One document matched: draft-ono-trust-path-discovery-00.txt
Network Working Group K. Ono
Internet-Draft NTT Corporation
Expires: January 11, 2006 H. Schulzrinne
Columbia University
July 10, 2005
Trust Path Discovery
draft-ono-trust-path-discovery-00
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Copyright (C) The Internet Society (2005).
Abstract
Chained or transitive trust can be used to determine whether incoming
communication is likely to be desirable or not. We can build a
chained trust relationship by introducing friends to out friends, for
example. We propose mechanisms for discovering trust paths and
binary responsive trustworthiness. The trust paths are based on a
chain of trust relationships between users, a user and a domain, and
domains. We apply this model to relatively low-value trust
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establishment, suitable for deciding whether to accept communication
requests such as emails, calls, or instant messages from strangers.
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].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Protection Mechanisms for Unsolicited Bulk Messages . . . . 4
3. Our Goal and Approach . . . . . . . . . . . . . . . . . . . 5
4. What Indicates a Trust Relationship? . . . . . . . . . . . . 5
5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1 Generic Requirements . . . . . . . . . . . . . . . . . . . 5
5.2 Security Requirements . . . . . . . . . . . . . . . . . . 6
6. Operations on Trust Paths . . . . . . . . . . . . . . . . . 6
6.1 Generating Trust Paths . . . . . . . . . . . . . . . . . . 6
6.2 Propagating Trust Paths . . . . . . . . . . . . . . . . . 7
6.3 Aggregating Trust Paths . . . . . . . . . . . . . . . . . 8
7. Network Architecture . . . . . . . . . . . . . . . . . . . . 9
7.1 Peering Model . . . . . . . . . . . . . . . . . . . . . . 9
7.2 Client-server Model . . . . . . . . . . . . . . . . . . . 9
8. Message Formats . . . . . . . . . . . . . . . . . . . . . . 10
8.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 12
10. Security Considerations . . . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1 Normative References . . . . . . . . . . . . . . . . . . 12
11.2 Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . 14
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1. Introduction
When dealing with strangers in electronic interactions, establishing
trust is the core challenge as mere authentication does not yield
more than a name or URI. Trust depends on the interaction.
Different levels of trust are required based on the potential impact
and risk of the interaction. We focus here on relatively low-risk
interactions where more potential parties are trustworthy.
There are some methods to determine the trustworthiness of a stranger
in networks. One method is to ask the third party, such as a
reputation system that rates the entity by some numerical scale such
as "trust points". The trust points are based on evaluations that
other anonymous entities have performed, including the experience
from past transactions. Such trust points are often used for
e-commerce, such as auction sites or small sellers aggregated by a
large e-commerce site. Another method is to ask trusted friends for
their evaluation of the stranger. Their opinions are subjective.
We generally trust our own friends more than the unknown third party,
but the circle of our friends is small and none of them may know the
stranger directly. For communication interactions, instead of a
trust scale, only a simple question needs to be answered, namely
whether the person making the trust statement would be willing to
accept communications from the stranger.
The underlying model is that the number of individuals generating
unsolicited bulk mails or spam, spit (Spam over Internet Telephony)
and other undesirable communications is very small compared to the
total population, and thus the likelihood that even a friend-of-a-
friend would know or trust such a spammer is also very low. Also,
communications often occur in subsets of the total human population
that share common values or profession, making it likely that
legitimate strangers are known indirectly.
In this document, we are not too concerned with establishing trust
or bona fides within the spammer community itself. They are
invited to address this problem in appropriate fora.
Instead of determining the reputation of an individual, it is often
sufficient to gauge the trustworthiness of a whole DNS domain. If
the domain has a positive reputation and maintains strict rules for
minting identifiers for its users, as is common for many large
enterprises, we can trust users within the domain without having to
establish trust to each individual.
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This is not likely to be true for high-value and high-risk
transactions such as selling a used car, but as noted above, we
are focusing on lower-risk transactions in this document.
For gathering trustworthy opinions of our friends or community, we
need to find trust paths where we can apply transitive trust. This
document proposes mechanisms for discovering trust paths and binary
responsive trustworthiness. The trust paths are based on a chain of
trust relationship between users, a user and a domain, and domains,
in terms of receiving messages, such as emails, calls, or instant
messages. We believe that it can provide one component of a system
to reduce the amount of unsolicited bulk communication.
