Network Working Group C. Luck
Internet-Draft pEp Foundation
Intended status: Informational July 05, 2019
Expires: January 6, 2020

pretty Easy privacy (pEp): Progressive Header Disclosure
draft-luck-lamps-pep-header-protection-03

Abstract

Issues with email header protection in S​/​MIME have been recently raised in the IETF LAMPS Working Group. The need for amendments to the existing specification regarding header protection was expressed.

The pretty Easy privacy (pEp) implementations currently use a mechanism quite similar to the currently standardized message wrapping for S​/​MIME. The main difference is that pEp is using PGP/MIME instead, and adds space for carrying public keys next to the protected message.

In LAMPS, it has been expressed that whatever mechanism will be chosen, it should not be limited to S​/​MIME, but also applicable to PGP/MIME.

This document aims to contribute to this discussion and share the pEp implementation experience with email header protection.

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 6, 2020.

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

A range of protocols for the protection of electronic mail (email) exist, which allow to assess the authenticity and integrity of the email headers section, or selected header fields, but limited to a domain-level perspective. Specifically these are DomainKeys Identified Mail (DKIM) [RFC6376] and Sender Policy Framework (SPF) [RFC7208] and Domain-based Message Authentication, Reporting, and Conformance (DMARC) [RFC7489]. These protocols, while essential to responding to a range of attacks on email, do not offer full End-to-End protection to the headers section and are not capable of providing privacy regarding the information represented therein.

The need for means of Data Minimization, which includes data spareness and hiding of all information technically hideable, has grown in importance over the past years.

A standard for End-to-End protection of the email headers section exists for S​/​MIME since version 3.1. (cf. [RFC8551]):

No mechanism for header protection has been standardized for [PGPMIME] yet.

End-to-End protection for the email headers section is currently not widely implemented – neither for messages protected by means of S​/​MIME nor PGP/MIME. At least two variants of header protection are known to be implemented. A recently submitted Internet-Draft [I-D.melnikov-lamps-header-protection] discusses the two variants and the challenges with header protection for S​/​MIME. The two variants are referred to as:

pEp (pretty Easy privacy) [I-D.birk-pep] for email [I-D.marques-pep-email] already implements an option quite similar to Option 2, adapting the S​/​MIME standards to PGP/MIME (cf. Section 2, ff.). Existing implementations of pEp have also added inbound support for “Memory Hole” referred to above as Option 1, thus being able to study the differences and the implementator’s challenges.

pEp now also supports the “forwarded=no” attribute proposed in [I-D.melnikov-lamps-header-protection], and further described in Section 2.3.

Interoperability and implementation symmetry between PGP/MIME and S​/​MIME is planned by pEp, but still in an early stage of development.

This document describes Progressive Header Disclosure as implemented in the “pEp message format version 2”. This format inherently offers header protection as a side effect, but is only intended for use only with signed-and-encrypted messages. The feature of protecting headers may be used independently by mail user agents otherwise not wanting to conform to pEp standards (Section 2, ff.).

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:

2. Message Format for Progressive Header Disclosure

2.1. Compatibility

The pEp message format version 2 is designed such that a receiving Mail User Agent (MUA), which is OpenPGP-compliant but not pEp-compliant, still has built-in capability to properly verify the integrity of the mail, decode it and display all information of the original mail to the user. The recovered protected message is selfsufficiently described, including all protected header fields.

The pEp message format version 2 (as used by all the various pEp implementations, cf. Section 9) is similar to what is standardized for S​/​MIME in [RFC8551]:

2.2. Inner Message

Note: this section is for your information only. It does not add requirements for the header protection feature to work.

The pEp message format requires the innermost protected message to follow a fixed MIME structure and to consist of exactly one human-readable message which is represented in plain or HTML format. Both plain and html entities MUST represent the same message to the user. Any attachment to the message must be laid out in a flat list. No additional multipart entities are allowed in the pEp message.

These restrictions permit to build mail user agents which are immune to the EFAIL attacks.

This message is herein further referred to as the “pEp inner message”.

A mail user agent wanting to follow this standard, SHOULD transform any “original message” into a “pEp inner message” for safe representation on the receiving side.

2.3. Content-Type Parameter “forwarded”

One caveat of the design is that the user interaction with message/rfc822 entities varies considerably across different mail user agents. No standard is currently available which enables MUAs to reliably determine whenever a nested message/rfc822 entity is meant to blend the containing message, or if it was effectively intended to be forwarded as a file document. pEp currently implements the proposal described by [I-D.melnikov-lamps-header-protection], 3.2, which defines a new Content-Type header field parameter with name “forwarded”, for the MUA to distinguish between a forwarded message and a nested message for the purpose of header protection, i.e., using “forwarded=no”.

