QUIC M. Thomson, Ed.
Internet-Draft Mozilla
Intended status: Standards Track S. Turner, Ed.
Expires: March 26, 2018 sn3rd
September 22, 2017
Using Transport Layer Security (TLS) to Secure QUIC
draft-ietf-quic-tls-06
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 .
Working Group information can be found at https://github.com/quicwg ;
source code and issues list for this draft can be found at
https://github.com/quicwg/base-drafts/labels/tls .
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 http://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 March 26, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 4
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4
3.1. TLS Overview . . . . . . . . . . . . . . . . . . . . . . 5
3.2. TLS Handshake . . . . . . . . . . . . . . . . . . . . . . 6
4. TLS Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Handshake and Setup Sequence . . . . . . . . . . . . . . 7
4.2. Interface to TLS . . . . . . . . . . . . . . . . . . . . 9
4.2.1. Handshake Interface . . . . . . . . . . . . . . . . . 9
4.2.2. Source Address Validation . . . . . . . . . . . . . . 10
4.2.3. Key Ready Events . . . . . . . . . . . . . . . . . . 11
4.2.4. Secret Export . . . . . . . . . . . . . . . . . . . . 12
4.2.5. TLS Interface Summary . . . . . . . . . . . . . . . . 12
4.3. TLS Version . . . . . . . . . . . . . . . . . . . . . . . 13
4.4. ClientHello Size . . . . . . . . . . . . . . . . . . . . 13
4.5. Peer Authentication . . . . . . . . . . . . . . . . . . . 13
4.6. TLS Errors . . . . . . . . . . . . . . . . . . . . . . . 14
5. QUIC Packet Protection . . . . . . . . . . . . . . . . . . . 14
5.1. Installing New Keys . . . . . . . . . . . . . . . . . . . 14
5.2. QUIC Key Expansion . . . . . . . . . . . . . . . . . . . 15
5.2.1. 0-RTT Secret . . . . . . . . . . . . . . . . . . . . 15
5.2.2. 1-RTT Secrets . . . . . . . . . . . . . . . . . . . . 15
5.2.3. Packet Protection Key and IV . . . . . . . . . . . . 17
5.3. QUIC AEAD Usage . . . . . . . . . . . . . . . . . . . . . 17
5.4. Packet Numbers . . . . . . . . . . . . . . . . . . . . . 18
5.5. Receiving Protected Packets . . . . . . . . . . . . . . . 19
5.6. Packet Number Gaps . . . . . . . . . . . . . . . . . . . 19
6. Unprotected Packets . . . . . . . . . . . . . . . . . . . . . 19
6.1. Integrity Check Processing . . . . . . . . . . . . . . . 19
6.2. The 64-bit FNV-1a Algorithm . . . . . . . . . . . . . . . 20
7. Key Phases . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.1. Packet Protection for the TLS Handshake . . . . . . . . . 21
7.1.1. Initial Key Transitions . . . . . . . . . . . . . . . 21
7.1.2. Retransmission and Acknowledgment of Unprotected
Packets . . . . . . . . . . . . . . . . . . . . . . . 22
7.2. Key Update . . . . . . . . . . . . . . . . . . . . . . . 23
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8. Client Address Validation . . . . . . . . . . . . . . . . . . 24
8.1. HelloRetryRequest Address Validation . . . . . . . . . . 25
8.1.1. Stateless Address Validation . . . . . . . . . . . . 25
8.1.2. Sending HelloRetryRequest . . . . . . . . . . . . . . 26
8.2. NewSessionTicket Address Validation . . . . . . . . . . . 26
8.3. Address Validation Token Integrity . . . . . . . . . . . 27
9. Pre-handshake QUIC Messages . . . . . . . . . . . . . . . . . 27
9.1. Unprotected Packets Prior to Handshake Completion . . . . 28
9.1.1. STREAM Frames . . . . . . . . . . . . . . . . . . . . 28
9.1.2. ACK Frames . . . . . . . . . . . . . . . . . . . . . 28
9.1.3. Updates to Data and Stream Limits . . . . . . . . . . 29
9.1.4. Denial of Service with Unprotected Packets . . . . . 29
9.2. Use of 0-RTT Keys . . . . . . . . . . . . . . . . . . . . 30
9.3. Receiving Out-of-Order Protected Frames . . . . . . . . . 30
10. QUIC-Specific Additions to the TLS Handshake . . . . . . . . 31
10.1. Protocol and Version Negotiation . . . . . . . . . . . . 31
10.2. QUIC Transport Parameters Extension . . . . . . . . . . 32
10.3. Priming 0-RTT . . . . . . . . . . . . . . . . . . . . . 32
11. Security Considerations . . . . . . . . . . . . . . . . . . . 33
11.1. Packet Reflection Attack Mitigation . . . . . . . . . . 33
11.2. Peer Denial of Service . . . . . . . . . . . . . . . . . 33
12. Error codes . . . . . . . . . . . . . . . . . . . . . . . . . 34
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
14.1. Normative References . . . . . . . . . . . . . . . . . . 34
14.2. Informative References . . . . . . . . . . . . . . . . . 35
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 36
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . 36
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 36
C.1. Since draft-ietf-quic-tls-05 . . . . . . . . . . . . . . 36
C.2. Since draft-ietf-quic-tls-04 . . . . . . . . . . . . . . 36
C.3. Since draft-ietf-quic-tls-03 . . . . . . . . . . . . . . 36
C.4. Since draft-ietf-quic-tls-02 . . . . . . . . . . . . . . 36
C.5. Since draft-ietf-quic-tls-01 . . . . . . . . . . . . . . 37
C.6. Since draft-ietf-quic-tls-00 . . . . . . . . . . . . . . 37
C.7. Since draft-thomson-quic-tls-01 . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38
1. Introduction
This document describes how QUIC [QUIC-TRANSPORT] is secured using
Transport Layer Security (TLS) version 1.3 [I-D.ietf-tls-tls13]. TLS
1.3 provides critical latency improvements for connection
establishment over previous versions. Absent packet loss, most new
connections can be established and secured within a single round
trip; on subsequent connections between the same client and server,
the client can often send application data immediately, that is,
using a zero round trip setup.
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This document describes how the standardized TLS 1.3 acts a security
component of QUIC. The same design could work for TLS 1.2, though
few of the benefits QUIC provides would be realized due to the
handshake latency in versions of TLS prior to 1.3.
2. Notational Conventions
The words "MUST", "MUST NOT", "SHOULD", and "MAY" are used in this
document. It's not shouting; when they are capitalized, they have
the special meaning defined in [RFC2119].
This document uses the terminology established in [QUIC-TRANSPORT].
For brevity, the acronym TLS is used to refer to TLS 1.3.
TLS terminology is used when referring to parts of TLS. Though TLS
assumes a continuous stream of octets, it divides that stream into
_records_. Most relevant to QUIC are the records that contain TLS
_handshake messages_, which are discrete messages that are used for
key agreement, authentication and parameter negotiation. Ordinarily,
TLS records can also contain _application data_, though in the QUIC
usage there is no use of TLS application data.
3. Protocol Overview
QUIC [QUIC-TRANSPORT] assumes responsibility for the confidentiality
and integrity protection of packets. For this it uses keys derived
from a TLS 1.3 connection [I-D.ietf-tls-tls13]; QUIC also relies on
TLS 1.3 for authentication and negotiation of parameters that are
critical to security and performance.
