draft-ietf-tls-falsestart-00.txt   draft-ietf-tls-falsestart-01.txt 
TLS Working Group A. Langley TLS Working Group A. Langley
Internet-Draft N. Modadugu Internet-Draft N. Modadugu
Intended status: Experimental B. Moeller Intended status: Experimental B. Moeller
Expires: November 8, 2015 Google Expires: May 5, 2016 Google
May 7, 2015 November 2, 2015
Transport Layer Security (TLS) False Start Transport Layer Security (TLS) False Start
draft-ietf-tls-falsestart-00 draft-ietf-tls-falsestart-01
Abstract Abstract
This document specifies an optional behavior of TLS implementations, This document specifies an optional behavior of TLS client
dubbed False Start. It affects only protocol timing, not on-the-wire implementations, dubbed False Start. It affects only protocol
protocol data, and can be implemented unilaterally. The TLS False timing, not on-the-wire protocol data, and can be implemented
Start feature leads to a latency reduction of one round trip for unilaterally. A TLS False Start reduces handshake latency to one
certain handshakes. round trip.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 8, 2015. This Internet-Draft will expire on May 5, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Requirements Notation . . . . . . . . . . . . . . . . . . . . 2 1. Requirements Notation . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
3. False Start Compatibility . . . . . . . . . . . . . . . . . . 5 3. False Start Compatibility . . . . . . . . . . . . . . . . . . 4
4. Client-side False Start . . . . . . . . . . . . . . . . . . . 5 4. Client-side False Start . . . . . . . . . . . . . . . . . . . 4
5. Server-side False Start . . . . . . . . . . . . . . . . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 5.1. Symmetric Cipher . . . . . . . . . . . . . . . . . . . . 6
6.1. Symmetric Cipher . . . . . . . . . . . . . . . . . . . . 7 5.2. Protocol Version . . . . . . . . . . . . . . . . . . . . 7
6.2. Protocol Version . . . . . . . . . . . . . . . . . . . . 8 5.3. Key Exchange and Client Certificate Type . . . . . . . . 7
6.3. Key Exchange and Client Certificate Type . . . . . . . . 8 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 8.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 9 8.2. Informative References . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 10 Appendix A. Implementation Notes . . . . . . . . . . . . . . . . 9
Appendix A. Implementation Notes . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Requirements Notation 1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
2. Introduction 2. Introduction
A full TLS handshake as specified in [RFC5246] requires two full A full handshake in TLS protocol versions up to TLS 1.2 [RFC5246]
protocol rounds (four flights) before the handshake is complete and requires two full protocol rounds (four flights) before the handshake
the protocol parties may begin to send application data. Thus, using is complete and the protocol parties may begin to send application
TLS can add a latency penalty of two network round-trip times for data. Thus, using TLS can add a latency penalty of two network
application protocols in which the client sends data first, such as round-trip times for application protocols in which the client sends
HTTP [RFC2616]. An abbreviated handshake (resuming an earlier TLS data first, such as HTTP [RFC2616].
session) is complete after three flights, thus adding just one round-
trip time if the client sends application data first.
