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Keywords: domain, security, messaging, dkim, spf, authentication, reputation
Internet Engineering Task Force (IETF) N. Borenstein
Request for Comments: 7070 Mimecast
Category: Standards Track M. Kucherawy
ISSN: 2070-1721 November 2013
An Architecture for Reputation Reporting
Abstract
This document describes a general architecture for a reputation-based
service, allowing one to request reputation-related data over the
Internet, where "reputation" refers to predictions or expectations
about an entity or an identifier such as a domain name. The document
roughly follows the recommendations of RFC 4101 for describing a
protocol model.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7070.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Borenstein & Kucherawy Standards Track [Page 1]
RFC 7070 Reputation Architecture November 2013
Table of Contents
1. Introduction ....................................................3
2. Overview ........................................................4
3. Related Documents ...............................................5
4. High-Level Architecture .........................................5
4.1. Example of a Reputation Service Being Used .................6
5. Terminology and Definitions .....................................7
5.1. Application ................................................7
5.2. Response Set ...............................................7
5.3. Assertions and Ratings .....................................8
5.4. Reputon ....................................................9
6. Information Represented in the Protocol .........................9
7. Information Flow in the Reputation Query Protocol ..............10
8. Privacy Considerations .........................................10
8.1. Data in Transit ...........................................10
8.2. Aggregation ...............................................11
8.3. Collection of Data ........................................11
8.4. Queries Can Reveal Information ............................11
8.5. Compromised Relationships .................................11
9. Security Considerations ........................................12
9.1. Biased Reputation Agents ..................................12
9.2. Malformed Messages ........................................12
9.3. Further Discussion ........................................13
10. Informative References ........................................13
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RFC 7070 Reputation Architecture November 2013
1. Introduction
Historically, many Internet protocols have operated between
unauthenticated entities. For example, an email message's author
field (From:) [MAIL] can contain any display name or address and is
not verified by the recipient or other agents along the delivery
path. Similarly, a server that sends email using the Simple Mail
Transfer Protocol [SMTP] trusts that the Domain Name System [DNS] has
led it to the intended receiving server. Both kinds of trust are
easily betrayed, opening the operation to subversion of some kind,
which makes spam, phishing, and other attacks even easier than they
would otherwise be.
In recent years, explicit identity authentication mechanisms have
begun to see wider deployment. For example, the DomainKeys
Identified Mail [DKIM] protocol permits associating a validated
identifier to a message. This association is cryptographically
strong, and is an improvement over the prior state of affairs, but it
does not distinguish between identifiers of good actors and bad.
Even when it is possible to validate the domain name in an author
field (e.g., "trustworthy.example.com" in
"john.doe@trustworthy.example.com"), there is no basis for knowing
whether it is associated with a good actor who is worthy of trust.
As a practical matter, both bad actors and good adopt basic
authentication mechanisms like DKIM. In fact, bad actors tend to
adopt them even more rapidly than the good actors do in the hope that
some receivers will confuse identity authentication with identity
assessment. The former merely means that the name is being used by
its owner or their agent, while the latter makes a statement about
the quality of the owner.
With the advent of these authentication protocols, it is possible to
satisfy the requirement for a mechanism by which mutually trusted
parties can exchange assessment information about other actors. For
these purposes, we may usefully define "reputation" as "the
estimation in which an identifiable actor is held, especially by the
community or the Internet public generally". (This is based on the
definition of "reputation" in [RANDOMHOUSE].) We may call an
aggregation of individual assessments "reputation input".
While the need for reputation services has been perhaps especially
clear in the email world, where abuses are commonplace, other
Internet services are coming under attack and may have a similar
need. For instance, a reputation mechanism could be useful in rating
the security of web sites, the quality of service of an Internet
Service Provider (ISP), or an Application Service Provider (ASP).
More generally, there are many different opportunities for use of
reputation services, such as customer satisfaction at e-commerce
Borenstein & Kucherawy Standards Track [Page 3]
RFC 7070 Reputation Architecture November 2013
sites, and even things unrelated to Internet protocols, such as
plumbers, hotels, or books. Just as human beings traditionally rely
on the recommendations of trusted parties in the physical world, so
too they can be expected to make use of such reputation services in a
variety of applications on the Internet.
