Network Working Group N. Williams
Request for Comments: 5056 Sun
Category: Standards Track November 2007 On the Use of Channel Bindings to Secure Channels
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
The concept of channel binding allows applications to establish that
the two end-points of a secure channel at one network layer are the
same as at a higher layer by binding authentication at the higher
layer to the channel at the lower layer. This allows applications to
delegate session protection to lower layers, which has various
This document discusses and formalizes the concept of channel binding
to secure channels.
In a number of situations, it is useful for an application to be able
to handle authentication within the application layer, while
simultaneously being able to utilize session or transport security at
a lower network layer. For example, IPsec [RFC4301] [RFC4303]
[RFC4302] is amenable to being accelerated in hardware to handle very
high link speeds, but IPsec key exchange protocols and the IPsec
architecture are not as amenable to use as a security mechanism
within applications, particularly applications that have users as
clients. A method of combining security at both layers is therefore
attractive. To enable this to be done securely, it is necessary to
"bind" the mechanisms together -- so as to avoid man-in-the-middle
vulnerabilities and enable the mechanisms to be integrated in a
seamless way. This is the objective of "Channel Bindings".
The term "channel binding", as used in this document, derives from
the Generic Security Service Application Program Interface (GSS-API)
[RFC2743], which has a channel binding facility that was intended for
binding GSS-API authentication to secure channels at lower network
layers. The purpose and benefits of the GSS-API channel binding
facility were not discussed at length, and some details were left
unspecified. Now we find that this concept can be very useful,
therefore we begin with a generalization and formalization of
"channel binding" independent of the GSS-API.
Although inspired by and derived from the GSS-API, the notion of
channel binding described herein is not at all limited to use by GSS-
API applications. We envision use of channel binding by applications
that utilize other security frameworks, such as Simple Authentication
and Security Layer (SASL) [RFC4422] and even protocols that provide
their own authentication mechanisms (e.g., the Key Distribution
Center (KDC) exchanges of Kerberos V [RFC4120]). We also envision
use of the notion of channel binding in the analysis of security
The main goal of channel binding is to be able to delegate
cryptographic session protection to network layers below the
application in hopes of being able to better leverage hardware
implementations of cryptographic protocols. Section 5 describes some
intended uses of channel binding. Also, some applications may
benefit by reducing the amount of active cryptographic state, thus
reducing overhead in accessing such state and, therefore, the impact
of security on latency.
The critical security problem to solve in order to achieve such
delegation of session protection is ensuring 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.
There may well be an MITM, particularly if either the lower network
layer provides no authentication or there is no strong connection
between the authentication or principals used at the application and
those used at the lower network layer.
Even if such MITM attacks seem particularly difficult to effect, the
attacks must be prevented for certain applications to be able to make
effective use of technologies such as IPsec [RFC2401] [RFC4301] or
HTTP with TLS [RFC4346] in certain contexts (e.g., when there is no
authentication to speak of, or when one node's set of trust anchors
is too weak to believe that it can authenticate its peers).
Additionally, secure channels that are susceptible to MITM attacks
because they provide no useful end-point authentication are useful
when combined with application-layer authentication (otherwise they
are only somewhat "better than nothing" -- see Better Than Nothing
Security (BTNS) [BTNS-AS]).
For example, Internet Small Computer Systems Interface (iSCSI)
[RFC3720] provides for application-layer authentication (e.g., using
Kerberos V), but relies on IPsec for transport protection; iSCSI does
not provide a binding between the two. iSCSI initiators have to be
careful to make sure that the name of the server authenticated at the
application layer and the name of the peer at the IPsec layer match
-- an informal form of channel binding.
This document describes a solution: the use of "channel binding" to
bind authentication at application layers to secure sessions at lower
layers in the network stack.
1.1. 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].
o Secure channel: a packet, datagram, octet stream connection, or
sequence of connections between two end-points that affords
cryptographic integrity and, optionally, confidentiality to data
exchanged over it. We assume that the channel is secure -- if an
attacker can successfully cryptanalyze a channel's session keys,
for example, then the channel is not secure.
o Channel binding: the process of establishing that no man-in-the-
middle exists between two end-points that have been authenticated
at one network layer but are using a secure channel at a lower
network layer. This term is used as a noun.
o Channel bindings: [See historical note below.]
