comments on draft-balfanz-tls-channelid
=JeffH <Jeff.Hodges <at> KingsMountain.com>
2013-05-01 00:11:21 GMT
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Here's some comments on draft-balfanz-tls-channelid, I hope they're helpful.
=JeffH
------
High-level comments:
1. EncryptedExtensions modification to the TLS handshake ought to be split out
into a separate spec.
2. What are the ramifications if the TLS WG does not adopt an
EncryptedExtensions mechanism? I.e., what are the downsides to the "channel id"
being publicly exposed, i.e., the security ones as well as the privacy
implications?
3. In my view, this spec defines how a TLS client generates and subsequently
wields a unique client identifier/key with a particular TLS server over multiple
TLS connections (in parallel and serially). Thus it is more about creating a
"security association" between the client and server (eg, see definitions for
the latter, as well as "channel", in RFC4949). Thus I would not use the term
"channel" for this mechanism. Also, TLS unto itself is creating a cryptographic
channel between client and server and thus using the channel term for this mech
seems ripe for confusion. This comment implies some re-writing of the
introduction (I've made some modest suggestions below). Perhaps "TLS Security
Association ID" (TLS-SAIDs) ?
4. The lifecycle for the unique client identifier (aka "channel id") ought to be
more fully discussed/specified in main portion of the spec. Presently it's
sprinkled in the introduction and the privacy considerations.
5. There would seem to be considerations for applications in terms of reliance
on the persistence of a given "channel id" and provisions for the "channel id"
changing or disappearing, e.g. if an app leverages "channel id" to bind
long-lived app-layer objects (eg cookies) to the TLS client, and these
implications/issues should be discussed.
6. This mechanism should be compared/contrasted with TLS Channel Binding
RFC5929. They don't appear to be mutually exclusive on first thought. What are
the semantics if they are both employed by an application? For what might one
employ one or the other or both concurrently?
7. This is a "trust on first use (TOFU)" mechanism. This should be
mentioned/discussed explicitly.
Detailed comments/questions:
> 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.
>
snip
>
> 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
>
snip
>
> 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.
^^^^^^^ ^
security association , denoted by an ECC public key.
> A server can
^
For example,
> choose to bind authentication tokens to this channel, thus rendering
^^^^^^^
association
> 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.
suggest..
..tokens bound to the security association will be ignored by the
if they are not sent by the legitimate client.
>
> 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.
^^^^^^^
security association
> Such a channel
^^^^^^^^^
an association
> 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.
>
snip
>
> 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
what are the threats if a long-term, asymmetric-key-based client identifier is
sent in the clear during TLS handshake? Is it mostly a privacy concern? Or is
it because of anticipated use cases of binding app-layer info to the TLS layer?
> 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
^^^^^
bearer
> tokens; completely defeating our objectives.
I'm curious here... RFC5246 Appendix F.1.4. indicates..
When a connection is established by resuming a session, new
ClientHello.random and ServerHello.random values are hashed with the
session's master_secret. Provided that the master_secret has not
been compromised and that the secure hash operations used to produce
the encryption keys and MAC keys are secure, the connection should be
secure and effectively independent from previous connections.
..so I'm not sure how "session resumption secrets" are "equivalent bearer tokens" ?
>
> 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.
>
snip
>
> 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
>
>
>
> Balfanz & Hamilton Expires May 12, 2013 [Page 6]
>
> Internet-Draft TLS Channel ID November 2012
>
>
> 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]
the below should be separate section...
> If the server included a "channel_id" extension in its "ServerHello"
> message, the client MUST include an "Extension" with "extension_type"
suggest..
..the client MUST include, within the EncryptedExtensions message, 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.
you mean that the "x" and "y" fields (of the ChannelIDExtension struct) contain
the affine coordinates of a P-256 [DSS] curve point comprising an ECC public
key "Q" ?
[ where Q = dG, d = ECC private key, G is the curve base point, and other
elliptic curve "domain parameters" are as given in [DSS] for curve P-256, and
the key pair is generated according to appendix B.4 in [DSS] aka FIPS-186-3 ? ]
So Q = (x,y) is the "channel id" per se ? It should be made explicit.
