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Differences from draft-pritikin-ttimodel-00.txt
Network Working Group M. Pritikin
Internet-Draft Cisco Systems, Inc.
Expires: January 3, 2005 July 5, 2004
Trusted Transitive Introduction Model
draft-pritikin-ttimodel-01.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on January 3, 2005.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
We describe the 'out-of-band' exchange of data in a classical
enrollment protocol as a Trusted Transitive Introduction (TTI)
between two end entities and an introducer, thus distinguishing
introduction from enrollment. This document describes the three
system entities in the trusted transitive introduction model and the
data exchanges between them. Three introduction stages are defined
and examined in the context of a 'TTI over HTTP' introduction.
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Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
3. TTI Entities . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Deployment Scenarios . . . . . . . . . . . . . . . . . . . . 8
4.1 Existing Infrastructures and Authorization . . . . . . . . . 10
4.2 Storyboard 1: Deployment of a VPN device . . . . . . . . . . 10
4.3 Storyboard 2: Purchase of a new device from a service . . . 12
4.4 Storyboard 3: Imprint of a new device . . . . . . . . . . . 13
4.5 Storyboard 4: Establishing a 'pay for use' account . . . . . 14
5. TTI transports . . . . . . . . . . . . . . . . . . . . . . . 17
5.1 TTI over HTTP . . . . . . . . . . . . . . . . . . . . . . . 17
5.1.1 TTI over HTTP Welcome . . . . . . . . . . . . . . . . . . . 18
5.1.2 TTI over HTTP Introduction . . . . . . . . . . . . . . . . . 18
5.1.3 TTI over HTTP Completion . . . . . . . . . . . . . . . . . . 19
6. Where is the User in all this? . . . . . . . . . . . . . . . 20
7. Security Considerations . . . . . . . . . . . . . . . . . . 21
References . . . . . . . . . . . . . . . . . . . . . . . . . 23
Author's Address . . . . . . . . . . . . . . . . . . . . . . 23
Intellectual Property and Copyright Statements . . . . . . . 24
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1. 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 [RFC2119].
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2. Introduction
When adding a device into a security domain the first task is to
exchange cryptographic and configuration information between the
security domain and the device. This process we term an Introduction.
Prior to a successful Introduction there are no security associations
between the device and the target security domain. After an
introduction there is enough of a security association for the device
and the domain to communicate securely at least once. It is our
expectation that this initial secure communication will be used to
enroll and receive longer term credentials as appropriate.
Classically this enrollment process has been handled by the
'out-of-band' communication of cryptographic and configuration
information. Although this works well in the simple case, and is
common practice, complex authentication and authorization
infrastructures often require complex cryptographic keys and
configurations to be exchanged. For example PKI deployments often
involve involve manual verification of RSA public key material
hashes, complex configuration tasks, and/or specific authorization
tasks that must occur in a particular sequence. This process is a
burden on the administrator and complicates deployment scenarios
tremendously. The intuitive nature of the introduction has been lost
in the details of the complex cryptographic and configuration
material.
Here we describe the introduction process as a communication protocol
that leverages transitive trust across a third party, the introducer,
to complete the 'out-of-band' exchange of data. This maintains the
intuitive nature of an introduction while allowing complex
configuration and cryptographic material to be exchanged, thus
allowing simple enrollment procedures to be leveraged to support more
complex enrollments. This process is Trusted Transitive Introduction
(TTI).
The basic concepts follow from a real life introduction:
"Alice, meet Bob. Bob, this is Alice."
The third party in this exchange is the Introducer, trusted by both
Alice and Bob.
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3. TTI Entities
An introduction involves at least three logical entities. For example
the new device, the security domain, and the 'out-of-band' system
administrator(s). TTI defines these entities as,
(P) Petitioner - a new VPN client device.
(I) Introducer - a user at their browser.
(R) Registrar - the VPN network hub.
Although described here as a discrete entities an instantiation of
the TTI model may include situations where the logical entities are
themselves complex systems. For example,
(P) Petitioner - the product.
(I) Introducer - the product vendor.
(R) Registrar - a service provider.
