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   o  Plaintext

   o  Early Data (0-RTT) Keys

   o  Handshake Keys

   o  Application Data (1-RTT) Keys

   Application data may appear only in the early data and application
   data levels.  Handshake and Alert messages may appear in any level.

   The 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.

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 handshake [TLS13], but instead of carrying TLS records
   over QUIC (as with TCP), TLS Handshake and Alert messages are carried
   directly over the QUIC transport, which takes over the
   responsibilities of the TLS record layer, as shown below.

   +--------------+--------------+ +-------------+
   |     TLS      |     TLS      | |    QUIC     |
   |  Handshake   |    Alerts    | | Applications|
   |              |              | | (h2q, etc.) |
   +--------------+--------------+-+-------------+
   |                                             |
   |                QUIC Transport               |
   |   (streams, reliability, congestion, etc.)  |
   |                                             |
   +---------------------------------------------+
   |                                             |
   |            QUIC Packet Protection           |
   |                                             |
   +---------------------------------------------+

   QUIC also relies on TLS 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, ordered
   delivery, and record layer provided by QUIC.

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   At a high level, there are two main interactions between the TLS and
   QUIC components:

   o  The TLS component sends and receives messages via the QUIC
      component, with QUIC providing a reliable stream abstraction to
      TLS.

   o  The TLS component provides a series of updates to the QUIC
      component, including (a) new packet protection keys to install (b)
      state changes such as handshake completion, the server
      certificate, etc.

   Figure 2 shows these interactions in more detail, with the QUIC
   packet protection being called out specially.

   +------------+                        +------------+
   |            |<- Handshake Messages ->|            |
   |            |<---- 0-RTT Keys -------|            |
   |            |<--- Handshake Keys-----|            |
   |   QUIC     |<---- 1-RTT Keys -------|    TLS     |
   |            |<--- Handshake Done ----|            |
   +------------+                        +------------+
    |         ^
    | Protect | Protected
    v         | Packet
   +------------+
   |   QUIC     |
   |  Packet    |
   | Protection |
   +------------+

                    Figure 2: QUIC and TLS Interactions

   Unlike TLS over TCP, QUIC applications which want to send data do not
   send it through TLS "application_data" records.  Rather, they send it
   as QUIC STREAM frames which are then carried in QUIC packets.

4.  Carrying TLS Messages

   QUIC carries TLS handshake data in CRYPTO frames, each of which
   consists of a contiguous block of handshake data identified by an
   offset and length.  Those frames are packaged into QUIC packets and
   encrypted under the current TLS encryption level.  As with TLS over
   TCP, once TLS handshake data has been delivered to QUIC, it is QUIC's
   responsibility to deliver it reliably.  Each chunk of data that is
   produced by TLS is associated with the set of keys that TLS is
   currently using.  If QUIC needs to retransmit that data, it MUST use
   the same keys even if TLS has already updated to newer keys.

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   One important difference between TLS records (used with TCP) and QUIC
   CRYPTO frames is that in QUIC multiple frames may appear in the same
   QUIC packet as long as they are associated with the same encryption
   level.  For instance, an implementation might bundle a Handshake
   message and an ACK for some Handshake data into the same packet.

   Each encryption level has a specific list of frames which may appear
   in it.  The rules here generalize those of TLS, in that frames
   associated with establishing the connection can usually appear at any
   encryption level, whereas those associated with transferring data can
   only appear in the 0-RTT and 1-RTT encryption levels:

   o  CRYPTO frames MAY appear in packets of any encryption level except
      0-RTT.

   o  CONNECTION_CLOSE MAY appear in packets of any encryption level
      other than 0-RTT.

   o  PADDING frames MAY appear in packets of any encryption level.

   o  ACK frames MAY appear in packets of any encryption level other
      than 0-RTT, but can only acknowledge packets which appeared in
      that packet number space.

   o  STREAM frames MUST ONLY appear in the 0-RTT and 1-RTT levels.

   o  All other frame types MUST only appear at the 1-RTT levels.

   Because packets could be reordered on the wire, QUIC uses the packet
   type to indicate which level a given packet was encrypted under, as
   shown in Table 1.  When multiple packets of different encryption
   levels need to be sent, endpoints SHOULD use coalesced packets to
   send them in the same UDP datagram.

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            +-----------------+------------------+-----------+
            | Packet Type     | Encryption Level | PN Space  |
            +-----------------+------------------+-----------+
            | Initial         | Initial secrets  | Initial   |
            |                 |                  |           |
            | 0-RTT Protected | 0-RTT            | 0/1-RTT   |
            |                 |                  |           |
            | Handshake       | Handshake        | Handshake |
            |                 |                  |           |
            | Retry           | N/A              | N/A       |
            |                 |                  |           |
            | Short Header    | 1-RTT            | 0/1-RTT   |
            +-----------------+------------------+-----------+

                 Table 1: Encryption Levels by Packet Type

   Section 17 of [QUIC-TRANSPORT] shows how packets at the various
   encryption levels fit into the handshake process.

4.1.  Interface to TLS

   As shown in Figure 2, the interface from QUIC to TLS consists of
   three primary functions:

   o  Sending and receiving handshake messages

   o  Rekeying (both transmit and receive)

   o  Handshake state updates

   Additional functions might be needed to configure TLS.

4.1.1.  Sending and Receiving Handshake Messages

   In order to drive the handshake, TLS depends on being able to send
   and receive handshake messages.  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 8.2) that it wishes to carry.