A full trust architecture encompasses a range of actors and
activities, to enable an end-to-end service for creating, exchanging,
and consuming trust-related information. One component of that is a
query mechanism, to permit retrieval of a reputation. Not all such
reputation services will need to convey the same information. Some
need only to produce a basic rating, while others need to provide
underlying detail. This is akin to the difference between check
approval and a credit report.
An overall reckoning of goodness versus badness can be defined
generically, but specific applications are likely to want to describe
reputations for multiple attributes: an e-commerce site might be
rated on price, speed of delivery, customer service, etc., and might
receive very different ratings on each. Therefore, the architecture
defines a generic query mechanism and basic format for reputation
retrieval, but allows extensions for each application.
Omitted from this architecture is the means by which a reputation-
reporting agent goes about collecting such data and the method for
creating an evaluation. The mechanism defined here merely enables
asking a question and getting an answer; the remainder of an overall
service provided by such a reputation agent is specific to the
implementation of that service and is out of scope here.
2. Overview
The basic premise of this reputation system involves a client that is
seeking to evaluate content based on an identifier associated with
the content, and a reputation service provider that collects,
aggregates, and makes available for consumption, scores based on the
collected data. Typically, client and service operators enter into
some kind of agreement during which some parameters are exchanged,
such as: the location at which the reputation service can be reached,
the nature of the reputation data being offered, possibly some client
authentication details, and the like.
Upon receipt of some content the client operator wishes to evaluate
(an Internet message, for example), the client extracts from the
content one or more identifiers of interest to be evaluated.
Examples of this include the domain name found in the From: field of
a message, or the domain name extracted from a valid DKIM signature.
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RFC 7070 Reputation Architecture November 2013
Next, the goal is to ask the reputation service provider what the
reputation of the extracted identifier is. The query will contain
the identifier to be evaluated and possibly some context-specific
information (such as to establish the context of the query, e.g., an
email message) or client-specific information. The client typically
folds the data in the response into whatever local evaluation logic
it applies to decide what disposition the content deserves.
3. Related Documents
This document presents a high-level view of the reputation
architecture.
For the purposes of sending and receiving reputation information,
[RFC7071] defines a media type for containing responses to reputation
queries, and a serialization format for these data (with examples).
It also creates the registry for specific reputation contexts and the
parameters related to them.
[RFC7072] describes how to construct and issue reputation queries and
replies in the context of this architecture using the HyperText
Transport Protocol (HTTP) as the query protocol.
Finally, [RFC7073] defines (and registers) a first, common,
reputation application, namely the evaluation of portions of an email
message as subjects for reputation queries and replies.
4. High-Level Architecture
This document outlines the reputation query and response mechanism.
It provides the following definitions:
o Vocabulary for the current work and work of this type;
o The types and content of queries that can be supported;
o The extensible range of response information that can be provided;
o Query/response transport conventions.
It provides an extremely simple query/response model that can be
carried over a variety of transports, including the Domain Name
System. (Although not typically thought of as a 'transport', the DNS
provides generic capabilities and can be thought of as a mechanism
for transporting queries and responses that have nothing to do with
Internet addresses, such as is done with a DNS BlockList [DNSBL].)
Each specification for Repute transport is independent of any other
specification.
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RFC 7070 Reputation Architecture November 2013
The precise syntaxes of both the query and response are application
specific. An application within this architecture defines the
parameters available to queries of that type, and it also defines the
data returned in response to any query.