Rather than a strict layering, these two protocols are co-dependent:
QUIC uses the TLS handshake; TLS uses the reliability and ordered
delivery provided by QUIC streams.
This document defines how QUIC interacts with TLS. This includes a
description of how TLS is used, how keying material is derived from
TLS, and the application of that keying material to protect QUIC
packets. Figure 1 shows the basic interactions between TLS and QUIC,
with the QUIC packet protection being called out specially.
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+------------+ +------------+
| |------ Handshake ------>| |
| |<-- Validate Address ---| |
| |-- OK/Error/Validate -->| |
| |<----- Handshake -------| |
| QUIC |------ Validate ------->| TLS |
| | | |
| |<------ 0-RTT OK -------| |
| |<------ 1-RTT OK -------| |
| |<--- Handshake Done ----| |
+------------+ +------------+
| ^ ^ |
| Protect | Protected | |
v | Packet | |
+------------+ / /
| QUIC | / /
| Packet |-------- Get Secret -------' /
| Protection |<-------- Secret -----------'
+------------+
Figure 1: QUIC and TLS Interactions
The initial state of a QUIC connection has packets exchanged without
any form of protection. In this state, QUIC is limited to using
stream 0 and associated packets. Stream 0 is reserved for a TLS
connection. This is a complete TLS connection as it would appear
when layered over TCP; the only difference is that QUIC provides the
reliability and ordering that would otherwise be provided by TCP.
At certain points during the TLS handshake, keying material is
exported from the TLS connection for use by QUIC. This keying
material is used to derive packet protection keys. Details on how
and when keys are derived and used are included in Section 5.
3.1. TLS Overview
TLS provides two endpoints with a way to establish a means of
communication over an untrusted medium (that is, the Internet) that
ensures that messages they exchange cannot be observed, modified, or
forged.
TLS features can be separated into two basic functions: an
authenticated key exchange and record protection. QUIC primarily
uses the authenticated key exchange provided by TLS but provides its
own packet protection.
The TLS authenticated key exchange occurs between two entities:
client and server. The client initiates the exchange and the server
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responds. If the key exchange completes successfully, both client
and server will agree on a secret. TLS supports both pre-shared key
(PSK) and Diffie-Hellman (DH) key exchanges. PSK is the basis for
0-RTT; the latter provides perfect forward secrecy (PFS) when the DH
keys are destroyed.
After completing the TLS handshake, the client will have learned and
authenticated an identity for the server and the server is optionally
able to learn and authenticate an identity for the client. TLS
supports X.509 [RFC5280] certificate-based authentication for both
server and client.
The TLS key exchange is resistent to tampering by attackers and it
produces shared secrets that cannot be controlled by either
participating peer.
3.2. TLS Handshake
TLS 1.3 provides two basic handshake modes of interest to QUIC:
o A full 1-RTT handshake in which the client is able to send
application data after one round trip and the server immediately
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 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
ClientHello
(0-RTT Application Data) -------->
ServerHello
{EncryptedExtensions}
{Finished}
<-------- [Application Data]
(EndOfEarlyData)
{Finished} -------->
[Application Data] <-------> [Application Data]
Figure 2: TLS Handshake with 0-RTT
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This 0-RTT handshake is only possible if the client and server have
previously communicated. In the 1-RTT handshake, the client is
unable to send protected application data until it has received all
of the handshake messages sent by the server.
Two additional variations on this basic handshake exchange are
relevant to this document:
o The server can respond to a ClientHello with a HelloRetryRequest,
which adds an additional round trip prior to the basic exchange.
This is needed if the server wishes to request a different key
exchange key from the client. HelloRetryRequest is also used to
verify that the client is correctly able to receive packets on the
address it claims to have (see [QUIC-TRANSPORT]).
o A pre-shared key mode can be used for subsequent handshakes to
reduce the number of public key operations. This is the basis for
0-RTT data, even if the remainder of the connection is protected
by a new Diffie-Hellman exchange.
4. TLS Usage
QUIC reserves stream 0 for a TLS connection. Stream 0 contains a
complete TLS connection, which includes the TLS record layer. Other
than the definition of a QUIC-specific extension (see Section 10.2),
TLS is unmodified for this use. This means that TLS will apply
confidentiality and integrity protection to its records. In
particular, TLS record protection is what provides confidentiality
protection for the TLS handshake messages sent by the server.
QUIC permits a client to send frames on streams starting from the
first packet. The initial packet from a client contains a stream
frame for stream 0 that contains the first TLS handshake messages
from the client. This allows the TLS handshake to start with the
first packet that a client sends.
QUIC packets are protected using a scheme that is specific to QUIC,
see Section 5. Keys are exported from the TLS connection when they
become available using a TLS exporter (see Section 7.5 of
[I-D.ietf-tls-tls13] and Section 5.2). After keys are exported from
TLS, QUIC manages its own key schedule.
4.1. Handshake and Setup Sequence
The integration of QUIC with a TLS handshake is shown in more detail
in Figure 3. QUIC "STREAM" frames on stream 0 carry the TLS
handshake. QUIC performs loss recovery [QUIC-RECOVERY] for this
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stream and ensures that TLS handshake messages are delivered in the
correct order.
Client Server
@C QUIC STREAM Frame(s) <0>:
ClientHello
+ QUIC Extension
-------->
0-RTT Key => @0
@0 QUIC STREAM Frame(s) <any stream>:
Replayable QUIC Frames
-------->
QUIC STREAM Frame <0>: @C
ServerHello
{TLS Handshake Messages}
<--------
1-RTT Key => @1
QUIC Frames <any> @1
<--------
@C QUIC STREAM Frame(s) <0>:
(EndOfEarlyData)
{Finished}
-------->
@1 QUIC Frames <any> <-------> QUIC Frames <any> @1
Figure 3: QUIC over TLS Handshake
In Figure 3, symbols mean:
o "<" and ">" enclose stream numbers.
o "@" indicates the keys that are used for protecting the QUIC
packet (C = cleartext, with integrity only; 0 = 0-RTT keys; 1 =
1-RTT keys).
o "(" and ")" enclose messages that are protected with TLS 0-RTT
handshake or application keys.
o "{" and "}" enclose messages that are protected by the TLS
Handshake keys.
If 0-RTT is not attempted, then the client does not send packets
protected by the 0-RTT key (@0). In that case, the only key
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transition on the client is from cleartext packets (@C) to 1-RTT
protection (@1), which happens after it sends its final set of TLS
handshake messages.
Note: the client uses two different types of cleartext packet during
the handshake. The Client Initial packet carries a TLS ClientHello
message; the remainder of the TLS handshake is carried in Client
Cleartext packets.
The server sends TLS handshake messages without protection (@C). The
server transitions from no protection (@C) to full 1-RTT protection
(@1) after it sends the last of its handshake messages.
Some TLS handshake messages are protected by the TLS handshake record
protection. These keys are not exported from the TLS connection for
use in QUIC. QUIC packets from the server are sent in the clear
until the final transition to 1-RTT keys.
The client transitions from cleartext (@C) to 0-RTT keys (@0) when
sending 0-RTT data, and subsequently to to 1-RTT keys (@1) after its
second flight of TLS handshake messages. This creates the potential
for unprotected packets to be received by a server in close proximity
to packets that are protected with 1-RTT keys.
More information on key transitions is included in Section 7.1.
4.2. Interface to TLS
As shown in Figure 1, the interface from QUIC to TLS consists of four
primary functions: Handshake, Source Address Validation, Key Ready
Events, and Secret Export.