Generally, some data that "names" a channel or one or both of
its end-points such that if this data can be shown, at a higher
network layer, to be the same at both ends of a channel, then
there are no MITMs between the two end-points at that higher
network layer. This term is used as a noun.
More formally, there are two types of channel bindings:
+ unique channel bindings:
channel bindings that name a channel in a cryptographically
secure manner and uniquely in time;
+ end-point channel bindings:
channel bindings that name the authenticated end-points, or
even a single end-point, of a channel which are, in turn,
securely bound to the channel, but which do not identify a
channel uniquely in time.
o Cryptographic binding: (e.g., "cryptographically bound") a
cryptographic operation that causes an object, such as a private
encryption or signing key, or an established secure channel, to
"speak for" [Lampson91] some principal, such as a user, a
computer, etcetera. For example, a Public Key Infrastructure for
X.509 Certificates (PKIX) certificate binds a private key to the
name of a principal in the trust domain of the certificate's
issuer such that a possessor of said private key can act on behalf
of the user (or other entity) named by the certificate.
Cryptographic bindings are generally asymmetric in nature (not to
be confused with symmetric or asymmetric key cryptography) in that
an object is rendered capable of standing for another, but the
reverse is not usually the case (we don't say that a user speaks
for their private keys, but we do say that the user's private keys
speak for the user).
Note that there may be many instances of "cryptographic binding" in
an application of channel binding. The credentials that authenticate
principals at the application layer bind private or secret keys to
the identities of those principals, such that said keys speak for
them. A secure channel typically consists of symmetric session keys
used to provide confidentiality and integrity protection to data sent
over the channel; each end-point's session keys speak for that end-
point of the channel. Finally, each end-point of a channel bound to
authentication at the application layer speaks for the principal
authenticated at the application layer on the same side of the
The terms defined above have been in use for many years and have been
taken to mean, at least in some contexts, what is stated below.
Unfortunately this means that "channel binding" can refer to the
channel binding operation and, sometimes to the name of a channel,
and "channel bindings" -- a difference of only one letter --
generally refers to the name of a channel.
Note that the Extensible Authentication Protocol (EAP) [RFC3748] uses
"channel binding" to refer to a facility that may appear to be
similar to the one decribed here, but it is, in fact, quite
different. See Section 2.2 for mode details.
2.1. Properties of Channel Binding
Applications, authentication frameworks (e.g., the GSS-API, SASL),
security mechanisms (e.g., the Kerberos V GSS-API mechanism
[RFC1964]), and secure channels must meet the requirements and should
follow the recommendations that are listed below.
o In order to use channel binding, applications MUST verify that the
same channel bindings are observed at either side of the channel.
To do this, the application MUST use an authentication protocol at
the application layer to authenticate one, the other, or both
application peers (one at each end of the channel).
* If the authentication protocol used by the application supports
channel binding, the application SHOULD use it.
* An authentication protocol that supports channel binding MUST
provide an input slot in its API for a "handle" to the channel,
or its channel bindings.
* If the authentication protocol does not support a channel
binding operation, but provides a "security layer" with at
least integrity protection, then the application MUST use the
authentication protocol's integrity protection facilities to
exchange channel bindings, or cryptographic hashes thereof.
* The name of the type of channel binding MUST be used by the
application and/or authentication protocol to avoid ambiguity
about which of several possible types of channels is being
bound. If nested instances of the same type of channel are
available, then the innermost channel MUST be used.
o Specifications of channel bindings for any secure channels MUST
provide for a single, canonical octet string encoding of the
channel bindings. Under this framework, channel bindings MUST
start with the channel binding unique prefix followed by a colon
o The channel bindings for a given type of secure channel MUST be
constructed in such a way that an MITM could not easily force the
channel bindings of a given channel to match those of another.
o Unique channel bindings MUST bind not only the key exchange for
the secure channel, but also any negotiations and authentication
that may have taken place to establish the channel.
