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---
coding: utf-8
title: "Motivation and Requirements for Decentralized Usable Privacy"
abbrev: MEDUP Motivation and Requirements
docname: draft-symeonidis-medup-requirements-00
category: std
stand_alone: yes
pi: [toc, sortrefs, symrefs, comments]
author:
{::include ../shared/author_tags/iraklis_symeonidis.mkd}
{::include ../shared/author_tags/bernie_hoeneisen.mkd}
normative:
RFC4949:
RFC7435:
{::include ../shared/references/unger-sok.mkd}
{::include ../shared/references/pfitzmann-terminology-privacy.mkd}
{::include ../shared/references/tor-timing-attacks.mkd}
{::include ../shared/references/diaz-measuring-anonymity.mkd}
informative:
RFC4880:
# RFC6973:
# RFC7258:
# RFC7942:
RFC8280:
I-D.birk-pep:
# I-D.marques-pep-email:
I-D.birk-pep-trustwords:
# I-D.marques-pep-rating:
# I-D.marques-pep-handshake:
# {::include ../shared/references/ed-keysync.mkd}
# {::include ../shared/references/isoc-btn.mkd}
# {::include ../shared/references/implementation-status.mkd}
--- abstract
{{RFC8280}} has identified and documented important principles in such
as Data Minimization, End-to-End and Interoperability in order to
enable access to Human Rights. While (partial) implementations of
these concepts are already available, today's applications widely lack
Privacy support that ordinary users can easily handle.
This document covers analysis of threats and requirements.
--- middle
# Introduction
{{RFC8280}} has identified and documented important principles in such
as Data Minimization, End-to-End and Interoperability in order to
enable access to Human Rights. While (partial) implementations of
these concepts are already available, today's applications widely lack
Privacy support that ordinary users can easily handle.
In MEDUP these issues are addressed based on Opportunistic Security
{{RFC7435}} principles.
This document covers analysis of threats and requirements.
{::include ../shared/text-blocks/key-words-rfc2119.mkd}
{::include ../shared/text-blocks/terms-intro.mkd}
<!-- {::include ../shared/text-blocks/handshake.mkd} -->
{::include ../shared/text-blocks/trustwords.mkd}
{::include ../shared/text-blocks/tofu.mkd}
{::include ../shared/text-blocks/mitm.mkd}
# Motivation and Background
## Objectives
* An open standard for secure messaging requirements
* Unified evaluation framework: unified goals and threat models
* Common pitfalls
* Future directions on requirements and technologies
* Misleading products on the wild (EFF secure messaging scorecard)
## Known Implementations
### Pretty Easy Privacy (pEp) {#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}}:
{::include ../shared/ascii-arts/basic-msg-flow.mkd}
<vspace blankLines="10" />
<!--
pEp is using the paradigm to have online and offline transports.
On offline transport is transporting messages by store and forward. The
connection status of the receiver is not important while sending, and it
may be not available at all. Examples are Internet Mail and SMS.
An online transport is transporting messages synchronously to receivers
if those are online. If receivers are offline, no message can be
transported. The connection status of an receiver is available to the
sender. Examples are Jabber and IRC.
-->
pEp is supposed to solve three problems <!-- for both types of transports -->:
* Key management
* Trust management
* Identity management
pEp is supposed to provide Privacy by Default at least for message
content. In addition, pEp is providing meta data protection. pEp is
meant to be used in already existing messaging solutions.
And pEp is supposed to provide technical data protection by
implementing mix network capabilities.
Additionally, there are use cases for enterprise environments, where
e.g. some instance at the enterprise may need to look into the
messages. Reasons for this include compliance requirements or virus /
malware checking
\[\[ TODO: Decide whether enterprise requirements will be covered
herein \]\]
### Autocrypt
The Autocrypt approach is also a known project following the above
mentioned principles, though - compared to pEp (cf. {{pEp}}) - the
goals slightly differ, for example regarding support of
legacy PGP {{RFC4880}} implementations.
More information on Autocypt can be found on:
https://autocrypt.org/background.html
\[\[ TODO: Input from autocrypt group \]\]
# Basic Functional Requirements
This section outlines the functional requirements. We follow the
requirements extracted from the literature on private emails and
instant messaging {{Unger}}.
* Message: send and receive message(s)
* Multi-device support: synchronisation across multiple devices
* Group messaging: communication of more than 2 users
\[\[ TODO: Add more text on Group Messaging requirements. \]\]
# Threat Analyses
This section describes a set of possible threats. Note that not all threats
can be addressed, due to conflicting requirements.
## Establish Evaluation Criteria for:
* Security and privacy requirements
* Usability (little work on usability and trust establishment)
* Adoption implications
## Focus Areas (Design Challenges):
* Trust establishment: some human interaction
* Conversation security: no human interaction
* Transport privacy: no human interaction
# System Model
## Entities
* Users: Sender and receiver(s)
There are the communicating parties such as the sender and receiver of messages.
* Messaging operators and network nodes
Are the servers and network nodes who are responsible for message delivery and synchronization.
