Internet DRAFT - draft-pep-handshake
draft-pep-handshake
Network Working Group H. Marques
Internet-Draft B. Hoeneisen
Intended status: Standards Track pEp Foundation
Expires: 19 June 2023 16 December 2022
pretty Easy privacy (pEp): Contact and Channel Authentication through
Handshake
draft-pep-handshake-00
Abstract
In interpersonal messaging, end-to-end encryption means for public
key distribution and verification of its authenticity are needed; the
latter to prevent man-in-the-middle (MITM) attacks.
This document proposes a new method to easily verify a public key is
authentic by a Handshake process that allows users to easily
authenticate their communication channel. The new method is targeted
to Opportunistic Security scenarios and is already implemented in
several applications of pretty Easy privacy (pEp).
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
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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 19 June 2023.
Copyright Notice
Copyright (c) 2022 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
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and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Existing Solutions . . . . . . . . . . . . . . . . . . . 4
3. The pEp Handshake Proposal . . . . . . . . . . . . . . . . . 6
3.1. Verification Process . . . . . . . . . . . . . . . . . . 6
3.1.1. Short, Long and Full Trustword Mapping . . . . . . . 7
3.1.2. Display Modes . . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
4.1. Pre-Generation of all Trustwords . . . . . . . . . . . . 9
4.2. Wrong Comparision of Trustwords . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Implementation Status . . . . . . . . . . . . . . . . . . . . 9
6.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 9
6.2. Current software implementations of pEp . . . . . . . . . 10
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Document Changelog . . . . . . . . . . . . . . . . . 13
Appendix B. Open Issues . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
In interpersonal messaging with end-to-end encryption, means for
public key distribution and verification of its authenticity are
needed.
Examples for key distribution include:
* Exchange public keys out-band before starting encrypted
communications
* Use of centralized public key stores (e.g., OpenPGP Key Servers)
* Ship public keys in-band when communicating
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To prevent man-in-the-middle (MITM) attacks, additionally the
authenticity of a public key needs to be verified. Methods to
authenticate public keys of peers include, e.g., to verify digital
signatures of public keys (which may be signed in a hierarchical or
flat manner, e.g., by a Web of Trust (WoT)), to compare the public
key's fingerprints via a suitable independent channel, or to scan a
QR mapping of the fingerprint (cf. Section 2).
This document proposes a new method to verify the authenticity of
public keys by a Handshake process that allows users to easily verify
their communication channel. Fingerprints of the involved peers are
combined and mapped to (common) Trustwords [I-D.pep-trustwords]. The
successful manual comparison of these Trustwords is used to consider
the communication channel as trusted.
The proposed method is already implemented and used in applications
of pretty Easy privacy (pEp) [I-D.pep-general]. This document is
targeted to applications based on the pEp framework and Opportunistic
Security [RFC7435]. However, it may be also used in other
applications as suitable.
Note: The pEp framework [I-D.pep-general] proposes to automatize the
use of end-to-end encryption for Internet users of email and other
messaging applications and introduces methods to easily allow
authentication.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Terms
The following terms are defined for the scope of this document:
* Trustwords: A representation of 16-bit natural numbers (0 to
65535) as natural language words: For each natural language a
fixed number-to-word map can be defined as convention and
registered with IANA. Trustwords are generated from the combined
public key fingerprints of a both communication partners.
Trustwords are used for verification and establishment of trust
(for the respective keys and communication partners).
[I-D.pep-trustwords]
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* 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."
* 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."
Note: Historically, MITM has stood for '_Man_-in-the-middle'.
However, to indicate that the entity in the middle is not always a
human attacker, MITM can also stand for 'Machine-in-the-middle' or
'Meddler-in-the-middle'.
2. Problem Statement
To secure a communication channel in public key cryptography each
involved peer needs a key pair. Its public key needs to be shared to
other peers by some means. However, the key obtained by the receiver
may have been substituted or tampered with to allow for re-encryption
attacks. To prevent such man-in-the-middle (MITM) attacks, an
important step is to verify the authenticity of a public key
obtained.
2.1. Use Cases
Such a verification process is useful in at least two scenarios:
* Verify channels to peers, e.g., to make sure opportunistically
(in-band) exchanged keys for end-to-end encryption are authentic.
* Verify channels between own devices (in multi-device contexts),
e.g., for the purpose of importing and synchronizing keys among
different devices belonging to the same user (cf.
[I-D.pep-keysync]). This scenario is comparable to Bluetooth
pairing before starting data transfers.