Or the TTI entities may all be devices in network without direct
human intervention,
(P) Petitioner - an 802.1x capable device.
(I) Introducer - an 802.1x switch.
(R) Registrar - the network Authentication Authorization
infrastructure).
Often associated with the Introducer, and clearly influential in the
resulting exchange, is the user/administrator. The user has an
interface to one or more elements of the introduction and either
initiates the introduction or configures the appropriate behaviors/
policy that an introduction can be automatically initiated at a later
time. Deployment discussions that minimize or ignore the role of the
Introducer often require an interface on both the Petitioner and
Registrar.
An important and likely component of at least one of these systems is
the existing Authentication and Authorization (AA) infrastructure
that the Petitioner is expecting to enroll with. In some
instantiations of the TTI model all entities may include complex AA
infrastructures. For clarity these are occasionally discussed as
distinct from the TTI entities themselves,
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(A) Authority (e.g. a radius server, CA, or other)
Graphically this looks like:
+------------+ +-------------------+
+------------+ | Petitioner | | P's Authority (A) |
| |<------>| (P) |--| (AAA, CA or other)|
| | | | +-------------------+
| | +------------+
| |
| Introducer | (there is no initial trust relationship
| (I) | between P and R)
| | +------------+
| | | Registrar | +-------------------+
| |<------>| (R) |--| R's Authority (A) |
+------------+ | | | (AAA, CA or other)|
+------------+ +-------------------+
Figure 1
The Petitioner is the entity obtaining new credentials after the TTI
exchange. The Registrar is the entity issuing the new credentials.
One possibility is that an entity may act as either a Registrar and a
Petitioner. The TTI protocols must consider such situations.
The specifics of the relationship between each entity and its own
Authority are out of the scope of the TTI model. The existence of
either a Petition's or a Registrar's Authority does not effect the
sequence of communication events, although it may effect the
enrollment and configuration protocols that depend on TTI for their
'out-of-band' data exchange and it may effect the configuration
information exchanged by TTI.
After introduction is complete the Petitioner and Registrar have
exchanged key and configuration material and can complete an
authenticated exchange with each other. Due to the transitive and
possibly manual nature of the introduction process TTI is not
envisioned as as a full featured policy, configuration distribution,
and enrollment mechanism. Instead peers are expected to conclude the
introduction with an appropriate 'in band' enrollment mechanism.
The peer entities now communicate directly to enroll:
+------------+ +-------------------+
+------------+ | Petitioner | | P's Authority (A) |
| | | (P) |--| (AAA, CA or other)|
| | | | +-------------------+
| | +------------+
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| | |
| Introducer | | (Now can run enrollment protocol)
| (I) | |
| | +------------+
| (No longer | | Registrar | +-------------------+
| involved) | | (R) |--| R's Authority (A) |
+------------+ | | | (AAA, CA or other)|
+------------+ +-------------------+
Figure 2
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4. Deployment Scenarios
The Introducer must authenticate to the Petitioner and Registrar
using the authentication and authorization mechanisms currently in
place. This recursive use of prior associations is the strength of
TTI. It allows complex authentication and authorization mechanisms to
be built from relatively simple default mechanisms. It is important
to note that current enrollment scenarios also depend on existing
authentication mechanisms without explicitely defining how they do
so. For example when a certificate request hash for PKI enrollment is
transfered 'out-of-band' it is obtained, sent, and checked using the
existing authentication/authorization mechanisms. This may be a
remote administrative connection to the new device secured with HTTPS
or it may be a direct physical connection.
The following example deployment scenarios help clarify this,
A highly knowledgeable administrator facilitates deployment at the
target location.
A physically secured staging location is used for deployment.
An external party provides the 'staging' area.
An end user facilitates deployment at the target location.
The simplest solution to code is to expect a highly trained and
knowledgeable administrator to travel to the new deployment site and
deploy the device manually. While this must be supported and care
should be taken to ease the deployment task this is the most costly
form of deployment and can not be used for wide scale deployments.
Using an in house staging area is well understood and can be
automated to a large extent. Unfortunately even the unpacking,
automated deployment deployment, and repacking, of equipment is also
a highly costly process and does not scale well.