What's the lifecycle management of this identifier (ie "Q")? Is the TLS server
(or the server-side app running on top of it) to remember this identifier? Is
it to be mapped to other client-side identifying information eg IP address, user
data (eg account name), etc ? Even though this I-D is just specifying the
identifier and proof-of-possession mechanism (aka "holder of key"), some modest
discussion of example use cases (beyond the mention of binding to
"authentication tokens" that appears in the Introduction) would be helpful.
Presumably the TLS client stores this identifier keyed by TLS server domain name
and re-uses it in future TLS connections to that server (the "storage model"
needs to be discussed/specified)? When might a TLS client change an established
identifier?
(Some mention of these lifecycle items is given in the PrivCons section below)
Is it to be remembered on first use? Is it to be updated in the server-side
store upon it changing? What are this identifier's overall semantics?
> The "r" and "s" fields contain an
> ECDSA [DSS] signature by the corresponding private key of "TLS
> Channel ID signature\x00"
suggest..
The "r" and "s" fields contain an ECDSA [DSS] signature by the
corresponding private key over this US-ASCII string (not including
quotes, and where "\x00" represents an octet containing all zero bits):
"TLS Channel ID signature\x00"
..although we should probably use ABNF (or the RFC5246 notation) to define this
string and its concatenation with the handshake message hashes.
> followed by the handshake hash(es) prior to
> the "EncryptedExtensions" message.
hashes of both the client-sent and server-sent handshake messages, as seen by
the client?
>
> 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.
but presumably the same (long-lived?) TLS client identifier Q = (x,y) used and
only (r,s) are different ?
Or are new values for all of {x,y,r,s} calculated? [ "no" is implied by the
Introduction and Priv. Cons. ]
> 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.
why? simply for privacy reasons?
Or is the main concern that the "channel id" will be used by app layers, eg in
HTTP cookies, in order to bind objects to a particular TLS client-server pair,
and thus the "channel id" should be protected as a secret?
If the "channel id" is simply included in cookies, and those cookies are leaked
in the clear (which is not uncommon for a variety of reasons), then the "channel
id" will potentially be divulged in any case.
Since the channel id is a public key, the server could include some nonce,
encrypted by the channel id key, in messages to the client, the client could
decrypt it, then encrypt the nonce with its private key, and include that on
messages back to the server, which the server could decrypt with the client's
public key and verify. doing something like that could help keep the channel id
itself out of these messages and their potentially leaky components (such as
cookies)
if servers who rely upon the Channel ID always do so with a corresponding
proof-of-possession of the private key on the part of the TLS client (and they
perform the requisite checks), then what are the security (as opposed to
privacy) issues if the Channel ID is observable by others? I suppose it depends
on the app-layer use cases employing the "channel id".
> 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.
do you mean in possession of the corresponding private key (to the cert 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.
these TLS cipher suites should be explicitly listed.
>
> 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
^
legitimate
> 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.
did you mean to say..
We assume that TLS provides sufficient security to prevent these attackers
from being able to hijack the TLS connection.
..?
> An active
> attacker with a misissued certificate can successfully terminate the
> TLS connection and decrypt the Channel ID.
need a definition and perhaps reference for "mis-issued" cert.
> However, as the signature
> covers the handshake hashes, and therefore the server's certificate,
> it wouldn't be accepted by the true server.
this essentially means that a attacker server with a mis-issued certificate
cannot act as a real-time TLS proxy between the TLS client and legit server, yes?
But otherwise such an attacker could potentially mount an impersonation of the
legitimate server, yes?
>
> 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.
here "connection" means the long-lived "security association" between TLS client
and legit server, yes?
the known forward-secret TLS cipher suites should perhaps be listed in an appendix.
>
snip
>
> 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.
There's ostensibly privacy implications worth mentioning in that each distinct
TLS client stack one wields with a particular TLS server will generate a
distinct "channel id" (TLS-SAID), even if they are "behind" one IP address from
the server's perspective.
> 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.
Yes, because otherwise this becomes a "super cookie" mech.
This implies dependent app-layers will need to be able to handle dynamically
changing "channel id" (TLS-SAID)s.
>
> 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.
>
snip
>
> 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".
>
snip
>
> 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".
^
Issued June, 2009
>
> 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.
>
>
<snip/>
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