Network Working Group                                      I. Symeonidis
Internet-Draft                                  University of Luxembourg
Intended status: Standards Track                            B. Hoeneisen
Expires: May 3, 2020                                   
                                                        October 31, 2019

       Privacy and Security Threat Analysis for Private Messaging


   Modern email and instant messaging applications offer private
   communications between users.  As IM and Email network designs become
   more similar, both share common concerns about security and privacy
   of the information exchanged.  However, the solutions available to
   mitigate these threats and to comply with the requirements may
   differ.  The two communication methods are, in fact, built on
   differing assumptions and technologies.  Assuming a scenario of
   untrusted servers, we analyze threats against message delivery and
   storage, the requirements that these systems need, and the solutions
   that exist in order to help implement secure and private messaging.
   From the discussed technological challenges and requirements, we aim
   to derive an open standard for private messaging.

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Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Terms . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  System Model  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Entities  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Assets and Functional Requirements  . . . . . . . . . . .   5
   3.  Threat Analyses and Requirements  . . . . . . . . . . . . . .   5
     3.1.  Adversarial Model . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Assumptions . . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Security Threats and Requirements . . . . . . . . . . . .   6
       3.3.1.  Spoofing and Entity Authentication  . . . . . . . . .   6
       3.3.2.  Information Disclosure and Confidentiality  . . . . .   7
       3.3.3.  Tampering With Data and Data Authentication . . . . .   7
       3.3.4.  Repudiation and Accountability (Non-Repudiation)  . .   7
       3.3.5.  Elevation of Privilege and Authorization  . . . . . .   8
     3.4.  Privacy Threats and Requirements  . . . . . . . . . . . .   8
       3.4.1.  Identifiability - Anonymity . . . . . . . . . . . . .   8
       3.4.2.  Linkability - Unlinkability . . . . . . . . . . . . .   8
       3.4.3.  Detectability and Observability - Undetectability . .   9
     3.5.  Information Disclosure - Confidentiality  . . . . . . . .   9
     3.6.  Non-repudiation and Deniability . . . . . . . . . . . . .   9
       3.6.1.  Policy Non-compliance and Policy compliance . . . . .  10
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  10
   6.  Future Key Challenges . . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Document Changelog . . . . . . . . . . . . . . . . .  12
   Appendix B.  Open Issues  . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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1.  Introduction

   Private messaging should ensure that, in an exchange of messages
   between (two) peers, no one but the sender and the receiver of the
   communication will be capable of reading the messages exchanged at
   any (current, future or past) time.  Essentially, no one but the
   communicating peers should ever have access to the messages during
   transit such as Telecom, Internet providers, or intermediary parties,
   and storage such as messaging servers.  As private messaging, we are
   referring to Instant Messaging (IM) [RFC2779], such as WhatsApp and
   Signal, and Emailing applications, such as the centralized Protonmail
   and the fully decentralized pEp [I-D.birk-pep].

   The aim of this document is to provide an open standard for private
   messaging requirements, as well as a unified evaluation framework.
   The framework catalogues security and privacy threats and the
   corresponding, to threats, requirements.  IM and Email applications
   have common feature design characteristics and support a common set
   of information assets for transmission during communication between
   peers.  For example, applications for both systems should support
   message exchange of text and files (e.g., attachments) in a private
   messaging manner.

   Despite having common characteristics, IM and Email have network
   design divergences in areas such as responsiveness and synchronicity.
   For example, low-latency and synchronous were the common features for
   instant messaging and high-latency and asynchronous for email.  As IM
   and Email network designs become more similar, approaches to security
   and privacy should be able to address both types of communications.
   Current IM applications tend to be asynchronous, allowing delivery of
   messages when the communicating parties are not at the same time

   Solutions available to implement private messaging in the two types
   of applications may call for different mitigation mechanisms and
   design choices.  For instance, confidentiality can be preserved in
   multiple ways and with various cryptographic primitives.  As design
   choices, it depends on the expected level of protection and the
   background of the user.  For instance, for users whose lives may be
   at stake, such as journalists, whistleblowers, or political
   dissidents, the design choices for requirements and mitigation
   mechanisms can be (and often are) much more advanced than those for
   organizations and general end-users.  Despite this distinction,
   privacy and security on the internet are Human Rights, and easily-
   enabled means to protect these rights need to exist.  But in cases
   where stronger protections are required, usability may come second to
   more robust protection.

