Network Working Group K. Ono Internet-Draft NTT Corporation Expires: April 25, 2006 H. Schulzrinne Columbia University October 22, 2005 Trust Path Discovery draft-ono-trust-path-discovery-01 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 25, 2006. Copyright Notice 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 Ono & Schulzrinne Expires April 25, 2006 [Page 1] Internet-Draft Trust Path Discovery October 2005 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 4.1 Trust Indicators . . . . . . . . . . . . . . . . . . . . . 5 4.2 Usage of Trust Indicators . . . . . . . . . . . . . . . . 6 5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 6 5.1 Generic Requirements . . . . . . . . . . . . . . . . . . . 6 5.2 Security Requirements . . . . . . . . . . . . . . . . . . 7 6. Operations on Trust Paths . . . . . . . . . . . . . . . . . 7 6.1 Push-base Model vs. Query-base Model . . . . . . . . . . . 7 7. Generating Trust Paths and Destination List . . . . . . . . 8 7.1 Trust Paths . . . . . . . . . . . . . . . . . . . . . . . 8 7.2 Destination List . . . . . . . . . . . . . . . . . . . . . 9 8. Propagating Trust Paths . . . . . . . . . . . . . . . . . . 10 8.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 10 8.2 Network Model . . . . . . . . . . . . . . . . . . . . . . 11 8.3 Propagation of Users' Trust Paths via Opinion Servers . . 13 8.4 Propagation of Domains' Trust Paths . . . . . . . . . . . 13 9. Aggregating Trust Paths . . . . . . . . . . . . . . . . . . 13 10. Opinion Event Package . . . . . . . . . . . . . . . . . . . 13 10.1 Publication and Subscription . . . . . . . . . . . . . . 13 10.2 Destination List . . . . . . . . . . . . . . . . . . . . 14 11. Message Formats of Trust Paths . . . . . . . . . . . . . . . 14 11.1 Examples of Trust Paths . . . . . . . . . . . . . . . . 14 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . 16 13. Security Considerations . . . . . . . . . . . . . . . . . . 16 14. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . 17 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 15.1 Normative References . . . . . . . . . . . . . . . . . . 17 15.2 Informative References . . . . . . . . . . . . . . . . . 18 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 19 Intellectual Property and Copyright Statements . . . . . . . 20 Ono & Schulzrinne Expires April 25, 2006 [Page 2] Internet-Draft Trust Path Discovery October 2005 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. Ono & Schulzrinne Expires April 25, 2006 [Page 3] Internet-Draft Trust Path Discovery October 2005 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 [10]. Sender-based filtering performs based on user's name or URI or server's. For example of server-based filtering, Certified Server Validation [11] uses DNS to provide indications what kind of assertions a domain offers for its users. Third-party accreditation services [12] 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 [13] uses DNS to provide the proper IP addressees of an SMTP sender. DomainKeys [14] 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 [2], SIP identity [15] 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 Ono & Schulzrinne Expires April 25, 2006 [Page 4] Internet-Draft Trust Path Discovery October 2005 the originator via the authentication server. 3. Our Goal and Approach Our goal is to help a recipient of a communication attempt, i.e., a mail transfer agent, 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". Our approach requires that the identities of the friends are authenticated using some of the anti-spoofing mechanisms shown in Section 2. 4. What Indicates a Trust Relationship? 4.1 Trust Indicators 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 [16] with 'active' status, 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 for emails, calls or messages. 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. * Alice has trust relationship to many users belonging to the same domain which maintains strict rules for minting identifiers for its users. [Note: The domain which mints the users' identifiers for Ono & Schulzrinne Expires April 25, 2006 [Page 5] Internet-Draft Trust Path Discovery October 2005 anybody does not deserve to be trusted, even if a large number of users at the same domain entry at Alice's whitelist.] o Trust relationship between domains. e.g., "A.com" trusts "B.com". * History of transactions. * Contracts or agreements. 4.2 Usage of Trust Indicators The trust indicators are used to generate "trust paths" that contains the entries whom the generator trusts. The entries are used in two phases: on exchanging their own trust indicators with others via a network, as the destination list, and on judging whether to accept the message from a stranger at local. In the exchange phase, users and domains limit the entries to peers with having closer relationship than those in the judging phase, because the exchange has potentially more risk (i.e., requiring more efforts or a breach of privacy). For users, the trust paths generated from their watcher list generally show closer relationship than those from their mail log, because disclosing their presence requires the closer relationship than sending only emails. In the judging phase, we need to consider the type of an identifier in a trust path, especially for users. The type of an identifier is either SIP-URI, tel-URI, presence URI, or email address, depending on the trust indicators to generate the trust paths. The appropriate type is determined by the type of application which needs the judgement. For example, an email address in a trust path is useful to judge whether to accept an email from a stranger, but not for a SIP voice call. Therefore, a single user can have multiple chains of trust relationship that unify the type of identifiers, and can apply each chain for an application separately. 