o End-point channel bindings MUST be bound into the secure channel
and all its negotiations. For example, a public key as an end-
point channel binding should be used to verify a signature of such
negotiations (or to encrypt them), including the initial key
exchange and negotiation messages for that channel -- such a key
would then be bound into the channel. A certificate name as end-
point channel binding could also be bound into the channel in a
similar way, though in the case of a certificate name, the binding
also depends on the strength of the authentication of that name
(that is, the validation of the certificate, the trust anchors,
the algorithms used in the certificate path construction and
o End-point channel bindings MAY be identifiers (e.g., certificate
names) that must be authenticated through some infrastructure,
such as a public key infrastructure (PKI). In such cases,
applications MUST ensure that the channel provides adequate
authentication of such identifiers (e.g., that the certificate
validation policy and trust anchors used by the channel satisfy
the application's requirements). To avoid implementation
difficulties in addressing this requirement, applications SHOULD
use cryptographic quantities as end-point channel bindings, such
as certificate-subject public keys.
o Applications that desire confidentiality protection MUST use
application-layer session protection services for confidentiality
protection when the bound channel does not provide confidentiality
o The integrity of a secure channel MUST NOT be weakened should
their channel bindings be revealed to an attacker. That is, the
construction of the channel bindings for any type of secure
channel MUST NOT leak secret information about the channel. End-
point channel bindings, however, MAY leak information about the
end-points of the channel (e.g., their names).
o The channel binding operation MUST be at least integrity protected
in the security mechanism used at the application layer.
o Authentication frameworks and mechanisms that support channel
binding MUST communicate channel binding failure to applications.
o Applications MUST NOT send sensitive information, requiring
confidentiality protection, over the underlying channel prior to
completing the channel binding operation.
o End-point channel bindings where the end-points are meaningful
names SHOULD NOT be used when the channel does not provide
confidentiality protection and privacy protection is desired.
Alternatively, channels that export such channel bindings SHOULD
provide for the use of a digest and SHOULD NOT introduce new
digest/hash agility problems as a result.
o Authentication frameworks and mechanisms that support channel
binding MAY fail to establish authentication if channel binding
o Applications MAY send information over the underlying channel and
without integrity protection from the application-layer
authentication protocol prior to completing the channel binding
operation if such information requires only integrity protection.
This could be useful for optimistic negotiations.
o A security mechanism MAY exchange integrity-protected channel
o A security mechanism MAY exchange integrity-protected digests of
channel bindings. Such mechanisms SHOULD provide for hash/digest
o A security mechanism MAY use channel bindings in key exchange,
authentication, or key derivation, prior to the exchange of
2.2. EAP Channel Binding
This section is informative. This document does not update EAP
[RFC3748], it neither normatively describes, nor does it impose
requirements on any aspect of EAP or EAP methods.
EAP [RFC3748] includes a concept of channel binding described as
The communication within an EAP method of integrity-protected
channel properties such as endpoint identifiers which can be
compared to values communicated via out of band mechanisms (such
as via a AAA or lower layer protocol).
Section 7.15 of [RFC3748] describes the problem as one where a
Network Access Server (NAS) (a.k.a. "authenticator") may lie to the
peer (client) and cause the peer to make incorrect authorization
decisions (e.g., as to what traffic may transit through the NAS).
This is not quite like the purpose of generic channel binding (MITM
Section 7.15 of [RFC3748] calls for "a protected exchange of channel
properties such as endpoint identifiers" such that "it is possible to
match the channel properties provided by the authenticator via out-
of-band mechanisms against those exchanged within the EAP method".
This has sometimes been taken to be very similar to the generic
notion of channel binding provided here. However, there is a very
subtle difference between the two concepts of channel binding that
makes it much too difficult to put forth requirements and
recommendations that apply to both. The difference is about the
o In the generic channel binding case, the identities of either end
of this channel are irrelevant to anything other than the
construction of a name for that channel, in which case the
identities of the channel's end-points must be established a
o Whereas in the EAP case, the identity of the NAS end of the
channel, and even security properties of the channel itself, may
be established during or after authentication of the EAP peer to
the EAP server.
In other words: there is a fundamental difference in mechanics
(timing of lower-layer channel establishment) and in purpose
(authentication of lower-layer channel properties for authorization
purposes vs. MITM detection).