   A QUIC client starts TLS by requesting TLS handshake bytes from TLS.
   The client acquires handshake bytes before sending its first packet.
   A QUIC server starts the process by providing TLS with the client's
   handshake bytes.

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   At any given time, the TLS stack at an endpoint will have a current
   sending encryption level and receiving encryption level.  Each
   encryption level is associated with a different flow of bytes, which
   is reliably transmitted to the peer in CRYPTO frames.  When TLS
   provides handshake bytes to be sent, they are appended to the current
   flow and any packet that includes the CRYPTO frame is protected using
   keys from the corresponding encryption level.

   QUIC takes the unprotected content of TLS handshake records as the
   content of CRYPTO frames.  TLS record protection is not used by QUIC.
   QUIC assembles CRYPTO frames into QUIC packets, which are protected
   using QUIC packet protection.

   When an endpoint receives a QUIC packet containing a CRYPTO frame
   from the network, it proceeds as follows:

   o  If the packet was in the TLS receiving encryption level, sequence
      the data into the input flow as usual.  As with STREAM frames, the
      offset is used to find the proper location in the data sequence.
      If the result of this process is that new data is available, then
      it is delivered to TLS in order.

   o  If the packet is from a previously installed encryption level, it
      MUST not contain data which extends past the end of previously
      received data in that flow.  Implementations MUST treat any
      violations of this requirement as a connection error of type
      PROTOCOL_VIOLATION.

   o  If the packet is from a new encryption level, it is saved for
      later processing by TLS.  Once TLS moves to receiving from this
      encryption level, saved data can be provided.  When providing data
      from any new encryption level to TLS, if there is data from a
      previous encryption level that TLS has not consumed, this MUST be
      treated as a connection error of type PROTOCOL_VIOLATION.

   Each time that TLS is provided with new data, new handshake bytes are
   requested from TLS.  TLS might not provide any bytes 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 bytes 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

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   might wish to provide additional or updated session tickets to a
   client.

   When the handshake is complete, QUIC only needs to provide TLS with
   any data that arrives in CRYPTO streams.  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 CRYPTO frame that carries the
      Finished message in multiple packets.  This enables immediate
      server processing for those packets.

4.1.2.  Encryption Level Changes

   As keys for new encryption levels become available, TLS provides QUIC
   with those keys.  Separately, as TLS starts using keys at a given
   encryption level, TLS indicates to QUIC that it is now reading or
   writing with keys at that encryption level.  These events are not
   asynchronous; they always occur immediately after TLS is provided
   with new handshake bytes, or after TLS produces handshake bytes.

   TLS provides QUIC with three items as a new encryption level becomes
   available:

   o  A secret

   o  An Authenticated Encryption with Associated Data (AEAD) function

   o  A Key Derivation Function (KDF)

   These values are based on the values that TLS negotiates and are used
   by QUIC to generate packet and header protection keys (see Section 5
   and Section 5.4).

   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 bytes, the TLS stack
   might signal the change to 0-RTT keys.  On the server, after

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   receiving handshake bytes that contain a ClientHello message, a TLS
   server might signal that 0-RTT keys are available.

   Although TLS only uses one encryption level at a time, QUIC may use
   more than one level.  For instance, after sending its Finished
   message (using a CRYPTO frame at the Handshake encryption level) an
   endpoint can send STREAM data (in 1-RTT encryption).  If the Finished
   message is lost, the endpoint uses the Handshake encryption level to
   retransmit the lost message.  Reordering or loss of packets can mean
   that QUIC will need to handle packets at multiple encryption levels.
   During the handshake, this means potentially handling packets at
   higher and lower encryption levels than the current encryption level
   used by TLS.

   In particular, server implementations need to be able to read packets
   at the Handshake encryption level at the same time as the 0-RTT
   encryption level.  A client could interleave ACK frames that are
   protected with Handshake keys with 0-RTT data and the server needs to
   process those acknowledgments in order to detect lost Handshake
   packets.

4.1.3.  TLS Interface Summary

   Figure 3 summarizes the exchange between QUIC and TLS for both client
   and server.  Each arrow is tagged with the encryption level used for
   that transmission.

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   Client                                                    Server

   Get Handshake
                        Initial ------------->
   Rekey tx to 0-RTT Keys
                        0-RTT --------------->
                                                 Handshake Received
                                                      Get Handshake
                        <------------- Initial
                                             Rekey rx to 0-RTT keys
                                                 Handshake Received
                                         Rekey rx to Handshake keys
                                                      Get Handshake
                        <----------- Handshake
                                             Rekey tx to 1-RTT keys
                        <--------------- 1-RTT
   Handshake Received
   Rekey rx to Handshake keys
   Handshake Received
   Get Handshake
   Handshake Complete
                        Handshake ----------->
   Rekey tx to 1-RTT keys
                        1-RTT --------------->
                                                 Handshake Received
                                             Rekey rx to 1-RTT keys
                                                      Get Handshake
                                                 Handshake Complete
                        <--------------- 1-RTT
   Handshake Received

            Figure 3: Interaction Summary between QUIC and TLS

4.2.  TLS Version

   This document describes how TLS 1.3 [TLS13] is used with QUIC.

   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.

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4.3.  ClientHello Size

   QUIC requires that the first Initial packet from a client contain an
   entire cryptographic handshake message, which for TLS is the
   ClientHello.  Though a packet larger than 1200 bytes might be
   supported by the path, a client improves the likelihood that a packet
   is accepted if it ensures that the first ClientHello message is small
   enough to stay within this limit.