5. Requirements 5.1 Generic Requirements Below are some of generic requirements for mechanisms to discover trust paths. REQ-GEN-1: The solution SHOULD be simple and scalable because the number of users and connections of their friends are huge. Ono & Schulzrinne Expires April 25, 2006 [Page 6] Internet-Draft Trust Path Discovery October 2005 REQ-GEN-2: It SHOULD enable entities to obtain the trust path prior to being needed, for quick determination at starting a session. 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 the destination list based on its trust indicators, and recursively exchanges them with the others in the destination list. Although there are two ways of the exchange, push-base model and pull-base (query-model) model, we prefer push-base model where entities propagate trust paths immediately after generating or aggregating trust paths for the following reasons. 6.1 Push-base Model vs. Query-base Model Push-base model enables entities to quickly determine the trustworthiness of the stranger, since they discovered the trust paths in advance. Additionally, this model enables them to free from guessing the appropriate query based on the name of stranger. This model, however, may have some disadvantages of the efficiency in the procedure and privacy, since entities propagate trust paths independently of the stranger's name to be judged. Ono & Schulzrinne Expires April 25, 2006 [Page 7] Internet-Draft Trust Path Discovery October 2005 In query-base model, if the name of the stranger contains a good hint for the query, entities can efficiently query the trustworthiness to limited users or domains. If not, they need to ask all users or domains whom they trust and then the trustees need to ask their trustees. This query procedure needs to be recursive in certain cycles. It would not so efficient. We estimate that the possibility containing some hints at the name of the stranger is very low, because the user part of the name contains few hint, although the domain part of the name often contains some hints. Therefore, query-base model has no advantage of efficiency in the query procedure, comparing to the propagation procedure in push- base model. Trust paths are potentially privacy-sensitive, especially when they contain trust relationships between users. In push-base model, entities need to exclude privacy-sensitive information from their trust paths to be propagated. In query-base model, entities need to respond to queries on the basis of their trust paths as basis. The queries and responses are also potentially privacy-sensitive, if they are related to their privacy-sensitive relationship. Therefore, entities need to exclude privacy-sensitive information from their trust paths to be used as the basis for the responses. In the both model, trust paths can only contain public information. 7. Generating Trust Paths and Destination List Both of trust paths and destination list are generated from an entity's trust indicators described in Section 4.1. A user SHOULD generate a single pair of trust paths and the destination list for its own URI. A user MAY generate multiple pairs of trust paths and destination list separately for some categories, such as private and business, considering with a privacy issue. 7.1 Trust Paths Trust paths MUST consist of the following information. The message format is shown in Section 11. o Owner of trust paths: The owner's URI. This is used as the identifier of a user or a domain for the authentication and authorization in the exchange/propagation phase. o Pairs of entities' identifiers: Originator's URI and the list of neighbors' URIs. A user URI pairs with the list of domains' URIs or the list of users' URIs that have the same type of URI. A domain URI pairs with the list of domains' URIs. Ono & Schulzrinne Expires April 25, 2006 [Page 8] Internet-Draft Trust Path Discovery October 2005 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. o Time stamp: The time in GMT when the trust path is generated. In addition, the trust path MAY contain the expiry time and the weight on the trustworthiness of individuals or domains. Expiry time: the time duration for validity. For example, when trust paths are extracted from watcher list that contains the subscription duration, entities MAY set the expiry time of the trust paths based on the duration. Weight on the trustworthiness: Trustworthiness of individuals or domains. [Note: Recipients of trust paths may weigh them differently depending on who has forwarded them. However, we decided against including weights as a mandatory data in the trust paths, since this appears difficult to make commensurate among participants.] 7.2 Destination List The destination list is a subset of neighbor's URIs in the trust paths. As described in Section 4.2, users exchange only with limited entries extracted from the watcher list. Therefore, the destination list for users contains only SIP-URIs. On the other hand, the destination list for domains contains domain names. Users and domains have to translate from the destination list to the network addresses for the exchange. Users' SIP-URIs are Address-of- Records (AoRs), from which a location service translates to contact addresses that contains FQDNs or IP addresses. However, only SIP proxy/redirect servers can invoke the location service. Therefore, a user needs another service such as a presence service in order to obtain the network addresses corresponding to the friends' AoRs. Since how to obtain the addresses depends on the network model for propagation, we describe the detail in the later discussion on the Ono & Schulzrinne Expires April 25, 2006 [Page 9] Internet-Draft Trust Path Discovery October 2005 network model in Section 8.2. 