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Network Working Group A. Langley
Internet-Draft Google Inc
Expires: December 20, 2010 June 18, 2010
Transport Layer Security (TLS) Snap Start
draft-agl-tls-snapstart-00
Abstract
This document describes a Transport Layer Security (TLS) extension
for eliminating the latency of handshakes when the client has prior
knowledge about the server. Unlike resumption, this prior knowledge
is not secret and may be obtained from third parties and stored on
disk for significant periods of time.
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 December 20, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Details . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Requirements Notation . . . . . . . . . . . . . . . . . . . . 7
5. Snap Start Extension . . . . . . . . . . . . . . . . . . . . . 8
6. Active attack considerations . . . . . . . . . . . . . . . . . 11
7. Requirements on the application . . . . . . . . . . . . . . . 12
8. Interactions with the Session Tickets extension . . . . . . . 13
9. Interactions with OCSP stapling . . . . . . . . . . . . . . . 14
10. Interactions with client certificates . . . . . . . . . . . . 15
11. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
12. Prior work . . . . . . . . . . . . . . . . . . . . . . . . . . 17
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
15. Normative References . . . . . . . . . . . . . . . . . . . . . 20
Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
Snap Start aims to remove the latency overhead of TLS handshakes in
the case that the application protocol involves the client speaking
first. Currently, TLS handshaking imposes additional latency and is
costly for time-sensitive applications.
In order to achieve this, the initial flow from the client must
contain application data and, therefore, everything needed for the
server to complete a handshake and process it. Starting from this
premise we can derive the essential features of Snap Start.
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2. Design
At first we are only considering the case of a full handshake.
Assume, for the sake of argument, a client that is capable of
predicting the contents of a server's first handshake flow (i.e. the
"ServerHello" message through to the "ServerHelloDone"). Such a
client could send "ClientKeyExchange", "ChangeCipherSpec" and
"Finished" messages immediately following its "ClientHello". It
would then be able to transmit application data records and we have
successfully eliminated the TLS handshake latency.
However, several elements of the server's first handshake flow are
unpredictable. Fundamentally, any ephemeral Diffie-Hellman based
cipher suite is incompatible with Snap Start so the following assumes
that the server is using non-ephemeral key agreement.
The chosen cipher suite, compression method, supported extensions,
ordering of those extensions, certificate etc are all somewhat
unpredictable for a given server but are highly correlated across
time. Given a previous handshake with the same server, assuming that
it will make the same choices when presented with a similar
"ClientHello" is sufficiently accurate to expect a very high rate of
prediction of these elements of the handshake.
The server's chosen session id is unpredictable. However, this can
be eliminated by using Session Tickets [RFC5077]. When Session
Tickets are in use the "ServerHello" doesn't include a session id and
the "NewSessionTicket" message itself is not part of the first flow.
Lastly, the "server_random" is unpredictable. The "server_random"
exists to provide uniqueness and freshness. When the server picks a
random value it can be assured that no previous TLS connection has
ever used the same value. Therefore the connection cannot be a
replay of the client's traffic. Additionally, the server knows when
its unpredictable random value was created, relative to its local
clock, and therefore knows that handshake hasn't been delayed for an
arbitrary amount of time.
In order for the "server_random" to be predictable by the client, it
will have to be chosen by the client and suggested to the server. In
order for the server to be assured of uniqueness, it will have to
remember every "server_random" value that has been used so that it
may reject duplicates. (Several methods of limiting the amount of
state required for this are introduced below.)
Even without Snap Start, an attacker can delay an application data
record in an established connection. However, both parties to the
connection are likely to timeout after some period of inactivity,
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bounding the amount of possible delay introduced. With Snap Start,
since we are assuming that the client's initial flow includes a full
handshake and application data, an attacker could arbitrarily delay
the flow and have the server process the application data at a time
of their choosing. As the server lacks an nonce it has no way of
detecting this.