2.2. Existing Solutions
Current methods to authenticate public keys of peers include:
* Digitally signed public keys are verified by a chain of trust.
Two trust models are common in today's implementations.
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- Signing is carried out hierarchically, e.g., in a Public Key
Infrastructure (PKI) [RFC5280], in which case the verification
is based on a chain of trust with a Trust Anchor (TA) at the
root.
- Signing of public keys is done in a flat manner (by a Web of
Trust), e.g., key signing in OpenPGP [RFC4880], where users
sign the public keys of other users. Verification may be based
on transitive trust.
* Peers are expected to directly compare the public key's
fingerprints by any suitable independent channel - e.g, by phone
or with a face-to-face meeting. This method is often used in
OpenPGP [RFC4880] contexts.
* The public keys' fingerprints are mapped into a QR code, which is
expected to be scanned between the peers when they happen to meet
face-to-face. This method is, e.g., used in the chat application
Threema [threema].
* The public keys' fingerprints are mapped into numerical codes
which decimal digits only (so-called "safety numbers"), which
makes the strings the humans need to compare easier in respect to
hexadecimal numbers, but longer and thus nevertheless cumbersome.
This method is, e.g., used in the chat application Signal
[signal].
Some of the methods can even be used in conjunction with Trustwords
[I-D.pep-trustwords] or the PGP Word list [PGP.wl].
None of the existing solutions meet all requirements set up by pEp
[I-D.pep-general], e.g.:
* Easy solution that can be handled easily by ordinary users, also
for users which are physically distant from each other
* In case privacy and security contradict each other, privacy is
always preferred over security (e.g., the Web of Trust contradicts
privacy)
* No central entities to be used
Most of today's systems lack easy ways for users to authenticate
their communication channel. Some methods leak private data (e.g.,
their social graph) or depend on central entities. Thus, none of
today's systems fulfills all of the pEp requirements (cf. above).
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3. The pEp Handshake Proposal
In pretty Easy privacy (pEp), the proposed approach for peers to
authenticate each other is to engage in the pEp Handshake.
In current pEp implementations (cf. Section 6.2), the same kinds of
keys as in OpenPGP are used. Such keys include a fingerprint as
cryptographic hash over the public key. This fingerprint is normally
represented in a hexadecimal form, consisting of ten 4-digit
hexadecimal blocks, e.g.:
8E31 EF52 1D47 5183 3E9D EADC 0FFE E7A5 7E5B AD19
Each block may also be represented in decimal numbers from 0 to 65535
or in other numerical forms, e.g.
* Hexadecimal: 8E31
* Decimal: 36401
* Binary: 1000111000110001
3.1. Verification Process
In the pEp Handshake the fingerprints of any two peers involved are
combined and displayed in form of Trustwords [I-D.pep-trustwords] for
easy comparison by the involved parties.
The default verification process involves three steps:
1. Combining fingerprints by XOR function
Any two peers' fingerprints are combined bit-by-bit using the
Exclusive-OR (XOR) function resulting in the Combined Fingerprint
(CFP).
2. Mapping result to Trustwords:
The CFP is then mapped to 16-bit Trustwords (i.e., every 4-digit
hexadecimal block is mapped to a given Trustword) resulting in
the Trustword Mapping (TWM).
3. Verify Trustwords over independent channel
The resulting Trustwords (TWM) are compared and verified over an
independent channel, e.g., a phone line. If this step was
successful, the channel can be marked as authenticated.
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Note: In prior implementations of pEp the fingerprints of any two
peers were concatenated. While this has the advantage that the own
identity's Trustwords can be printed on a business card (like with
fingerprints) or on contact sites or in signature texts of emails,
this at the same time has the drawback that users might not carefully
compare the words as they start to remember and recognize their
Trustwords in the concatenated mapping. The avoid this undesired
training effect, Trustwords for any peer-to-peer combination shall
(very likely) differ.
3.1.1. Short, Long and Full Trustword Mapping
The more an ordinary user needs to contribute to a process, the less
likely a user will carry out all steps carefully. In particular, it
was observed that a simple (manual) comparison of OpenPGP
fingerprints is rarely carried out to the full extent, i.e., mostly
only parts of the fingerprint are compared, if at all.
For usability reasons and to create incentives for people to actually
carry out a Handshake (while maintaining a certain level of entropy),
pEp allows for different entropy levels, i.e.:
1. Full Trustword Mapping (F-TWM) MUST represent the maximum entropy
achievable by the mapping. This means all Trustwords of a TWM
MUST be displayed and compared.