   QUIC packet and framing add at least 36 bytes of overhead to the
   ClientHello message.  That overhead increases if the client chooses a
   connection ID without zero length.  Overheads also do not include the
   token or a connection ID longer than 8 bytes, both of which might be
   required if a server sends a Retry packet.

   A typical TLS ClientHello can easily fit into a 1200 byte packet.
   However, in addition to the overheads added by QUIC, there are
   several variables that could cause this limit to be exceeded.  Large
   session tickets, 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, in addition to connection IDs and tokens, the size of
   TLS session tickets can have an effect on a client's ability to
   connect.  Minimizing the size of these values increases the
   probability that they can be successfully used by a client.

   A client is not required to fit the ClientHello that it sends in
   response to a HelloRetryRequest message into a single UDP datagram.

   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.

4.4.  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
   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

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   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 [TLS13]).

4.5.  Enabling 0-RTT

   In order to be usable for 0-RTT, TLS MUST provide a NewSessionTicket
   message that contains the "max_early_data" extension with the value
   0xffffffff; the amount of data which the client can send in 0-RTT is
   controlled by the "initial_max_data" transport parameter supplied by
   the server.  A client MUST treat receipt of a NewSessionTicket that
   contains a "max_early_data" extension with any other value as a
   connection error of type PROTOCOL_VIOLATION.

   Early data within the TLS connection MUST NOT be used.  As it is for
   other TLS application data, a server MUST treat receiving early data
   on the TLS connection as a connection error of type
   PROTOCOL_VIOLATION.

4.6.  Rejecting 0-RTT

   A server rejects 0-RTT by rejecting 0-RTT at the TLS layer.  This
   also prevents QUIC from sending 0-RTT data.  A server will always
   reject 0-RTT if it sends a TLS HelloRetryRequest.

   When 0-RTT is rejected, all connection characteristics that the
   client assumed might be incorrect.  This includes the choice of
   application protocol, transport parameters, and any application
   configuration.  The client therefore MUST reset the state of all
   streams, including application state bound to those streams.

   A client MAY attempt to send 0-RTT again if it receives a Retry or
   Version Negotiation packet.  These packets do not signify rejection
   of 0-RTT.

4.7.  HelloRetryRequest

   In TLS over TCP, the HelloRetryRequest feature (see Section 4.1.4 of
   [TLS13]) can be used to correct a client's incorrect KeyShare
   extension as well as for a stateless round-trip check.  From the
   perspective of QUIC, this just looks like additional messages carried
   in the Initial encryption level.  Although it is in principle
   possible to use this feature for address verification in QUIC, QUIC
   implementations SHOULD instead use the Retry feature (see Section 8.1
   of [QUIC-TRANSPORT]).  HelloRetryRequest is still used to request key
   shares.

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4.8.  TLS Errors

   If TLS experiences an error, it generates an appropriate alert as
   defined in Section 6 of [TLS13].

   A TLS alert is turned into a QUIC connection error by converting the
   one-byte alert description into a QUIC error code.  The alert
   description is added to 0x100 to produce a QUIC error code from the
   range reserved for CRYPTO_ERROR.  The resulting value is sent in a
   QUIC CONNECTION_CLOSE frame.

   The alert level of all TLS alerts is "fatal"; a TLS stack MUST NOT
   generate alerts at the "warning" level.

4.9.  Discarding Unused Keys

   After QUIC moves to a new encryption level, packet protection keys
   for previous encryption levels can be discarded.  This occurs several
   times during the handshake, as well as when keys are updated (see
   Section 6).  Initial packet protection keys are treated specially,
   see Section 4.10.

   Packet protection keys are not discarded immediately when new keys
   are available.  If packets from a lower encryption level contain
   CRYPTO frames, frames that retransmit that data MUST be sent at the
   same encryption level.  Similarly, an endpoint generates
   acknowledgements for packets at the same encryption level as the
   packet being acknowledged.  Thus, it is possible that keys for a
   lower encryption level are needed for a short time after keys for a
   newer encryption level are available.

   An endpoint cannot discard keys for a given encryption level unless
   it has both received and acknowledged all CRYPTO frames for that
   encryption level and when all CRYPTO frames for that encryption level
   have been acknowledged by its peer.  However, this does not guarantee
   that no further packets will need to be received or sent at that
   encryption level because a peer might not have received all the
   acknowledgements necessary to reach the same state.

   After all CRYPTO frames for a given encryption level have been sent
   and all expected CRYPTO frames received, and all the corresponding
   acknowledgments have been received or sent, an endpoint starts a
   timer.  For 0-RTT keys, which do not carry CRYPTO frames, this timer
   starts when the first packets protected with 1-RTT are sent or
   received.  To limit the effect of packet loss around a change in
   keys, endpoints MUST retain packet protection keys for that
   encryption level for at least three times the current Retransmission
   Timeout (RTO) interval as defined in [QUIC-RECOVERY].  Retaining keys

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   for this interval allows packets containing CRYPTO or ACK frames at
   that encryption level to be sent if packets are determined to be lost
   or new packets require acknowledgment.

   Though an endpoint might retain older keys, new data MUST be sent at
   the highest currently-available encryption level.  Only ACK frames
   and retransmissions of data in CRYPTO frames are sent at a previous
   encryption level.  These packets MAY also include PADDING frames.

