[Docs] [txt|pdf] [Tracker] [Email] [Nits]

Versions: 00

Network Working Group                                      I. Symeonidis
Internet-Draft                                  University of Luxembourg
Intended status: Standards Track                            B. Hoeneisen
Expires: January 9, 2020                                         Ucom.ch
                                                           July 08, 2019


   Privacy and Security Threat Analysis and Requirements for Private
                               Messaging
                 draft-symeonidis-medup-requirements-00

Abstract

   [RFC8280] has identified and documented important principles, such as
   Data Minimization, End-to-End, and Interoperability in order to
   enable access to fundamental Human Rights.  While (partial)
   implementations of these concepts are already available, many current
   applications lack Privacy support that the average user can easily
   navigate.  This document covers analysis of threats to privacy and
   security and derives requirements from this threat analysis.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on January 9, 2020.

Copyright Notice

   Copyright (c) 2019 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
   (https://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



Symeonidis & Hoeneisen   Expires January 9, 2020                [Page 1]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 2019


   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terms . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Motivation and Background . . . . . . . . . . . . . . . . . .   4
     2.1.  Objectives  . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Known Implementations . . . . . . . . . . . . . . . . . .   4
       2.2.1.  Pretty Easy Privacy (pEp) . . . . . . . . . . . . . .   4
       2.2.2.  Autocrypt . . . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Focus Areas (Design Challenges):  . . . . . . . . . . . .   6
   3.  System Model  . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Entities  . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Basic Functional Requirements . . . . . . . . . . . . . .   7
   4.  Threat Analyses . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Adversarial model . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Security Threats and Requirements . . . . . . . . . . . .   8
       4.2.1.  Spoofing and Entity Authentication  . . . . . . . . .   8
       4.2.2.  Information Disclosure and Confidentiality  . . . . .   8
       4.2.3.  Tampering With Data and Data Authentication . . . . .   8
       4.2.4.  Repudiation and Accountability (Non-Repudiation)  . .   9
     4.3.  Privacy Threats and Requirements  . . . . . . . . . . . .   9
       4.3.1.  Identifiability - Anonymity . . . . . . . . . . . . .   9
       4.3.2.  Linkability - Unlinkability . . . . . . . . . . . . .  10
       4.3.3.  Detectability and Observability - Undetectability . .  10
     4.4.  Information Disclosure - Confidentiality  . . . . . . . .  10
     4.5.  Non-repudiation and Deniability . . . . . . . . . . . . .  10
   5.  Specific Security and Privacy Requirements  . . . . . . . . .  11
     5.1.  Messages Exchange . . . . . . . . . . . . . . . . . . . .  11
       5.1.1.  Send Message  . . . . . . . . . . . . . . . . . . . .  11
       5.1.2.  Receive Message . . . . . . . . . . . . . . . . . . .  11
     5.2.  Trust Management  . . . . . . . . . . . . . . . . . . . .  12
     5.3.  Key Management  . . . . . . . . . . . . . . . . . . . . .  12
     5.4.  Synchronization Management  . . . . . . . . . . . . . . .  12
     5.5.  Identity Management . . . . . . . . . . . . . . . . . . .  13
     5.6.  User Interface  . . . . . . . . . . . . . . . . . . . . .  13
   6.  Subcases  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Interaction States  . . . . . . . . . . . . . . . . . . .  13
     6.2.  Subcases for Sending Messages . . . . . . . . . . . . . .  14
     6.3.  Subcases for Receiving Messages . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  16
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16



Symeonidis & Hoeneisen   Expires January 9, 2020                [Page 2]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 2019


   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     11.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Appendix A.  Document Changelog . . . . . . . . . . . . . . . . .  18
   Appendix B.  Open Issues  . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   [RFC8280] has identified and documented important principles, such as
   Data Minimization, End-to-End, and Interoperability in order to
   enable access to fundamental Human Rights.  While (partial)
   implementations of these concepts are already available, many current
   applications lack Privacy support that the average user can easily
   navigate.

   In MEDUP these issues are addressed based on Opportunistic Security
   [RFC7435] principles.

   This documents covers analysis of threats to privacy and security and
   derives requirements from this threat analysis.

1.1.  Requirements Language

   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].

1.2.  Terms

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

   o  Trustwords: A scalar-to-word representation of 16-bit numbers (0
      to 65535) to natural language words.  When doing a Handshake,
      peers are shown combined Trustwords of both public keys involved
      to ease the comparison.  [I-D.birk-pep-trustwords]

   o  Trust On First Use (TOFU): cf. [RFC7435], which states: "In a
      protocol, TOFU calls for accepting and storing a public key or
      credential associated with an asserted identity, without
      authenticating that assertion.  Subsequent communication that is
      authenticated using the cached key or credential is secure against
      an MiTM attack, if such an attack did not succeed during the
      vulnerable initial communication."

   o  Man-in-the-middle (MITM) attack: cf. [RFC4949], which states: "A
      form of active wiretapping attack in which the attacker intercepts



Symeonidis & Hoeneisen   Expires January 9, 2020                [Page 3]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 2019


      and selectively modifies communicated data to masquerade as one or
      more of the entities involved in a communication association."