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   The objectives of this document are to create an open standard for
   secure messaging requirements.  The open standard for private
   messaging aims to serve as a unified evaluation framework, including
   an adversarial model, threats, and requirements.  With this document,
   we catalogue the threats and requirements for implementing secure and
   private messaging systems.  In this current version, we discuss two
   key design features of IM and Email, message delivery and storage/
   archival.  This draft is an ongoing work in progress, and the list of
   requirements discussed here are not exhaustive.  However, our work
   already shows an emerging and rich set of security and privacy

   Of course, IM additionally can support voice/video calls, which is an
   additional feature/asset under which a threat assessment and
   requirements can be evaluated.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

1.2.  Terms

   The following terms are defined for the scope of this document:

   o  Man-in-the-middle (MITM) attack: cf. [RFC4949], which states: "A
      form of active wiretapping attack in which the attacker intercepts
      and selectively modifies communicated data to masquerade as one or
      more of the entities involved in a communication association."

2.  System Model

2.1.  Entities

   o  Users: The communicating parties who exchange messages, typically
      referred to as senders and receivers.

   o  Messaging operators and network nodes: The communicating service
      providers and network nodes that are responsible for message
      delivery and synchronization.

   o  Third parties: Any other entity who interacts with the messaging

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2.2.  Assets and Functional Requirements

   This section outlines a private messaging system.  It describes the
   functionalities that needs to support and the information that can be
   collected by the system as assets from users.  We follow the
   requirements extracted from real world systems and applications as
   well as from the academic literature for email and instant messaging
   [Unger] [Ermoshina] [Clark].


   o  Content: text, files (e.g., attachments), voice/video

   o  Identities: sender/receiver identity, contact list

   o  Metadata: sender/receiver, timing, frequency, packet size


   o  [Email/IM] Messages: send and receive text + attachments

      *  Peer or group: more than 2 participants communicating

   o  [IM] Voice / video call

   o  [Email/IM] Archive and search: of messages and attachments

   o  [Email/IM] Contacts: synchronisation and matching

   o  [Email/IM] Multi-device support: synchronisation across multiple

3.  Threat Analyses and Requirements

   This section describes a set of possible threats.  Note that
   typically not all threats can be addressed in a system, due to
   conflicting requirements.

3.1.  Adversarial Model

   An adversary is any entity who leverages threats against the
   communication system, whose goal is to gain improper access to the
   message content and users' information.  They can be anyone who is
   involved in communication, such as users of the system, message
   operators, network nodes, or even third parties.

   o  Internal - external: An adversary can seize control of entities
      within the system, such as extracting information from a specific

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      entity or preventing a message from being sent.  An external
      adversary can only compromise the communication channels
      themselves, eavesdropping and tampering with messaging such as
      performing Man-in-the-Middle (MitM) attacks.

   o  Local - global: A local adversary can control one entity that is
      part of a system, while a global adversary can seize control of
      several entities in a system.  A global adversary can also monitor
      and control several parts of the network, granting them the
      ability to correlate network traffic, which is crucial in
      performing timing attacks.

   o  Passive - active: A passive attacker can only eavesdrop and
      extract information, while an active attacker can tamper with the
      messages themselves, such as adding, removing, or even modifying

   Attackers can combine these adversarial properties in a number of
   ways, increasing the effectiveness - and probable success - of their
   attacks.  For instance, an external global passive attacker can
   monitor multiple channels of a system, while an internal local active
   adversary can tamper with the messages of a targeted messaging
   provider [Diaz].