Without a similar mechanism to bound the delay of a Snap Start
handshake, an attacker could perform a pseudo-replay attack: a
handshake is delayed until the client retries, but the first
handshake can still be used to deliver the same application data a
second time. Because of this (and for other reasons other reasons
given below) we require some degree of clock synchronisation between
the client and server with respect the the timestamp in the
"ClientHello". The level of synchronisation required is left to the
application layer to determine although it's recommended that the
permitted clock skew be shorter than the application's retry timeout.
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3. Details
The essential features of Snap Start are now established: the client
predicts the server's handshake flow using a client suggested
"server_random", SessionTickets and knowledge from a previous
connection to the same server.
We now note that this functions for both full and abbreviated
handshakes. The abbreviated handshake retains its lower
computational requirements but both now complete in the same number
of round trips.
In order to limit the amount of state required for the server to
reject repeated "server_random"s, we allow the server to bound this
state temporally and spatially. In the temporal dimension, we define
that the "gmt_unix_time" of the server random is taken from
"gmt_unix_time" of the client's random value. The server may use
this timestamp to reject all suggested random values outside some
window around the current time.
Spatially, we define that the subsequent eight bytes of the server's
random value are the server's 'orbit' value. This value must be
discovered by the client from a previous (non Snap Start) handshake.
In the event that several, geographically separated servers share the
same certificates, they may use different orbit values. This allows
one to reject "server_random" values for the other without any
communication between them. (The term 'orbit' was chosen only to be
short and otherwise reasonably meaningless in this context.)
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4. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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5. Snap Start Extension
A new extension type ("snap_start(TBD)") is defined and MAY be
included by the client in its "ClientHello" message. If, and only
if, the server sees this extension in the "ClientHello", it MAY
choose to include the extension in its "ServerHello".
enum {
snap_start(TBD), (65535)
} ExtensionType;
The "extension_data" field of a "snap_start" extension in a
"ClientHello" MAY be empty.
If the server chooses to echo a "snap_start" extension then it is
indicating that it MAY support Snap Start on future connections. The
contents of the "extension_data" in this case MUST be:
struct {
opaque orbit[8];
CipherSuite snap_start_cipher_suite;
} ServerSnapStart;
"orbit" is the server's current orbit value and
"snap_start_cipher_suite" contains the CipherSuite value that the
client should assume that the server will use in a Snap Start
handshake.
If the client wishes to attempt a Snap Start connection then it
includes a non-empty "snap_start" extension in its "ClientHello". If
the extension is not empty, then its contents MUST be:
struct {
opaque orbit[8];
opaque random_bytes[20];
opaque predicted_server_handshake[8];
// TLSCiphertext structures follow
} ClientSnapStart;
Following this, without a length prefix, the client may include one
or more "TLSCiphertext" structures to be processed by the server in
the case that the Snap Start is successful. These records are as
described in RFC 5246 [RFC5246] section 6.2
The "orbit" MUST contain an orbit value obtained from a previous
connection to the same server. "random_bytes" MUST contain 20 random
bytes from a cryptographic random source equal in strength to the one
used for the "client_random".
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"predicted_server_handshake" MUST contain an FNV1a64 [fnv1a] hash of
the server's predicted response flow. This hash is taken over the
bytes of the "Handshake" structures, as defined in RFC 5246 section
7.4. If the client is attempting to resume a connection then this is
calculated over the server's "ServerHello" message. Otherwise, this
is calculated over all handshake messages from the "ServerHello" to
the "ServerHelloDone" (inclusive). The client's prediction MUST
assume that the server chooses its server random as detailed below.
The client's prediction MUST also assume that the server includes a
Snap Start extension with the same "ServerSnapStart" contents as
previously observed.
If the client's prediction is correct then the server MAY perform a
Snap Start handshake. If the server wishes to perform a Snap Start
handshake then it MUST form its "random" value from the
"gmt_unix_time" of the client's "random", followed by the server's
orbit value, followed by the contents of "random_bytes" from the
client's Snap Start extension.