Additional functions might be needed to configure TLS.
4.2.1. Handshake Interface
In order to drive the handshake, TLS depends on being able to send
and receive handshake messages on stream 0. There are two basic
functions on this interface: one where QUIC requests handshake
messages and one where QUIC provides handshake packets.
Before starting the handshake QUIC provides TLS with the transport
parameters (see Section 10.2) that it wishes to carry.
A QUIC client starts TLS by requesting TLS handshake octets from TLS.
The client acquires handshake octets before sending its first packet.
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A QUIC server starts the process by providing TLS with stream 0
octets.
Each time that an endpoint receives data on stream 0, it delivers the
octets to TLS if it is able. Each time that TLS is provided with new
data, new handshake octets are requested from TLS. TLS might not
provide any octets if the handshake messages it has received are
incomplete or it has no data to send.
Once the TLS handshake is complete, this is indicated to QUIC along
with any final handshake octets that TLS needs to send. TLS also
provides QUIC with the transport parameters that the peer advertised
during the handshake.
Once the handshake is complete, TLS becomes passive. TLS can still
receive data from its peer and respond in kind, but it will not need
to send more data unless specifically requested - either by an
application or QUIC. One reason to send data is that the server
might wish to provide additional or updated session tickets to a
client.
When the handshake is complete, QUIC only needs to provide TLS with
any data that arrives on stream 0. In the same way that is done
during the handshake, new data is requested from TLS after providing
received data.
Important: Until the handshake is reported as complete, the
connection and key exchange are not properly authenticated at the
server. Even though 1-RTT keys are available to a server after
receiving the first handshake messages from a client, the server
cannot consider the client to be authenticated until it receives
and validates the client's Finished message.
The requirement for the server to wait for the client Finished
message creates a dependency on that message being delivered. A
client can avoid the potential for head-of-line blocking that this
implies by sending a copy of the STREAM frame that carries the
Finished message in multiple packets. This enables immediate
server processing for those packets.
4.2.2. Source Address Validation
During the processing of the TLS ClientHello, TLS requests that the
transport make a decision about whether to request source address
validation from the client.
An initial TLS ClientHello that resumes a session includes an address
validation token in the session ticket; this includes all attempts at
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0-RTT. If the client does not attempt session resumption, no token
will be present. While processing the initial ClientHello, TLS
provides QUIC with any token that is present. In response, QUIC
provides one of three responses:
o proceed with the connection,
o ask for client address validation, or
o abort the connection.
If QUIC requests source address validation, it also provides a new
address validation token. TLS includes that along with any
information it requires in the cookie extension of a TLS
HelloRetryRequest message. In the other cases, the connection either
proceeds or terminates with a handshake error.
The client echoes the cookie extension in a second ClientHello. A
ClientHello that contains a valid cookie extension will always be in
response to a HelloRetryRequest. If address validation was requested
by QUIC, then this will include an address validation token. TLS
makes a second address validation request of QUIC, including the
value extracted from the cookie extension. In response to this
request, QUIC cannot ask for client address validation, it can only
abort or permit the connection attempt to proceed.
QUIC can provide a new address validation token for use in session
resumption at any time after the handshake is complete. Each time a
new token is provided TLS generates a NewSessionTicket message, with
the token included in the ticket.
See Section 8 for more details on client address validation.
4.2.3. Key Ready Events
TLS provides QUIC with signals when 0-RTT and 1-RTT keys are ready
for use. These events are not asynchronous, they always occur
immediately after TLS is provided with new handshake octets, or after
TLS produces handshake octets.
When TLS completed its handshake, 1-RTT keys can be provided to QUIC.
On both client and server, this occurs after sending the TLS Finished
message.
This ordering means that there could be frames that carry TLS
handshake messages ready to send at the same time that application
data is available. An implementation MUST ensure that TLS handshake
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messages are always sent in cleartext packets. Separate packets are
required for data that needs protection from 1-RTT keys.
If 0-RTT is possible, it is ready after the client sends a TLS
ClientHello message or the server receives that message. After
providing a QUIC client with the first handshake octets, the TLS
stack might signal that 0-RTT keys are ready. On the server, after
receiving handshake octets that contain a ClientHello message, a TLS
server might signal that 0-RTT keys are available.
1-RTT keys are used for packets in both directions. 0-RTT keys are
only used to protect packets sent by the client.
4.2.4. Secret Export
Details how secrets are exported from TLS are included in
Section 5.2.
4.2.5. TLS Interface Summary
Figure 4 summarizes the exchange between QUIC and TLS for both client
and server.
Client Server
Get Handshake
0-RTT Key Ready
--- send/receive --->
Handshake Received
0-RTT Key Ready
Get Handshake
1-RTT Keys Ready
<--- send/receive ---
Handshake Received
Get Handshake
Handshake Complete
1-RTT Keys Ready
--- send/receive --->
Handshake Received
Get Handshake
Handshake Complete
<--- send/receive ---
Handshake Received
Get Handshake
Figure 4: Interaction Summary between QUIC and TLS
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4.3. TLS Version
This document describes how TLS 1.3 [I-D.ietf-tls-tls13] is used with
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.
4.4. ClientHello Size
QUIC requires that the initial handshake packet from a client fit
within the payload of a single packet. The size limits on QUIC
packets mean that a record containing a ClientHello needs to fit
within 1171 octets.
A TLS ClientHello can fit within this limit with ample space
remaining. However, there are several variables that could cause
this limit to be exceeded. Implementations are reminded that large
session tickets or HelloRetryRequest cookies, multiple or large key
shares, and long lists of supported ciphers, signature algorithms,
versions, QUIC transport parameters, and other negotiable parameters
and extensions could cause this message to grow.
For servers, the size of the session tickets and HelloRetryRequest
cookie extension can have an effect on a client's ability to connect.
Choosing a small value increases the probability that these values
can be successfully used by a client.
The TLS implementation does not need to ensure that the ClientHello
is sufficiently large. QUIC PADDING frames are added to increase the
size of the packet as necessary.
4.5. Peer Authentication
The requirements for authentication depend on the application
protocol that is in use. TLS provides server authentication and
permits the server to request client authentication.
A client MUST authenticate the identity of the server. This
typically involves verification that the identity of the server is
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included in a certificate and that the certificate is issued by a
trusted entity (see for example [RFC2818]).
A server MAY request that the client authenticate during the
handshake. A server MAY refuse a connection if the client is unable
to authenticate when requested. The requirements for client
authentication vary based on application protocol and deployment.
A server MUST NOT use post-handshake client authentication (see
Section 4.6.2 of [I-D.ietf-tls-tls13]).
4.6. TLS Errors
Errors in the TLS connection SHOULD be signaled using TLS alerts on
stream 0. A failure in the handshake MUST be treated as a QUIC
connection error of type TLS_HANDSHAKE_FAILED. Once the handshake is
complete, an error in the TLS connection that causes a TLS alert to
be sent or received MUST be treated as a QUIC connection error of
type TLS_FATAL_ALERT_GENERATED or TLS_FATAL_ALERT_RECEIVED
respectively.
5. QUIC Packet Protection
QUIC packet protection provides authenticated encryption of packets.
This provides confidentiality and integrity protection for the
content of packets (see Section 5.3). Packet protection uses keys
that are exported from the TLS connection (see Section 5.2).
Different keys are used for QUIC packet protection and TLS record
protection. TLS handshake messages are protected solely with TLS
record protection, but post-handshake messages are redundantly
proteted with both both the QUIC packet protection and the TLS record
protection. These messages are limited in number, and so the
additional overhead is small.