* Third parties
It is any other entity who is interacting with the system.
# Problem Areas
## Security Threats and Requirements
### Spoofing and Entity Authentication
An adversary can spoof and impersonate a profile of a user. It may
attempt to send or receive a message on behalf of a legitimate
user. An adversary can be a user of the system gaining access as an
imposter sending or receiving a message. For example, an adversary can
impersonate a valid sender of a message and send it on their
behalf. The capabilities of an adversary are usually local controlling
one entity or a set of entities, in the sense that each spoofed
identity will be used to communicate with different end users. To
mitigate spoofing threats is essential to have entity authentication
mechanisms safeguarding that a user is the legitimate owner of a
messaging service account. For example, it can prove that he/she knows
something such as passwords, posses something such as public key and
have specific features such as biometrics.
### Information Disclosure and Confidentiality
An adversary aims to retrieve and disclose information about the
content of a message. It can attempt to perform a man-in-the-middle
attack (MitM) eavesdropping and forwarding messages as an intermediary
between the communicating users. For example, an adversary can try to
position itself between two communicating parties such as the
messaging server and remain undetectable collecting information
transmitted to the intended users. The capabilities of an adversary
can be from local controlling one point of the communication channel
such as an entity or a communication link of the network. It can also
be a global adversary controlling several entities and communication
links of the channel, gaining the capability of correlating traffic
such as in timing attacks even for end-to-end communication systems
{{Tor}}. Therefore, confidentiality of messages exchanged in the
system should be guaranteed with the use of encryption schemes such as
symmetric, asymmetric, or homomorphic encryption.
### Tampering With Data and Data Authentication
An adversary can tamper with the messages aiming to modify the
information stored or 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. It can be
anyone but the users who are communicating such as the message
operators, the network node, and third parties. The capabilities of an
adversary can be local controlling an entity that can alter messages
usually performing MitM attack for an encrypted channel. Therefore, no
honest party should accept a message that was modified in transit.
Data authentication of messages needs to be guaranteed such as with
the use of MAC algorithms and digital signatures.
### Repudiation and Accountability (Non-Repudiation)
An adversary can repudiate an email sent or received by providing
falsified information about the status of the message to users of the
system. For instance, an adversary may attempt to state 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 be of origin, submission,
delivery, and receipt providing proof of actions performed to the
intended recipient. It can be achieved with the use of cryptographic
schemes such as digital signatures and audit trails such as
timestamps.
## Privacy Threats and Requirements
### Identifiability -- Anonymity
An adversary can identify a specific user associated with an Items of
Interest (IOI), i.e., an ID of a subject, a message sent, and an
action performed. Identifiability is the state under which a specific
user can be identified from a set of users defined as the
identifiability set. For instance, it may identify the sender of a
message by examining the headers of an email exchanged within a
system. An adversary can be anyone but the users who are communicating
such as the message operators, the network node or third parties. To
mitigate identifiability threats, the anonymity of users must be
guaranteed. It is defined as the "Anonymity of a subject from an
attacker’s perspective means that the attacker cannot sufficiently
identify the subject within a set of subjects, the anonymity set"
{{Pfitzmann}}. Essentially, to enable anonymity, there is always need
to be a set of possible subjects such that for an adversary the
communicating user can be equally likely of any other user in the set
{{Diaz}}. Thus, an adversary cannot deduce who is the originator 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.
### Linkability -- Unlinkability
An adversary can sufficiently distinguish within the system, whether
two or more Items of Interest (IOI) such as subjects, objects,
messages, and actions are linked to the same user. For instance, an
adversary can relate pseudonyms from messages exchanged and deduce
whether it is the same user who sent the messages. It can be anyone
but the users who are communicating such as the message operators, the
network node, or third parties. Therefore, unlinkability of IOIs
should be guaranteed with the use of pseudonyms and cryptographic
schemes such as anonymous credentials.
### Detectability and observatility -- Unditectability
An adversary can sufficiently detect an IOI such as messages exchanged
within the system from random noise. For instance, an adversary can
detect a specific IOI when a user is sending a message from a set of
communicating users. An adversary can be anyone but the users who are
communicating such as the message operators, the network node or third
parties. In contrast to anonymity and unlinkability, where the
relationship from an IOI to a user is preserved, undetectability is
defined as "Undetectability of an item of interest (IOI) from an
attacker’s perspective means that the attacker cannot sufficiently
distinguish whether it exists or not." {{Pfitzmann}}. Undetectability
of IOIs can be guaranteed with the use of cryptographic schemes such
as Mix-nets and obfuscation mechanisms such as dummy traffic.
## Information disclosure -- confidentiality
An adversary can disclose information exchanged within the system
about users. It can perform a MitM aiming to learn the contents of a
message the metadata information such as with whom someone is
communicating with and with which frequency. It can be anyone but the
users who are communicating such as the messaging server, and the
network nodes. The capabilities of an adversary can be local
controlling one entity or channel of the network to a global adversary
which can control several entities and communication
links. Confidentiality of messages, together with security, needs to
be guaranteed with the use of cryptographic operations such as secret
sharing, symmetric, asymmetric, or homomorphic encryption.
