Linkability Considered Harmful
draft-johansson-linkability-bad-00

General                                                     L. Johansson
Internet-Draft                                                     SUNET
Intended status: Informational                          January 15, 2014
Expires: July 19, 2014

                     Linkability Considered Harmful
                   draft-johansson-linkability-bad-00

Abstract

   Current debate on pervasive monitoring often focus on passive attacks
   on the protocol and transport layers but even if these issues were
   eliminated through the judicious use of encryption, roughly the same
   information would still be available to an attacker who is able to
   (legally or otherwize) obtain access to linked data sets which are
   being maintained by large content and service providers.

Status of This Memo

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   This Internet-Draft will expire on July 19, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of
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1.  Introduction

   This I-D is submitted as a position paper for the joint IAB/W3C
   STRINT workshop 2014.  The author wishes to call attention to the
   fact that linked data sets are a source of information, sometimes
   every bit as useful as anything that can be gleaned from passive
   monitoring of Internet traffic.  Such data sets are routinely
   generated and maintained by service and content providers and are
   often a source of secondary (or even primary) income for those that
   own and generate them.

   In the current discussion on pervasive monitoring we often overlook
   the fact that even as more encryption is used, making passive attacks
   harder, focus may simply shift to active attacks on owners of linked
   data sets.  We should strike at the root of this problem by making it
   less appealing to maintain these data sets and by offering users a
   measure of control over how their information is used and shared.

2.  A Simple Example

   Service providers (we use this term in a general sense, and not with
   a view to any particular protocols etc) typically manage users and
   billing records.  This leads to a data set being created for every
   user of that service.  Most services employ a simple pattern for user
   enrollment which relies on an email address as a means of
   (supposedly) uniquely identifying the user.  The email address has
   become the defacto user identifier on the Internet.

   When a user pays for the service a pair of linked data sets is
   created: the user data at the service provider is associated (via the
   credit card information) to the user data held by the credit card
   company.  The value of the linked data, aswell as the risk to the
   user, is higher than the value/risk involved in the two data sets
   taken as separate entities.  For instance the linked data says
   something about the buying habits of the user (based on the use of
   the particular credit card) which in itself is valuable information.

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   Linking increases the risk to the user aswell.  With every service
   that stores the users credit card the risk of exposure to active
   attacks increase as events in recent years have made it painfully
   clear.

   If this example seems overly simplified or even naive to bring up,
   consider the simple observation that when we visit a store in the
   physical world we have the ability to "browse", i.e to view and
   select among the offered goods without having to identify ourselves
   or prove our ability to pay for any of the goods in the store.  This
   aspect of the real world has not been translated into the online
   world where prospective customers are routinely fingerprinted and our
   behaviour tracked even when we have shown no intention of engaging in
   a business transaction with the store owner.

   Naturally there must be ways to "conduct business on the Internet",
   but there are ways to enable business without the need for linkable
   attributes.  In fact there are ways to enable business using non-
   linkable attributes in such a way that the risk to business owners
   are reduced.

3.  From Linkable Identifiers to Attributes

   The way to avoid linking is simple (and yet so hard in practice):
   avoid the use of linkable attributes.  In our e-commerce example
   above, the credit card number is a linkable attribute.  However in
   this case the credit card is strictly speaking not needed at the
   service provider.  When the user provides her credit card information
   to the service provider she is actually providing an authorization to
   the service giving the service provider the right to obtain payment
   from the credit card company.

   Instead of using the credit card number as an implicit grant (of a
   right to obtain payment), a token that isn't linkable across
   identifier domains could be used to represent an explicit grant
   issued on behalf of the user by the credit card company to the
   service provider.  This is a simple example of a general pattern:
   instead of using a linkable user identifier, provide access to an
   attribute representing some property of the user that used to grant
   specific access.

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   Some credit card companies have actually taken first steps towards
   this by involving the user directly in a second factor authentication
   (typically to reduce the risk of fraud).  This practice follows a
   model for 3:rd party authentication services (aka identity providers)
   commonly used in the enterprise and R&E community.  Experience from
   the R&E identity federation community shows that access control using
   identity providers and non-linkable pseudonomous identifiers is by no
   means problem free, but can be made to work in many situations.

4.  Incentives

   There are strong incentives for service providers to enrich the value
   of their data set using attribute linking.  The value of the
   attribute naturally increase with the inverse of the size of the set
   of users who share that attribute: the more specific the attribute
   the more valuable it is, because it can be used to identify a user
   with a higher degree of certainty.

   Unfortunately there seem to be few costs associated with keeping
   large linked data sets around - stolen user credentials in the 10s of
   thousands rarely result in more than a brief notice in the news
   anymore.  To date the IETF community have focused on how to avoid the
   use of long-term credentials (passwords) to reduce the effects of
   such attacks.  We need to broaden our scope to find ways to
   disincentivize the (over)use of linkable attributes.

5.  "Conclusions"

   Part of the Internet economy seems to be based on linked data sets
   and linkable attributes.  Changing this will require creating
   negative incentives for service providers, making it less attractive
   to keep data around aswell as establishing technical mechanisms that
   allow service providers access to the attributes they do need in
   order to conduct their business wo having to rely on linkable
   attributes.  Success will depend on carefully engineering the
   negative incentives to match the technical mechanisms in order to
   promote good behaviour.

   Last but not least the Internet community needs to decide what it
   considers acceptable behaviour wrt to attribute linking.

6.  Acknowledgements

   Many thanks to Linus Nordberg for important contributions and lots of
   interesting discussions on this topic.  Many thanks also to Lucy
   Lynch who is the source of much wisdom, the "I'm just browsing"
   response to identification on the web in particular.

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7.  References

Author's Address

   Leif Johansson
   SUNET

   Email: leifj@sunet.se

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