External parties can amortize the cost of staging across multiple
customer deployments and drive the cost down to a point where it is
less of an issue. Particularly when the external party is the device
manufacturer who already has the device out of the box and on an
automated provisioning system (such as is used for initial device
configuration, possibly including initial configuration with a
customer supplied configuration). Support for this must be provided
in such a way that secret cryptographic key material is not exposed.
Even in a resulting trust and identity infrastructure where PKI is
minimized it is normally leveraged here (e.g. X509 certificates
signed by the manufacturer, possibly including an initial
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configuration of the customer supplied X509 root certificates).
We can restate the various deployment scenarios as a relationship
between the following aspects:
A) The deployment or enrollment requirements and process. Many
organizations will focus initially on costs. Although interested
in 'security' they'll be looking for a cheap solution that is not
insecure (emphasis theirs). A highly secure but very manual
(costly) process may be available but not acceptable. For example
training all users/administrators to check certificate
fingerprints doesn't work.
B) Physical securities in the "out-of-band" communications channel
used by an administrative element to communicate with the device.
Although the extreme ideal is to leverage quantum cryptography
across a physical link the currently accepted practice is to use a
local console cable or local physical access. Due to management/
usability issues (see A) it is currently common for a local LAN
connection to suffice - and occasionally this LAN connection may
even be wireless! One example of a tradeoff for this area is to
specify in (A) that ssh (even un-authenticated) instead of telnet
should be used for device to device communication.
C) Trust in existing previously configured credentials. The
device's existing credentials are leveraged to provide virtual
security for (B) in response to pressures from (A). For example an
authenticated ssh connection, an HTTPS connection or some other
secured link
Where lowered requirements for (A) and (B) result in higher
requirements for (C).
In some instantiations of the model the Introducer may be co-located
with either the Petitioner or the Registrar. One could thus describe
current enrollment scenarios that involve an administrator on the new
device as an instantiation of the TTI model.
In a password based enrollment mechanism a user drives enrollment by
providing a password to the new device (which then optionally uses
this password to communicate with the Registrar system to obtain more
complex credentials). This may be described using the TTI model by
including the Introducer system on the same basic platform as the
Petitioner. In this case the authentication mechanism between (I) and
(P) is the direct physical/logical integration of these components
(e.g. They are running on the same system) (See B above):
+-------------------------------+
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| New Device/System |
|+----------+ +------------+|
||Introducer| | Petitioner ||
|| |<--->| (P) ||
|| | | ||
|+----|-----+ +------------+|
+-----|-------------------------+
| (there is no initial trust relationship
| between P and R)
| +------------+
| | Registrar |
+---------->| (R) |
(password) | |
+------------+
Figure 3
4.1 Existing Infrastructures and Authorization
As discussed above the TTI model can depend on an existing security
association between the Introducer and the Petitioner (and between
the Introducer and the Registrar).
Even when it is acceptable to re-use existing credentials the TTI
model is used to configure authorization. Consider the case when an
existing credential, say a manufacturing ID certificate, is
available. The device can uniquely identify itself, but this does not
preclude the need for an enrollment/deployment scenario. The existing
credential has been issued by a 3rd party but must be associated with
appropriate authorization before it can be generally accepted on the
target network. This process of configuring the authorization is an
Introduction (wherein the Introducer obtains the new device's ID from
the Petitioner and provides it to the Registrar system thus
authorizing the device).
4.2 Storyboard 1: Deployment of a VPN device
In this situation a corporate VPN user with existing corporate
credentials wishes to enable a remote access hardware VPN device.
This device allows the a SOHO office to behaves much like a corporate
office. For example direct access to corporate printers, IP phones
work correctly, multiple computers/servers can be deployed, 802.1x
security scenarios apply etc.
The TTI entities are defined as:
(P) Petitioner: The hardware VPN device.
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(I) Introducer: The user with a web browser.
(R) Registrar: The corporate VPN service.
We assume the user had IP connectivity and has purchased a hardware
VPN device at a local retail store. The hardware VPN device is a
consumer electronics equipment with a default configuration and
behavior according to the designer specifications. It has no
knowledge of the corporate credentials, nor does it have special
credentials for Enrollment:
User connects VPN device to the home network as appropriate.