The server MUST NOT transmit any predicted handshake messages and
MUST start processing records from the client's Snap Start extension.
For the purposes of the Finished calculation, the client's
"ClientHello" is hashed as if the "snap_start" extension were not
included (the "length" field of the "Handshake" structure from RFC
5246 section 7.4, and the prefixed length of the "extensions" member
of the "ClientHello" are updated accordingly). When processing
records from the extension, they are hashed as usual. Once the set
of records embedded in the "ClientHello" has been exhausted, the
server resumes reading records from the network. Records must not be
partially contained within the "ClientHello" and partially read from
the network.
(Since the "ClientHello" is likely to have a "Finished" message
embedded within it, it cannot be hashed into the Finished calculation
as normal as its contents would then depend in its own hash. One
could consider hashing with the embedded hash zeroed out, however a
"Finished" message is encrypted under the negotiated cipher suite and
a CBC-based ciphersuite would spread the effects of the embedded hash
beyond its apparent boundaries. Likewise, the "Finished" message is
compressed using the negotiated compression algorithm thus the length
of the record, and thus the extension, depend on the contents of the
hash. Given these limitations, removing the extension altogether is
the simplest way to break the cycle.)
If the server chooses not to perform a Snap Start handshake (for any
reason, including that the application rejected the suggested random
value or that the client mispredicted the handshake) then the
handshake MUST continue as normal. The server MUST NOT change the
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way that it hashes the "ClientHello". The server MAY echo a
"snap_start" extension.
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6. Active attack considerations
Given that this draft makes changes to the Finished hash, it's
required to show that no active attack can cause a handshake to
complete which differs from that which would have occurred otherwise.
Since the "snap_start" extension is not included in the Finished hash
of a Snap Start handshake, we have to consider the results of an
attacker manipulating its contents.
Firstly, since the embedded records are hashed as usual, the same
security properties hold: any manipulation will be detected by the
Finished hash, or by a MAC verification failure.
An attacker could manipulate the orbit. This would typically cause
the server to reject the suggested random value and a normal
handshake would detect the manipulation. In the case that the server
still proceeds with a Snap Start handshake, the orbit is copied into
the "server_random" in the "ServerHello", which is then hashed into
the Finished calculation (although not transmitted).
An attacker could manipulate the suggested server random. The same
argument as for the orbit holds here.
An attacker could manipulate the hash of the predicted messages.
Assuming that the client correctly predicted the hash, the
manipulation would cause the server to not perform a Snap Start
handshake and the manipulation would be detected. Assuming that the
client mispredicted, and that the manipulation results in a
different, but also incorrect, value then the same argument applies.
Assuming that the client mispredicted and the manipulation corrects
the hash, the server could perform a Snap Start handshake, but the
differing contents of the predicted handshake messages will be hashed
into the Finished calculation and the manipulation will be detected.
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7. Requirements on the application
Applications are required to ensure that no suggested random value is
accepted twice within the scope of any given certificate. In
general, validation of the suggested random value is outside the
scope a TLS implementation (although it may handle simple cases and
provide utility code for others). Applications may use the orbit
value and client timestamp to aid them in this. Applications may
always safely reject a suggested random value. Applications SHOULD
limit the allowed difference between the timestamp in the suggested
random value and the current time in order to prevent arbitrary
delays, as detailed in the design section of this document.
For a single server deployment, the server may generate a new, random
orbit value each time that it starts and, thereafter, maintain an in-
memory data structure. Each random value seen should be "struck off"
by recording it in this data structure (the "strike register"). The
strike register's size can be bound by fixed limits and by rejecting
all random values where the timestamp is outside a certain window
around the current time.
For a multi-server deployment in a single location, the servers
should share an orbit value and a strike register. The strike
register is likely to be a held in a single location which the TLS
servers access over an internal network.
For a multi-cluster deployment, where the clusters are geographically
separated, each cluster should have its own orbit value and shared
strike register. The effectiveness of Snap Start in this setup is
limited by the probability of a given client repeatedly being served
by the same cluster. With pure round robin scheduling, a Snap Start
handshake is unlikely to be successful.