Client Server Client Server
ClientHello --------> ClientHello -------->
ServerHello ServerHello
Certificate* Certificate*
ServerKeyExchange* ServerKeyExchange*
CertificateRequest* CertificateRequest*
<-------- ServerHelloDone <-------- ServerHelloDone
Certificate* Certificate*
ClientKeyExchange ClientKeyExchange
CertificateVerify* CertificateVerify*
[ChangeCipherSpec] [ChangeCipherSpec]
Finished --------> Finished -------->
[ChangeCipherSpec] [ChangeCipherSpec]
<-------- Finished <-------- Finished
Application Data <-------> Application Data Application Data <-------> Application Data
Figure 1 [RFC5246]. Message flow for a full handshake Figure 1 [RFC5246]. Message flow for a full handshake
Client Server
ClientHello -------->
ServerHello
[ChangeCipherSpec]
<-------- Finished
[ChangeCipherSpec]
Finished -------->
Application Data <-------> Application Data
Figure 2 [RFC5246]. Message flow for an abbreviated handshake
This document describes a technique that alleviates the latency This document describes a technique that alleviates the latency
burden imposed by TLS: the TLS False Start. If certain conditions burden imposed by TLS: the client-side TLS False Start. If certain
are met, application data can be sent when the handshake is only conditions are met, the client can start to send application data
partially complete -- i.e., when the sender has sent its own when the full handshake is only partially complete, namely, when the
"ChangeCipherSpec" and "Finished" messages (thus having updated its client has sent its own "ChangeCipherSpec" and "Finished" messages
TLS Record Protocol write state as negotiated in the handshake), but (thus having updated its TLS Record Protocol write state as
has yet to receive the other side's "ChangeCipherSpec" and "Finished" negotiated in the handshake), but has yet to receive the server's
messages. (By section 7.4.9 of [RFC5246], each party would have to "ChangeCipherSpec" and "Finished" messages. (By section 7.4.9 of
delay sending application data until it has received and validated [RFC5246], after a full handshake, the client would have to delay
the other side's "Finished" message.) This achieves an improvement sending application data until it has received and validated the
of one round-trip time server's "Finished" message.) Accordingly, the latency penalty for
using TLS with HTTP can be kept at one round-trip time.
o for full handshakes if the client sends application data first,
o for abbreviated handshakes if the server sends application data When an earlier TLS session is resumed, TLS uses an abbreviated
first. handshake with only three protocol flights. For application
protocols in which the client sends data first, this abbreviated
handshake adds just one round-trip time to begin with, so there is no
need for a client-side False Start. However, if the server sends
application data first, the abbreviated handshake adds two round-trip
times, and this could be reduced to just one added round-trip time by
doing a server-side False Start. There is little need for this in
practice, so this document does not consider server-side False Starts
further.
Accordingly, the latency penalty for using TLS with HTTP can be kept Note also that TLS versions 1.3 [tls13] and beyond are out of scope
at one round-trip time regardless of whether a full handshake or an for this document. False Start will not be needed with these newer
abbreviated handshake takes place. versions since protocol flows minimizing the number of round trips
have become a first-order design goal.
In a False Start, when a party sends application data before it has In a False Start, when the client sends application data before it
received and verified the other party's "Finished" message, there are has received and verified the server's "Finished" message, there are
two possible outcomes: two possible outcomes:
o The handshake completes successfully: Once both "Finished" o The handshake completes successfully: Once both "Finished"
messages have been received and verified, this retroactively messages have been received and verified, this retroactively
validates the handshake. In this case, the transcript of protocol validates the handshake. In this case, the transcript of protocol
data carried over the transport underlying TLS will look as usual, data carried over the transport underlying TLS will look as usual,
apart from the different timing. apart from the different timing.
o The handshake fails: If a party does not receive the other side's o The handshake fails: If a party does not receive the other side's
"Finished" message, or if the "Finished" message's contents are "Finished" message, or if the "Finished" message's contents are
not correct, the handshake never gets validated. This means that not correct, the handshake never gets validated. This means that
an attacker may have removed, changed, or injected handshake an attacker may have removed, changed, or injected handshake
messages. In this case, data has been sent over the underlying messages. In this case, data has been sent over the underlying
transport that would not have been sent without the False Start. transport that would not have been sent without the False Start.
The latter scenario makes it necessary to restrict when a False Start The latter scenario makes it necessary to restrict when a False Start
is allowed, as described in this document. Section 3 considers basic is allowed, as described in this document. Section 3 considers basic
requirements for using False Start. Section 4 and Section 5 specify requirements for using False Start. Section 4 specifies the behavior
the behavior for clients and servers, respectively, referring to for clients, referring to important security considerations in
important security considerations in Section 6. Section 5.
3. False Start Compatibility 3. False Start Compatibility
TLS False Start as described in detail in the subsequent sections, if TLS False Start as described in detail in the subsequent sections, if
implemented, is an optional feature. implemented, is an optional feature.