   Once this timer expires, an endpoint MUST NOT either accept or
   generate new packets using those packet protection keys.  An endpoint
   can discard packet protection keys for that encryption level.

   Key updates (see Section 6) can be used to update 1-RTT keys before
   keys from other encryption levels are discarded.  In that case,
   packets protected with the newest packet protection keys and packets
   sent two updates prior will appear to use the same keys.  After the
   handshake is complete, endpoints only need to maintain the two latest
   sets of packet protection keys and MAY discard older keys.  Updating
   keys multiple times rapidly can cause packets to be effectively lost
   if packets are significantly delayed.  Because key updates can only
   be performed once per round trip time, only packets that are delayed
   by more than a round trip will be lost as a result of changing keys;
   such packets will be marked as lost before this, as they leave a gap
   in the sequence of packet numbers.

4.10.  Discarding Initial Keys

   Packets protected with Initial secrets (Section 5.2) are not
   authenticated, meaning that an attacker could spoof packets with the
   intent to disrupt a connection.  To limit these attacks, Initial
   packet protection keys can be discarded more aggressively than other
   keys.

   The successful use of Handshake packets indicates that no more
   Initial packets need to be exchanged, as these keys can only be
   produced after receiving all CRYPTO frames from Initial packets.
   Thus, a client MUST discard Initial keys when it first sends a
   Handshake packet and a server MUST discard Initial keys when it first
   successfully processes a Handshake packet.  Endpoints MUST NOT send
   Initial packets after this point.

   This results in abandoning loss recovery state for the Initial
   encryption level and ignoring any outstanding Initial packets.

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5.  Packet Protection

   As with TLS over TCP, QUIC protects packets with keys derived from
   the TLS handshake, using the AEAD algorithm negotiated by TLS.

5.1.  Packet Protection Keys

   QUIC derives packet protection keys in the same way that TLS derives
   record protection keys.

   Each encryption level has separate secret values for protection of
   packets sent in each direction.  These traffic secrets are derived by
   TLS (see Section 7.1 of [TLS13]) and are used by QUIC for all
   encryption levels except the Initial encryption level.  The secrets
   for the Initial encryption level are computed based on the client's
   initial Destination Connection ID, as described in Section 5.2.

   The keys used for packet protection are computed from the TLS secrets
   using the KDF provided by TLS.  In TLS 1.3, the HKDF-Expand-Label
   function described in Section 7.1 of [TLS13]) is used, using the hash
   function from the negotiated cipher suite.  Other versions of TLS
   MUST provide a similar function in order to be used QUIC.

   The current encryption level secret and the label "quic key" are
   input to the KDF to produce the AEAD key; the label "quic iv" is used
   to derive the IV, see Section 5.3.  The header protection key uses
   the "quic hp" label, see Section 5.4).  Using these labels provides
   key separation between QUIC and TLS, see Section 9.4.

   The KDF used for initial secrets is always the HKDF-Expand-Label
   function from TLS 1.3 (see Section 5.2).

5.2.  Initial Secrets

   Initial packets are protected with a secret derived from the
   Destination Connection ID field from the client's first Initial
   packet of the connection.  Specifically:

   initial_salt = 0xef4fb0abb47470c41befcf8031334fae485e09a0
   initial_secret = HKDF-Extract(initial_salt,
                                 client_dst_connection_id)

   client_initial_secret = HKDF-Expand-Label(initial_secret,
                                             "client in", "",
                                             Hash.length)
   server_initial_secret = HKDF-Expand-Label(initial_secret,
                                             "server in", "",
                                             Hash.length)

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   The hash function for HKDF when deriving initial secrets and keys is
   SHA-256 [SHA].

   The connection ID used with HKDF-Expand-Label is the Destination
   Connection ID in the Initial packet sent by the client.  This will be
   a randomly-selected value unless the client creates the Initial
   packet after receiving a Retry packet, where the Destination
   Connection ID is selected by the server.

   The value of initial_salt is a 20 byte sequence shown in the figure
   in hexadecimal notation.  Future versions of QUIC SHOULD generate a
   new salt value, thus ensuring that the keys are different for each
   version of QUIC.  This prevents a middlebox that only recognizes one
   version of QUIC from seeing or modifying the contents of handshake
   packets from future versions.

   The HKDF-Expand-Label function defined in TLS 1.3 MUST be used for
   Initial packets even where the TLS versions offered do not include
   TLS 1.3.

   Note:  The Destination Connection ID is of arbitrary length, and it
      could be zero length if the server sends a Retry packet with a
      zero-length Source Connection ID field.  In this case, the Initial
      keys provide no assurance to the client that the server received
      its packet; the client has to rely on the exchange that included
      the Retry packet for that property.

5.3.  AEAD Usage

   The Authentication Encryption with Associated Data (AEAD) [AEAD]
   function used for QUIC packet protection is the 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.

   Packets are protected prior to applying header protection
   (Section 5.4).  The unprotected packet header is part of the
   associated data (A).  When removing packet protection, an endpoint
   first removes the header protection.

   All QUIC packets other than Version Negotiation and Retry packets are
   protected with an AEAD algorithm [AEAD].  Prior to establishing a
   shared secret, packets are protected with AEAD_AES_128_GCM and a key
   derived from the destination connection ID in the client's first
   Initial packet (see Section 5.2).  This provides protection against
   off-path attackers and robustness against QUIC version unaware
   middleboxes, but not against on-path attackers.