5.1. Installing New Keys
As TLS reports the availability of keying material, the packet
protection keys and initialization vectors (IVs) are updated (see
Section 5.2). The selection of AEAD function is also updated to
match the AEAD negotiated by TLS.
For packets other than any unprotected handshake packets (see
Section 7.1), once a change of keys has been made, packets with
higher packet numbers MUST be sent with the new keying material. The
KEY_PHASE bit on these packets is inverted each time new keys are
installed to signal the use of the new keys to the recipient (see
Section 7 for details).
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An endpoint retransmits stream data in a new packet. New packets
have new packet numbers and use the latest packet protection keys.
This simplifies key management when there are key updates (see
Section 7.2).
5.2. QUIC Key Expansion
QUIC uses a system of packet protection secrets, keys and IVs that
are modelled on the system used in TLS [I-D.ietf-tls-tls13]. The
secrets that QUIC uses as the basis of its key schedule are obtained
using TLS exporters (see Section 7.5 of [I-D.ietf-tls-tls13]).
QUIC uses HKDF with the same hash function negotiated by TLS for key
derivation. For example, if TLS is using the TLS_AES_128_GCM_SHA256,
the SHA-256 hash function is used.
5.2.1. 0-RTT Secret
0-RTT keys are those keys that are used in resumed connections prior
to the completion of the TLS handshake. Data sent using 0-RTT keys
might be replayed and so has some restrictions on its use, see
Section 9.2. 0-RTT keys are used after sending or receiving a
ClientHello.
The secret is exported from TLS using the exporter label "EXPORTER-
QUIC 0-RTT Secret" and an empty context. The size of the secret MUST
be the size of the hash output for the PRF hash function negotiated
by TLS. This uses the TLS early_exporter_secret. The QUIC 0-RTT
secret is only used for protection of packets sent by the client.
client_0rtt_secret
= TLS-Exporter("EXPORTER-QUIC 0-RTT Secret"
"", Hash.length)
5.2.2. 1-RTT Secrets
1-RTT keys are used by both client and server after the TLS handshake
completes. There are two secrets used at any time: one is used to
derive packet protection keys for packets sent by the client, the
other for packet protection keys on packets sent by the server.
The initial client packet protection secret is exported from TLS
using the exporter label "EXPORTER-QUIC client 1-RTT Secret"; the
initial server packet protection secret uses the exporter label
"EXPORTER-QUIC server 1-RTT Secret". Both exporters use an empty
context. The size of the secret MUST be the size of the hash output
for the PRF hash function negotiated by TLS.
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client_pp_secret_0
= TLS-Exporter("EXPORTER-QUIC client 1-RTT Secret"
"", Hash.length)
server_pp_secret_0
= TLS-Exporter("EXPORTER-QUIC server 1-RTT Secret"
"", Hash.length)
These secrets are used to derive the initial client and server packet
protection keys.
After a key update (see Section 7.2), these secrets are updated using
the HKDF-Expand-Label function defined in Section 7.1 of
[I-D.ietf-tls-tls13]. HKDF-Expand-Label uses the PRF hash function
negotiated by TLS. The replacement secret is derived using the
existing Secret, a Label of "QUIC client 1-RTT Secret" for the client
and "QUIC server 1-RTT Secret" for the server, an empty HashValue,
and the same output Length as the hash function selected by TLS for
its PRF.
client_pp_secret_<N+1>
= HKDF-Expand-Label(client_pp_secret_<N>,
"QUIC client 1-RTT Secret",
"", Hash.length)
server_pp_secret_<N+1>
= HKDF-Expand-Label(server_pp_secret_<N>,
"QUIC server 1-RTT Secret",
"", Hash.length)
This allows for a succession of new secrets to be created as needed.
HKDF-Expand-Label uses HKDF-Expand [RFC5869] with a specially
formatted info parameter, as shown:
HKDF-Expand-Label(Secret, Label, HashValue, Length) =
HKDF-Expand(Secret, HkdfLabel, Length)
Where HkdfLabel is specified as:
struct {
uint16 length = Length;
opaque label<10..255> = "tls13 " + Label;
uint8 hashLength; // Always 0
} HkdfLabel;
For example, the client packet protection secret uses an info
parameter of:
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info = (HashLen / 256) || (HashLen % 256) || 0x1f ||
"tls13 QUIC client 1-RTT secret" || 0x00
5.2.3. Packet Protection Key and IV
The complete key expansion uses an identical process for key
expansion as defined in Section 7.3 of [I-D.ietf-tls-tls13], using
different values for the input secret. QUIC uses the AEAD function
negotiated by TLS.
The packet protection key and IV used to protect the 0-RTT packets
sent by a client are derived from the QUIC 0-RTT secret. The packet
protection keys and IVs for 1-RTT packets sent by the client and
server are derived from the current generation of client_pp_secret
and server_pp_secret respectively. The length of the output is
determined by the requirements of the AEAD function selected by TLS.
The key length is the AEAD key size. As defined in Section 5.3 of
[I-D.ietf-tls-tls13], the IV length is the larger of 8 or N_MIN (see
Section 4 of [RFC5116]). For any secret S, the corresponding key and
IV are derived as shown below:
key = HKDF-Expand-Label(S, "key", "", key_length)
iv = HKDF-Expand-Label(S, "iv", "", iv_length)
The QUIC record protection initially starts without keying material.
When the TLS state machine reports that the ClientHello has been
sent, the 0-RTT keys can be generated and installed for writing.
When the TLS state machine reports completion of the handshake, the
1-RTT keys can be generated and installed for writing.
5.3. QUIC AEAD Usage
The Authentication Encryption with Associated Data (AEAD) [RFC5116]
function used for QUIC packet protection is AEAD that is negotiated
for use with the TLS connection. For example, if TLS is using the
TLS_AES_128_GCM_SHA256, the AEAD_AES_128_GCM function is used.
Regular QUIC packets are protected by an AEAD algorithm [RFC5116].
Version negotiation and public reset packets are not protected.
Once TLS has provided a key, the contents of regular QUIC packets
immediately after any TLS messages have been sent are protected by
the AEAD selected by TLS.
The key, K, is either the client packet protection key
(client_pp_key_n) or the server packet protection key
(server_pp_key_n), derived as defined in Section 5.2.
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The nonce, N, is formed by combining the packet protection IV (either
client_pp_iv_n or server_pp_iv_n) with the packet number. The 64
bits of the reconstructed QUIC packet number in network byte order is
left-padded with zeros to the size of the IV. The exclusive OR of
the padded packet number and the IV forms the AEAD nonce.
The associated data, A, for the AEAD is the contents of the QUIC
header, starting from the flags octet in either the short or long
header.
The input plaintext, P, for the AEAD is the content of the QUIC frame
following the header, as described in [QUIC-TRANSPORT].
The output ciphertext, C, of the AEAD is transmitted in place of P.
Prior to TLS providing keys, no record protection is performed and
the plaintext, P, is transmitted unmodified.
5.4. Packet Numbers
QUIC has a single, contiguous packet number space. In comparison,
TLS restarts its sequence number each time that record protection
keys are changed. The sequence number restart in TLS ensures that a
compromise of the current traffic keys does not allow an attacker to
truncate the data that is sent after a key update by sending
additional packets under the old key (causing new packets to be
discarded).
QUIC does not assume a reliable transport and is required to handle
attacks where packets are dropped in other ways. QUIC is therefore
not affected by this form of truncation.