A TLS implementation (not necessarily offering the False Start option A TLS server implementation is defined to be "False Start compatible"
itself) is defined to be "False Start compatible" if it tolerates if it tolerates receiving TLS records on the transport connection
receiving TLS records on the transport connection early, before the early, before the protocol has reached the state to process these.
protocol has reached the state to process these. To successfully use For successful use of client-side False Start in a TLS connection,
False Start in a TLS connection, the other side has to be False Start the server has to be False Start compatible. Out-of-band knowledge
compatible. Out-of-band knowledge that the peer is False Start that the server is False Start compatible may be available, e.g. if
compatible may be available, e.g. if this is mandated by specific this is mandated by specific application profile standards. As
application profile standards. As discussed in Appendix A, the discussed in Appendix A, the requirement for False Start
requirement for False Start compatibility does not pose a hindrance compatibility does generally not pose a hindrance in practice.
in practice.
4. Client-side False Start 4. Client-side False Start
This section specifies a change to the behavior of TLS client This section specifies a change to the behavior of TLS client
implementations in full TLS handshakes. implementations in full TLS handshakes.
When the client has sent its "ChangeCipherSpec" and "Finished" When the client has sent its "ChangeCipherSpec" and "Finished"
messages, its default behavior following [RFC5246] is to not send messages, its default behavior following [RFC5246] is to not send
application data until it has received the server's application data until it has received the server's
"ChangeCipherSpec" and "Finished" messages, which completes the "ChangeCipherSpec" and "Finished" messages, which completes the
handshake. With the False Start protocol modification, the client handshake. With the False Start protocol modification, the client
MAY send application data earlier (under the new Cipher Spec) if each MAY send application data earlier (under the new Cipher Spec) if each
of the following conditions is satisfied: of the following conditions is satisfied:
o The application layer has requested the TLS False Start option. o The application layer has requested the TLS False Start option.
o The symmetric cipher defined by the cipher suite negotiated in o The symmetric cipher defined by the cipher suite negotiated in
this handshake has been whitelisted for use with False Start this handshake has been whitelisted for use with False Start
according to the Security Considerations in Section 6.1. according to the Security Considerations in Section 5.1.
o The protocol version chosen by ServerHello.server_version has been o The protocol version chosen by ServerHello.server_version has been
whitelisted for use with False Start according to the Security whitelisted for use with False Start according to the Security
Considerations in Section 6.2. Considerations in Section 5.2.
o The key exchange method defined by the cipher suite negotiated in o The key exchange method defined by the cipher suite negotiated in
this handshake has been whitelisted for use with False Start this handshake and, if applicable, its parameters have been
according to the Security Considerations in Section 6.3. whitelisted for use with False Start according to the Security
Considerations in Section 5.3.
o In the case of a handshake with client authentication, the client o In the case of a handshake with client authentication, the client
certificate type has been whitelisted for use with False Start certificate type has been whitelisted for use with False Start
according to the Security Considerations in Section 6.3. according to the Security Considerations in Section 5.3.
The rules for receiving application data from the server remain The rules for receiving data from the server remain unchanged.
unchanged.
Note that the TLS client cannot infer the presence of an Note that the TLS client cannot infer the presence of an
authenticated server until all handshake messages have been received. authenticated server until all handshake messages have been received.
With False Start, unlike with the default handshake behavior, With False Start, unlike with the default handshake behavior,
applications are able to send data before this point has been applications are able to send data before this point has been
reached: from an application point of view, being able to send data reached: from an application point of view, being able to send data
does not imply that an authenticated peer is present. Accordingly, does not imply that an authenticated peer is present. Accordingly,
it is recommended that TLS implementations allow the application it is recommended that TLS implementations allow the application
layer to query whether the handshake has completed. layer to query whether the handshake has completed.
5. Server-side False Start 5. Security Considerations
This section specifies a change to the behavior of TLS server
implementations in abbreviated TLS handshakes.
When the server has sent its "ChangeCipherSpec" and "Finished"
messages, its default behavior following [RFC5246] is not to send
application data until it has received the client's
"ChangeCipherSpec" and "Finished" messages, which completes the
handshake. With the False Start protocol modification, the server
MAY send application data earlier (under the new Cipher Spec) if each
of the following conditions is satisfied:
o The application layer has requested the TLS False Start option.
o The symmetric cipher defined by the cipher suite of the session
being resumed has been whitelisted for use with False Start
according to the Security Considerations in Section 6.1.