8. Propagating Trust Paths 8.1 Overview Propagating trust paths is somewhat similar to propagating path vectors in routing protocol such as BGP [17]. 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 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} Ono & Schulzrinne Expires April 25, 2006 [Page 10] Internet-Draft Trust Path Discovery October 2005 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} 8.2 Network Model Trust paths can be processed and propagated in a peering model or client-server model, both of which we described below. 8.2.1 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. * Note: It require all user entities to use presence service in order to obtain the contact addresses of peers on the destination list. Domain entities can obtain the contact addresses of peers by DNS. Disadvantages: o Difficulties in peer authentication for users: Each entity needs to authenticate each other when propagating its trust paths. This requires pre-shared key or self-signed certificate of all user entities that propagate their trust paths. For domain server entities, this is not a disadvantage since they have their public key certificates that can be used for authentication. * 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 [3], 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[18]. As a result, it requires servers for this purpose. Ono & Schulzrinne Expires April 25, 2006 [Page 11] Internet-Draft Trust Path Discovery October 2005 8.2.2 Client-server Model In a client-server model, users connect to opinion servers as shown in Figure 2, which in turn peer with each other. Since a single server is more likely to accommodate many users, the number of user entities that need to connect with each other is smaller. As for domain server entities, they might connect to "root" or "super" opinion servers beyond their own domains, only if the domains form some alliance. They usually have their own opinion servers. 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 public key certificate. Each opinion server also authenticates each other directly or via SIP proxy severs 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 put 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. o Note: In order to obtain the name of his friend's opinion server, it requires that the presence service provides it as an extension element. Ono & Schulzrinne Expires April 25, 2006 [Page 12] Internet-Draft Trust Path Discovery October 2005 To sum up, the client-server model has some advantages for users, while these two models are almost the same for domain servers where they have their own opinion servers. It is RECOMMENDED for entities to use opinion servers that store their trust paths and propagate them on behalf of users and domains. 8.3 Propagation of Users' Trust Paths via Opinion Servers Propagation via opinion servers benefits user entities by being free from the presence status of peer (i.e., online or offline). A user can access to his friends' trust paths at their opinion servers at anytime when the servers run. Although their trust paths themselves are not as frequently changed as their presence status, they often add some trust paths by exchanging them with their friends. Therefore, they need to access their friends' opinion sever to check if the trust paths are updated or not. There are two ways to know the update: one is a periodical access and another is the subscription mechanism [4]. We apply the subscription mechanism to the propagation, since this can provide rapid update notification. We also apply the publication mechanism [5] between users and their opinion servers. The specification is described in Section 10. 8.4 Propagation of Domains' Trust Paths The propagation way of users' trust path is also applicable to that of domain servers. They have their own trust paths and propagate them each other. The difference to the users' way is that domain servers usually have their own opinion servers inside their domains. Therefore, domain servers MAY use other way than the publication mechanism for inner-domain communication, while they MUST use the subscription mechanism for inter-domains communication. 9. 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 MAY 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 prioritize the negative opinion, un-trustworthy. 10. Opinion Event Package 10.1 Publication and Subscription When a user generates or aggregates new trust paths, he MUST send his Ono & Schulzrinne Expires April 25, 2006 [Page 13] Internet-Draft Trust Path Discovery October 2005 own trust paths to his opinion server in the PUBLISH request. When a user needs to know his friends' trust paths, he MUST subscribe each of his friends' opinion servers to know their trust paths by sending the SUBSCRIBE requests. The opinion servers that accept the subscriptions, MUST send the NOTIFY requests containing updated trust paths to the subscribers. For scalability, a user and an opinion server SHOULD support both of the partial publication mechanism and partial notification mechanism, such as [6] [7]. 10.1.1 Package Definition The name of this event package is "opinion". The Content-Type is 'application/tpdf+xml' for the publication and notification of trust paths, and 'application/tpdf-diff+xml for the partial publish and notification. 10.2 Destination List For the authorization of accessing user's trust paths at an opinion server, a user MUST set the destination list as the authorization list for his trust paths. Since the destination list is only updated by the user, the watcher list mechanism for this is not needed. 11. Message Formats of Trust Paths The message format of trust paths uses the XML [8] data format. The data size of the trust paths depends on the number of its friends and the length of the chain. 