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8. Interactions with the Session Tickets extension
A successful Snap Start abbreviated handshake can occur without the
use of session tickets. A successful Snap Start full handshake,
without session tickets, can only occur if the server doesn't
generate a random session id. A server MAY choose not to generate a
session id if the client presents a Snap Start extension but not a
session tickets extension. However, all TLS implementations of Snap
Start SHOULD implement session tickets. TLS clients which send a
Snap Start extension SHOULD also send a Session Tickets extension.
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9. Interactions with OCSP stapling
Clients attempting a Snap Start handshake MUST trust the server's
cached certificate. This includes validating revocation information
(via OCSP [RFC2560], CRLs [RFC5280] etc) as the local policy
dictates.
TLS clients which send a Snap Start extension SHOULD NOT send a
"status_request" extension as defined in RFC 4366 [RFC4366] section
3.6. A client may be able to predict the contents of a
"CertificateStatus" message but, if it can predict it, then it
doesn't need it and, if it needs fresh OCSP information, then it
shouldn't have attempted a Snap Start handshake using a certificate
that it cannot validate.
This does preclude the case where the client has cached a valid OCSP
response that is still timely, but the server has a response valid
further into the future. We can only suggest that opportunistic OCSP
stapling additionally be included in application level protocols for
this situation.
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10. Interactions with client certificates
A Snap Start handshake can include client-side authentication. In
this case the client must predict that the server will send a
"CertificateRequest" message, calculate its
"predicted_server_handshake" accordingly and embed "Certificate" and
"CertificateVerify" messages in the Snap Start extension.
The "handshake_messages" over which the "CertificateVerify" is
calculated MUST omit the Snap Start extension as detailed for the
Finished calculation, above.
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11. Examples
Firstly, a client contacting a previously unknown server for the
first time may include an empty Snap Start extension in its
ClientHello. The server, if so capable, could reply with:
01 02 03 04 05 06 07 08 (Orbit value)
00 2f (Cipher suite)
A future connection may now attempt a Snap Start by including a Snap
Start extension in the ClientHello with the following contents:
01 02 03 04 05 06 07 08 (Orbit value)
88 c0 1e 1c a9 4e ... (Random bytes)
aa bb cc dd ee ff 00 11 (predicted server handshake)
16 03 03 00 84 10 00 00 80 ... (ClientKeyExchange)
14 03 03 00 01 01 (ChangeCipherSpec)
16 03 03 00 20 ... (Finished)
17 03 03 00 50 ... (Application data)
If the Snap Start is successful, then the message flow looks like
this:
ClientHello -------->
ChangeCipherSpec
Finished
<-------- Application data
If the client mispredicts the server's handshake, however, then the
flow is unaltered from Figure 1 in RFC 5246 section 7.3.
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12. Prior work
The idea of cache-side caching of long lived server parameters has
been discussed in Client Side Caching for TLS [fasttrack] and
specified in draft-ietf-tls-cached-info [cached-info]. Client Side
Caching for TLS [fasttrack] also considered including an
opportunistic "ClientKeyExchange" message in the client's initial
flow.
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13. IANA Considerations
This document requires IANA to update its registry of TLS extensions
to assign an entry, referred herein as "snap_start".
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14. Acknowledgements
This document benefited specifically from discussions with Wan-Teh
Chang, Bodo Moeller and Nagendra Modadugu.
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15. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560, June 1999.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, April 2006.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[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, May 2008.
[fnv1a] Noll, L., "FNV hash".
[cached-info]
Santesson, S., "Transport Layer Security (TLS) Cached
Information Extension", Internet Draft (work in progress),
April 2010.
[fasttrack]
Shacham, H., Boneh, D., and E. Rescorla, "Client Side
Caching for TLS", Nov 2004.
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Appendix A. Changes
To be removed by RFC Editor before publication
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Author's Address
Adam Langley
Google Inc
Email: agl@google.com
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