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   QUIC can use any of the ciphersuites defined in [TLS13] with the
   exception of TLS_AES_128_CCM_8_SHA256.  The AEAD for that
   ciphersuite, AEAD_AES_128_CCM_8 [CCM], does not produce a large
   enough authentication tag for use with the header protection designs
   provided (see Section 5.4).  All other ciphersuites defined in
   [TLS13] have a 16-byte authentication tag and produce an output 16
   bytes larger than their input.

   The key and IV for the packet are computed as described in
   Section 5.1.  The nonce, N, is formed by combining the packet
   protection IV with the packet number.  The 64 bits of the
   reconstructed QUIC packet number in network byte order are 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 byte in either the short or long
   header, up to and including the unprotected packet number.

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

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

   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 6) prior to exceeding any limit set for the AEAD
   that is in use.

5.4.  Header Protection

   Parts of QUIC packet headers, in particular the Packet Number field,
   are protected using a key that is derived separate to the packet
   protection key and IV.  The key derived using the "quic hp" label is
   used to provide confidentiality protection for those fields that are
   not exposed to on-path elements.

   This protection applies to the least-significant bits of the first
   byte, plus the Packet Number field.  The four least-significant bits
   of the first byte are protected for packets with long headers; the
   five least significant bits of the first byte are protected for
   packets with short headers.  For both header forms, this covers the
   reserved bits and the Packet Number Length field; the Key Phase bit
   is also protected for packets with a short header.

   The same header protection key is used for the duration of the
   connection, with the value not changing after a key update (see

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   Section 6).  This allows header protection to be used to protect the
   key phase.

   This process does not apply to Retry or Version Negotiation packets,
   which do not contain a protected payload or any of the fields that
   are protected by this process.

5.4.1.  Header Protection Application

   Header protection is applied after packet protection is applied (see
   Section 5.3).  The ciphertext of the packet is sampled and used as
   input to an encryption algorithm.  The algorithm used depends on the
   negotiated AEAD.

   The output of this algorithm is a 5 byte mask which is applied to the
   protected header fields using exclusive OR.  The least significant
   bits of the first byte of the packet are masked by the least
   significant bits of the first mask byte, and the packet number is
   masked with the remaining bytes.  Any unused bytes of mask that might
   result from a shorter packet number encoding are unused.

   Figure 4 shows a sample algorithm for applying header protection.
   Removing header protection only differs in the order in which the
   packet number length (pn_length) is determined.

   mask = header_protection(hp_key, sample)

   pn_length = (packet[0] & 0x03) + 1
   if (packet[0] & 0x80) == 0x80:
      # Long header: 4 bits masked
      packet[0] ^= mask[0] & 0x0f
   else:
      # Short header: 5 bits masked
      packet[0] ^= mask[0] & 0x1f

   # pn_offset is the start of the Packet Number field.
   packet[pn_offset:pn_offset+pn_length] ^= mask[1:1+pn_length]

                  Figure 4: Header Protection Pseudocode

   Figure 5 shows the protected fields of long and short headers marked
   with an E.  Figure 5 also shows the sampled fields.

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   Long Header:
   +-+-+-+-+-+-+-+-+
   |1|1|T T|E E E E|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Version -> Length Fields                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Short Header:
   +-+-+-+-+-+-+-+-+
   |0|1|S|E E E E E|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Destination Connection ID (0/32..144)         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Common Fields:
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |E E E E E E E E E  Packet Number (8/16/24/32) E E E E E E E E...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   [Protected Payload (8/16/24)]             ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Sampled part of Protected Payload (128)         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Protected Payload Remainder (*)             ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 5: Header Protection and Ciphertext Sample

   Before a TLS ciphersuite can be used with QUIC, a header protection
   algorithm MUST be specified for the AEAD used with that ciphersuite.
   This document defines algorithms for AEAD_AES_128_GCM,
   AEAD_AES_128_CCM, AEAD_AES_256_GCM, AEAD_AES_256_CCM (all AES AEADs
   are defined in [AEAD]), and AEAD_CHACHA20_POLY1305 [CHACHA].  Prior
   to TLS selecting a ciphersuite, AES header protection is used
   (Section 5.4.3), matching the AEAD_AES_128_GCM packet protection.

5.4.2.  Header Protection Sample

   The header protection algorithm uses both the header protection key
   and a sample of the ciphertext from the packet Payload field.

   The same number of bytes are always sampled, but an allowance needs
   to be made for the endpoint removing protection, which will not know
   the length of the Packet Number field.  In sampling the packet
   ciphertext, the Packet Number field is assumed to be 4 bytes long
   (its maximum possible encoded length).

   An endpoint MUST discard packets that are not long enough to contain
   a complete sample.

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   To ensure that sufficient data is available for sampling, packets are
   padded so that the combined lengths of the encoded packet number and
   protected payload is at least 4 bytes longer than the sample required
   for header protection.  For the AEAD functions defined in [TLS13],
   which have 16-byte expansions and 16-byte header protection samples,
   this results in needing at least 3 bytes of frames in the unprotected
   payload if the packet number is encoded on a single byte, or 2 bytes
   of frames for a 2-byte packet number encoding.

   The sampled ciphertext for a packet with a short header can be
   determined by the following pseudocode:

   sample_offset = 1 + len(connection_id) + 4

   sample = packet[sample_offset..sample_offset+sample_length]

   For example, for a packet with a short header, an 8 byte connection
   ID, and protected with AEAD_AES_128_GCM, the sample takes bytes 13 to
   28 inclusive (using zero-based indexing).

