Network Working Group D. Balfanz
Internet-Draft R. Hamilton
Expires: May 12, 2013 Google Inc
November 8, 2012
Transport Layer Security (TLS) Channel IDs
draft-balfanz-tls-channelid-00
Abstract
This document describes a Transport Layer Security (TLS) extension
for identifying client machines at the TLS layer without using bearer
tokens.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Why not client certificates . . . . . . . . . . . . . . . . . 4
3. Requirements Notation . . . . . . . . . . . . . . . . . . . . 5
4. Channel ID Extension . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
Many applications on the Internet use _bearer tokens_ to authenticate
clients to servers. The most prominent example is the HTTP-based
World Wide Web, which overwhelmingly uses HTTP cookies to
authenticate client requests. Other examples include OpenID or SAML
assertions, and OAuth tokens. All these have in common that the
_bearer_ of the HTTP cookie or authentication token is granted access
to a protected resource, regardless of the channel over which the
token is presented, or who presented it.
As a result, an adversary that manages to steal a bearer token from a
client can impersonate that client to services that require the
token.
This document describes a light-weight mechanism for establishing a
_cryptographic channel_ between client and server. A server can
choose to bind authentication tokens to this channel, thus rendering
the theft of authentication tokens fruitless - tokens must be sent
over the channel to which they are bound (i.e., by the client to
which they were issued) or else they will be ignored.
This document does not prescribe _how_ authentication tokens are
bound to the underlying channel. Rather, it prescribes how a client
can establish a long-lived channel with a server. Such a channel
persists across HTTP requests, TLS connections, and even multiple TLS
sessions, as long as the same client communicates with the same
server.
The basic idea is that the client proves, during the TLS handshake,
possession of a private key. The corresponding public key becomes
the "Channel ID" that identifies this TLS connection. Clients should
re-use the same private/public key pair across subsequent TLS
connections to the same server, thus creating TLS connections that
share the same Channel ID.
Using private/public key pairs to define a channel (as opposed to,
say, an HTTP session cookie) has several advantages: One, the
credential establishing the channel (the private key) is never sent
from client to server, thus removing it from the reach of
eavesdroppers in the network. Two, clients can choose to implement
cryptographic operations in a secure hardware module, which further
removes the private key from the reach of eavesdroppers residing on
the client itself.
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2. Why not client certificates
TLS already supports a means of identifying clients without using
bearer tokens: client certificates. However, a number of problems
with using client certificates motivated the development of an
alternative.
Most importantly, it's not acceptable for a client identifier to be
transmitted in the clear and client certificates in TLS are sent
unencrypted. Although we could also define a change to the TLS state
machine to move the client certificates under encryption, such
changes eliminate most of the benefits of reusing something that's
already defined.
TLS client certificates are also defined to be part of the session
state. This turns session resumption secrets into equivalent barer
tokens; completely defeating our objectives.
Client-certificates typically identify a user, while we seek to
identify machines. Since they are not, conceptually, mutually
exclusive and as only a single client certificate can be provided in
TLS, we don't want to consume that single slot and eliminate the
possibility of also using existing client certificates.
Client certificates are implemented in TLS as X.509 certificates and
we don't wish to require servers to parse arbitrary ASN.1. ASN.1 is
a complex encoding that has been the source of several security
vulnerabilities in the past and typical TLS servers can currently
avoid doing ASN.1 parsing.
X.509 certificates always include a signature, which would be a self-
signature in this case. Calculating and transmitting the self-
signature is a waste of computation and network traffic in our use.
Although we could define a null signature algorithm with an empty
signature, such deviations from X.509 eliminate many of the benefits
of reusing something that is already implemented.
Finally, client certificates trigger significant server-side
processing by default and often need to be stored in their entirety
for the duration of the connection. Since this design is intended to
be widely used, it allows servers to retain only a cryptographic hash
of the client's public key after the handshake completes.