4.1. Example of a Reputation Service Being Used
A reputation mechanism functions as a component of an overall
service. A current example is that of an email system that uses DKIM
[DKIM] to affix a stable identifier to a message and then uses that
as a basis for evaluation:
+-------------+ +------------+
| Sender | | Recipient |
+-------------+ +------------+
| ^
V |
+-------------+ +------------+
| MSA | | MDA |
+-------------+ +------------+
| ^
| |
| +------------+
| | Handling |
| | Filter |
| +------------+
| ^
| |
| +------------+ +------------+
| | Reputation |<=====>| Identifier |
| | Service | | Assessor |
| +------------+ +------------+
| ^
V |
+------------+ Responsible Identifier +------------+
| Identifier |. . . . . . . . . . . . . .>| Identifier |
| Signer | (DKIM) | Verifier |
+------------+ +------------+
| ^
V |
+-------------+ /~~~~~~~~~~\ +------+-----+
| MTA |----->( other MTAs )------>| MTA |
+-------------+ \~~~~~~~~~~/ +------------+
Figure 1: Actors in a Trust Sequence Using DKIM
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RFC 7070 Reputation Architecture November 2013
See [EMAIL-ARCH] for a general description of the Internet messaging
architecture. In particular, the terms Message Submission Agent
(MSA), Message Delivery Agent (MDA), and Message Transfer Agent (MTA)
are described there.
In this figure, the solid lines indicate the flow of a message; the
dotted line indicates transfer of validated identifiers within the
message content; and the double line shows the query and response of
the reputation information.
Here, the DKIM Service provides one or more stable identifiers that
is the basis for the reputation query. On receipt of a message from
an MTA, the DKIM Service provides a (possibly empty) set of validated
identifiers -- domain names, in this case -- that are the subjects of
reputation queries made by the Identifier Assessor. The Identifier
Assessor queries a Reputation Service to determine the reputation of
the provided identifiers, and delivers the identifiers and their
reputations to the Handling Filter. The Handling Filter makes a
decision about whether and how to deliver the message to the
recipient based on these and other inputs about the message, possibly
including evaluation mechanisms in addition to DKIM.
5. Terminology and Definitions
This section defines terms used in the rest of the document.
5.1. Application
An "Application" is a specific context in which reputation queries
are made. Some obvious popular examples include restaurants, movies,
or providers of various services.
Applications have different sets of attributes of interest, and so
the subjects of queries and the resulting responses will vary in
order to describe the reputations of entities in their respective
contexts. For example, the Application "movies" would have a
different set of properties of interest and associated ratings (see
below) from "restaurants". It is therefore necessary for them to be
formally defined.
5.2. Response Set
A "Response Set" is a representation for data that are returned in
response to a reputation query about a particular entity within the
context of an Application. A Response Set will always contain at
least the following components:
o the name of the entity being rated;
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o the Assertion (see Section 5.3);
o the Rating (see Section 5.3).
The full content of the Response Set is specific to the Application;
though all Applications have these few key Response Set fields in
common, some of the reputation data returned in the evaluation of
email senders would be different than that returned about a movie,
restaurant, or baseball player. The specific meaning of a Rating is
also specific to an Application.
A Response Set is declared in a specification document, along with a
symbolic name representing the Application. The specifying documents
will include the details of query parameters and responses particular
to that Application. The symbolic names and corresponding specifying
documents are registered with IANA in the "Reputation Applications"
registry in order to prevent name collisions and provide convenient
references to the documents.
IANA registries are created in [RFC7071].
5.3. Assertions and Ratings
One of the key properties of a Response Set is called an "Assertion".
Assertions are claims made about the subject of a reputation query.
For example, one might assert that a particular restaurant serves
good food. In the context of this architecture, the assertion would
be "serves good food".
Assertions are coupled with a numeric value called a "Rating", which
is an indication of how much the party generating the Response Set
agrees with the assertion being made. Ratings are typically
expressed as a floating point value between 0.0 and 1.0 inclusive,
with the former indicating no support for the assertion and the
latter indicating total agreement with the assertion.
The documents that define future applications will also specify the
type of scale in use when generating ratings, to which all reputation
service providers for that application space must adhere. This will
allow a client to change which reputation service provider is being
queried without having to learn through some out-of-band method what
the new provider's ratings mean. For example, a registration might
state that ratings are linear, which would mean a score of "x" is
twice as strong as a value of "x/2". It also allows easier
aggregation of ratings collected from multiple reputation service
providers.
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5.4. Reputon
A "reputon" is an object that comprises the basic response to a
reputation query. It contains the Response Set relevant to the
subject of the query in a serialized form. Its specific encoding is
left to documents that implement this architecture.