The QUIC packet number is not reset and it is not permitted to go
higher than its maximum value of 2^64-1. This establishes a hard
limit on the number of packets that can be sent.
Some AEAD functions have limits for how many packets can be encrypted
under the same key and IV (see for example [AEBounds]). This might
be lower than the packet number limit. An endpoint MUST initiate a
key update (Section 7.2) prior to exceeding any limit set for the
AEAD that is in use.
TLS maintains a separate sequence number that is used for record
protection on the connection that is hosted on stream 0. This
sequence number is not visible to QUIC.
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5.5. Receiving Protected Packets
Once an endpoint successfully receives a packet with a given packet
number, it MUST discard all packets with higher packet numbers if
they cannot be successfully unprotected with either the same key, or
- if there is a key update - the next packet protection key (see
Section 7.2). Similarly, a packet that appears to trigger a key
update, but cannot be unprotected successfully MUST be discarded.
Failure to unprotect a packet does not necessarily indicate the
existence of a protocol error in a peer or an attack. The truncated
packet number encoding used in QUIC can cause packet numbers to be
decoded incorrectly if they are delayed significantly.
5.6. Packet Number Gaps
[QUIC-TRANSPORT]; Section 7.5.1.1 also requires a secret to compute
packet number gaps on connection ID transitions. That secret is
computed as:
packet_number_secret
= TLS-Exporter("EXPORTER-QUIC Packet Number Secret"
"", Hash.length)
6. Unprotected Packets
QUIC adds an integrity check to all cleartext packets. Cleartext
packets are not protected by the negotiated AEAD (see Section 5), but
instead include an integrity check. This check does not prevent the
packet from being altered, it exists for added resilience against
data corruption and to provide added assurance that the sender
intends to use QUIC.
Cleartext packets all use the long form of the QUIC header and so
will include a version number. For this version of QUIC, the
integrity check uses the 64-bit FNV-1a hash (see Section 6.2). The
output of this hash is appended to the payload of the packet.
The integrity check algorithm MAY change for other versions of the
protocol.
6.1. Integrity Check Processing
An endpoint sending a packet that has a long header and a type that
does not indicate that the packet will be protected (that is, 0-RTT
Encrypted (0x05), 1-RTT Encrypted (key phase 0) (0x06), or 1-RTT
Encrypted (key phase 1) (0x07)) first constructs the packet that it
sends without the integrity check.
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The sender then calculates the integrity check over the entire
packet, starting from the type field. The output of the hash is
appended to the packet.
A receiver that receives an unprotected packet first checks that the
version is correct, then removes the trailing 8 octets. It
calculates the integrity check over the remainder of the packet.
Unprotected packets that do not contain a valid integrity check MUST
be discarded.
6.2. The 64-bit FNV-1a Algorithm
QUIC uses the 64-bit version of the alternative Fowler/Noll/Vo hash
(FNV-1a) [FNV].
FNV-1a can be expressed in pseudocode as:
hash := offset basis
for each input octet:
hash := hash XOR input octet
hash := hash * prime
That is, a 64-bit unsigned integer is initialized with an offset
basis. Then, for each octet of the input, the exclusive binary OR of
the value is taken, then multiplied by a prime. Any overflow from
multiplication is discarded.
The offset basis for the 64-bit FNV-1a is the decimal value
14695981039346656037 (in hex, 0xcbf29ce484222325). The prime is
1099511628211 (in hex, 0x100000001b3; or as an expression 2^40 + 2^8
+ 0xb3).
Once all octets have been processed in this fashion, the final
integer value is encoded as 8 octets in network byte order.
7. Key Phases
As TLS reports the availability of 0-RTT and 1-RTT keys, new keying
material can be exported from TLS and used for QUIC packet
protection. At each transition during the handshake a new secret is
exported from TLS and packet protection keys are derived from that
secret.
Every time that a new set of keys is used for protecting outbound
packets, the KEY_PHASE bit in the public flags is toggled. 0-RTT
protected packets use the QUIC long header, they do not use the
KEY_PHASE bit to select the correct keys (see Section 7.1.1).
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Once the connection is fully enabled, 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 bit can update keys and
decrypt the packet that contains the changed bit, see Section 7.2.
The KEY_PHASE bit is included as the 0x20 bit of the QUIC short
header, or is determined by the packet type from the long header (a
type of 0x06 indicates a key phase of 0, 0x07 indicates key phase 1).
Transitions between keys during the handshake are complicated by the
need to ensure that TLS handshake messages are sent with the correct
packet protection.
7.1. Packet Protection for the TLS Handshake
The initial exchange of packets are sent without protection. These
packets use a cleartext packet type.
TLS handshake messages MUST NOT be protected using QUIC packet
protection. All TLS handshake messages up to the TLS Finished
message sent by either endpoint use cleartext packets.
Any TLS handshake messages that are sent after completing the TLS
handshake do not need special packet protection rules. Packets
containing these messages use the packet protection keys that are
current at the time of sending (or retransmission).
Like the client, a server MUST send retransmissions of its
unprotected handshake messages or acknowledgments for unprotected
handshake messages sent by the client in cleartext packets.
7.1.1. Initial Key Transitions
Once the TLS handshake is complete, keying material is exported from
TLS and QUIC packet protection commences.
Packets protected with 1-RTT keys initially have a KEY_PHASE bit set
to 0. This bit inverts with each subsequent key update (see
Section 7.2).
If the client sends 0-RTT data, it uses the 0-RTT packet type. The
packet that contains the TLS EndOfEarlyData and Finished messages are
sent in cleartext packets.
Using distinct packet types during the handshake for handshake
messages, 0-RTT data, and 1-RTT data ensures that the server is able
to distinguish between the different keys used to remove packet
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protection. All of these packets can arrive concurrently at a
server.
A server might choose to retain 0-RTT packets that arrive before a
TLS ClientHello. The server can then use those packets once the
ClientHello arrives. However, the potential for denial of service
from buffering 0-RTT packets is significant. These packets cannot be
authenticated and so might be employed by an attacker to exhaust
server resources. Limiting the number of packets that are saved
might be necessary.
The server transitions to using 1-RTT keys after sending its first
flight of TLS handshake messages. From this point, the server
protects all packets with 1-RTT keys. Future packets are therefore
protected with 1-RTT keys. Initially, these are marked with a
KEY_PHASE of 0.
7.1.2. Retransmission and Acknowledgment of Unprotected Packets
TLS handshake messages from both client and server are critical to
the key exchange. The contents of these messages determines the keys
used to protect later messages. If these handshake messages are
included in packets that are protected with these keys, they will be
indecipherable to the recipient.
Even though newer keys could be available when retransmitting,
retransmissions of these handshake messages MUST be sent in cleartext
packets. An endpoint MUST generate ACK frames for these messages and
send them in cleartext packets.
A HelloRetryRequest handshake message might be used to reject an
initial ClientHello. A HelloRetryRequest handshake message is sent
in a Server Stateless Retry packet; any second ClientHello that is
sent in response uses a Client Initial packet type. Neither packet
is protected. This is natural, because no new keying material will
be available when these messages need to be sent. Upon receipt of a
HelloRetryRequest, a client SHOULD cease any transmission of 0-RTT
data; 0-RTT data will only be discarded by any server that sends a
HelloRetryRequest.
The packet type ensures that protected packets are clearly
distinguished from unprotected packets. Loss or reordering might
cause unprotected packets to arrive once 1-RTT keys are in use,
unprotected packets are easily distinguished from 1-RTT packets using
the packet type.