3.2.  Assumptions

   In this current work, we assume that end points are secure such that
   the mobile devices of the users.  Moreover, we assume that an
   adversary cannot break any of the underline cryptographic primitives.

3.3.  Security Threats and Requirements

3.3.1.  Spoofing and Entity Authentication

   Spoofing occurs when an adversary gains improper access to the system
   upon successfully impersonating the profile of a valid user.  The
   adversary may also attempt to send or receive messages on behalf of
   that user.  The threat posed by an adversary's spoofing capabilities
   is typically based on the local control of one entity or a set of
   entities, with each compromised account typically is used to
   communicate with different end-users.  In order to mitigate spoofing
   threats, it is essential to have entity authentication mechanisms in
   place that will verify that a user is the legitimate owner of a
   messaging service account.  The entity authentication mechanisms
   typically rely on the information or physical traits that only the
   valid user should know/possess, such as passwords, valid public keys,
   or biometric data like fingerprints.

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3.3.2.  Information Disclosure and Confidentiality

   An adversary aims to eavesdrop and disclose information about the
   content of a message.  They can attempt to perform a man-in-the-
   middle attack (MitM).  For example, an adversary can attempt to
   position themselves between two communicating parties, such as
   gaining access to the messaging server and remain undetectable while
   collecting information transmitted between the intended users.  The
   threat posed by an adversary can be from local gaining control of one
   point of a communication channel such as an entity or a communication
   link within the network.  The adversarial threat can also be broader
   in scope, such as seizing global control of several entities and
   communication links within the channel.  That grants the adversary
   the ability to correlate and control traffic in order to execute
   timing attacks, even in the end-to-end communication systems [Tor].
   Therefore, confidentiality of messages exchanged within a system
   should be guaranteed with the use of encryption schemes

3.3.3.  Tampering With Data and Data Authentication

   An adversary can also modify the information stored and exchanged
   between the communication entities in the system.  For instance, an
   adversary may attempt to alter an email or an instant message by
   changing the content of them.  As a result, it can be anyone but the
   users who are communicating, such as the message operators, the
   network node, or third parties.  The threat posed by an adversary can
   be in gaining local control of an entity which can alter messages,
   usually resulting in a MitM attack on an encrypted channel.
   Therefore, no honest party should accept a message that was modified
   in transit.  Data authentication of messages exchanged needs to be
   guaranteed, such as with the use of Message Authentication Code (MAC)
   and digital signatures.

3.3.4.  Repudiation and Accountability (Non-Repudiation)

   Adversaries can repudiate, or deny, the status of the message to
   users of the system.  For instance, an adversary may attempt to
   provide inaccurate information about an action performed, such as
   about sending or receiving an email.  An adversary can be anyone who
   is involved in communicating, such as the users of the system, the
   message operators, and the network nodes.  To mitigate repudiation
   threats, accountability, and non-repudiation of actions performed
   must be guaranteed.  Non-repudiation of action can include proof of
   origin, submission, delivery, and receipt between the intended users.
   Non-repudiation can be achieved with the use of cryptographic schemes
   such as digital signatures and audit trails such as timestamps.

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3.3.5.  Elevation of Privilege and Authorization

   An adversary may attempt to elevate privileges aiming to gain access
   to the assets of other users or the resources of the system.  For
   instance, an adversary may attempt to become an administrator of a
   message group or a superuser of the system aiming at retrieving
   users' messages or executing operations as a superuser.  Therefore,
   authorization mechanisms such as access control lists that comply
   with the principle of least privilege for user accounts and processes
   should be applied.