   A packet with a long header is sampled in the same way, noting that
   multiple QUIC packets might be included in the same UDP datagram and
   that each one is handled separately.

   sample_offset = 6 + len(destination_connection_id) +
                       len(source_connection_id) +
                       len(payload_length) + 4
   if packet_type == Initial:
       sample_offset += len(token_length) +
                        len(token)

   sample = packet[sample_offset..sample_offset+sample_length]

5.4.3.  AES-Based Header Protection

   This section defines the packet protection algorithm for
   AEAD_AES_128_GCM, AEAD_AES_128_CCM, AEAD_AES_256_GCM, and
   AEAD_AES_256_CCM.  AEAD_AES_128_GCM and AEAD_AES_128_CCM use 128-bit
   AES [AES] in electronic code-book (ECB) mode.  AEAD_AES_256_GCM, and
   AEAD_AES_256_CCM use 256-bit AES in ECB mode.

   This algorithm samples 16 bytes from the packet ciphertext.  This
   value is used as the counter input to AES-ECB.  In pseudocode:

   mask = AES-ECB(pn_key, sample)

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5.4.4.  ChaCha20-Based Header Protection

   When AEAD_CHACHA20_POLY1305 is in use, header protection uses the raw
   ChaCha20 function as defined in Section 2.4 of [CHACHA].  This uses a
   256-bit key and 16 bytes sampled from the packet protection output.

   The first 4 bytes of the sampled ciphertext are interpreted as a
   32-bit number in little-endian order and are used as the block count.
   The remaining 12 bytes are interpreted as three concatenated 32-bit
   numbers in little-endian order and used as the nonce.

   The encryption mask is produced by invoking ChaCha20 to protect 5
   zero bytes.  In pseudocode:

   counter = DecodeLE(sample[0..3])
   nonce = DecodeLE(sample[4..7], sample[8..11], sample[12..15])
   mask = ChaCha20(pn_key, counter, nonce, {0,0,0,0,0})

5.5.  Receiving Protected Packets

   Once an endpoint successfully receives a packet with a given packet
   number, it MUST discard all packets in the same packet number space
   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 6).  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.  Use of 0-RTT Keys

   If 0-RTT keys are available (see Section 4.5), 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, except that
   it MUST NOT send ACKs with 0-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

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   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; it uses 1-RTT
   keys to protect acknowledgements of 0-RTT packets.  A client MUST NOT
   attempt to decrypt 0-RTT packets it receives and instead MUST discard
   them.

   Note:  0-RTT data can be acknowledged by the server as it receives
      it, but any packets containing acknowledgments of 0-RTT data
      cannot have packet protection removed by the client until the TLS
      handshake is complete.  The 1-RTT keys necessary to remove packet
      protection cannot be derived until the client receives all server
      handshake messages.

5.7.  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
   client.

   However, a server MUST NOT process data from incoming 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 4.2.11 of [TLS13]).  Verifying
   these values provides the server with an assurance that the
   ClientHello has not been modified.  Packets protected with 1-RTT keys
   MAY be stored and later decrypted and used once the handshake is
   complete.

   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.

6.  Key Update

   Once the 1-RTT keys are established and the short header is in use,
   it is possible to update the keys.  The KEY_PHASE bit in the short
   header is used to indicate whether key updates have occurred.  The
   KEY_PHASE bit is initially set to 0 and then inverted with each key
   update.

   The KEY_PHASE bit allows a recipient to detect a change in keying
   material without necessarily needing to receive the first packet that
   triggered the change.  An endpoint that notices a changed KEY_PHASE

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   bit can update keys and decrypt the packet that contains the changed
   bit.

   This mechanism replaces the TLS KeyUpdate message.  Endpoints MUST
   NOT send a TLS KeyUpdate message.  Endpoints MUST treat the receipt
   of a TLS KeyUpdate message as a connection error of type 0x10a,
   equivalent to a fatal TLS alert of unexpected_message (see
   Section 4.8).

   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.

   A receiving endpoint detects an update when the KEY_PHASE bit does
   not match what it is expecting.  It creates a new secret (see
   Section 7.2 of [TLS13]) and the corresponding read key and IV using
   the KDF function provided by TLS.  The header protection key is not
   updated.

   If the packet can be decrypted and authenticated using the updated
   key and IV, then the keys the endpoint uses for packet protection are
   also updated.  The next packet sent by the endpoint will then use the
   new keys.

   An endpoint does not always need to send packets 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 period of no more than three
   times the Probe Timeout (PTO, see [QUIC-RECOVERY]).  After this
   period, old keys and their corresponding secrets SHOULD be discarded.
   Retaining keys allow endpoints to process packets that were sent with
   old keys and delayed in the network.  Packets with higher packet
   numbers always use the updated keys and MUST NOT be decrypted with
   old keys.

   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.

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      Initiating Peer                    Responding Peer

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

                           Figure 6: Key Update

   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.

   In deciding when to update keys, endpoints MUST NOT exceed the limits
   for use of specific keys, as described in Section 5.5 of [TLS13].

7.  Security of Initial Messages

   Initial packets are not protected with a secret key, so they are
   subject to potential tampering by an attacker.  QUIC provides
   protection against attackers that cannot read packets, but does not
   attempt to provide additional protection against attacks where the
   attacker can observe and inject packets.  Some forms of tampering -
   such as modifying the TLS messages themselves - are detectable, but
   some - such as modifying ACKs - are not.