After some discussion we have concluded that there is no simple way
to obtain requirements and recommendations that apply to both generic
and EAP channel binding. Therefore, EAP is out of the scope of this
3. Authentication and Channel Binding Semantics
Some authentication frameworks and/or mechanisms provide for channel
binding, such as the GSS-API and some GSS-API mechanisms, whereas
others may not, such as SASL (however, ongoing work is adding channel
binding support to SASL). Semantics may vary with respect to
negotiation, how the binding occurs, and handling of channel binding
failure (see below).
Where suitable channel binding facilities are not provided,
application protocols MAY include a separate, protected exchange of
channel bindings. In order to do this, the application-layer
authentication service must provide message protection services (at
least integrity protection).
3.1. The GSS-API and Channel Binding
The GSS-API [RFC2743] provides for the use of channel binding during
initialization of GSS-API security contexts, though GSS-API
mechanisms are not required to support this facility.
This channel binding facility is described in [RFC2743] and
GSS-API mechanisms must fail security context establishment when
channel binding fails, and the GSS-API provides no mechanism for the
negotiation of channel binding. As a result GSS-API applications
must agree a priori, through negotiation or otherwise, on the use of
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 binding within their
existing methods for negotiating GSS-API mechanisms. For example,
NFSv4 [RFC3530] provides its own GSS-API mechanism negotiation, as
does the SSHv2 protocol [RFC4462]. Such pseudo-mechanisms are being
proposed separately, see [STACKABLE].
3.2. SASL and Channel Binding
SASL [RFC4422] does not yet provide for the use of channel binding
during initialization of SASL contexts.
Work is ongoing [SASL-GS2] to specify how SASL, particularly its new
bridge to the GSS-API, performs channel binding. SASL will likely
differ from the GSS-API in its handling of channel binding failure
(i.e., when there may be an MITM) in that channel binding
success/failure will only affect the negotiation of SASL security
layers. That is, when channel binding succeeds, SASL should select
no security layers, leaving session cryptographic protection to the
secure channel that SASL authentication has been bound to.
4. Channel Bindings Specifications
Channel bindings for various types of secure channels are not
described herein. Some channel bindings specifications can be found
| Secure Channel | Reference |
| Type | |
| SSHv2 | [SSH-CB] |
| | |
| TLS | [TLS-CB] |
| | |
| IPsec | There is no specification for IPsec channel |
| | bindings yet, but the IETF Better Than |
| | Nothing Security (BTNS) WG is working to |
| | specify IPsec channels, and possibly IPsec |
| | channel bindings. |
4.1. Examples of Unique Channel Bindings
The following text is not normative, but is here to show how one
might construct channel bindings for various types of secure
For SSHv2 [RFC4251] the SSHv2 session ID should suffice as it is a
cryptographic binding of all relevant SSHv2 connection parameters:
key exchange and negotiation.
The TLS [RFC4346] session ID is simply assigned by the server. As
such, the TLS session ID does not have the required properties to be
useful as a channel binding because any MITM, posing as the server,
can simply assign the same session ID to the victim client as the
server assigned to the MITM. Instead, the initial, unencrypted TLS
finished messages (the client's, the server's, or both) are
sufficient as they are the output of the TLS pseudo-random function,
keyed with the session key, applied to all handshake material.
4.2. Examples of End-Point Channel Bindings
The following text is not normative, but is here to show how one
might construct channel bindings for various types of secure
For SSHv2 [RFC4251] the SSHv2 host public key, when present, should
suffice as it is used to sign the algorithm suite negotiation and
Diffie-Hellman key exchange; as long the client observes the host
public key that corresponds to the private host key that the server
used, then there cannot be an MITM in the SSHv2 connection. Note
that not all SSHv2 key exchanges use host public keys; therefore,
this channel bindings construction is not as useful as the one given
in Section 4.1.
