Network Working Group Nicolas Williams
INTERNET-DRAFT Sun Microsystems
November 2004
On the Use of Channel Bindings to Secure Channels
<draft-ietf-nfsv4-channel-bindings-01.txt>
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document defines and formalizes the concept of channel bindings
to secure layers and defines the actual contents of channel bindings
for several secure channels.
The concept of channel bindings allows applications to prove that the
end-points of two secure channels are the same by binding
authentication at one network layer to the session protection
negotiation at a lower network layer. The use of channel bindings
allows applications to delegate session protection to lower layers.
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1. Introduction pg. 3
2. Definitions pg. 4
3. Authentication protocols and channel bindings pg. 6
3.1. The GSS-API and channel bindings pg. 6
3.2. SASL and channel bindings pg. 7
3.3. Kerberos V and channel bindings pg. 7
4. Channel bindings to secure layers pg. 7
4.1. Bindings to SSHv2 channels pg. 7
4.2. Bindings to TLS channels pg. 7
4.3. Bindings to IPsec pg. 8
4.3.1. Interfaces for creating IPsec channels pg. 8
4.4. Bindings to other types of channels pg. 9
5. Benefits of channel bindings to secure channels pg. 9
6. Security considerations pg. 10
7. References pg. 10
7.1. Informative references pg. 10
7.2. Normative references pg. 11
8. Acknowledgements pg. 12
9. Author's Address pg. 12
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Conventions used in this document
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 [RFC2119].
1. Introduction
[NOTE: This I-D text has been split out from the "The Channel
Conjunction Mechanism (CCM) for GSS" I-D, which will be
updated soon to define only the CCM-BIND and CCM-MIC GSS-API
pseudo-mechanisms and describe their use. CCM-BIND is
particularly relevant to the use of channel bindings with
GSS-API applications. See draft-ietf-nfsv4-ccm-01.txt.]
Over the years several attempts have been made to delegate session
protection at one network layer to another, for performance and/or
scalability as well as for design elegance and also to avoid having
to reinvent the wheel for every new application or security layer.
The critical security problem to solve in order to achieve such
delegation of session protection is always the same: how to ensure
that there is no man-in-the-middle (MITM), from the point of view the
application, at the lower network layer to which session protection
is to be delegated.
Alternative statement of the problem: how does one ensure that the
end-points of two secure channels at different network layers are the
same?
And there may well be a MITM, particularly if the lower network layer
either provides no authentication or if there is no connection
between the authentication or principals used at the application and
those used at the lower network layer.
Such MITM attacks can be effected by, for example, spoofing IP
address lookups (which is possible, for example, when using DNS but
not DNSSEC) in a way that the application may not detect but which
directs the client application or network stack to connect to a
different host than had been intended (e.g., to the MITM's host).
Even if such MITM attacks seem particularly difficult to effect, the
problem must be solved.
For example: a user decides to use TELNET, with Kerberos V
authentication, over TLS to connect to some server but an attacker
spoofs the name service lookup and causes the TELNET client to be
redirected to some other host which TLS authenticates correctly and
where the attacker forwards the connection, with or without TLS, to
the server that the user had intended. In this example there is an
MITM from the point of view of the application (TELNET), even though
there is no MITM as far as TLS is concerned. The TELNET client and
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server cannot assume that there is no MITM and so cannot leverage the
protection afforded by the TLS channel, unless they prove to each
other that there is no MITM.
A solution to this problem is highly desirable, particularly where
multi-user applications are run over secure network layers (e.g., NFS
over IPsec). For such applications the authentication model used at
the application layer (usually user<->server) is generally very
different from that used by secure, lower network layers, such as
IPsec (usually client<->server or single-user<->server), and may even
use different authentication infrastructures altogether (e.g.,
Kerberos V for the application layer, x.509 certificates at the lower
layer). Such applications cannot generally leverage the security
provided by the lower network layers, which, if they could, would
allow them to offload session security to the secure lower layer.
One solution involves ensuring the use of secure name services for
hostname to network address translation and the use of secure
networks (e.g., IPsec). This approach can prevent the MITM attack
described above, but does not offer applications any guarantees that
there is no MITM in the lower layer.
Another solution is to use "channel bindings" (a GSS-API concept
[RFC2743]) to bind authentication at application layers to secure
transports at lower layers in the network stack. This solution is
only applicable to applications that provide for user authentication.
"Channel bindings" are data which securely identify a secure channel
such that, when verified to match on both endpoints of end-to-end
application connections, leave no doubt that the endpoints of two
secure channels (the one identified by the bindings and the one used
to exchange/verify the bindings) are the same.