Once 1-RTT keys are available to an endpoint, it no longer needs the
TLS handshake messages that are carried in unprotected packets.
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However, a server might need to retransmit its TLS handshake messages
in response to receiving an unprotected packet that contains ACK
frames. A server MUST process ACK frames in unprotected packets
until the TLS handshake is reported as complete, or it receives an
ACK frame in a protected packet that acknowledges all of its
handshake messages.
To limit the number of key phases that could be active, an endpoint
MUST NOT initiate a key update while there are any unacknowledged
handshake messages, see Section 7.2.
7.2. Key Update
Once the TLS handshake is complete, the KEY_PHASE bit allows for
refreshes of keying material by either peer. Endpoints start using
updated keys immediately without additional signaling; the change in
the KEY_PHASE bit indicates that a new key is in use.
An endpoint MUST NOT initiate more than one key update at a time. A
new key cannot be used until the endpoint has received and
successfully decrypted a packet with a matching KEY_PHASE. Note that
when 0-RTT is attempted the value of the KEY_PHASE bit will be
different on packets sent by either peer.
A receiving endpoint detects an update when the KEY_PHASE bit doesn't
match what it is expecting. It creates a new secret (see
Section 5.2) and the corresponding read key and IV. If the packet
can be decrypted and authenticated using these values, then the keys
it uses for packet protection are also updated. The next packet sent
by the endpoint will then use the new keys.
An endpoint doesn't need to send packets immediately when it detects
that its peer has updated keys. The next packet that it sends will
simply use the new keys. If an endpoint detects a second update
before it has sent any packets with updated keys it indicates that
its peer has updated keys twice without awaiting a reciprocal update.
An endpoint MUST treat consecutive key updates as a fatal error and
abort the connection.
An endpoint SHOULD retain old keys for a short period to allow it to
decrypt packets with smaller packet numbers than the packet that
triggered the key update. This allows an endpoint to consume packets
that are reordered around the transition between keys. Packets with
higher packet numbers always use the updated keys and MUST NOT be
decrypted with old keys.
Keys and their corresponding secrets SHOULD be discarded when an
endpoint has received all packets with sequence numbers lower than
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the lowest sequence number used for the new key. An endpoint might
discard keys if it determines that the length of the delay to
affected packets is excessive.
This ensures that once the handshake is complete, packets with the
same KEY_PHASE will have the same packet protection keys, unless
there are multiple key updates in a short time frame succession and
significant packet reordering.
Initiating Peer Responding Peer
@M QUIC Frames
New Keys -> @N
@N QUIC Frames
-------->
QUIC Frames @M
New Keys -> @N
QUIC Frames @N
<--------
Figure 5: Key Update
As shown in Figure 3 and Figure 5, there is never a situation where
there are more than two different sets of keying material that might
be received by a peer. Once both sending and receiving keys have
been updated,
A server cannot initiate a key update until it has received the
client's Finished message. Otherwise, packets protected by the
updated keys could be confused for retransmissions of handshake
messages. A client cannot initiate a key update until all of its
handshake messages have been acknowledged by the server.
A packet that triggers a key update could arrive after successfully
processing a packet with a higher packet number. This is only
possible if there is a key compromise and an attack, or if the peer
is incorrectly reverting to use of old keys. Because the latter
cannot be differentiated from an attack, an endpoint MUST immediately
terminate the connection if it detects this condition.
8. Client Address Validation
Two tools are provided by TLS to enable validation of client source
addresses at a server: the cookie in the HelloRetryRequest message,
and the ticket in the NewSessionTicket message.
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8.1. HelloRetryRequest Address Validation
The cookie extension in the TLS HelloRetryRequest message allows a
server to perform source address validation during the handshake.
When QUIC requests address validation during the processing of the
first ClientHello, the token it provides is included in the cookie
extension of a HelloRetryRequest. As long as the cookie cannot be
successfully guessed by a client, the server can be assured that the
client received the HelloRetryRequest if it includes the value in a
second ClientHello.
An initial ClientHello never includes a cookie extension. Thus, if a
server constructs a cookie that contains all the information
necessary to reconstruct state, it can discard local state after
sending a HelloRetryRequest. Presence of a valid cookie in a
ClientHello indicates that the ClientHello is a second attempt from
the client.
An address validation token can be extracted from a second
ClientHello and passed to the transport for further validation. If
that validation fails, the server MUST fail the TLS handshake and
send an illegal_parameter alert.
Combining address validation with the other uses of HelloRetryRequest
ensures that there are fewer ways in which an additional round-trip
can be added to the handshake. In particular, this makes it possible
to combine a request for address validation with a request for a
different client key share.
If TLS needs to send a HelloRetryRequest for other reasons, it needs
to ensure that it can correctly identify the reason that the
HelloRetryRequest was generated. During the processing of a second
ClientHello, TLS does not need to consult the transport protocol
regarding address validation if address validation was not requested
originally. In such cases, the cookie extension could either be
absent or it could indicate that an address validation token is not
present.
8.1.1. Stateless Address Validation
A server can use the cookie extension to store all state necessary to
continue the connection. This allows a server to avoid committing
state for clients that have unvalidated source addresses.
For instance, a server could use a statically-configured key to
encrypt the information that it requires and include that information
in the cookie. In addition to address validation information, a
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server that uses encryption also needs to be able recover the hash of
the ClientHello and its length, plus any information it needs in
order to reconstruct the HelloRetryRequest.
8.1.2. Sending HelloRetryRequest
A server does not need to maintain state for the connection when
sending a HelloRetryRequest message. This might be necessary to
avoid creating a denial of service exposure for the server. However,
this means that information about the transport will be lost at the
server. This includes the stream offset of stream 0, the packet
number that the server selects, and any opportunity to measure round
trip time.
A server MUST send a TLS HelloRetryRequest in a Server Stateless
Retry packet. Using a Server Stateless Retry packet causes the
client to reset stream offsets. It also avoids the need for the
server select an initial packet number, which would need to be
remembered so that subsequent packets could be correctly numbered.
A HelloRetryRequest message MUST NOT be split between multiple Server
Stateless Retry packets. This means that HelloRetryRequest is
subject to the same size constraints as a ClientHello (see
Section 4.4).
8.2. NewSessionTicket Address Validation
The ticket in the TLS NewSessionTicket message allows a server to
provide a client with a similar sort of token. When a client resumes
a TLS connection - whether or not 0-RTT is attempted - it includes
the ticket in the handshake message. As with the HelloRetryRequest
cookie, the server includes the address validation token in the
ticket. TLS provides the token it extracts from the session ticket
to the transport when it asks whether source address validation is
needed.
If both a HelloRetryRequest cookie and a session ticket are present
in the ClientHello, only the token from the cookie is passed to the
transport. The presence of a cookie indicates that this is a second
ClientHello - the token from the session ticket will have been
provided to the transport when it appeared in the first ClientHello.
A server can send a NewSessionTicket message at any time. This
allows it to update the state - and the address validation token -
that is included in the ticket. This might be done to refresh the
ticket or token, or it might be generated in response to changes in
the state of the connection. QUIC can request that a
NewSessionTicket be sent by providing a new address validation token.
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A server that intends to support 0-RTT SHOULD provide an address
validation token immediately after completing the TLS handshake.
8.3. Address Validation Token Integrity
TLS MUST provide integrity protection for address validation token
unless the transport guarantees integrity protection by other means.