2. Protection Mechanisms for Unsolicited Bulk Messages
A variety of mechanisms have been proposed to protect recipient
against undesirable communications:
o Anti-spam mechanisms:
Many existing anti-spam mechanisms rely on filtering messages at
the receiver, either based on content or sender. However,
content-based filtering has limited applicability to emails and
instant messages[3]. Sender-based filtering performs based on
user's name or URI or server's. For example of server-based
filtering, Certified Server Validation [4]uses DNS to provide
indications what kind of assertions a domain offers for its users.
Third-party accreditation services [5] can attest that an SMTP
sender follows certain policies or is otherwise trust worthy.
o Anti-spoofing mechanisms:
Anti-spoofing mechanisms authenticate originators of messages and
calls and nodes that relay such messages or calls. For example,
Sender ID [6] uses DNS to provide the proper IP addressees of an
SMTP sender. DomainKeys [7] uses a secure hash of the content
that a recipient can verify using the domain's public key that is
obtained by DNS.
For calls or instant messages in SIP [8], SIP identity [9]
provides an authentication mechanism of originators. The
authentication server located in the originator's domain
authenticates the originator by HTTP Digest authentication. The
authentication server generates a secure hash of several important
headers and the message body on behalf of the originator. The
recipient can verify the secure hash using the domain's public
key. The destination user authenticates the originator domain by
the verification. As a result, the destination user authenticate
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the originator via the authentication server.
3. Our Goal and Approach
Our goal is to help a recipient of a communication attempt, i.e., an
email receiver, callee or target of an instant message, judge whether
to accept the message from a stranger. This requires a binary
decision of trust. That stranger may then later be added to a
whitelist or blacklist, once the recipient has confirmed that future
communication is desirable or not.
Our approach is to find a chain of trust relationships that exist
among individuals or among domains. If a friend of ours tells us
that the stranger is his/her own friend, we can decide to accept the
communication attempt. If the stranger belongs to a certain trust
domain, we might accept it. We call the chains of trust
relationships, "trust paths".
4. What Indicates a Trust Relationship?
We distinguish trust relationships between users and between domains.
Below, we provide examples of how such trust relationships might be
established.
o Trust relationship between users. e.g., Alice trusts Bob.
* Alice has Bob in her watcher list [10], i.e., she has allowed
Bob to subscribe to her presence information. [Note: This does
not indicate that Bob trusts Alice. In other words, if Bob has
Alice in his buddy list, the entry does not indicate that Bob
trusts Alice.]
* A log contains an email, call, or message from Alice to Bob.
* Bob is listed in Alice's white list.
o Trust relationship between a user and a domain. e.g., Alice trusts
a domain, "A.com".
* Alice has registered her SIP contact address in the domain.
o Trust relationship between domains. e.g., "A.com" trusts "B.com".
* History of transactions.
* Contracts or agreements.
5. Requirements
5.1 Generic Requirements
Below are some of generic requirements for mechanisms to discover
trust paths.
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REQ-GEN-1: The solution SHOULD be simple and scalable because the
number of users and connections of their friends are huge.
REQ-GEN-2: It SHOULD enable entities to obtain the trust path prior
to being needed, for quick determination at starting a
session.
Open Issue: This requirement conflicts with one of the
security requirements caused by privacy-sensitivity of
the trust path information.
REQ-GEN-3: It SHOULD enable entities to set the maximum length of the
trust path. The reliability of trust paths diminishes as
their length increases.
5.2 Security Requirements
Below are security requirements for mechanisms to discover trust
paths.
REQ-SEC-1: The solution MUST enable entities to obtain a trust path
from a trusted and authenticated entity.
REQ-SEC-2: It MUST enable entities to obtain a trust path from a
trusted entity without revealing its content to
unauthorized third parties and while protecting its
integrity against modification by entities not on the
trust path.
REQ-SEC-3: It MUST enable entities to detect forgery of a trust path.
REQ-SEC-4: It SHOULD enable entities to reveal only parts of the
trust path to a recipient, particularly since trust path
information is privacy sensitive.
6. Operations on Trust Paths
Trust paths are chains of trust relationships between entities. Each
entity generates its own trust paths and exchanges them with each
other. In order to quickly discover trust paths, we propose that
entities propagate trust information immediately after generating or
aggregating trust paths.
6.1 Generating Trust Paths
Trust paths are generated from an entity's trust indicators. Trust
paths MUST consist of the following information. The message format
is shown in Figure 3.
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o Entities identity: Originator's URI and friends' URIs. URIs are
for users or domains.
o Binary opinion of trust: A true/false opinion whether the trustee
considers the individual or domain listed a desirable originator
of communication.
o Export policy: The export policy determines whether a recipient
should further propagate this information.