6. Information Represented in the Protocol
Regardless of the transport selected for the interchange, the basic
information to be represented in the protocol is fairly simple, and
normally includes at least the following data:
In the query:
o the subject of the query;
o the name of the reputation context ("Application"; see
Section 5.1);
o optionally, name(s) of the specific reputation assertions of
interest.
Different transports, or different reputation contexts, might need
additional query parameters.
In the response:
o the identity of the entity providing the reputation information;
o the identity of the entity being rated;
o the application context for the query (e.g., email address
evaluation);
o the overall rating score for that entity.
Beyond this, arbitrary amounts of additional information might be
included for specific uses of the service. The entire collection of
data found in the response is the Response Set for that application
and is defined in specifying documents as described above.
For example, a specification might be needed for a reputation
Response Set for an "email-sending-domain"; the Response Set might
include information on how often spam was received from that domain.
[RFC7071] defines a media type and format for reputation data, and
[RFC7072] describes a protocol for exchanging such data.
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7. Information Flow in the Reputation Query Protocol
The basic Response Set could be wrapped into a new MIME media type
[MIME] or a DNS Resource Record (RR), and transported accordingly.
It also could be the integral payload of a purpose-built protocol.
For a basic request/response scenario, one entity (the client) will
ask a second entity (the server) for reputation data about a third
entity (the subject), and the second entity will respond with those
data.
An application might benefit from an extremely lightweight mechanism,
supporting constrained queries and responses, while others might need
to support larger and more complex responses.
8. Privacy Considerations
8.1. Data in Transit
Some reputation exchanges can be sensitive, and should not be shared
publicly. A client making use of this framework is explicitly
revealing that it is interested in particular subjects, and the
server is revealing what its information sources have reported about
those subjects (in the aggregate). In the email context, for
example, a client is revealing from whom it receives email, and the
server is revealing what it (based on its aggregated data) believes
to be true about those subjects.
These can be sensitive things that need to be secured, particularly
when a client is talking to a server outside of its own
administrative domain. Furthermore, certain types of reputation
information are typically perceived as more sensitive than others;
movie ratings, for example, are much less damaging if leaked than a
person's credit rating.
For interchanges that are sensitive to such exposures, it is
imperative to protect the information from unauthorized access and
viewing, and possibly add the capability to do object-level integrity
and origin verification. Not all transport options can be adequately
secured in these ways. In particular, DNS queries and responses are
entirely insecure. Services need to use a transport method that
provides adequate security when privacy-sensitive data are involved.
The architecture described here neither suggests nor precludes any
particular transport mechanism for the data. An HTTP mechanism is
defined in [RFC7072], and email-based mechanisms are also envisioned.
For HTTP, use of HTTP over Transport Layer Security [HTTP-TLS] is
very strongly advised. For email, mechanisms such as OpenPGP
[OPENPGP] and S/MIME [SMIME] are similarly advised.
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RFC 7070 Reputation Architecture November 2013
8.2. Aggregation
The data that are collected as input to a reputation calculation are,
in essence, a statement by one party about the actions or output of
another. What one party says about another is often meant to be kept
in confidence. Accordingly, steps often need to be taken to secure
the submission of these input data to a reputation service provider.
Moreover, although the aggregated reputation is the product provided
by this service, its inadvertent exposure can have undesirable
effects. Just as the collection of data about a subject needs due
consideration to privacy and security, so too does the output and
storage of whatever aggregation the service provider applies.
8.3. Collection of Data
The basic notion of collection and storage of reputation data is
obviously a privacy issue in that the opinions of one party about
another are likely to be sensitive. Inadvertent or unauthorized
exposure of those data can lead to personal or commercial damage.
8.4. Queries Can Reveal Information
When a client asks a service provider about a particular subject, the
service provider can infer the existence of that subject and begin
observing which clients ask about it. This can be an unanticipated
leak of private information.