For a NewSessionTicket that includes confidential information - such
as the resumption secret - including the token under authenticated
encryption ensures that the token gains both confidentiality and
integrity protection without duplicating the overheads of that
protection.
9. Pre-handshake QUIC Messages
Implementations MUST NOT exchange data on any stream other than
stream 0 without packet protection. QUIC requires the use of several
types of frame for managing loss detection and recovery during this
phase. In addition, it might be useful to use the data acquired
during the exchange of unauthenticated messages for congestion
control.
This section generally only applies to TLS handshake messages from
both peers and acknowledgments of the packets carrying those
messages. In many cases, the need for servers to provide
acknowledgments is minimal, since the messages that clients send are
small and implicitly acknowledged by the server's responses.
The actions that a peer takes as a result of receiving an
unauthenticated packet needs to be limited. In particular, state
established by these packets cannot be retained once record
protection commences.
There are several approaches possible for dealing with
unauthenticated packets prior to handshake completion:
o discard and ignore them
o use them, but reset any state that is established once the
handshake completes
o use them and authenticate them afterwards; failing the handshake
if they can't be authenticated
o save them and use them when they can be properly authenticated
o treat them as a fatal error
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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 are made usable and authenticated as part of
the TLS handshake (see Section 10.2).
o Most unprotected messages are treated as fatal errors when
received except for the small number necessary to permit the
handshake to complete (see Section 9.1).
o Protected packets can either be discarded or saved and later used
(see Section 9.3).
9.1. Unprotected Packets Prior to Handshake Completion
This section describes the handling of messages that are sent and
received prior to the completion of the TLS handshake.
Sending and receiving unprotected messages is hazardous. Unless
expressly permitted, receipt of an unprotected message of any kind
MUST be treated as a fatal error.
9.1.1. STREAM Frames
"STREAM" frames for stream 0 are permitted. These carry the TLS
handshake messages. Once 1-RTT keys are available, unprotected
"STREAM" frames on stream 0 can be ignored.
Receiving unprotected "STREAM" frames for other streams MUST be
treated as a fatal error.
9.1.2. ACK Frames
"ACK" frames are permitted prior to the handshake being complete.
Information learned from "ACK" frames cannot be entirely relied upon,
since an attacker is able to inject these packets. Timing and packet
retransmission information from "ACK" frames is critical to the
functioning of the protocol, but these frames might be spoofed or
altered.
Endpoints MUST NOT use an "ACK" frame in an unprotected packet to
acknowledge packets that were protected by 0-RTT or 1-RTT keys. An
endpoint MUST treat receipt of an "ACK" frame in an unprotected
packet that claims to acknowledge protected packets as a connection
error of type OPTIMISTIC_ACK. An endpoint that can read protected
data is always able to send protected data.
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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.
An endpoint SHOULD use data from "ACK" frames carried in unprotected
packets or packets protected with 0-RTT keys only during the initial
handshake. All "ACK" frames contained in unprotected packets that
are received after successful receipt of a packet protected with
1-RTT keys MUST be discarded. An endpoint SHOULD therefore include
acknowledgments for unprotected and any packets protected with 0-RTT
keys until it sees an acknowledgment for a packet that is both
protected with 1-RTT keys and contains an "ACK" frame.
9.1.3. Updates to Data and Stream Limits
"MAX_DATA", "MAX_STREAM_DATA", "BLOCKED", "STREAM_BLOCKED", and
"MAX_STREAM_ID" frames MUST NOT be sent unprotected.
Though data is exchanged on stream 0, the initial flow control window
on that stream is sufficiently large to allow the TLS handshake to
complete. This limits the maximum size of the TLS handshake and
would prevent a server or client from using an abnormally large
certificate chain.
Stream 0 is exempt from the connection-level flow control window.
Consequently, there is no need to signal being blocked on flow
control.
Similarly, there is no need to increase the number of allowed streams
until the handshake completes.
9.1.4. Denial of Service with Unprotected Packets
Accepting unprotected - specifically unauthenticated - packets
presents a denial of service risk to endpoints. An attacker that is
able to inject unprotected packets can cause a recipient to drop even
protected packets with a matching sequence number. The spurious
packet shadows the genuine packet, causing the genuine packet to be
ignored as redundant.
Once the TLS handshake is complete, both peers MUST ignore
unprotected packets. From that point onward, unprotected messages
can be safely dropped.
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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.
In addition to causing valid packets to be dropped, an attacker can
generate packets with an intent of causing the recipient to expend
processing resources. See Section 11.2 for a discussion of these
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 that is received prior to the end of the handshake
MUST be treated as a fatal error.
9.2. Use of 0-RTT Keys
If 0-RTT keys are available, the lack of replay protection means that
restrictions on their use are necessary to avoid replay attacks on
the protocol.
A client MUST only use 0-RTT keys to protect data that is idempotent.
A client MAY wish to apply additional restrictions on what data it
sends prior to the completion of the TLS handshake. A client
otherwise treats 0-RTT keys as equivalent to 1-RTT keys.
A client that receives an indication that its 0-RTT data has been
accepted by a server can send 0-RTT data until it receives all of the
server's handshake messages. A client SHOULD stop sending 0-RTT data
if it receives an indication that 0-RTT data has been rejected.
A server MUST NOT use 0-RTT keys to protect packets.
9.3. Receiving Out-of-Order Protected Frames
Due to reordering and loss, protected packets might be received by an
endpoint before the final TLS handshake messages are received. A
client will be unable to decrypt 1-RTT packets from the server,
whereas a server will be able to decrypt 1-RTT packets from the
client.
Packets protected with 1-RTT keys MAY be stored and later decrypted
and used once the handshake is complete. A server MUST NOT use 1-RTT
protected packets before verifying either the client Finished message
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or - in the case that the server has chosen to use a pre-shared key -
the pre-shared key binder (see Section 4.2.8 of
[I-D.ietf-tls-tls13]). Verifying these values provides the server
with an assurance that the ClientHello has not been modified.
A server could receive packets protected with 0-RTT keys prior to
receiving a TLS ClientHello. The server MAY retain these packets for
later decryption in anticipation of receiving a ClientHello.
Receiving and verifying the TLS Finished message is critical in
ensuring the integrity of the TLS handshake. A server MUST NOT use
protected packets from the client prior to verifying the client
Finished message if its response depends on client authentication.
10. QUIC-Specific Additions to the TLS Handshake
QUIC uses the TLS handshake for more than just negotiation of
cryptographic parameters. The TLS handshake validates protocol
version selection, provides preliminary values for QUIC transport
parameters, and allows a server to perform return routeability checks
on clients.
10.1. Protocol and Version Negotiation
The QUIC version negotiation mechanism is used to negotiate the
version of QUIC that is used prior to the completion of the
handshake. However, this packet is not authenticated, enabling an
active attacker to force a version downgrade.
To ensure that a QUIC version downgrade is not forced by an attacker,
version information is copied into the TLS handshake, which provides
integrity protection for the QUIC negotiation. This does not prevent
version downgrade prior to the completion of the handshake, though it
means that a downgrade causes a handshake failure.
TLS uses Application Layer Protocol Negotiation (ALPN) [RFC7301] to
select an application protocol. The application-layer protocol MAY
restrict the QUIC versions that it can operate over. Servers MUST
select an application protocol compatible with the QUIC version that
the client has selected.