Note: Information propagated over many hops is likely to be
less reliable, so it is desirable to limit the length of the
chain. However, there is no single limit that works in all
circumstances, so we rely on including the number of hops that
a trust tuple has traversed and then having recipients make
decisions on whether to further propagate trust tuples that
have traveled far.
In addition, the trust path MAY contain the weight on the
trustworthiness of individuals or domains.
Recipients of trust paths may weigh them differently depending on
who has forwarded them. However, we decided against including
weights in the trust paths, since this appears difficult to make
commensurate among participants.
6.2 Propagating Trust Paths
Propagating trust paths is somewhat similar to propagating path
vectors in routing protocol such as BGP [11].
Figure 1 depicts an example of trust relationships among five people.
Alice mutually trusts Bob and Dave. Bob mutually trusts Alice and
Carol. Carol mutually trusts Bob and Ed.
Dave (D) <---------------------> Ed (E)
^ ^
| |
| |
* *
Alice (A) <---> Bob (B) <---> Carol (C)
A<->B or A*->B: A mutually trusts B.
Figure 1: Trust relationship and indicators
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1. Alice creates her own trust paths based on her own trust
indicators.
A: {A,B} and {A,D}
2. Alice sends out the trust paths to all entities that Alice
trusts, here Bob and Dave.
A->B: {A,B},{A,D}, A->D: {A,B},{A,D}
Note: Alice can vary her trust paths according to the
recipients.
3. Bob creates his own trust paths based on his own trust indicators
before receiving Alice's trust paths. He accepts her trust paths
because he trusts her. If not, he drops them.
B: {B,A},{B,C}
4. Bob adds his name to the trust path except ones that already
includes his name. He sends the modified trust paths to all
trusting entities except Alice.
B->C: {B,A},{B,C},{B,A,D}
5. Ed sends his trust path to Carol.
E->C: {E,D},{E,C}
6. Since Carol trusts Bob and Ed, she accepts these trust paths.
Although Carol doesn't directly know Alice nor Dave, now she
knows them via Bob and Ed. She receives two paths to Dave, {E,D}
from Ed and {B,A,D} from Bob, and then she selects shorter path,
{E,D}.
C: {C,B},{C,E},{C,B,A},{C,E,D}
6.3 Aggregating Trust Paths
As Carol selects shorter path in the example of Figure 1, an entity
needs to aggregate receiving trust paths and its own trust paths. An
entity SHOULD select the shortest path to the same entity. An entity
SHOULD select the most reliable path to the same entity according to
the local preference based on the weight of the trustworthiness. If
an entity receives different opinions on the same entity, trustworthy
and un-trustworthy, it is safer to prioritise the negative opinion,
un-trustworthy. If an entity receives a trust path that has any
conflict of trustworthiness, the entity MUST drop the trust path.
The number of opinions the same entity MAY be summed up in order to
indicate objectivity. The aggregation mechanism depends on local
policy.
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7. Network Architecture
Trust paths can be processed and propagated in a peering model or
client-server model, both of which we described below.
7.1 Peering Model
In a peering model, all entities support the same operations on trust
paths.
Advantages:
o Fewer types of operation messages:
To exchange trust paths, only a simple message sending mechanism
is needed, in addition to a mechanism to manage peering
connections.
Disadvantages:
o Difficulties in peer authentication:
Each entity needs to authenticate each other when propagating its
trust paths. This requires pre-shared key or self-signed
certificate of all entities.
* Option 1: Pre-shared key between entities.
Requiring a pre-shared key each between two users is not
feasible. However, a group shared-key among all her friends or
parts of them is feasible. The group shared-key could be
published as one of her events. If they directly connect to
each other by using TLS, this option is not appropriate,
because TLS requires their certificates.
* Option 2: Self-signed certificates.
This is feasible since self-signed certificates can be
exchanged among entities via credential servers[12]. As a
result, it requires servers for this purpose.
7.2 Client-server Model
In a client-server model, users connect to opinion server as shown in
Figure 2, which in turn peer with each other. Since multiple users
are likely to share the same server, the number of entities that need
to connect with each other is smaller.