11.1 Examples of Trust Paths 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. Ono & Schulzrinne Expires April 25, 2006 [Page 14] Internet-Draft Trust Path Discovery October 2005 sip:alice@A.com sip:bob@B.com, sip:dave@D.com yes yes 2005-07-04T20:57:29Z Figure 3: An example of one hop trust path of SIP-URIs alice@mail.A.com bob@mail.B.com, dave@mail.D.com yes yes 2005-07-04T20:57:29Z Figure 4: An example of one hop trust path of email addresses Ono & Schulzrinne Expires April 25, 2006 [Page 15] Internet-Draft Trust Path Discovery October 2005 sip:bob@B.com sip:alice@A.com yes yes 2005-07-05T20:57:29Z sip:alice@A.com sip:bob@B.com, sip:dave@D.com yes yes 2005-07-04T20:57:29Z Figure 5: An example of two hops trust paths of SIP-URIs 12. IANA Considerations This document proposes a new event package, "opinion", and new Content-Types, "application/tpdf+xml" and "application/ tpdf.diff+xml". 13. Security Considerations In the generation phase, some entities generate a wrong trust path that includes a hostile entity as a trustworthy one. Additionally, in the aggregation phase, some entities improperly add or modify trust paths propagated from other entities. Therefore, the traceability of the trust paths SHOULD be supported. Entities SHOULD attach the signature, before the propagation, using XML-signature [9] to the path element of the trust path data generated from their own trust indicators. Since the aggregation of the trust paths makes the signature invalid, entities SHOULD log the trust paths that contains generator's signature before the aggregation. Entities SHOULD check if the signature exists and is valid, when the trust path is propagated by the originator of the trust paths. In the propagation phase, we have to consider impersonating a user or domain entity, as well as an opinion server, by a malicious user or Ono & Schulzrinne Expires April 25, 2006 [Page 16] Internet-Draft Trust Path Discovery October 2005 server. Additionally, we have to consider tampering trust paths by a malicious one. To protect against these attacks, user entities and opinion servers MUST authenticate mutually, and the data integrity of transmitted trust paths MUST be protected. Since they use the publication and subscription mechanisms for the propagation of the trust paths, authentication and data integrity protection follow those of the mechanisms. A user and domain entities, and an opinion server MUST support TLS. A user and domain entities MUST authenticate an opinion server with the public key certificate during the TLS handshake protocol. If there is no direct TLS connection, transitive trust MUST be applied. An opinion server MUST authenticate a user who belongs to the server by HTTP Digest Authentication, whereas using by the SIP identity mechanism for a user who does not belong to. 14. Changes Changes from -00.txt o Added the usage of trust indicators. o Added the comparison between push-base model and query-base mode. o Added text for the destination list. o Applied the publication and subscription mechanism to propagate users' trust paths. o Clarified the difference between the operations of users' trust paths and domains'. o Simplified the data format for trust paths. o Added text in the Security Consideration. 15. References 15.1 Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [2] 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. [3] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [4] Roach, A., "Session Initiation Protocol (SIP)-Specific Event Notification", RFC 3265, June 2002. [5] Niemi, A., "Session Initiation Protocol (SIP) Extension for Event State Publication", RFC 3903, October 2004. Ono & Schulzrinne Expires April 25, 2006 [Page 17] Internet-Draft Trust Path Discovery October 2005 [6] Lonnfors, M., "Publication of Partial Presence Information", draft-ietf-simple-partial-publish-03 (work in progress), July 2005. [7] Lonnfors, M., "Session Initiation Protocol (SIP) extension for Partial Notification of Presence Information", draft-ietf-simple-partial-notify-06 (work in progress), October 2005. [8] Mealling, M., "The IETF XML Registry", RFC 3688, January 2004. [9] Eastlake, D., Reagle, J., and D. Solo, "(Extensible Markup Language) XML-Signature Syntax and Processing", RFC 3275, March 2002. 15.2 Informative References [10] Rosenberg, J., "The Session Initiation Protocol (SIP) and Spam", draft-ietf-sipping-spam-00 (work in progress), February 2005. [11] Crocker, D., "Certified Server Validation (CSV)", draft-ietf-marid-csv-intro-02 (work in progress), February 2005. [12] Leslie, J., "Domain Name Accreditation (DNA)", draft-ietf-marid-csv-dna-02 (work in progress), February 2005. [13] Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail", draft-lyon-senderid-code-01 (work in progress), May 2005. [14] Delany, M., "Domain-based Email Authentication Using Public- Keys Advertised in the DNS (DomainKeys)", draft-delany-domainkeys-base-02 (work in progress), March 2005. [15] 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. [16] Rosenberg, J., "A Watcher Information Event Template-Package for the Session Initiation Protocol (SIP)", RFC 3857, August 2004. [17] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC 1771, March 1995. [18] Jennings, C. and J. Peterson, "Certificate Management Service for The Session Initiation Protocol (SIP)", Ono & Schulzrinne Expires April 25, 2006 [Page 18] Internet-Draft Trust Path Discovery October 2005 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 Ono & Schulzrinne Expires April 25, 2006 [Page 19] Internet-Draft Trust Path Discovery October 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Ono & Schulzrinne Expires April 25, 2006 [Page 20]