2.  Motivation and Background

2.1.  Objectives

   o  An open standard for secure messaging requirements

   o  Unified evaluation framework: unified goals and threat models

   o  Common pitfalls

   o  Future directions on requirements and technologies

   o  Misleading products on the wild (EFF secure messaging scorecard)

2.2.  Known Implementations

2.2.1.  Pretty Easy Privacy (pEp)

   To achieve privacy of exchanged messages in an opportunistic way
   [RFC7435], the following model (simplified) is proposed by pEp
   (pretty Easy Privacy) [I-D.birk-pep]:



























Symeonidis & Hoeneisen   Expires January 9, 2020                [Page 4]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 2019


             -----                                       -----
             | A |                                       | B |
             -----                                       -----
               |                                           |
   +------------------------+                 +------------------------+
   | auto-generate key pair |                 | auto-generate key pair |
   |    (if no key yet)     |                 |    (if no key yet)     |
   +------------------------+                 +------------------------+
               |                                           |
   +-----------------------+                   +-----------------------+
   | Privacy Status for B: |                   | Privacy Status for A: |
   |     *Unencrypted*     |                   |     *Unencrypted*     |
   +-----------------------+                   +-----------------------+
               |                                           |
               |   A sends message to B (Public Key        |
               |   attached) / optionally signed, but      |
               |               NOT ENCRYPTED               |
               +------------------------------------------>|
               |                                           |
               |                               +-----------------------+
               |                               | Privacy Status for A: |
               |                               |      *Encrypted*      |
               |                               +-----------------------+
               |                                           |
               |      B sends message to A (Public Key     |
               |      attached) / signed and ENCRYPTED     |
               |<------------------------------------------+
               |                                           |
   +-----------------------+                               |
   | Privacy Status for B: |                               |
   |      *Encrypted*      |                               |
   +-----------------------+                               |
               |                                           |
               |   A and B successfully compare their      |
               |   Trustwords over an alternative channel  |
               |   (e.g., phone line)                      |
               |<-- -- -- -- -- -- -- -- -- -- -- -- -- -->|
               |                                           |
   +-----------------------+                   +-----------------------+
   | Privacy Status for B: |                   | Privacy Status for A: |
   |       *Trusted*       |                   |       *Trusted*       |
   +-----------------------+                   +-----------------------+
               |                                           |








Symeonidis & Hoeneisen   Expires January 9, 2020                [Page 5]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 2019


   pEp is intended to solve three problems :

   o  Key management

   o  Trust management

   o  Identity management

   pEp is intended to be used in pre-existing messaging solutions and
   provide Privacy by Default, at a minimum, for message content.  In
   addition, pEp provides technical data protection including metadata
   protection.

   An additional set of use cases applies to enterprise environments
   only.  In some instances, the enterprise may require access to
   message content.  Reasons for this may include the need to conform to
   compliance requirements or virus/malware defense.

2.2.2.  Autocrypt

   Another known approach in this area is Autocrypt.  Compared to pEp
   (cf.  Section 2.2.1) - there are certain differences, for example,
   regarding the prioritization of support for legacy PGP [RFC4880]
   implementations.

   More information on Autocrypt can be found on: https://autocrypt.org/
   background.html

   [[ TODO: Input from autocrypt group ]]

2.3.  Focus Areas (Design Challenges):

   o  Trust establishment: some human interaction

   o  Conversation security: no human interaction

   o  Transport privacy: no human interaction

3.  System Model

3.1.  Entities

   o  Users, sender and receiver(s): 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.



Symeonidis & Hoeneisen   Expires January 9, 2020                [Page 6]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 2019


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

3.2.  Basic Functional Requirements

   This section outlines the functional requirements.  We follow the
   requirements extracted from the literature on private emails and
   instant messaging [Unger] [Ermoshina] [Clark].

   o  Message: send and receive message(s)

   o  Multi-device support: synchronization across multiple devices

   o  Group messaging: communication of more than 2 users

   [[ TODO: Add more text on Group Messaging requirements. ]]

4.  Threat Analyses

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

4.1.  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
      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
      them.



Symeonidis & Hoeneisen   Expires January 9, 2020                [Page 7]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 2019


   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].

4.2.  Security Threats and Requirements

4.2.1.  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.

4.2.2.  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

4.2.3.  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



Symeonidis & Hoeneisen   Expires January 9, 2020                [Page 8]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 2019


   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.

4.2.4.  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.