If the server cannot select a compatible combination of application
protocol and QUIC version, it MUST abort the connection. A client
MUST abort a connection if the server picks an incompatible
combination of QUIC version and ALPN identifier.
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10.2. QUIC Transport Parameters Extension
QUIC transport parameters are carried in a TLS extension. Different
versions of QUIC might define a different format for this struct.
Including transport parameters in the TLS handshake provides
integrity protection for these values.
enum {
quic_transport_parameters(26), (65535)
} ExtensionType;
The "extension_data" field of the quic_transport_parameters extension
contains a value that is defined by the version of QUIC that is in
use. The quic_transport_parameters extension carries a
TransportParameters when the version of QUIC defined in
[QUIC-TRANSPORT] is used.
The quic_transport_parameters extension is carried in the ClientHello
and the EncryptedExtensions messages during the handshake. The
extension MAY be included in a NewSessionTicket message.
10.3. Priming 0-RTT
QUIC uses TLS without modification. Therefore, it is possible to use
a pre-shared key that was established in a TLS handshake over TCP to
enable 0-RTT in QUIC. Similarly, QUIC can provide a pre-shared key
that can be used to enable 0-RTT in TCP.
All the restrictions on the use of 0-RTT apply, with the exception of
the ALPN label, which MUST only change to a label that is explicitly
designated as being compatible. The client indicates which ALPN
label it has chosen by placing that ALPN label first in the ALPN
extension.
The certificate that the server uses MUST be considered valid for
both connections, which will use different protocol stacks and could
use different port numbers. For instance, HTTP/1.1 and HTTP/2
operate over TLS and TCP, whereas QUIC operates over UDP.
Source address validation is not completely portable between
different protocol stacks. Even if the source IP address remains
constant, the port number is likely to be different. Packet
reflection attacks are still possible in this situation, though the
set of hosts that can initiate these attacks is greatly reduced. A
server might choose to avoid source address validation for such a
connection, or allow an increase to the amount of data that it sends
toward the client without source validation.
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11. Security Considerations
There are likely to be some real clangers here eventually, but the
current set of issues is well captured in the relevant sections of
the main text.
Never assume that because it isn't in the security considerations
section it doesn't affect security. Most of this document does.
11.1. Packet Reflection Attack Mitigation
A small ClientHello that results in a large block of handshake
messages from a server can be used in packet reflection attacks to
amplify the traffic generated by an attacker.
Certificate caching [RFC7924] can reduce the size of the server's
handshake messages significantly.
QUIC requires that the packet containing a ClientHello be padded to a
minimum size. A server is less likely to generate a packet
reflection attack if the data it sends is a small multiple of this
size. A server SHOULD use a HelloRetryRequest if the size of the
handshake messages it sends is likely to significantly exceed the
size of the packet containing the ClientHello.
11.2. Peer Denial of Service
QUIC, TLS and HTTP/2 all contain a messages that have legitimate uses
in some contexts, but that can be abused to cause a peer to expend
processing resources without having any observable impact on the
state of the connection. If processing is disproportionately large
in comparison to the observable effects on bandwidth or state, then
this could allow a malicious peer to exhaust processing capacity
without consequence.
QUIC prohibits the sending of empty "STREAM" frames unless they are
marked with the FIN bit. This prevents "STREAM" frames from being
sent that only waste effort.
TLS records SHOULD always contain at least one octet of a handshake
messages or alert. Records containing only padding are permitted
during the handshake, but an excessive number might be used to
generate unnecessary work. Once the TLS handshake is complete,
endpoints SHOULD NOT send TLS application data records unless it is
to hide the length of QUIC records. QUIC packet protection does not
include any allowance for padding; padded TLS application data
records can be used to mask the length of QUIC frames.
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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.
12. Error codes
The portion of the QUIC error code space allocated for the crypto
handshake is 0xC0000000-0xFFFFFFFF. The following error codes are
defined when TLS is used for the crypto handshake:
TLS_HANDSHAKE_FAILED (0xC000001C): The TLS handshake failed.
TLS_FATAL_ALERT_GENERATED (0xC000001D): A TLS fatal alert was sent,
causing the TLS connection to end prematurely.
TLS_FATAL_ALERT_RECEIVED (0xC000001E): A TLS fatal alert was
received, causing the TLS connection to end prematurely.
13. IANA Considerations
This document does not create any new IANA registries, but it does
utilize the following registries:
o QUIC Transport Parameter Registry - IANA is to register the three
values found in Section 12.
o TLS ExtensionsType Registry - IANA is to register the
quic_transport_parameters extension found in Section 10.2.
Assigning 26 to the extension would be greatly appreciated. The
Recommended column is to be marked Yes.
o TLS Exporter Label Registry - IANA is requested to register
"EXPORTER-QUIC 0-RTT Secret" from Section 5.2.1; "EXPORTER-QUIC
client 1-RTT Secret" and "EXPORTER-QUIC server 1-RTT Secret" from
Section 5.2.2; "EXPORTER-QUIC Packet Number Secret" Section 5.6.
The DTLS column is to be marked No. The Recommended column is to
be marked Yes.
14. References
14.1. Normative References
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-21 (work in progress),
July 2017.
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[QUIC-TRANSPORT]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", draft-ietf-quic-
transport (work in progress), September 2017.
[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>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010, <https://www.rfc-
editor.org/info/rfc5869>.
[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>.
14.2. 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>.
[FNV] Fowler, G., Noll, L., Vo, K., Eastlake, D., and T. Hansen,
"The FNV Non-Cryptographic Hash Algorithm", draft-
eastlake-fnv-13 (work in progress), June 2017.
[QUIC-HTTP]
Bishop, M., Ed., "Hypertext Transfer Protocol (HTTP) over
QUIC", draft-ietf-quic-http (work in progress), September
2017.
[QUIC-RECOVERY]
Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
and Congestion Control", draft-ietf-quic-recovery (work in
progress), September 2017.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000, <https://www.rfc-
editor.org/info/rfc2818>.
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[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>.
[RFC7924] Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", RFC 7924,
DOI 10.17487/RFC7924, July 2016, <https://www.rfc-
editor.org/info/rfc7924>.
Appendix A. Contributors
Ryan Hamilton was originally an author of this specification.
Appendix B. Acknowledgments
This document has benefited from input from Dragana Damjanovic,
Christian Huitema, Jana Iyengar, Adam Langley, Roberto Peon, Eric
Rescorla, Ian Swett, and many others.
Appendix C. Change Log
*RFC Editor's Note:* Please remove this section prior to
publication of a final version of this document.
Issue and pull request numbers are listed with a leading octothorp.
C.1. Since draft-ietf-quic-tls-05
No significant changes.
C.2. Since draft-ietf-quic-tls-04
o Update labels used in HKDF-Expand-Label to match TLS 1.3 (#642)
C.3. Since draft-ietf-quic-tls-03
No significant changes.
C.4. Since draft-ietf-quic-tls-02
o Updates to match changes in transport draft
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C.5. 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)
C.6. Since draft-ietf-quic-tls-00
o Changed bit used to signal key phase
o Updated key phase markings during the handshake
o Added TLS interface requirements section
o Moved to use of TLS exporters for key derivation
o Moved TLS error code definitions into this document
C.7. Since draft-thomson-quic-tls-01
o Adopted as base for draft-ietf-quic-tls
o Updated authors/editors list
o Added status note
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Authors' Addresses
Martin Thomson (editor)
Mozilla
Email: martin.thomson@gmail.com
Sean Turner (editor)
sn3rd
Email: sean@sn3rd.com
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