The rules for receiving application data from the client remain
unchanged.
Note that the TLS server cannot infer the presence of an
authenticated client until all handshake messages have been received.
With False Start, unlike with the default handshake behavior,
applications are able to send data before this point has been
reached: from an application point of view, being able to send data
does not imply that an authenticated peer is present. Accordingly,
it is recommended that TLS implementations allow the application
layer to query whether the handshake has completed.
6. Security Considerations
In a TLS handshake, the "Finished" messages serve to validate the In a TLS handshake, the "Finished" messages serve to validate the
entire handshake. These messages are based on a hash of the entire handshake. These messages are based on a hash of the
handshake so far processed by a PRF keyed with the new master secret handshake so far processed by a PRF keyed with the new master secret
(serving as a MAC), and are also sent under the new Cipher Spec with (serving as a MAC), and are also sent under the new Cipher Spec with
its keyed MAC, where the MAC key again is derived from the master its keyed MAC, where the MAC key again is derived from the master
secret. The protocol design relies on the assumption that any server secret. The protocol design relies on the assumption that any server
and/or client authentication done during the handshake carries over and/or client authentication done during the handshake carries over
to this. While an attacker could, for example, have changed the to this. While an attacker could, for example, have changed the
cipher suite list sent by the client to the server and thus cipher suite list sent by the client to the server and thus
skipping to change at page 7, line 40 skipping to change at page 6, line 22
have already been sent before "Finished" validation confirms that the have already been sent before "Finished" validation confirms that the
handshake has not been tampered with -- so there is generally no hope handshake has not been tampered with -- so there is generally no hope
to be sure that communication with the expected peer is indeed taking to be sure that communication with the expected peer is indeed taking
place during the False Start. Instead, the security goal is to place during the False Start. Instead, the security goal is to
ensure that if anyone at all can decrypt the application data sent in ensure that if anyone at all can decrypt the application data sent in
a False Start, this must be the legitimate peer: while an attacker a False Start, this must be the legitimate peer: while an attacker
could be influencing the handshake (restricting cipher suite could be influencing the handshake (restricting cipher suite
selection, modifying key exchange messages, etc.), the attacker selection, modifying key exchange messages, etc.), the attacker
should not be able to benefit from this. The TLS protocol already should not be able to benefit from this. The TLS protocol already
relies on such a security property for authentication -- with False relies on such a security property for authentication -- with False
Start, the same is needed for encryption. This motivates the Start, the same is needed for encryption. This motivates the rules
following rules. put forth in the following subsections.
6.1. Symmetric Cipher It is prudent for applications to be even more restrictive. If
heuristically a small list of cipher suites and a single protocol
version is found to be sufficient for the majority of TLS handshakes
in practice, it could make sense to forego False Start for any
handshake that does not match this expected pattern, even if there is
no concrete reason to assume a cryptographic weakness. Similarly, if
handshakes almost always use ephemeral ECDH over one of a few named
curves, it could make sense to disallow False Start with any other
supported curve.
Clients and servers MUST NOT use the False Start protocol 5.1. Symmetric Cipher
modification in a handshake unless the cipher suite uses a symmetric
cipher that is considered cryptographically strong. Clients MUST NOT use the False Start protocol modification in a
handshake unless the cipher suite uses a symmetric cipher that is
considered cryptographically strong.
Implementations may have their own classification of ciphers (and may Implementations may have their own classification of ciphers (and may
additionally allow the application layer to provide a additionally allow the application layer to provide a
classification), but generally only symmetric ciphers with an classification), but generally only symmetric ciphers with an
effective key length of 128 bits or more can be considered strong. effective key length of 128 bits or more can be considered strong.