Because many applications exist which provide for authentication at
the application layer, because many such applications use generic
authentication frameworks, such as the GSS-API and SASL and are
already deployed along with a common authentication infrastructure
(e.g., Kerberos V, PKI, etc...), because such applications exist
which multiplex multiple users onto a single session (and so cannot
leverage network [e.g., IKE] authentication), the use of channel
bindings is an elegant solution even where secure name services and
networks are deployed.
A formal definition of the channel bindings concept is given below,
as well as the specific formulation of channel bindings for various
protocols that provide for session security.
2. Definitions
The GSS-API [RFC2743] is a generic interface to GSS-API security
mechanisms which provides for authentication and session
cryptographic protection. One facility provided by the GSS-API is a
concept of "channel bindings" which consists of some data which must
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be provided, if at all, by initiators and acceptors and which the
GSS-API security mechanisms ensure are the same for both, the
initiator and acceptor of any given GSS-API security context - if
the channel bindings provided by them do not match then the mechanism
fails to establish a security context.
o Channel bindings
Generally some data which names a channel or its end-points.
o Channel bindings to secure channels
Channel bindings that securely identify a secure channel or its
end-points.
Applications can exchange authenticated, integrity-protected
verifiers of their same channel bindings data to prove that the
end-points of the channel identified by the channel bindings are
the same as the application endpoints and thus, there can be no
MITM at the lower layer.
More formally, there are two types of channel bindings:
- bindings that name a channel in a cryptographically secure
manner (e.g., the session ID in SSHv2; see below)
- bindings that name the authenticated end-points of a channel
(e.g., as in IPsec; see below)
Bindings that name a channel
- MUST be cryptographically bound to the key exchange of the
secure session
- MUST be cryptographically bound to all potentially un-
authenticated plaintext used for negotiation of the secure
session (e.g., algorithm negotiations)
and
- users of channel bindings MUST exchange authenticated,
integrity protected channel bindings data or signatures thereof
(such exchanges MAY also be confidentiality protected)
Additionally, the channel represented by the bindings MUST provide
a cryptographically secure key exchange (and re-keying) and
channel setup negotiation, and it MUST provide at least
cryptographically secure data integrity protection services.
Channel bindings data SHOULD NOT be constructed in such a way that
their exchange requires confidentiality protection.
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No channel bindings described herein require confidentiality
protected exchanges.
The security of channel bindings depends on the security of:
- the authentication and integrity protection technology used to
protect the channel bindings exchanges at the application
layers
- the security of the channels identified by the channel bindings
- the security of the channel bindings construction
o Channel bindings to network addresses
The GSS-API originally defined only channel bindings to network
addresses. Such channel bindings, of course, are generally not
cryptographically secure.
For channel bindings to network addresses to be secure the
application peers MUST be able to verify and ensure that network
communications between them are secured and that there is no MITM
- which generally means that the application peers MUST be able to
interpret and authorize identities authenticated by the network
and MUST be able to protect the methods by which they obtain the
network addresses in the first place.
In practice channel bindings to network addresses have mostly just
caused trouble with Network Address Translation (NAT).
3. Authentication protocols and channel bindings
Some authentication services provide for channel bindings, such as
the GSS-API and some GSS-API mechanisms - others do not, such as
SASL. Where suitable channel bindings facilities are not provided
application protocol designers may include a separate, protected
(where the authentication service provides message protection
services) exchange of channel bindings material
3.1. The GSS-API and channel bindings
The GSS-API provides for the use of channel bindings during
initialization of GSS-API security contexts, though GSS-API
mechanisms are not required to support this facility.
This channel bindings facility is defined in detail in RFC2744.
Unfortunately, the use of GSS-API channel bindings is generally not
negotiated by GSS-API mechanisms, therefore GSS-API applications must
agree a priori on the use of channel bindings or otherwise negotiate
the use of channel bindings.
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Fortunately, it is possible to design GSS-API pseudo-mechanisms that
simply wrap around existing mechanisms for the purpose of allowing
applications to negotiate the use of channel bindings within their
existing methods for negotiating GSS-API mechanisms. For example,
NFSv4 [RFC3530] provides its own GSS-API mechanism negotiation, as
does the MOUNT protocol for NFSv2/3 [RFC....]. [NOTE: This is an
indirect reference to the Channel Conjunction Mechanism (CCM).]
3.2. SASL and channel bindings
SASL does not provide for the use of channel bindings during
initialization of SASL contexts.
SASL applications MAY define their own exchange of integrity-
protected channel bindings using established SASL integrity layers.
Alternatively, SASL applications MAY use the GSS-* SASL mechanisms
(which correspond to GSS-API mechanisms) to ensure the use of channel
bindings through the GSS-API's facilities.