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3. 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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4. Channel ID Extension
A new extension type ("channel_id(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 echo the extension in its "ServerHello". In both cases,
the "extension_data" field MUST be empty.
enum {
channel_id(TBD), (65535)
} ExtensionType;
A new handshake message type ("encrypted_extensions(TBD)") is
defined. If the server included a "channel_id" extension in its
"ServerHello" message, the client MUST verify that the selected
cipher suite is sufficiently strong. If the cipher suite provides <
80-bits of security, the client MUST abort the handshake with a fatal
"illegal_parameter" alert. Otherwise, the client MUST send an
"EncryptedExtensions" message after its "ChangeCipherSpec" and before
its "Finished" message.
enum {
encrypted_extensions(TBD), (65535)
} HandshakeType;
Therefore a full handshake with "EncryptedExtensions" has the
following flow (contrast with section 7.3 of RFC 5246 [RFC5246]):
Client Server
ClientHello (ChannelID extension) -------->
ServerHello
(ChannelID extension)
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
EncryptedExtensions
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data
An abbreviated handshake with "EncryptedExtensions" has the following
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flow:
Client Server
ClientHello (ChannelID extension) -------->
ServerHello
(ChannelID extension)
[ChangeCipherSpec]
<-------- Finished
[ChangeCipherSpec]
EncryptedExtensions
Finished -------->
Application Data <-------> Application Data
The "EncryptedExtensions" message contains a series of "Extension"
structures (see section 7.4.1.4 of RFC 5246 [RFC5246]
If the server included a "channel_id" extension in its "ServerHello"
message, the client MUST include an "Extension" with "extension_type"
equal to "channel_id(TBD)". The "extension_data" of which has the
following format:
struct {
opaque x[32];
opaque y[32];
opaque r[32];
opaque s[32];
} ChannelIDExtension;
The contents of each of "x", "y", "r" and "s" is a 32-byte, big-
endian number. The "x" and "y" fields contain the affine coordinates
of a P-256 [DSS] curve point. The "r" and "s" fields contain an
ECDSA [DSS] signature by the corresponding private key of "TLS
Channel ID signature\x00" followed by the handshake hash(es) prior to
the "EncryptedExtensions" message.
Unlike many other TLS extensions, this extension does not establish
properties of the session, only of the connection. When session
resumption or session tickets [RFC5077] are used, the previous
contents of this extension are irrelevant and only the values in the
new handshake messages are considered.
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5. Security Considerations
There are four classes of attackers against which we consider our
security guarantees: passive network attackers, active network
attackers, active network attackers with misissued certificates and
attackers in possession of the legitimate server's private key.
First, we wish to guarantee that we don't disclose the Channel ID to
passive or active network attackers. We do this by sending a
constant-length Channel ID under encryption. However, since the
Channel ID may be transmitted before the server's Finished message is
received, it's possible that the server isn't in possession of the
certificate that it presented. In this situation, an active attacker
could cause a Channel ID to be transmitted under a random key in a
cipher suite of their choosing. Therefore we limit the permissible
cipher suites to those where decrypting the message is infeasible.
Even with this limit, an active attacker can cause the Channel ID to
be transmitted in a non-forward-secure manner. Subsequent disclosure
of the server's private key would allow previously recorded Channel
IDs to be decrypted.
Second, we wish to guarantee that none of the first three attackers
can terminate/hijack a TLS connection and impersonate a Channel ID
from that connection when connecting to the legitimate server. We
assume that TLS provides sufficient security to prevent a connection
from being hijacked once established by these attackers. An active
attacker with a misissued certificate can successfully terminate the
TLS connection and decrypt the Channel ID. However, as the signature
covers the handshake hashes, and therefore the server's certificate,
it wouldn't be accepted by the true server.
Against an attacker with the legitimate server's private key we can
provide the second guarantee only if the legitimate server uses a
forward-secret cipher suite, otherwise the attacker can hijack the
connection.
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6. Privacy Considerations
The TLS layer does its part in protecting user privacy by
transmitting the Channel ID public key under encryption. Higher
levels of the stack must ensure that the same Channel ID is not used
with different servers in such a way as to provide a linkable
identifier. For example, a user-agent must use different Channel IDs
for communicating with different servers. User-agents must also
ensure that Channel ID state can be reset by the user in the same way
as other identifiers, i.e. cookies.
However, there are some security concerns that could result in the
disclosure of a client's Channel ID to a network attacker. This is
covered in the Security Considerations section.
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7. IANA Considerations
This document requires IANA to update its registry of TLS extensions
to assign an entry referred to here as "channel_id".
This document also requires IANA to update its registry of TLS
handshake types to assign an entry referred to here as
"encrypted_extensions".
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[DSS] National Institute of Standards and Technology, "FIPS
186-3: Digital Signature Standard".
8.2. Informative References
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, January 2008.
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Appendix A. Acknowledgements
The following individuals contributed to this specification:
Dirk Balfanz, Wan-Teh Chang, Ryan Hamilton, Adam Langley, and Mayank
Upadhyay.
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Authors' Addresses
Dirk Balfanz
Google Inc
Email: balfanz@google.com
Ryan Hamilton
Google Inc
Email: rch@google.com
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