Depending on the device purchased this may involve lengthy
configuration steps or may be a plug-and-go scenario.
The user establishes a secure connection to the new device. It is
expected that an HTTP(S) interface will be used for most headless
devices, and as such a default HTTPS certificate MAY be used by
the device to authenticate itself as per the "Storyboard: Imprint
of a new device"
Once the user is communicating securely with the new device they
Introduce it to the corporate network by indicating the corporate
VPN service's Registrar HTTP URL. During this exchange the
Petitioner's public key and provisioning information are
transferred to the user/browser (the user MAY not even be aware of
this).
The user's web browser is connected to Registrar's HTTP URL and
the user is authenticated. This authentication is between the user
(at their browser) and the corporate Registrar. It may be a
mutually authenticated connection with both server_auth and
client_auth (if the user has a local certificate and/or smartcard
system) or it may be only server authenticated with a username
password. The mechanisms used are between the user/browser and the
corporate Registrar and changes to these do not effect the
Petitioner device's requirements. The Registrar performs the
appropriate authorization exchanges such as determining if the
user has appropriate permissions to deploy a hardware VPN device.
During this exchange the Petitioner's public key and provisioning
information are transferred to the Registrar (the user MAY not
even be aware of this). In addition to the graphical user response
the Registrar's public key and provisioning information are
strangered to the user/browser (the user MAY not be aware of this
either).
The Introduction is completed when the user is redirected back to
the Petitioner HTTP interface. At this time the Registrar's public
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key and provisioning information are strangered to the Petitioner.
The Petitioner then instantiates the provisioning information.
Possibly this includes enrolling with the Registrar to obtain
Registrar significant credential material (certificates, symmetric
keys etc) and configuration information (possibly joining a
specified management framework etc).
Deployment is complete
4.3 Storyboard 2: Purchase of a new device from a service
Many services may with to provide value by performing the imprint
detailed above on behalf of the purchaser. In this case they perform
the above sequence of events but 'imprint' the device to the
purchases public key and provisioning material.
The TTI entities are defined as:
(P) Petitioner: The new device.
(I) Introducer: The manufacturer or 3rd party resaler.
(R) Registrar: The purchaser with their PC/web browser.
We assume the device is purchased from the manufacturer or 3rd party
web page:
The user orders the device by providing cryptographic and
provisioning information at the manufacturer/3rd party web site.
The manufacturer/3rd party performs the imprint sequence detailed
above and configures the device with the purchases cryptographic
and provisioning information. The device is shipped
As a result of the device being shipped a bill of sale is returned
with the devices cryptographic information to the purchaser.
The purchases sends this information into their Registrar system
(possibly manually, hopefully via an automated process etc).
Device arrives and is plugged in/turned on
The Petitioner then instantiates the provisioning information.
Possibly this includes enrolling with the Registrar to obtain
Registrar significant credential material (certificates, symmetric
keys etc) and configuration information (possibly joining a
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specified management framework etc).
Deployment is complete
4.4 Storyboard 3: Imprint of a new device
When a device is configured with factory default settings it does not
have a security association with an administrator, user, or
management system. Much like a duck it must Imprint on any entity
that presents itself as an administrator.
Although the device MAY have credentials of its own that it can
present (for example a certificate issued to the device during
manufacturing or a prior imprint process) this case also covers the
ubiquitous initial scenario where the device has no prior
credentials.
The TTI entities are defined as:
(P) Petitioner: The new device.
(I) Introducer: Very thin in this scenario - it is essentially the
transport mechanism providing secured connectivity to the new
device.
(R) Registrar: The initial configuration system. In an automated
corporate environment this may be a provisioning/configuration/
test rack. It may also be a PC/web browser etc.
We assume the device is directly off the manufacturing floor and has
absolutely minimal configuration:
The device comes up and performs accepts initial input(s) from the
introducer as having full administrative credentials.
The introducer detects the physical link is active and initiates
communication with the registrar.
The registrar provides provisioning information and cryptographic
information
The Petitioner then instantiates the provisioning information.
Possibly obtaining credentials such as a Device ID Certificate.