3.3. Kerberos V and channel bindings
Kerberos V does not provide for use of channel bindings, thus the
same general approach given above (post-authentication protected
channel bindings exchange) applies to Kerberos V as well.
However, Kerberos V AP client applications also MAY use the AP-REQ's
Authenticator's "checksum" field to send a hash of channel bindings
material to Kerberos V AP servers. Unfortunately, there is no slot
in the AP-REP message for carrying the AP server's channel bindings
(which justifies the statement that Kerberos V does not provide a
channel bindings facility), so Kerberos V applications MUST establish
a convention with regards to AP servers' handling of AP-REQ checksum
data - and such applications have to trust the servers to respond
with suitable error messages to AP-REQs bearing incorrect channel
bindings.
4. Channel bindings to secure layers
Not every secure session protocol or interface provides for secure
channels, and not every secure session protocol provides data
suitable for use as channel bindings.
4.1. Bindings to SSHv2 channels
SSHv2 provides both, a secure channel and material (the SSHv2
"session ID") that is suitable for use as channel bindings.
Thus it is RECOMMENDED that the SSHv2 "session ID" be used as the
channel bindings for SSHv2.
4.2. Bindings to TLS channels
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TLS provides both, a secure channel and material (the TLS "finished"
messages), that is suitable for use as channel bindings.
Thus it is RECOMMENDED that the concatenation of the client's and
server's "finished" messages, in that order, be used as the channel
bindings for TLS.
Note that the TLS "session ID," in spite of being named similarly to
the SSHv2 session ID, is not suitable for use as channel bindings
because it is assigned by the server, so a MITM could assign the same
session ID on the client side as it gets from the server.
4.3. Bindings to IPsec
IPsec does not provide a way to reliably name a channel regardless of
what key exchange protocol is used.
Therefore, the only reliable way to construct channel bindings to
IPsec is to use the identities authenticated by the IPsec key
exchange protocol for the given channel.
New interfaces [IPSP-APIREQ] are required by which applications can
create IPsec "channels."
The basic idea is to use the interfaces described in [IPSP-APIREQ] to
dynamically alter the SPD to reflect the requested bindings for the
requested connections and then use the authenticated IDs as the
identity of the channel.
This approach does not name the channel directly, but no MITM can
ensure that the authenticated IDs used as channel bindings match on
both end-points unless the MITM has stolen or broken the IPsec
credentials and/or authentication protocol. This is sufficient.
4.3.1. Interfaces for creating IPsec channels
In order to build an IPsec channel some additional application
programming interfaces are needed to:
- indicate that an as yet unconnected channel is to be bound to
IPsec IDs and
- explicitly specify one, the other or both of those IDs
- implicitly specify one, the other or both of those IDs (e.g.,
the ID corresponding to the current application program
instance)
- indirectly specify one, the other or both of those IDs (e.g., a
hostname)
and/or
- discover the IPsec IDs to which a channel is bound
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For connection-less datagram transports the IDs to be used need to be
specified/discovered on a per-datagram basis.
See [IPSP-APIREQ].
4.4. Bindings to other types of channels
For secure session protocols that do not provide material suitable
for use as channel bindings such material SHOULD be constructed by
concatenating the octets from the messages exchanged during the
initialization of a session in the chronological order in which they
were exchanged and processed (which requires synchronous session
initialization), or a strong hash thereof (such as SHA-1).
Some secure session protocols do not provide a secure channel but
which do provide per-message integrity or confidentiality protection
services. It is up to the network layers that use such protocols to
build channels from such services; applications MUST NOT delegate
session cryptographic protection to network layers that do not
provide a secure channel.
Kerberos V, certain GSS-API and SASL mechanisms, all provide session
cryptographic protection and the necessary key exchange, but they
provide neither a channel nor material suitable for use as channel
bindings.
Thus the RECOMMENDED channel bindings for channels protected by
Kerberos V consist of a SHA-1 hash of the concatenated octets of the
AP-REQ and AP-REP messages, in that order (or, for user-to-user
exchanges, the various messages exchanged, including the ticket
request, ticket and AP messages, in the order in which they were
generated and processed) used to initialize the channel's
cryptographic protection.
Similarly for channels protected by GSS-API security contexts the
RECOMMENDED channel bindings consist of a SHA-1 hash of the
concatenated octets of the context tokens exchanged to setup a
GSS-API security context in the order in which they were generated
and processed (i.e., starting with the initiator's initial context
token followed by the acceptor's reply token, if any, followed by the
initiator's reply token, if any, etc...).