For TLS [RFC4346]the server certificate should suffice for the same
reasons as above. Again, not all TLS cipher suites involve server
certificates; therefore, the utility of this construction of channel
bindings is limited to scenarios where server certificates are
5. Uses of Channel Binding
Uses for channel binding identified so far:
o Delegating session cryptographic protection to layers where
hardware can reasonably be expected to support relevant
* NFSv4 [RFC3530] with Remote Direct Data Placement (RDDP)
[NFS-DDP] for zero-copy reception where network interface
controllers (NICs) support RDDP. Cryptographic session
protection would be delegated to Encapsulating Security Payload
(ESP) [RFC4303] / Authentication Headers (AHs) [RFC4302].
* iSCSI [RFC3720] with Remote Direct Memory Access (RDMA)
[RFC5046]. Cryptographic session protection would be delegated
* HTTP with TLS [RFC2817] [RFC2818]. In situations involving
proxies, users may want to bind authentication to a TLS channel
between the last client-side proxy and the first server-side
proxy ("concentrator"). There is ongoing work to expand the
set of choices for end-to-end authentication at the HTTP layer,
that, coupled with channel binding to TLS, would allow for
proxies while not forgoing protection over public internets.
o Reducing the number of live cryptographic contexts that an
application must maintain:
* NFSv4 [RFC3530] multiplexes multiple users onto individual
connections. Each user is authenticated separately, and users'
remote procedure calls (RPCs) are protected with per-user GSS-
API security contexts. This means that large timesharing
clients must often maintain many cryptographic contexts per-
NFSv4 connection. With channel binding to IPsec, they could
maintain a much smaller number of cryptographic contexts per-
NFSv4 connection, thus reducing memory pressure and
interactions with cryptographic hardware.
For example, applications that wish to use RDDP to achieve zero-copy
semantics on reception may use a network layer understood by NICs to
offload delivery of application data into pre-arranged memory
buffers. Note that in order to obtain zero-copy reception semantics
either application data has to be in cleartext relative to this RDDP
layer, or the RDDP implementation must know how to implement
cryptographic session protection protocols used at the application
There are a multitude of application-layer cryptographic session
protection protocols available. It is not reasonable to expect that
NICs should support many such protocols. Further, some application
protocols may maintain many cryptographic session contexts per-
connection (for example, NFSv4 does). It is thought to be simpler to
push the cryptographic session protection down the network stack (to
IPsec), and yet be able to produce NICs that offload other operations
(i.e., TCP/IP, ESP/AH, and DDP), than it would be to add support in
the NIC for the many session cryptographic protection protocols in
use in common applications at the application layer.
The following figure shows how the various network layers are
| Application layer |<---+
| |<-+ | In cleartext, relative
+---------------------+ | | to each other.
| RDDP |<---+
| TCP/SCTP |<-+
+---------------------+ | Channel binding of app-layer
| ESP/AH |<-+ authentication to IPsec
| IP |
| ... |
6. Benefits of Channel Binding to Secure Channels
The use of channel binding to delegate session cryptographic
o Performance improvements by avoiding double protection of
application data in cases where IPsec is in use and applications
provide their own secure channels.
o Performance improvements by leveraging hardware-accelerated IPsec.
o Performance improvements by allowing RDDP hardware offloading to
be integrated with IPsec hardware acceleration.
Where protocols layered above RDDP use privacy protection, RDDP
offload cannot be done. Thus, by using channel binding 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 with RPCSEC_GSS
Delegation of session cryptographic protection to IPsec requires
features not yet specified. There is ongoing work to specify:
o IPsec channels [CONN-LATCH];
o Application programming interfaces (APIs) related to IPsec
o Channel bindings for IPsec channels;
o Low infrastructure IPsec authentication [BTNS-CORE].
7. IANA Considerations
IANA has created a new registry for channel bindings specifications
for various types of channels.
The purpose of this registry is not only to ensure uniqueness of
values used to name channel bindings, but also to provide a
definitive reference to technical specifications detailing each
channel binding available for use on the Internet.
There is no naming convention for channel bindings: any string
composed of US-ASCII alphanumeric characters, period ('.'), and dash
('-') will suffice.
The procedure detailed in Section 7.1 is to be used for registration
of a value naming a specific individual mechanism.
7.1. Registration Procedure
Registration of a new channel binding requires expert review as
defined in BCP 26 [RFC2434].