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4.5 Storyboard 4: Establishing a 'pay for use' account
In this situation a user wishes to use their credit card to establish
an account to access an online service such as a WLAN hot spot. There
are two models of this scenario that we must consider, the first
being a modeling of the scenario as it is currently implemented today
and the second being a model of a secure solution. The current
implementation of this scenario has been heavily influenced by the
existing authentication and authorization mechanisms available and is
not ideal. The user's credit card number is essentially a poorly
protected piece of "secret" key material surrendered to the provider
as a sort of authorization credential which the provider can then use
to obtain payment from the credit card provider.
[A paragraph here about how extensive and expensive $$ this fraud is
would be interesting. Some links to consider are The Internet Fraud
Complaint Center (http://www.ifccfbi.gov/strategy/statistics.asp)
concerning general Internet fraud issues indicate massive amounts of
moneys. Others like this random link (http://www.biz-sites.com/
merchant_accounts/credit_fraud.html) talk about $1billion a year. I'm
sure there are some good government estimates out there that I didn't
google my way to. The short answer is a lot of money is spent on this
broken security model.]
Regardless this is a scenario that we must consider and be prepared
to model. A direct modeling results in the strained definition of TTI
entities:
(P) Petitioner: The new user/laptop.
(I) Introducer: The credit card provider.
(R) Registrar: The WLAN hot spot.
The credit card and associated information could be considered a
template of introduction material already provided by the Introducer
to the Petitioner. Namely it is composed of cryptographic information
(the quasi-secret credit card number) and configuration information
(the type of card, expiration date, the 3numbers on the back etc).
The Registrar has likewise already established an ongoing
relationship with the Introducer. The introduction can be said to be
almost complete except that the Petitioner now triggers it to
completion (this is where the model is strained, previously we had
limited initiation of the introduction to the introducer).
The user initiates the introduction by the act of attempting an
enrollment. Which is to say by initiation the 'new account'
process at the Registrar.
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The Registrar optionally communicates with the Introducer to make
sure the Petitioner device is valid (e.g. Verifies the credit card
information). As per the existing relationship between the
merchant and the credit card provider this is optional in some
situations. Once the merchant is assured that appropriate
authorization is received (moneys in this scenario) the
introduction can proceed.
If the Introduction is successful the user is enrolled and issued
locally significant credentials. Depending on the WLAN hot spot
this may be an HTTP cookie authorizing use for a period of time or
it may be the establishment of a long term account (including
caching of the credit card information to use during later
authorization renewals (e.g. Monthly payments).
As shown above this works but is a strain on the model. Another way
of modeling this scenario presents itself:
(P) Petitioner: The WLAN hot spot.
(I) Introducer: The user/laptop.
(R) Registrar: The credit card provider.
This is description of TTI entities better matches what is important
here, the flow of moneys. And namely that the user is authorizing a
certain long term relationship between the WLAN hot spot and their
credit card provider (or bank):
The user accesses the WLAN hotspot (or any other Internet service
provider) and initiate establishment of an account. The Introducer
authenticates the Petitioner device by examining the HTTPS
certificates.
The user forwards the introduction material (e.g. The
cryptographic information identifying the Petitioner to the
Registrar). This is implemented as part of the HTML forms during
account establishment and involves an HTTPS connection between the
Introducer and their credit card provider. [As an implementation
detail this describes the WLAN hotspot allowing HTTPS traffic to
known credit card provider Registrar systems even prior to full
authentication/authorization of the user]
The user establishes with the Registrar what type of introduction
this is (should the result be a one time only authorization for a
certain amount of money or is it to allow enrollment of the
Petitioner for monthly access and withdrawal of a certain amount
or range? The details of this information are between the
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Introducer and the Registrar; the Petitioner is not required to
support them.
The introduction material is sent back to the Petitioner which
then uses it to enroll with the Registrar. Credentials are
obtained, moneys are transferred and/or accounts are established.
As a result the Petitioner now successfully provides a service to
the Introducer.
If this service is not acceptable or otherwise goes wrong the
Introducer can connect to the Registrar and revoke/change the
appropriate authorizations connected to the credentials issued to
the Petitioner.