E.g., the fingerprint
F482 E952 2F48 618B 01BC 31DC 5428 D7FA ACDC 3F13
is mapped to
dog house brother town fat bath school banana kite task
2. Long Trustword Mapping (L-TWM) requires a number of Trustwords
that MUST retain at least 128 bits of entropy. Thus, L-TWM
results into at least eight Trustwords to be compared by the
user.
E.g., the fingerprint
F482 E952 2F48 618B 01BC 31DC 5428 D7FA ACDC [ 3F13 ]
is mapped to
dog house brother town fat bath school banana kite [ remaining
Trustword(s) omitted ]
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3. Short Trustword Mapping (S-TWM) requires a number of Trustwords
that MUST retain at least 64 bits of entropy. Thus, S-TWM
results into at least four Trustwords to be compared by the user.
E.g., the fingerprint
F482 E952 2F48 618B 01BC [ 31DC 5428 D7FA ACDC 3F13 ]
is mapped to
dog house brother town fat [ remaining Trustwords omitted ]
3.1.2. Display Modes
The pEp Handshake has three display modes for the verification
process. All of the following modes MUST be implemented:
1. S-TWM mode (default)
By default the S-TWM SHOULD be displayed to the user for
comparison and verification. An easy way to switch to L-TWM mode
MUST be implemented. An easy way to switch to fingerprint mode
(see below) SHOULD be implemented. An easy way to switch to
F-TWM mode MAY be implemented
2. L-TWM mode
There are situations, where S-TWM is not sufficient (e.g.,
communication parties that are more likely under attack), in
which the L-TWM MAY be displayed to the user by default. An easy
way to switch to F-TWM mode MUST be implemented. An easy way to
switch to fingerprint mode (see below) SHOULD be implemented. An
easy way to switch to S-TWM mode MAY be implemented
3. F-TWM mode
The full F-TWM MUST be implemented too, to address high risk
scenarios. An easy way to switch to fingerprint mode (see below)
SHOULD be implemented. Easy ways to switch to L-TWM or S-TWM
mode MAY be implemented.
4. Fingerprint mode (fallback)
To retain compatibility to existing OpenPGP users (that know
nothing about Trustwords), the fingerprint mode, a fallback to
compare the original fingerprints (usually in hexadecimal form)
MAY be used. An easy way to switch to a least one of the TWM
modes MUST be implemented.
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If the verification process was successful, the user confirms it,
e.g., by setting a check mark. Once the user has confirmed it, the
Privacy Status [I-D.pep-rating] for this channel MUST be updated
accordingly.
4. Security Considerations
4.1. Pre-Generation of all Trustwords
A (global) adversary can pre-generate all Trustwords any two users
expect to compare and try to engage in MITM attacks which fit - it
MUST NOT be assumed public keys and thus fingerprints to be something
to stay secret, especially as in pEp public keys are aggressively
distributed to all peers. Also similar Trustwords can be generated,
which spelled on the phone might sound very similar.
Using time- or memory-intensive hash algorithms to create Trustwords
of any two fingerprints could be used to make extensive pre-
generation attacks more expensive.
4.2. Wrong Comparision of Trustwords
It might happen that users comparing Trustwords-similarly as it
happens to fingerprint comparisons-only compare the first (or, in any
case, to less) Trustwords, thus having way too less entropy in place.
5. IANA Considerations
This document has no actions for IANA.
6. Implementation Status
6.1. Introduction
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
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According to [RFC7942], "[...] this will allow reviewers and working
groups to assign due consideration to documents that have the benefit
of running code, which may serve as evidence of valuable
experimentation and feedback that have made the implemented protocols
more mature. It is up to the individual working groups to use this
information as they see fit."
6.2. Current software implementations of pEp
The following software implementations of the pEp protocols (to
varying degrees) already exists:
* pEp for Outlook as add-on for Microsoft Outlook, release
[SRC.pepforoutlook]
* pEp for iOS (implemented in a new MUA), release [SRC.pepforios]
* pEp for Android (based on a fork of the K9 MUA), release
[SRC.pepforandroid]
* pEp for Thunderbird as an add-on for Thunderbird, release
[SRC.pepforthunderbird]
Note: The former community project Enigmail/pEp as add-on for
Thunderbird was discontinued and replaced by pEp's own add-on for
Thunderbird [SRC.pepforthunderbird] in 2021.
pEp for Android, iOS, Outlook and Thunderbird are provided by pEp
Security, a commercial entity specializing in end-user pEp
implementations.