4.3.  Privacy Threats and Requirements

4.3.1.  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 "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.




Symeonidis & Hoeneisen   Expires January 9, 2020                [Page 9]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 2019


4.3.2.  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.

4.3.3.  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.

4.4.  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.

4.5.  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,



Symeonidis & Hoeneisen   Expires January 9, 2020               [Page 10]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 2019


   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.




   [[ TODO: Describe relation of the above introduced Problem Areas to
   scope of MEDUP ]]

5.  Specific Security and Privacy Requirements

   [[ This section is still in early draft state, to be substantially
   improved in future revisions.  Among other things, there needs to be
   clearer distinction between MEDUP requirements, and those of a
   specific implementation.  ]]

5.1.  Messages Exchange

5.1.1.  Send Message

   o  Send encrypted and signed message to another peer

   o  Send unencrypted and unsigned message to another peer

      Note: Subcases of sending messages are outlined in Section 6.2.

5.1.2.  Receive Message

   o  Receive encrypted and signed message from another peer

   o  Receive encrypted, but unsigned message from another peer

   o  Receive signed, but unencrypted message from another peer

   o  Receive unencrypted and unsigned message from another peer

      Note: Subcases of receiving messages are outlined in Section 6.3.







Symeonidis & Hoeneisen   Expires January 9, 2020               [Page 11]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 2019


5.2.  Trust Management

   o  Trust rating of a peer is updated (locally) when:

      *  Public Key is received the first time

      *  Trustwords have been compared successfully and confirmed by
         user (see above)

      *  Trust of a peer is revoked (cf.  Section 5.3, Key Reset)

   o  Trust of a public key is synchronized among different devices of
      the same user

      Note: Synchronization management (such as the establishment or
      revocation of trust) among a user's own devices is described in
      Section 5.4

5.3.  Key Management

   o  New Key pair is automatically generated at startup if none are
      found.

   o  Public Key is sent to peer via message attachment

   o  Once received, Public Key is stored locally

   o  Key pair is declared invalid and other peers are informed (Key
      Reset)

   o  Public Key is marked invalid after receiving a key reset message

   o  Public Keys of peers are synchronized among a user's devices

   o  Private Keys are synchronized among a user's devices

      Note: Synchronization management (such as establish or revoke
      trust) among a user's own devices is described in Section 5.4

5.4.  Synchronization Management

   A device group is comprised of devices belonging to one user, which
   share the same key pairs in order to synchronize data among them.  In
   a device group, devices of the same user mutually grant
   authentication.

   o  Form a device group of two (yet ungrouped) devices of the same
      user



Symeonidis & Hoeneisen   Expires January 9, 2020               [Page 12]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 2019


   o  Add another device of the same user to existing device group

   o  Leave device group

   o  Remove other device from device group

5.5.  Identity Management

   o  All involved parties share the same identity system

5.6.  User Interface

   [[ TODO ]]

6.  Subcases

6.1.  Interaction States

   The basic model consists of different interaction states:

   1.  Both peers have no public key of each other, no trust possible

   2.  Only one peer has the public key of the other peer, but no trust

   3.  Only one peer has the public key of the other peer and trusts
       that public key

   4.  Both peers have the public key of each other, but no trust

   5.  Both peers have exchanged public keys, but only one peer trusts
       the other peer's public key

   6.  Both peers have exchanged public keys, and both peers trust the
       other's public key

   The following table shows the different interaction states possible:















Symeonidis & Hoeneisen   Expires January 9, 2020               [Page 13]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 2019


   +-------+-----------------+-------------------+---------+-----------+
   | state |  Peer's Public  |   My Public Key   |   Peer  |    Peer   |
   |       |  Key available  | available to Peer | Trusted | trusts me |
   +-------+-----------------+-------------------+---------+-----------+
   | 1.    |        no       |         no        |   N/A   |    N/A    |
   |       |                 |                   |         |           |
   | 2a.   |        no       |        yes        |   N/A   |     no    |
   |       |                 |                   |         |           |
   | 2b.   |       yes       |         no        |    no   |    N/A    |
   |       |                 |                   |         |           |
   | 3a.   |        no       |        yes        |   N/A   |    yes    |
   |       |                 |                   |         |           |
   | 3b.   |       yes       |         no        |   yes   |    N/A    |
   |       |                 |                   |         |           |
   | 4.    |       yes       |        yes        |    no   |     no    |
   |       |                 |                   |         |           |
   | 5a.   |       yes       |        yes        |    no   |    yes    |
   |       |                 |                   |         |           |
   | 5b.   |       yes       |        yes        |   yes   |     no    |
   |       |                 |                   |         |           |
   | 6.    |       yes       |        yes        |   yes   |    yes    |
   +-------+-----------------+-------------------+---------+-----------+

   In the simplified model, only interaction states 1, 2, 4 and 6 are
   depicted.  States 3 and 5 may result from e.g. key mistrust or
   abnormal user behavior.  Interaction states 1, 2 and 4 are part of
   TOFU.  For a better understanding, you may consult the figure in
   Section 2.2.1 above.