   For example, an attacker could inject a packet containing an ACK
   frame that makes it appear that a packet had not been received or to
   create a false impression of the state of the connection (e.g., by
   modifying the ACK Delay).  Note that such a packet could cause a
   legitimate packet to be dropped as a duplicate.  Implementations
   SHOULD use caution in relying on any data which is contained in
   Initial packets that is not otherwise authenticated.

   It is also possible for the attacker to tamper with data that is
   carried in Handshake packets, but because that tampering requires
   modifying TLS handshake messages, that tampering will cause the TLS
   handshake to fail.

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8.  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.

8.1.  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.

8.2.  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(0xffa5), (65535)
      } ExtensionType;

   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

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   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.

   While the transport parameters are technically available prior to the
   completion of the handshake, they cannot be fully trusted until the
   handshake completes, and reliance on them should be minimized.
   However, any tampering with the parameters will cause the handshake
   to fail.

   Endpoints MUST NOT send this extension in a TLS connection that does
   not use QUIC (such as the use of TLS with TCP defined in [TLS13]).  A
   fatal unsupported_extension alert MUST be sent if this extension is
   received when the transport is not QUIC.

8.3.  Removing the EndOfEarlyData Message

   The TLS EndOfEarlyData message is not used with QUIC.  QUIC does not
   rely on this message to mark the end of 0-RTT data or to signal the
   change to Handshake keys.

   Clients MUST NOT send the EndOfEarlyData message.  A server MUST
   treat receipt of a CRYPTO frame in a 0-RTT packet as a connection
   error of type PROTOCOL_VIOLATION.

   As a result, EndOfEarlyData does not appear in the TLS handshake
   transcript.

9.  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.

9.1.  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.

   QUIC includes three defenses against this attack.  First, the packet
   containing a ClientHello MUST be padded to a minimum size.  Second,
   if responding to an unverified source address, the server is

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   forbidden to send more than three UDP datagrams in its first flight
   (see Section 8.1 of [QUIC-TRANSPORT]).  Finally, because
   acknowledgements of Handshake packets are authenticated, a blind
   attacker cannot forge them.  Put together, these defenses limit the
   level of amplification.

9.2.  Peer Denial of Service

   QUIC, TLS, and HTTP/2 all contain 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.

   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.

9.3.  Header Protection Analysis

   Header protection relies on the packet protection AEAD being a
   pseudorandom function (PRF), which is not a property that AEAD
   algorithms guarantee.  Therefore, no strong assurances about the
   general security of this mechanism can be shown in the general case.
   The AEAD algorithms described in this document are assumed to be
   PRFs.

   The header protection algorithms defined in this document take the
   form:

   protected_field = field XOR PRF(pn_key, sample)

   This construction is secure against chosen plaintext attacks (IND-
   CPA) [IMC].

   Use of the same key and ciphertext sample more than once risks
   compromising header protection.  Protecting two different headers
   with the same key and ciphertext sample reveals the exclusive OR of
   the protected fields.  Assuming that the AEAD acts as a PRF, if L
   bits are sampled, the odds of two ciphertext samples being identical
   approach 2^(-L/2), that is, the birthday bound.  For the algorithms
   described in this document, that probability is one in 2^64.

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   Note:  In some cases, inputs shorter than the full size required by
      the packet protection algorithm might be used.

   To prevent an attacker from modifying packet headers, the header is
   transitively authenticated using packet protection; the entire packet
   header is part of the authenticated additional data.  Protected
   fields that are falsified or modified can only be detected once the
   packet protection is removed.

   An attacker could guess values for packet numbers and have an
   endpoint confirm guesses through timing side channels.  Similarly,
   guesses for the packet number length can be trialed and exposed.  If
   the recipient of a packet discards packets with duplicate packet
   numbers without attempting to remove packet protection they could
   reveal through timing side-channels that the packet number matches a
   received packet.  For authentication to be free from side-channels,
   the entire process of header protection removal, packet number
   recovery, and packet protection removal MUST be applied together
   without timing and other side-channels.

   For the sending of packets, construction and protection of packet
   payloads and packet numbers MUST be free from side-channels that
   would reveal the packet number or its encoded size.

9.4.  Key Diversity

   In using TLS, the central key schedule of TLS is used.  As a result
   of the TLS handshake messages being integrated into the calculation
   of secrets, the inclusion of the QUIC transport parameters extension
   ensures that handshake and 1-RTT keys are not the same as those that
   might be produced by a server running TLS over TCP.  However, 0-RTT
   keys only include the ClientHello message and might therefore use the
   same secrets.  To avoid the possibility of cross-protocol key
   synchronization, additional measures are provided to improve key
   separation.

   The QUIC packet protection keys and IVs are derived using a different
   label than the equivalent keys in TLS.

   To preserve this separation, a new version of QUIC SHOULD define new
   labels for key derivation for packet protection key and IV, plus the
   packet number protection keys.

   The initial secrets also use a key that is specific to the negotiated
   QUIC version.  New QUIC versions SHOULD define a new salt value used
   in calculating initial secrets.

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10.  IANA Considerations

   This document does not create any new IANA registries, but it
   registers the values in the following registries:

   o  TLS ExtensionsType Registry [TLS-REGISTRIES] - IANA is to register
      the quic_transport_parameters extension found in Section 8.2.  The
      Recommended column is to be marked Yes.  The TLS 1.3 Column is to
      include CH and EE.