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Opinion Server ------------------- Opinion Server
(op.A.com) (op.B.com)
| |
| |
Alice (A) Bob (B)
Figure 2: Client-server model
Advantages:
o Easier user authentication:
Each entity does not need to authenticate each other when
propagating its trust paths. Entities can use transitive trust
for mutual user authentication. Each user mutually authenticates
the opinion server that he belong to. The opinion server
authenticates the user by using Digest authentication with his
credential such as a password, and the user authenticates the
opinion server by using TLS with its certificate. Each opinion
server also authenticates each other by using TLS with their
certificates.
o Higher service availability:
Trust paths remain available even if a user's end system is
temporarily unreachable.
o Less connections at users' side:
Users need to connect only to their own opinion servers. This
reduces connection cost at user's side, also makes firewall
traversal easier.
Disadvantages:
o Relatively more types of operation messages:
At least, two types of message are mandatory needed. One is for a
user to publish his trust paths to his opinion server, and another
is for a user to query its friend's trust paths to the friend's
opinion server.
* Open Issue: How can a user know his friend's opinion server?
Is this the same as to find his friend's presence server?
To sum up, the client-server model has some advantages. It is
RECOMMENDED to use opinion servers that store trust paths and
propagate them on behalf of users.
8. Message Formats
The message format of trust paths uses the XML [2] data format. The
data size of the trust paths depends on the number of its friends and
the length of the chain. When sending trust paths to neighbors,
entities SHOULD use TCP as a transport protocol.
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8.1 Examples
Below are examples of trust paths, one is generated locally and has
only one hop trust path, and another is propagated and has two hops.
<?xml version="1.0" encoding="UTF-8"?>
<trust-path xmlns="urn:ietf:params:xml:ns:tpf"
entity="sip:alice@A.com">
<path>
<tuple id="sf02sf">
<origin>sip:alice@A.com</origin>
<neighbor>sip:bob@B.com</neighbor>
<neighbor>sip:dave@D.com</neighbor>
<trustworthy>yes</trustworthy>
<export-allowed>yes</export-allowed>
<timestamp>2005-07-04T20:57:29Z</timestamp>
</tuple>
</path>
</trust-path>
Figure 3: An example of one hop trust path
<?xml version="1.0" encoding="UTF-8"?>
<trust-path xmlns="urn:ietf:params:xml:ns:tpf"
entity="sip:bob@B.com">
<path>
<tuple id="is02sf">
<origin>sip:bob@B.com</origin>
<neighbor>sip:alice@A.com</neighbor>
<trustworthy>yes</trustworthy>
<export-allowed>yes</export-allowed>
<timestamp>2005-07-05T20:57:29Z</timestamp>
</tuple>
<tuple id="sf02sf">
<origin>sip:alice@A.com</origin>
<neighbor>sip:bob@B.com</neighbor>
<neighbor>sip:dave@D.com</neighbor>
<trustworthy>yes</trustworthy>
<export-allowed>yes</export-allowed>
<timestamp>2005-07-04T20:57:29Z</timestamp>
</tuple>
</path>
</trust-path>
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Figure 4: An example of two hops trust paths
9. IANA Considerations
TBD
10. Security Considerations
TBD
11. References
11.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[2] Mealling, M., "The IETF XML Registry", RFC 3688, January 2004.
11.2 Informative References
[3] Rosenberg, J., "The Session Initiation Protocol (SIP) and
Spam", draft-ietf-sipping-spam-00 (work in progress),
February 2005.
[4] Crocker, D., "Certified Server Validation (CSV)",
draft-ietf-marid-csv-intro-02 (work in progress),
February 2005.
[5] Leslie, J., "Domain Name Accreditation (DNA)",
draft-ietf-marid-csv-dna-02 (work in progress), February 2005.
[6] Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
draft-lyon-senderid-code-01 (work in progress), May 2005.
[7] Delany, M., "Domain-based Email Authentication Using Public-
Keys Advertised in the DNS (DomainKeys)",
draft-delany-domainkeys-base-02 (work in progress), March 2005.
[8] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[9] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)",
draft-ietf-sip-identity-05 (work in progress), May 2005.
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[10] Rosenberg, J., "A Watcher Information Event Template-Package
for the Session Initiation Protocol (SIP)", RFC 3857,
August 2004.
[11] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
RFC 1771, March 1995.
[12] Jennings, C. and J. Peterson, "Certificate Management Service
for The Session Initiation Protocol (SIP)",
draft-ietf-sipping-certs-01 (work in progress), February 2005.
Authors' Addresses
Kumiko Ono
Network Service Systems Laboratories
NTT Corporation
9-11, Midori-Cho 3-Chome
Musashino-shi, Tokyo 180-8585
Japan
Email: kumiko@cs.columbia.edu, ono.kumiko@lab.ntt.co.jp
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
USA
Email: schulzrinne@cs.columbia.edu
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