3.4.  Privacy Threats and Requirements

3.4.1.  Identifiability - Anonymity

   Identifiability is defined as the extent to which a specific user can
   be identified from a set of users, which is the identifiability set.
   Identification is the process of linking information to allow the
   inference of a particular user's identity [RFC6973].  An adversary
   can identify a specific user associated with Items of Interest (IOI),
   which include items such as the ID of a subject, a sent message, or
   an action performed.  For instance, an adversary may identify the
   sender of a message by examining the headers of a message exchanged
   within a system.  To mitigate identifiability threats, the anonymity
   of users must be guaranteed.  Anonymity is defined from the attackers
   perspective as the "attacker cannot sufficiently identify the subject
   within a set of subjects, the anonymity set" [Pfitzmann].
   Essentially, in order to make anonymity possible, there always needs
   to be a set of possible users such that for an adversary the
   communicating user is equally likely to be of any other user in the
   set [Diaz].  Thus, an adversary cannot identify who is the sender of
   a message.  Anonymity can be achieved with the use of pseudonyms and
   cryptographic schemes such as anonymous remailers (i.e., mixnets),
   anonymous communications channels (e.g., Tor), and secret sharing.

3.4.2.  Linkability - Unlinkability

   Linkability occurs when an adversary can sufficiently distinguish
   within a given system that two or more IOIs such as subjects (i.e.,
   users), objects (i.e., messages), or actions are related to each
   other [Pfitzmann].  For instance, an adversary may be able to relate
   pseudonyms by analyzing exchanged messages and deduce that the
   pseudonyms belong to one user (though the user may not necessarily be
   identified in this process).  Therefore, unlinkability of IOIs should
   be guaranteed through the use of pseudonyms as well as cryptographic
   schemes such as anonymous credentials.

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3.4.3.  Detectability and Observability - Undetectability

   Detectability occurs when an adversary is able to sufficiently
   distinguish an IOI, such as messages exchanged within the system,
   from random noise [Pfitzmann].  Observability occurs when that
   detectability occurs along with a loss of anonymity for the entities
   within that same system.  An adversary can exploit these states in
   order to infer linkability and possibly identification of users
   within a system.  Therefore, undetectability of IOIs should be
   guaranteed, which also ensures unobservability.  Undetectability for
   an IOI is defined as that "the attacker cannot sufficiently
   distinguish whether it exists or not."  [Pfitzmann].  Undetectability
   can be achieved through the use of cryptographic schemes such as mix-
   nets and obfuscation mechanisms such as the insertion of dummy
   traffic within a system.

3.5.  Information Disclosure - Confidentiality

   Information disclosure - or loss of confidentiality - about users,
   message content, metadata or other information is not only a security
   but also a privacy threat that a communicating system can face.  For
   example, a successful MitM attack can yield metadata that can be used
   to determine with whom a specific user communicates with, and how
   frequently.  To guarantee the confidentiality of messages and prevent
   information disclosure, security measures need to be guaranteed with
   the use of cryptographic schemes such as symmetric, asymmetric or
   homomorphic encryption and secret sharing.

3.6.  Non-repudiation and Deniability

   Non-repudiation can be a threat to a user's privacy for private
   messaging systems, in contrast to security.  As discussed in section
   6.1.4, non-repudiation should be guaranteed for users.  However, non-
   repudiation carries a potential threat vector in itself when it is
   used against a user in certain instances.  For example, whistle-
   blowers may find non-repudiation used against them by adversaries,
   particularly in countries with strict censorship policies and in
   cases where human lives are at stake.  Adversaries in these
   situations may seek to use shreds of evidence collected within a
   communication system to prove to others that a whistle-blowing user
   was the originator of a specific message.  Therefore, plausible
   deniability is essential for these users, to ensure that an adversary
   can neither confirm nor contradict that a specific user sent a
   particular message.  Deniability can be guaranteed through the use of
   cryptographic protocols such as off-the-record messaging.