5. Benefits of channel bindings to secure channels
The use of channel bindings to delegate session cryptographic
protection include:
o Performance improvements by avoiding double protection in cases
where IPsec is in use and applications provide their own secure
channels.
o Performance improvements by leveraging hardware-accelerated IPsec.
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o Performance improvements by allowing RDDP hardware offloading to
be integrated with IPsec hardware acceleration. If protocols
layered above RDDP use privacy protection then RDDP offload cannot
be done, thus by using channel bindings to IPsec the privacy
protection is moved to IPsec, which is layered below RDDP, so RDDP
can address application protocol data that's in cleartext relative
to the RDDP headers.
o Latency improvements for applications that multiplex multiple
users onto a single channel, such as NFS w/ RPCSEC_GSS.
6. Security considerations
When delegating session protection from one layer to another, one
will almost certainly be making some session security trade-offs,
such as using weaker data encryption/authentication modes.
Implementors and administrators SHOULD understand these trade-offs.
Channel bindings cannot and MUST NOT be used without mutual
authentication (of client/user/initiator and server/user/acceptor)
and/or without integrity-protected, authenticated exchange of channel
bindings material.
Anonymous secure channels SHOULD NOT be used without authentication
and corresponding use of channel bindings (to the anonymous secure
channels) at higher network layers, or for any purposes other than
opportunistic encryption, since such channels provide no
authenticated protection on their own.
The security of channel bindings depends on the security of the
channels, the construction of the bindings and the security of the
authentication and integrity protection used to exchange channel
bindings.
7. References
7.1. Informative references
[Needs references to NFSv2/3 use of RPCSEC_GSS, to NFSv4, to SCTP,
and, possibly, to DNS, DNSSEC, TELNET, SPNEGO, SSHv2 gss keyex, and
CCM.]
[TELNET]
J. Postel, J.K. Reynolds, RFC0854 (STD0008): "Telnet Protocol
Specification," May 1993, Status: Standard.
[DNS]
P.V. Mockapetris, RFC1035 (STD0013): "Domain names -
implementation and specification," November 1987, Status:
Standard.
[DNSSEC]
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B. Wellington, RFC3008: "Domain Name System Security (DNSSEC)
Signing Authority," November 2000, Status: Proposed Standard.
[RFC2203]
M. Eisler, A. Chiu, L. Ling, RFC2203: "RPCSEC_GSS Protocol
Specification," September 1997, Status: Proposed Standard.
[RFC2623]
M. Eisler, "NFS Version 2 and Version 3 Security Issues and the
NFS Protocol's Use of RPCSEC_GSS and Kerberos V5," June 1999,
Status: Proposed Standard.
[NFSv4]
S. Shepler, et. al., RFC3530: "Network File System (NFS) version 4
Protocol," April 2003, Status: Proposed Standard.
[SPNEGO]
E. Baize, D. Pinkas, RFC2478: "The Simple and Protected GSS-API
Negotiation Mechanism," December 1998, Status: Proposed Standard.
[CCM]
M. Eisler, N. Williams, Internet-Draft: "The Channel Conjunction
Mechanism (CCM) for GSS," May 2003, Status: Internet-Draft.
...
7.2. Normative references
[Needs references to RFC2119, RFC2026, the GSS-API (RFCs 2743 &
2744), SASL, SSHv2, IKEv2, IPsec, Kerberos V, ...]
[RFC2026]
S. Bradner, RFC2026: "The Internet Standard Process - Revision
3," October 1996, Obsoletes - RFC 1602, Status: Best Current
Practice.
[RFC2119]
S. Bradner, RFC2119 (BCP14): "Key words for use in RFCs to
Indicate Requirement Levels," March 1997, Status: Best Current
Practice.
[RFC2743]
J. Linn, RFC2743: "Generic Security Service Application Program
Interface Version 2, Update 1," January 2000, Status: Proposed
Standard.
[RFC2744]
J. Wray, RFC2744: "Generic Security Service API Version 2 :
C-bindings," January 2000, Status: Proposed Standard.
[IPSP-APIREQ]
W. Sommerfeld, draft-ietf-ipsp-ipsec-apireq-00: "Requirements for
an IPsec API," June 2003, Status: Draft.
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...
8. Acknowledgements
The author would like to thank Mike Eisler for his work on the
Channel Conjunction Mechanism I-D and for bringing the problem to a
head, Sam Hartman for pointing out that channel bindings provide a
general solution to the channel binding problem, Jeff Altman for his
suggestion of using the TLS finished messages as the TLS channel
bindings, as well as Bill Sommerfeld, Radia Perlman for their most
helpful comments.
9. Author's Address
Nicolas Williams
Sun Microsystems
5300 Riata Trace Ct
Austin, TX 78727
Email: nicolas.williams@sun.com
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