11.  References

11.1.  Normative References

   [AEAD]     McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <https://www.rfc-editor.org/info/rfc5116>.

   [AES]      "Advanced encryption standard (AES)", National Institute
              of Standards and Technology report,
              DOI 10.6028/nist.fips.197, November 2001.

   [CHACHA]   Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
              Protocols", RFC 8439, DOI 10.17487/RFC8439, June 2018,
              <https://www.rfc-editor.org/info/rfc8439>.

   [QUIC-RECOVERY]
              Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
              and Congestion Control", draft-ietf-quic-recovery-17 (work
              in progress), December 2018.

   [QUIC-TRANSPORT]
              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", draft-ietf-quic-
              transport-17 (work in progress), December 2018.

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

   [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, <https://www.rfc-editor.org/info/rfc7301>.

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [SHA]      Dang, Q., "Secure Hash Standard", National Institute of
              Standards and Technology report,
              DOI 10.6028/nist.fips.180-4, July 2015.

   [TLS-REGISTRIES]
              Salowey, J. and S. Turner, "IANA Registry Updates for TLS
              and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
              <https://www.rfc-editor.org/info/rfc8447>.

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

11.2.  Informative References

   [AEBounds]
              Luykx, A. and K. Paterson, "Limits on Authenticated
              Encryption Use in TLS", March 2016,
              <http://www.isg.rhul.ac.uk/~kp/TLS-AEbounds.pdf>.

   [CCM]      McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
              Transport Layer Security (TLS)", RFC 6655,
              DOI 10.17487/RFC6655, July 2012,
              <https://www.rfc-editor.org/info/rfc6655>.

   [IMC]      Katz, J. and Y. Lindell, "Introduction to Modern
              Cryptography, Second Edition", ISBN 978-1466570269,
              November 2014.

   [QUIC-HTTP]
              Bishop, M., Ed., "Hypertext Transfer Protocol (HTTP) over
              QUIC", draft-ietf-quic-http-17 (work in progress),
              December 2018.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <https://www.rfc-editor.org/info/rfc2818>.

   [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,
              <https://www.rfc-editor.org/info/rfc5280>.

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11.3.  URIs

   [1] https://mailarchive.ietf.org/arch/search/?email_list=quic

   [2] https://github.com/quicwg

   [3] https://github.com/quicwg/base-drafts/labels/-tls

Appendix A.  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.

A.1.  Since draft-ietf-quic-tls-14

   o  Update the salt used for Initial secrets (#1970)

   o  Clarify that TLS_AES_128_CCM_8_SHA256 isn't supported (#2019)

   o  Change header protection

      *  Sample from a fixed offset (#1575, #2030)

      *  Cover part of the first byte, including the key phase (#1322,
         #2006)

   o  TLS provides an AEAD and KDF function (#2046)

      *  Clarify that the TLS KDF is used with TLS (#1997)

      *  Change the labels for calculation of QUIC keys (#1845, #1971,
         #1991)

   o  Initial keys are discarded once Handshake are avaialble (#1951,
      #2045)

A.2.  Since draft-ietf-quic-tls-13

   o  Updated to TLS 1.3 final (#1660)

A.3.  Since draft-ietf-quic-tls-12

   o  Changes to integration of the TLS handshake (#829, #1018, #1094,
      #1165, #1190, #1233, #1242, #1252, #1450)

      *  The cryptographic handshake uses CRYPTO frames, not stream 0

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      *  QUIC packet protection is used in place of TLS record
         protection

      *  Separate QUIC packet number spaces are used for the handshake

      *  Changed Retry to be independent of the cryptographic handshake

      *  Limit the use of HelloRetryRequest to address TLS needs (like
         key shares)

   o  Changed codepoint of TLS extension (#1395, #1402)

A.4.  Since draft-ietf-quic-tls-11

   o  Encrypted packet numbers.

A.5.  Since draft-ietf-quic-tls-10

   o  No significant changes.

A.6.  Since draft-ietf-quic-tls-09

   o  Cleaned up key schedule and updated the salt used for handshake
      packet protection (#1077)

A.7.  Since draft-ietf-quic-tls-08

   o  Specify value for max_early_data_size to enable 0-RTT (#942)

   o  Update key derivation function (#1003, #1004)

A.8.  Since draft-ietf-quic-tls-07

   o  Handshake errors can be reported with CONNECTION_CLOSE (#608,
      #891)

A.9.  Since draft-ietf-quic-tls-05

   No significant changes.

A.10.  Since draft-ietf-quic-tls-04

   o  Update labels used in HKDF-Expand-Label to match TLS 1.3 (#642)

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A.11.  Since draft-ietf-quic-tls-03

   No significant changes.

A.12.  Since draft-ietf-quic-tls-02

   o  Updates to match changes in transport draft

A.13.  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,
      #337)

   o  The QUIC header is included as AEAD Associated Data (#226, #243,
      #302)

   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)

A.14.  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

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A.15.  Since draft-thomson-quic-tls-01

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

   o  Updated authors/editors list

   o  Added status note

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.

Contributors

   Ryan Hamilton was originally an author of this specification.

Authors' Addresses

   Martin Thomson (editor)
   Mozilla

   Email: martin.thomson@gmail.com

   Sean Turner (editor)
   sn3rd

   Email: sean@sn3rd.com

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