Registration of a channel binding is requested by filling in the
o Subject: Registration of channel binding X
o Channel binding unique prefix (name):
o Channel binding type: (One of "unique" or "end-point")
o Channel type: (e.g., TLS, IPsec, SSH, etc.)
o Published specification (recommended, optional):
o Channel binding is secret (requires confidentiality protection):
o Description (optional if a specification is given; required if no
published specification is specified):
o Intended usage: (one of COMMON, LIMITED USE, or OBSOLETE)
o Person and email address to contact for further information:
o Owner/Change controller name and email address:
o Expert reviewer name and contact information: (leave blank)
o Note: (Any other information that the author deems relevant may be
and sending it via electronic mail to <firstname.lastname@example.org> (a
public mailing list) and carbon copying IANA at <email@example.com>.
After allowing two weeks for community input on the mailing list to
be determined, an expert will determine the appropriateness of the
registration request and either approve or disapprove the request
with notice to the requestor, the mailing list, and IANA.
If the expert approves registration, it adds her/his name to the
The expert has the primary responsibility of making sure that channel
bindings for IETF specifications go through the IETF consensus
process and that prefixes are unique.
The review should focus on the appropriateness of the requested
channel binding for the proposed use, the appropriateness of the
proposed prefix, and correctness of the channel binding type in the
registration. The scope of this request review may entail
consideration of relevant aspects of any provided technical
specification, such as their IANA Considerations section. However,
this review is narrowly focused on the appropriateness of the
requested registration and not on the overall soundness of any
provided technical specification.
Authors are encouraged to pursue community review by posting the
technical specification as an Internet-Draft and soliciting comment
by posting to appropriate IETF mailing lists.
7.2. Comments on Channel Bindings Registrations
Comments on registered channel bindings should first be sent to the
"owner" of the channel bindings and to the channel binding mailing
Submitters of comments may, after a reasonable attempt to contact the
owner, request IANA to attach their comment to the channel binding
type registration itself by sending mail to <firstname.lastname@example.org>. At
IANA's sole discretion, IANA may attach the comment to the channel
7.3. Change Control
Once a channel bindings registration has been published by IANA, the
author may request a change to its definition. The change request
follows the same procedure as the registration request.
The owner of a channel bindings may pass responsibility for the
channel bindings to another person or agency by informing IANA; this
can be done without discussion or review.
The IESG may reassign responsibility for a channel bindings
registration. The most common case of this will be to enable changes
to be made to mechanisms where the author of the registration has
died, has moved out of contact, or is otherwise unable to make
changes that are important to the community.
Channel bindings registrations may not be deleted; mechanisms that
are no longer believed appropriate for use can be declared OBSOLETE
by a change to their "intended usage" field. Such channel bindings
will be clearly marked in the lists published by IANA.
The IESG is considered to be the owner of all channel bindings that
are on the IETF standards track.
8. Security Considerations
Security considerations appear throughout this document. In
particular see Section 2.1.
When delegating session protection from one layer to another, one
will almost certainly be making some session security trade-offs,
such as using weaker cipher modes in one layer than might be used in
the other. Evaluation and comparison of the relative cryptographic
strengths of these is difficult, may not be easily automated, and is
far out of scope for this document. Implementors and administrators
should understand these trade-offs. Interfaces to secure channels
and application-layer authentication frameworks and mechanisms could
provide some notion of security profile so that applications may
avoid delegation of session protection to channels that are too weak
to match a required security profile.
Channel binding makes "anonymous" channels (where neither end-point
is strongly authenticated to the other) useful. Implementors should
avoid making it easy to use such channels without channel binding.
The security of channel binding depends on the security of the
channels, the construction of their channel bindings, and the
security of the authentication mechanism used by the application and
its channel binding method.
Channel bindings should be constructed in such a way that revealing
the channel bindings of a channel to third parties does not weaken
the security of the channel. However, for end-point channel bindings
disclosure of the channel bindings may disclose the identities of the
8.1. Non-Unique Channel Bindings and Channel Binding Re-Establishment
Application developers may be tempted to use non-unique channel
bindings for fast re-authentication following channel re-
establishment. Care must be taken to avoid the possibility of
attacks on multi-user systems.