8.5. Compromised Relationships
Reputation services that limit queries to authorized clients can
cause private information, such as the reputations themselves or the
data used to compute them, to be revealed if the client credentials
are compromised. It is critical to safeguard not only the
interchange of reputation information, and the information once it
has been delivered to the client, but the ability to issue requests
for information as well.
An important consideration here is that compromised credentials are
mainly an exposure of some third party (whose reputation is
improperly revealed) rather than the client or the server.
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RFC 7070 Reputation Architecture November 2013
9. Security Considerations
This document introduces an overall protocol architecture, but no
implementation details. As such, the security considerations
presented here are very high level. The detailed analysis of the
various specific components of the protocol can be found in the
documents that instantiate this architecture.
9.1. Biased Reputation Agents
As with [VBR], an agent seeking to make use of a reputation reporting
service is placing some trust that the service presents an unbiased
"opinion" of the object about which reputation is being returned.
The result of trusting the data is, presumably, to guide action taken
by the reputation client. It follows, then, that bias in the
reputation service can adversely affect the client. Clients
therefore need to be aware of this possibility and the effect it
might have. For example, a biased system returning a reputation
about a DNS domain found in email messages could result in the
admission of spam, phishing, or malware through a mail gateway (by
rating the domain name more favorably than warranted) or could result
in the needless rejection or delay of mail (by rating the domain more
unfavorably than warranted). As a possible mitigation strategy,
clients might seek to interact only with reputation services that
offer some disclosure of the computation methods for the results they
return. Such disclosure and evaluation is beyond the scope of the
present document.
Similarly, a client placing trust in the results returned by such a
service might suffer if the service itself is compromised, returning
biased results under the control of an attacker without the knowledge
of the agency providing the reputation service. This might result
from an attack on the data being returned at the source, or from a
man-in-the-middle attack. Protocols, therefore, need to be designed
so as to be as resilient against such attacks as possible.
9.2. Malformed Messages
Both clients and servers of reputation systems need to be resistant
to attacks that involve malformed messages, deliberate or otherwise.
Malformations can be used to confound clients and servers alike in
terms of identifying the party or parties responsible for the content
under evaluation. This can result in delivery of undesirable or even
dangerous content.
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RFC 7070 Reputation Architecture November 2013
9.3. Further Discussion
Involving a third party (in this case, a reputation service provider)
that can influence the handling of incoming content involves ceding
some amount of control to that third party. Numerous other topics
related to the management, operation, and safe use of reputation
systems can be found in [CONSIDERATIONS].
10. Informative References
[CONSIDERATIONS]
Kucherawy, M., "Operational Considerations Regarding
Reputation Services", Work in Progress, May 2013.
[DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy,
Ed., "DomainKeys Identified Mail (DKIM) Signatures",
STD 76, RFC 6376, September 2011.
[DNS] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[DNSBL] Levine, J., "DNS Blacklists and Whitelists", RFC 5782,
February 2010.
[EMAIL-ARCH]
Crocker, D., "Internet Mail Architecture", RFC 5598,
July 2009.
[HTTP-TLS] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[OPENPGP] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
November 2007.
[RANDOMHOUSE]
"Random House Webster's Dictionary, Revised Edition",
ISBN 978-0-345-44725-8, June 2001.
Borenstein & Kucherawy Standards Track [Page 13]
RFC 7070 Reputation Architecture November 2013
[RFC7071] Borenstein, N. and M. Kucherawy, "A Media Type for
Reputation Interchange", RFC 7071, November 2013.
[RFC7072] Borenstein, N. and M. Kucherawy, "A Reputation Query
Protocol", RFC 7072, November 2013.
[RFC7073] Borenstein, N. and M. Kucherawy, "A Reputation Response
Set for Email Identifiers", RFC 7073, November 2013.
[SMIME] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[VBR] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
Reference", RFC 5518, April 2009.
Authors' Addresses
Nathaniel Borenstein
Mimecast
203 Crescent St., Suite 303
Waltham, MA 02453
USA
Phone: +1 781 996 5340
EMail: nsb@guppylake.com
Murray S. Kucherawy
270 Upland Drive
San Francisco, CA 94127
USA
EMail: superuser@gmail.com
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