Also, various ciphers specified for use with TLS are known to have Also, various ciphers specified for use with TLS are known to have
cryptographic weaknesses regardless of key length (none of the cryptographic weaknesses regardless of key length (none of the
ciphers specified in [RFC4492] and [RFC5246] can be recommended for ciphers specified in [RFC4492] and [RFC5246] can be recommended for
use with False Start). The AES_128_GCM_SHA256 or AES_256_GCM_SHA384 use with False Start). The AES_128_GCM_SHA256 or AES_256_GCM_SHA384
ciphers specified in [RFC5288] and [RFC5289] can be considered ciphers specified in [RFC5288] and [RFC5289] can be considered
sufficiently strong for most uses. Implementations that support sufficiently strong for most uses. Implementations that support
additional cipher suites have to be careful to whitelist only additional cipher suites have to be careful to whitelist only
suitable symmetric ciphers; if in doubt, False Start should not be suitable symmetric ciphers; if in doubt, False Start should not be
used with a given symmetric cipher. used with a given symmetric cipher.
While an attacker can change handshake messages to force a downgrade While an attacker can change handshake messages to force a downgrade
to a less secure symmetric cipher than otherwise would have been to a less secure symmetric cipher than otherwise would have been
chosen, this rule ensures that in such a downgrade attack no chosen, this rule ensures that in such a downgrade attack no
application data will be sent under an insecure symmetric cipher. application data will be sent under an insecure symmetric cipher.
With respect to server-side False Start, if a client has negotiated a
TLS session using weak symmetric cryptography, this rule prevents
attackers from seeing the server encrypt more data under this session
than normally (if an attacker makes up a "ClientHello" message asking
to resume such a session, no False Start will happen).
6.2. Protocol Version 5.2. Protocol Version
Clients MUST NOT use the False Start protocol modification in a Clients MUST NOT use the False Start protocol modification in a
handshake unless the protocol version chosen by handshake unless the protocol version chosen by
ServerHello.server_version has been whitelisted for this use. ServerHello.server_version has been whitelisted for this use.
Generally, implementations should whitelist only the protocol Generally, to avoid potential protocol downgrade attacks,
version(s) for which they would not send TLS_FALLBACK_SCSV [RFC7507]. implementations should whitelist only their latest (highest-valued)
supported TLS protocol version (and, if applicable, any earlier
protocol versions that they would use in fallback retries without
TLS_FALLBACK_SCSV [RFC7507]).
The details of nominally identical cipher suites can differ between The details of nominally identical cipher suites can differ between
protocol versions, so this reinforces Section 6.1. protocol versions, so this reinforces Section 5.1.
6.3. Key Exchange and Client Certificate Type 5.3. Key Exchange and Client Certificate Type
Clients MUST NOT use the False Start protocol modification in a Clients MUST NOT use the False Start protocol modification in a
handshake unless the cipher suite uses a key exchange method that has handshake unless the cipher suite uses a key exchange method that has
been whitelisted for this use. Furthermore, when using client been whitelisted for this use. Also, clients MUST NOT use the False
Start protocol modification unless any parameters to the key exchange
methods (such as ServerDHParams, ServerECDHParams) have been
whitelisted for this use. Furthermore, when using client
authentication, clients MUST NOT use the False Start protocol authentication, clients MUST NOT use the False Start protocol
modification unless the client certificate type has been whitelisted modification unless the client certificate type has been whitelisted
for this use. for this use.
Implementations may have their own whitelists of key exchange methods Implementations may have their own whitelists of key exchange
and client certificate types (and may additionally allow the methods, parameters, and client certificate types (and may
application layer to specify whitelists). Generally, out of the additionally allow the application layer to specify whitelists).
options from [RFC5246] and [RFC4492], the following whitelists are Generally, out of the options from [RFC5246] and [RFC4492], the
recommended: following whitelists are recommended:
o Key exchange methods: DHE_RSA, ECDHE_RSA, DHE_DSS, ECDHE_ECDSA o Key exchange methods: DHE_RSA, ECDHE_RSA, DHE_DSS, ECDHE_ECDSA
o Client certificate types: rsa_sign, dss_sign, ecdsa_sign (or no o Parameters: well-known DH groups (at least 3,072 bits), named
client authentication) curves (at least 256 bits)
o Client certificate types: none
However, if an implementation that supports only key exchange methods However, if an implementation that supports only key exchange methods
from [RFC5246] and [RFC4492] does not support any of the above key from [RFC5246] and [RFC4492] does not support any of the above key
exchange methods, all of its supported key exchange methods can be exchange methods, all of its supported key exchange methods can be
whitelisted for False Start use. Care is required with any whitelisted for False Start use. Care is required with any
additional key exchange methods or client certificate types, as these additional key exchange methods, as these may not have similar
may not have similar properties. properties.