This latter scenario more accurately represents what we want out of
these types of transactions. With full acceptance of the TTI model
online credit card fraud could be significantly curtailed with
extremely minimal change in the user experience (providing equivalent
usability) without changing the format or specification for user
credit cards themselves (thus non-online fraud is still a problem).
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5. TTI transports
TTI should be transport independent. It is envisioned that the
different TTI protocol instantiations of the TTI model may run on a
variety of systems and protocols and should not be locked into a
particular transport mechanism.
Some areas that should be considered include:
IP Layer 2 (e.g. over an 802.1x EAP or EAP/TLS exchange)
IP Layer 3 (e.g. over an HTTP web/user based exchange)
Storage medium (e.g. an introduction should be 'distributable'
over a cd, smartcard, or other physical transport device)
5.1 TTI over HTTP
The use of HTTP mechanisms for the TTI transport allows a common HTML
web browser to be used as the Introducer in a TTI exchange. This
provides support for an extremely important and useful example; where
a user with their web browser initiates and completes the intuitive
introduction of two network entities without having to know or
understand the details of an introduction or enrollment.
This document provides a brief overview of what this would look like
to help explain the TTI model. The specific details of the TTI over
HTTP redirections, encoding of cryptographic information,
configuration information and resulting enrollment are out of scope
of this document. For a simple implementation of TTI over HTTP the
TTI entities are,
1. Petitioner - the device or product running a simple HTTP[S]
server used by the TTI exchange.
2. Introducer - a user at their web browser. This may be on the same
system as the Petitioner or it may be on a different system.
3. Registrar - the service or provider running a simple HTTP[S]
server used by the TTI exchange.
There are three essential phases to TTI over HTTP, each of these
include an HTTP GET or an HTTP POST of data as described in the
following sections.
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5.1.1 TTI over HTTP Welcome
The Welcome Phase:
+------------+ +-------------------+
+------------+ | Petitioner | | P's Authority (A) |
| |<-------| (P) |--| (AAA, CA or other)|
| | | | +-------------------+
| | +------------+
| |
| Introducer |
| (I) |
| Web | +------------+
| Browser | | Registrar | +-------------------+
| | | (R) |--| R's Authority (A) |
+------------+ | | | (AAA, CA or other)|
+------------+ +-------------------+
Figure 4
The user initiates their web browser (Introducer) to do an HTTP[S]
GET from the Petitioner. This requires that the user, or the users
web browser, log into the Petitioner according to the authentication
and authorization infrastructure currently in place. The web page
received includes 'hidden' input elements with cryptographic
information that identifies the Petitioner device and and input field
for the Registrar's HTTP address. The HTML input type 'hidden' avoids
displaying complex cryptographic information and confusing the naive
user, it is not a security mechanism. The user inputs the Registrar's
HTTP[S] URL and clicks the 'next' button.
5.1.2 TTI over HTTP Introduction
The Introduction phase:
+------------+ +-------------------+
+------------+ | Petitioner | | P's Authority (A) |
| | | (P) |--| (AAA, CA or other)|
| | | | +-------------------+
| | +------------+
| |
| Introducer |
| (I) |
| Web | +------------+
| Browser |------->| Registrar | +-------------------+
| |<-------| (R) |--| R's Authority (A) |
+------------+ | | | (AAA, CA or other)|
+------------+ +-------------------+
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(Note that an HTML page is recieved after doing an HTTP POST)
Figure 5
The Petitioner's cryptographic information is HTTP[S] POSTed to the
Registrar's server. This requires that the user, or the user's web
browser, log into the Registrar's server. As a result of the HTTP[S]
POST, and authorization of the Introducer, the resulting web page
includes, as hidden attributes, the cryptographic and configuration
information for the Petitioner device. The user clicks the 'next'
button.
5.1.3 TTI over HTTP Completion
Introducer does an HTTP POST back to the Petitioner:
+------------+ +-------------------+
+------------+ | Petitioner | | P's Authority (A) |
| |------->| (P) |--| (AAA, CA or other)|
| | | | +-------------------+
| | +------------+
| |
| Introducer |
| (I) |
| Web | +------------+
| Browser | | Registrar | +-------------------+
| | | (R) |--| R's Authority (A) |
+------------+ | | | (AAA, CA or other)|
+------------+ +-------------------+
Figure 6
The cryptographic and configuration information from the registrar is
HTTP[S] POSTed to the Petitioner.