## Non-repudiation and deniability
In contrast to security, non-repudiation can be a threat to a user's
privacy for messaging systems. An adversary may attempt to collect
evidence exchanged in the system aiming to prove to others that a
specific user is the originator of a specific message. That can be
problematic for users as whistle-blowers in countries where censorship
is a daily routine and to countries where human life can be at
stake. Therefore plausible deniability, unlike non-repudiation, must
be guaranteed where the system guarantees that an adversary cannot
confirm either contradict that a specific user has sent a
message. Deniability can be guaranteed with the use of cryptographic
schemes such as off-the-record messages.
<!-- =================================================================== -->
# Specific Security and Privacy Requirements
## Messages Exchange
### Send Message
* Send encrypted and signed message to another peer
* Send unencrypted and unsigned message to another peer
Note: Subcases of sending messages are outlined in
{{subcases-for-sending-messages}}.
### Receive Message
* Receive encrypted and signed message from another peer
* Receive encrypted, but not signed message from another peer
* Receive signed, but not encrypted message from another peer
* Receive unencrypted and unsigned message from another peer
Note: Subcases of receiving messages are outlined in
{{subcases-for-receiving-messages}}.
## Trust Management
* 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. {{key-management}}, Key Reset)
* Trust of a public key is synchronized among different devices of the
same user
Note: Synchronization management (such as establish or revoke trust)
among a user's own devices is described in
{{synchronization-management}}
## Key Management
* New Key pair is generated automatically (if none found) at startup
* Public Key is sent to peer by attaching it to messages
* Public Key received by a peer is stored locally
* Key pair is declared invalid and other peers are informed (Key Reset)
* Public Key is marked invalid after receiving a key reset message
* Public Keys are are of peers are synchronized among different
devices of the same user
* Private Key is synchronized among different devices of the same user
Note: Synchronization management (such as establish or revoke trust)
among a user's own devices is described in
{{synchronization-management}}
## Synchronization Management
A device group is a group of devices of the same user that share the
same key pairs in order to synchronize data among them. In a device
group devices of the same user mutually grant authentication.
* Form a device group of two (yet ungrouped) devices of the same user
<!--
Note: Preconditions for forming a device group are outlined in
{{preconditions-for-forming-a-device-group}}.
-->
* Join another device of the same user to existing device group
* Leave device group
* Remove other device from device group
## Identity Management
* All involved parties share the same identity system
## User Interface
* Need for user interaction is kept to the minimum necessary
* The privacy status of a peer is presented to the user by a color rating
* The privacy status of a message is presented to the user by a color rating
* The color rating is defined by a traffic-light semantics
# Subcases
<!-- Do we need this section at all? -->
## Interaction States
The basic model consists of three different interaction states, i.e.:
1. Both peers have got no public key of each other, no trust possible
2. Only one peer has got the public key of the other peer, but no trust
3. Only one peer has got the public key of the other peer and trusts
that public key
4. Both peers have got the public key of each other, but no trust
5. Both peers have got the public key of each other, but only one peer
trusts the other peer's public key
6. Both peers have got the public key of each other and both peers
trust each others public key
The following table shows the different interaction states possible:
| state | Peer's Public Key available | My Public Key available to Peer | Peer Trusted | Peer 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.
Note: As one peer may have several keys or if group conversations are
involved, things will get more complex. For the time being, we focus on
bilateral interactions, whereas group interactions are split up into
several bilateral interactions.
## Subcases for Sending Messages
* If peer's Public Key not available (Interaction States 1, 2a, and 3a)
* Send message Unencrypted (and not Signed)
* If peer's Public Key available (Interaction States 2b, 3b, 4, 5a, 5b, 6)
* Send message Encrypted and Signed
## Subcases for Receiving Messages
* 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 not encrypted
* No further processing regarding encryption
* 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 not encrypted
* No further processing regarding encryption
* If message not signed
* If message is encrypted
* exception
* If message not encrypted
* No further processing regarding encryption
<!-- =================================================================== -->
# Security Considerations
Relevant security considerations are outlined in {{security-threats-and-requirements}}.
# Privacy Considerations
Relevant privacy considerations are outlined in {{privacy-threats-and-requirements}}.
# IANA Considerations
This document requests no action from IANA.
\[\[ RFC Editor: This section may be removed before publication. \]\]
# Acknowledgements
The authors would like to thank the following people who have provided
feedback or significant contributions to the development of this
document: Volker Birk
\[\[ TODO: Forename Surname, Forename2 Surname2, ...\]\]
lorem ipsum
--- back
# Document Changelog
\[\[ RFC Editor: This section is to be removed before publication \]\]
* draft-symeonidis-medup-requirements-00:
* Initial version
# Open Issues
\[\[ RFC Editor: This section should be empty and is to be removed
before publication \]\]
* Add references to used materials (in particular threat analyses part)
* Get content from Autocrypt ({{autocrypt}})
* Add more text on Group Messaging requirements
* Decide on whether or not "enterprise requirement" will go to this
document
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