The recommended whitelists are such that if cryptographic algorithms The recommended whitelists are such that if cryptographic algorithms
suitable for forward secrecy would possibly be negotiated, no False suitable for forward secrecy would possibly be negotiated, no False
Start will take place if the current handshake fails to provide Start will take place if the current handshake fails to provide
forward secrecy. (Forward secrecy can be achieved using ephemeral forward secrecy. (Forward secrecy can be achieved using ephemeral
Diffie-Hellman or ephemeral Elliptic-Curve Diffie-Hellman; there is Diffie-Hellman or ephemeral Elliptic-Curve Diffie-Hellman; there is
no forward secrecy when a using key exchange method of RSA, RSA_PSK, no forward secrecy when a using key exchange method of RSA, RSA_PSK,
DH_DSS, DH_RSA, ECDH_ECDSA, or ECDH_RSA, or a client certificate type DH_DSS, DH_RSA, ECDH_ECDSA, or ECDH_RSA, or a client certificate type
of rsa_fixed_dh, dss_fixed_dh, rsa_fixed_ecdh, or ecdsa_fixed_ecdh.) of rsa_fixed_dh, dss_fixed_dh, rsa_fixed_ecdh, or ecdsa_fixed_ecdh.)
As usual, the benefits of forward secrecy may need to be balanced As usual, the benefits of forward secrecy may need to be balanced
against efficiency, and accordingly even implementations that support against efficiency, and accordingly even implementations that support
the above key exchange methods might whitelist further key exchange the above key exchange methods might whitelist further key exchange
methods and client certificate types from [RFC5246] and [RFC4492]. methods and client certificate types.
7. Acknowledgments Client certificate types rsa_sign, dss_sign, and ecdsa_sign do allow
forward security, but using False Start with any of these means
sending application data tied to the client's signature before the
server's authenticity (and, thus, the CertificateRequest message) has
been completely verified, so these too are not generally suitable for
the client certificate type whitelist.
The authors wish to thank Wan-Teh Chang, Ben Laurie, Eric Rescorla, 6. Acknowledgments
and Brian Smith for their input.
8. IANA Considerations The authors wish to thank Wan-Teh Chang, Ben Laurie, Martin Thomson,
Eric Rescorla, and Brian Smith for their input.
7. IANA Considerations
None. None.
9. References 8. References
9.1. Normative References 8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B. [RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492, May 2006. for Transport Layer Security (TLS)", RFC 4492, May 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008. (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois [RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288, Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
August 2008. August 2008.
[RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with [RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with
SHA-256/384 and AES Galois Counter Mode (GCM)", RFC 5289, SHA-256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
August 2008. August 2008.
9.2. Informative References 8.2. Informative References
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher [RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", RFC 7507, April 2015. Attacks", RFC 7507, April 2015.
[tls13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", Work in Progress, draft-ietf-tls-tls13-10,
October 2015.
Appendix A. Implementation Notes Appendix A. Implementation Notes
TLS False Start is a modification to the TLS protocol, and some TLS False Start is a modification to the TLS protocol, and some
implementations that conform to [RFC5246] may have problems implementations that conform to [RFC5246] may have problems
interacting with implementations that use the False Start interacting with implementations that use the False Start
modification. If the peer uses a False Start, application data modification. If the peer uses a False Start, application data
records may be received directly following the peer's "Finished" records may be received directly following the peer's "Finished"
message, before the TLS implementation has sent its own "Finished" message, before the TLS implementation has sent its own "Finished"
message. False Start compatibility as defined in Section 3 ensures message. False Start compatibility as defined in Section 3 ensures
that these records with application data will simply remain buffered that these records with application data will simply remain buffered
 End of changes. 40 change blocks. 
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