Consider a user multiplexing protocol like NFSv4 using channel
binding to IPsec on a multi-user client. If another user can connect
directly to port 2049 (NFS) on some server using IPsec and merely
assert RPCSEC_GSS credential handles, then this user will be able to
impersonate any user authenticated by the client to the server. This
is because the new connection will have the same channel bindings as
the NFS client's! To prevent this, the server must require that at
least a host-based client principal, and perhaps all the client's
user principals, re-authenticate and perform channel binding before
the server will allow the clients to assert RPCSEC_GSS context
handles. Alternatively, the protocol could require a) that secure
channels provide confidentiality protection and b) that fast re-
authentication cookies be difficult to guess (e.g., large numbers
In other contexts there may not be such problems, for example, in the
case of application protocols that don't multiplex users over a
single channel and where confidentiality protection is always used in
the secure channel.
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[BTNS-AS] Touch, J., Black, D., and Y. Wang, "Problem and
Applicability Statement for Better Than Nothing Security
(BTNS)", Work in Progress, October 2007.
[BTNS-CORE] Richardson, M. and N. Williams, "Better-Than-Nothing-
Security: An Unauthenticated Mode of IPsec", Work in
Progress, September 2007.
[BTNS-IPSEC] Richardson, M. and B. Sommerfeld, "Requirements for an
IPsec API", Work in Progress, April 2006.
[CONN-LATCH] Williams, N., "IPsec Channels: Connection Latching",
Work in Progress, September 2007.
[Lampson91] Lampson, B., Abadi, M., Burrows, M., and E. Wobber,
"Authentication in Distributed Systems: Theory and
Practive", October 1991.
[NFS-DDP] Callaghan, B. and T. Talpey, "NFS Direct Data
Placement", Work in Progress, July 2007.
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996.
[RFC2203] Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
Specification", RFC 2203, September 1997.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC2744] Wray, J., "Generic Security Service API Version 2 :
C-bindings", RFC 2744, January 2000.
[RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within
HTTP/1.1", RFC 2817, May 2000.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3530] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
Beame, C., Eisler, M., and D. Noveck, "Network File
System (NFS) version 4 Protocol", RFC 3530, April 2003.
[RFC3720] Satran, J., Meth, K., Sapuntzakis, C., Chadalapaka, M.,
and E. Zeidner, "Internet Small Computer Systems
Interface (iSCSI)", RFC 3720, April 2004.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
H. Levkowetz, "Extensible Authentication Protocol
(EAP)", RFC 3748, June 2004.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, December 2005.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.1", RFC 4346, April
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
[RFC4462] Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch,
"Generic Security Service Application Program Interface
(GSS-API) Authentication and Key Exchange for the Secure
Shell (SSH) Protocol", RFC 4462, May 2006.
[RFC5046] Ko, M., Chadalapaka, M., Hufferd, J., Elzur, U., Shah,
H., and P. Thaler, "Internet Small Computer System
Interface (iSCSI) Extensions for Remote Direct Memory
Access (RDMA)", RFC 5046, October 2007.
[SASL-GS2] Josefsson, S., "Using GSS-API Mechanisms in SASL: The
GS2 Mechanism Family", Work in Progress, October 2007.
[SSH-CB] Williams, N., "Channel Binding Identifiers for Secure
Shell Channels", Work in Progress, November 2007.
[STACKABLE] Williams, N., "Stackable Generic Security Service
Pseudo-Mechanisms", Work in Progress, June 2006.
[TLS-CB] Altman, J. and N. Williams, "Unique Channel Bindings for
TLS", Work in Progress, November 2007.
Appendix A. Acknowledgments
Thanks to Mike Eisler for his work on the Channel Conjunction
Mechanism document and for bringing the problem to a head, Sam
Hartman for pointing out that channel binding provides a general
solution to the channel binding problem, and Jeff Altman for his
suggestion of using the TLS finished messages as the TLS channel
bindings. Also, thanks to Bill Sommerfeld, Radia Perlman, Simon
Josefsson, Joe Salowey, Eric Rescorla, Michael Richardson, Bernard
Aboba, Tom Petch, Mark Brown, and many others.
5300 Riata Trace Ct.
Austin, TX 78727
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