All software is available as Free and Open Source Software and
published also in source form.
Handshake is already implemented in all platforms listed above.
7. Acknowledgments
Special thanks to Volker Birk and Leon Schumacher who developed the
original concept of the pEp Handshake.
This work was initially created by pEp Foundation, and then reviewed
and extended with funding by the Internet Society's Beyond the Net
Programme on standardizing pEp. [ISOC.bnet]
8. References
8.1. Normative References
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[I-D.pep-general]
Birk, V., Marques, H., and B. Hoeneisen, "pretty Easy
privacy (pEp): Privacy by Default", Work in Progress,
Internet-Draft, draft-pep-general-01, 21 October 2022,
<https://www.ietf.org/archive/id/draft-pep-general-
01.txt>.
[I-D.pep-trustwords]
Hoeneisen, B. and H. Marques, "IANA Registration of
Trustword Lists: Guide, Template and IANA Considerations",
Work in Progress, Internet-Draft, draft-pep-trustwords-00,
16 December 2022, <https://www.ietf.org/archive/id/draft-
pep-trustwords-00.txt>.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informative References
[I-D.pep-keysync]
Birk, V., Hoeneisen, B., and K. Bristol, "pretty Easy
privacy (pEp): Key Synchronization Protocol (KeySync)",
Work in Progress, Internet-Draft, draft-pep-keysync-02, 13
July 2020, <https://www.ietf.org/archive/id/draft-pep-
keysync-02.txt>.
[I-D.pep-rating]
Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp):
Mapping of Privacy Rating", Work in Progress, Internet-
Draft, draft-pep-rating-00, 16 December 2022,
<https://www.ietf.org/archive/id/draft-pep-rating-00.txt>.
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[ISOC.bnet]
Simao, I., "Beyond the Net. 12 Innovative Projects
Selected for Beyond the Net Funding. Implementing Privacy
via Mass Encryption: Standardizing pretty Easy privacy's
protocols", June 2017, <https://www.internetsociety.org/
blog/2017/06/12-innovative-projects-selected-for-beyond-
the-net-funding/>.
[PGP.wl] "PGP word list", November 2017,
<https://en.wikipedia.org/w/
index.php?title=PGP_word_list&oldid=749481933>.
[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>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
[signal] "Signal", n.d., <https://signal.org/>.
[SRC.pepforandroid]
"Source code for pEp for Android", December 2022,
<https://pep-security.lu/gitlab/android/pep>.
[SRC.pepforios]
"Source code for pEp for iOS", December 2022,
<https://pep-security.lu/gitlab/iOS/pep4ios>.
[SRC.pepforoutlook]
"Source code for pEp for Outlook", December 2022,
<https://pep-security.lu/gitlab/win/pEpForOutlook>.
[SRC.pepforthunderbird]
"Source code for pEp for Thunderbird", December 2022,
<https://pep-security.lu/gitlab/thunderbird/
pEpForThunderbird>.
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[threema] "Threema - Seriously secure messaging", n.d.,
<https://threema.ch>.
Appendix A. Document Changelog
[[ RFC Editor: This section is to be removed before publication ]]
* draft-pep-handshake-00:
- Extend Security Considerations
- Minor changes (mostly editorial)
* draft-marques-pep-handshake-05:
- Typos and update references
* draft-marques-pep-handshake-04:
- Updated terms and references
* draft-marques-pep-handshake-03:
- Updated terms and references
* draft-marques-pep-handshake-02:
- Update Sections "Display modes" and "Short, Long and Full
Trustword Mapping"
- Add Privacy and IANA Considerations sections
- Minor editorial changes
* draft-marques-pep-handshake-01:
- Fix references
- Rewrite Sections "Display modes" and "Short, Long and Full
Trustword Mapping"
- Add reason why not to concatenate and map fingerprints instead
- Minor editorial changes
* draft-marques-pep-handshake-00:
- Initial version
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Appendix B. Open Issues
[[ RFC Editor: This section should be empty and is to be removed
before publication ]]
* Add description for further processes to change the trust level,
e.g., to remove trust or even mistrust a peer and alike.
Authors' Addresses
Hernani Marques
pEp Foundation
Oberer Graben 4
CH- 8400 Winterthur
Switzerland
Email: hernani.marques@pep.foundation
URI: https://pep.foundation/
Bernie Hoeneisen
pEp Foundation
Oberer Graben 4
CH- 8400 Winterthur
Switzerland
Email: bernie.hoeneisen@pep.foundation
URI: https://pep.foundation/
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