   Note: In situations where one peer has multiple key pairs, or group
   conversations are occurring, interaction states become increasingly
   complex.  For now, we will focus on a single bilateral interaction
   between two peers, each possessing a single key pair.

   [[ Note: Future versions of this document will address more complex
   cases ]]

6.2.  Subcases for Sending Messages

   o  If peer's Public Key not available (Interaction States 1, 2a, and
      3a)

      *  Send message Unencrypted (and unsigned)

   o  If peer's Public Key available (Interaction States 2b, 3b, 4, 5a,
      5b, 6)

      *  Send message Encrypted and Signed



Symeonidis & Hoeneisen   Expires January 9, 2020               [Page 14]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 2019


6.3.  Subcases for Receiving Messages

   o  If peer's Public Key not available (Interaction States 1, 2a, and
      3a)

      *  If message is signed

         +  ignore signature

      *  If message is encrypted

         +  decrypt with caution

      *  If message unencrypted

         +  No further processing regarding encryption

   o  If peer's Public Key available or can be retrieved from received
      message (Interaction States 2b, 3b, 4, 5a, 5b, 6)

      *  If message is signed

         +  verify signature

         +  If message is encrypted

            -  Decrypt

         +  If message unencrypted

            -  No further processing regarding encryption

      *  If message unsigned

         +  If message is encrypted

            -  exception

         +  If message unencrypted

            -  No further processing regarding encryption

7.  Security Considerations

   Relevant security considerations are outlined in Section 4.2.






Symeonidis & Hoeneisen   Expires January 9, 2020               [Page 15]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 2019


8.  Privacy Considerations

   Relevant privacy considerations are outlined in Section 4.3.

9.  IANA Considerations

   This document requests no action from IANA.

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

10.  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.

11.  References

11.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <https://www.rfc-editor.org/info/rfc7435>.

11.2.  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.







Symeonidis & Hoeneisen   Expires January 9, 2020               [Page 16]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 2019


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

   [I-D.birk-pep]
              Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp):
              Privacy by Default", draft-birk-pep-03 (work in progress),
              March 2019.

   [I-D.birk-pep-trustwords]
              Birk, V., Marques, H., and B. Hoeneisen, "IANA
              Registration of Trustword Lists: Guide, Template and IANA
              Considerations", draft-birk-pep-trustwords-03 (work in
              progress), March 2019.

   [Pfitzmann]
              Pfitzmann, A. and M. Hansen, "A terminology for talking
              about privacy by data minimization: Anonymity,
              unlinkability, undetectability, unobservability,
              pseudonymity, and identity management", 2010,
              <https://nyuscholars.nyu.edu/en/publications/
              sok-secure-messaging>.

   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880,
              DOI 10.17487/RFC4880, November 2007,
              <https://www.rfc-editor.org/info/rfc4880>.

   [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,
              <https://www.rfc-editor.org/info/rfc6973>.

   [RFC8280]  ten Oever, N. and C. Cath, "Research into Human Rights
              Protocol Considerations", RFC 8280, DOI 10.17487/RFC8280,
              October 2017, <https://www.rfc-editor.org/info/rfc8280>.

   [Tor]      Project, T., "One cell is enough to break Tor's
              anonymity", June 2019, <https://blog.torproject.org/
              one-cell-enough-break-tors-anonymity/>.









Symeonidis & Hoeneisen   Expires January 9, 2020               [Page 17]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 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,
              <https://nyuscholars.nyu.edu/en/publications/
              sok-secure-messaging>.

Appendix A.  Document Changelog

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

   o  draft-symeonidis-medup-requirements-00:

      *  Initial version

Appendix B.  Open Issues

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

   o  Add references to used materials (in particular threat analyses
      part)

   o  Get content from Autocrypt (Section 2.2.2)

   o  Add more text on Group Messaging requirements

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

Authors' Addresses

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

   Email: iraklis.symeonidis@uni.lu
   URI:   https://wwwen.uni.lu/snt/people/iraklis_symeonidis











Symeonidis & Hoeneisen   Expires January 9, 2020               [Page 18]


Internet-Draft   Priv. Messaging: Threat Analysis & Req.       July 2019


   Bernie Hoeneisen
   Ucom Standards Track Solutions GmbH
   CH-8046 Zuerich
   Switzerland

   Phone: +41 44 500 52 40
   Email: bernie@ietf.hoeneisen.ch (bernhard.hoeneisen AT ucom.ch)
   URI:   https://ucom.ch/











































Symeonidis & Hoeneisen   Expires January 9, 2020               [Page 19]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/