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3.6.1.  Policy Non-compliance and Policy compliance

   Policy non-compliance can be a threat to the privacy of users in a
   private messaging system.  An adversary, can attempt to process
   information about users unlawfully and not-compliant to regulations.
   It may attempt to collect and process information of users exchanged
   in emails without the users' notification and explicit consent.  That
   can result in unauthorized processing of users information under the
   General Data Protection Regulation resulting in of such as profiling,
   advertisement and censorship.  Therefore, data protection policy
   compliance must be guaranteed.  It can be achieved with auditing such
   as with Data Protection Impact Assessment considering [GDPR].

4.  Security Considerations

   Relevant security considerations are outlined in Section 3.3.

5.  Privacy Considerations

   Relevant privacy considerations are outlined in Section 3.4.

6.  Future Key Challenges

   Reducing metadata leakage and standardization (i.e. prevent further

7.  IANA Considerations

   This document requests no action from IANA.

   [[ RFC Editor: This section may be removed before publication. ]]

8.  Acknowledgments

   The authors would like to thank the following people who have
   provided feedback or significant contributions to the development of
   this document: Athena Schumacher, Claudio Luck, Hernani Marques,
   Kelly Bristol, Krista Bennett, and Nana Karlstetter.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,

9.2.  Informative References

   [Clark]    Clark, J., van Oorschot, P., Ruoti, S., Seamons, K., and
              D. Zappala, "Securing Email", CoRR abs/1804.07706, 2018.

   [Diaz]     Diaz, C., Seys, St., Claessens, J., and B. Preneel,
              "Towards Measuring Anonymity", PET Privacy Enhancing
              Technologies, Second International Workshop, San
              Francisco, CA, USA, April 14-15, 2002, Revised Papers, pp.
              54-68, 2002.

              Ermoshina, K., Musiani, F., and H. Halpin, "End-to-End
              Encrypted Messaging Protocols: An Overview", INSCI 2016:
              pp. 244-254, 2016.

   [GDPR]     "General Data Protection Regulation 2016/680 of the
              European Parliament and of the Council (GDPR).", Official
              Journal of the European Union, L 119/89, 4.5.2016 , April
              2016, <>.

              Marques, H., Luck, C., and B. Hoeneisen, "pretty Easy
              privacy (pEp): Privacy by Default", draft-birk-pep-04
              (work in progress), July 2019.

              Pfitzmann, A. and M. Hansen, "A terminology for talking
              about privacy by data minimization: Anonymity,
              unlinkability, undetectability, unobservability,
              pseudonymity, and identity management", 2010,

   [RFC2779]  Day, M., Aggarwal, S., Mohr, G., and J. Vincent, "Instant
              Messaging / Presence Protocol Requirements", RFC 2779,
              DOI 10.17487/RFC2779, February 2000,

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,

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   [Tor]      Project, T., "One cell is enough to break Tor's
              anonymity", June 2019, <

   [Unger]    Unger, N., Dechand, S., Bonneau, J., Fahl, S., Perl, H.,
              Goldberg, I., and M. Smith, "SoK: Secure Messaging",
              IEEE Proceedings - 2015 IEEE Symposium on Security and
              Privacy, SP 2015, pages 232-249, July 2015,

Appendix A.  Document Changelog

   [[ RFC Editor: This section is to be removed before publication ]]

   o  draft-symeonidis-pearg-private-messaging-threats-00:

      *  Initial version

      *  this document partially replaces draft-symeonidis-medup-

Appendix B.  Open Issues

   [[ RFC Editor: This section should be empty and is to be removed
   before publication ]]

   o  Add more text on Group Messaging requirements

   o  Decide on whether or not "enterprise requirement" will go to this

Authors' Addresses

   Iraklis Symeonidis
   University of Luxembourg
   29, avenue JF Kennedy
   L-1855 Luxembourg


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   Bernie Hoeneisen
   Ucom Standards Track Solutions GmbH
   CH-8046 Zuerich

   Phone: +41 44 500 52 40
   Email: (bernhard.hoeneisen AT

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