At this point the Petitioner and the Registrar have exchanged the
appropriate information to engage in a secure enrollment protocol as
in Figure 2.
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6. Where is the User in all this?
The user drives an introduction by initiating events in an intuitive
order. What they do not do is manually exchange or process data. The
software tools they are using perform the actual work of the data
exchange.
In the above TTI over HTTP scenario the user drives the introduction
process using a generic web browser. The user, at their web browser,
is involved in every aspect of the TTI exchange. They are playing the
role of the Introducer with only the very thin layer of their web
browser to 'protect' them from being exposed to the raw cryptographic
and configuration material. In the example presented they could
examine this material by viewing the source of the HTML pages. Other
uses of the TTI model can provide a thicker layer on top of the user
experience. For example, custom introduction software, or tools,
might be used.
The user driving the introduction might not be at the Introducer at
all. There is nothing in the TTI model to preclude a Petitioner from
requesting an introduction from an Introducer. For example consider a
user on their corporate laptop as the Petitioner. The user might
initiate an introduction to one corporate resource by contacting the
corporate Introducer resource. The Introducer, after authenticating
and authorizing the user and/or laptop, proceeds to introduce the
user's laptop to the resource. The laptop enrolls with the ultimate
resource. Note that this is a transitive operation, the Introducer no
longer needs to be involved during subsequent communications between
the laptop and the resource.
A common scenario may involve the user working from the Registrar to
establish a security association with a new entity. From the user's
perspective this should be the same as initiating an introduction
from the Petitioner, with the only distinction being that data
traffic (which AA infrastructure will be used).
Instantiations of the TTI model should consider the possibilities
that the initiating user may be initiating the process through a TTI
entities other than the Introducer.
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7. Security Considerations
This document discusses models for deploying security
infrastructures. Any resulting TTI protocols will need to carefully
consider how the protocol elements will be protected.
The TTI model assumes that cryptographic material can be produced on
the Petitioner and Registrar, passed over third parties (the
Introducer) and later be be used to secure an enrollment between the
Petitioner and Registrar. It is expected that asymmetric key material
should be used for this portion of the exchange.
This document develops the concept of the Introducer, a new construct
in the normally bi-entity discussion of secure enrollment. This
serves as an alternative to the historically loosely defined
'out-of-band' security measures. The security implications of a
transitive relationship still apply. A compromised introducer could
act to enable enrollment of unexpected elements into a secure domain
in much the same way that a compromised 'out-of-band' mechanism can
be used to subvert a classic enrollment scenario. Care must be taken
not to overlook this when designing the TTI protocols.
The transitive nature of TTI (and any 'out-of-band' data exchange)
means that the Registrar does not know how secure the communication
channel between the Petitioner and the Introducer is.
An example of this is a Petitioner device, a wireless camera, that is
being deployed over a wireless connection. Assuming the camera was
purchased as a consumer product it might supports something like the
Imprinting model above, the first consumer to activate and connect to
it can establish an initial security association with the device.
Possibly by configuring the administrator password on the device (A
more complex scenario may involve an initial introduction to their
home network).
When the consumer then Introduces the device to a service, such as a
video conferencing provider, the provider may be concerned that such
a wireless imprinting provides too great of an attack risk. In such a
situation the Registrar may require that the camera assert an
identity (e.g. a manufacturer ID certificate) which can be verified
by having the Introducer confirm some characteristic of the device
(e.g. inputing a serial number off the device).
In such scenarios the TTI protocol should provide a clear mechanism
by which the Registrar can coach the user through the appropriate
sequence of events without undue confusion. The above examples of TTI
over HTTP provides for this by providing each element in the exchange
a web interface to the user. Similar care should be taken to consider
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when the process is being initiated by automated deployment systems.
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References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Author's Address
Max Pritikin
Cisco Systems, Inc.
EMail: pritikin@cisco.com
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