2.4. Outer Message

With pEp message format version 2, the pEp standardized message is equally wrapped in a message/rfc822 entity, but this time being in turn wrapped in a multipart/mixed entity. The purpose of the additional nesting is to allow for public keys of the sender to be stored alongside the original message while being protected by the same mechanism.

For the case of PGP/MIME, the currently only implemented MIME encryption protocol implemented in pEp, the top-level entity called the “outer message” MUST contain:

Notes on the current pEp client implementations:

This is an example of the top-level MIME entity, before being encrypted and signed:

    MIME-Version: 1.0
    Content-Type: multipart/mixed;
                  boundary="6b8b4567327b23c6643c986966334873"


    --6b8b4567327b23c6643c986966334873
    Content-Type: message/rfc822; forwarded="no"

    From: John Doe <jdoe@machine.example>
    To: Mary Smith <mary@example.net>
    Subject: Example
    Date: Fri, 30 Jun 2018 09:55:06 +0200
    Message-ID: <05d0526e-41c4-11e9-8828@pretty.Easy.privacy>
    X-Pep-Version: 2.0
    MIME-Version: 1.0
    Content-Type: multipart/alternative;
                  boundary="29fe9d2b2d7f6a703c1bffc47c162a8c"

    --29fe9d2b2d7f6a703c1bffc47c162a8c
    Content-Type: text/plain; charset="utf-8"
    Content-Transfer-Encoding: quoted-printable
    Content-Disposition: inline; filename="msg.txt"

    p=E2=89=A1p for Privacy by Default.
    -- =20
    Sent from my p=E2=89=A1p for Android.

    --29fe9d2b2d7f6a703c1bffc47c162a8c
    Content-Type: text/html; charset="utf-8"
    Content-Transfer-Encoding: quoted-printable

    p=E2=89=A1p for Privacy by Default.<br>
    -- <br>
    Sent from my p=E2=89=A1p for Android.<br>

    --29fe9d2b2d7f6a703c1bffc47c162a8c--
    --6b8b4567327b23c6643c986966334873
    Content-Type: application/pgp-keys
    Content-Disposition: attachment; filename="pEpkey.asc"

    -----BEGIN PGP PUBLIC KEY BLOCK-----

    ...
    -----END PGP PUBLIC KEY BLOCK-----

    --6b8b4567327b23c6643c986966334873--

2.5. Transport Message

In pEp message format 2 the “outer message” consists of a full RFC822 message with body and a minimal set of header fields, just those necessary to conform to MIME multipart standards.

The “outer message” should be encrypted and carry a signature according to the MIME encryption standards. The resulting message is the transport message which a root entity of type multipart/encrypted.

A minimal set of header fields should be set on the “transport message”, as to permit delivery, without disclosing private information.

The structure of the transport message may be altered in-transit, e.g. through mailing list agents, or inspection gateways.

Signing and encrypting a message with MIME Security with [PGPMIME], yields a message with the following complete MIME structure, seen across the encryption layer:

    = multipart/encrypted; protocol="application/pgp-encrypted";
      + application/pgp-encrypted [ Version: 1 ]
      + application/octet-stream; name="msg.asc"
        {
          Content-Disposition: inline; filename="msg.asc";
        }
           |
          [ opaque encrypted structure ]
           |
           { minimal headers }
           + multipart/mixed
             + message/rfc822; forwarded="no";
                  |
                 { protected message headers }
                 + multipart/mixed
                   + multipart/alternate
                     + text/plain
                     + text/html
                   + application/octet-stream [ attachmet_1 ]
                   + application/octet-stream [ attachmet_2 ]
             + application/pgp-keys

The header fields of the sub-part of type application/octet-stream SHOULD be modified to ensure that:

2.6. S/MIME Compatibility

Interoperability and implementation symmetry between PGP/MIME and S​/​MIME is on the roadmap of pEp.

3. Candidate Header Fields for Header Protection

By default, all headers of the original message SHOULD be wrapped with the original message, with one exception:

4. Stub Outside Headers

The outer message requires a minimal set of headers to be in place for being eligible for transport. This includes the “From”, “To”, “Cc”, “Bcc”, “Subject” and “Message-ID” header fields. The protocol hereby defined also depends on the “MIME-Version”, “Content-Type”, “Content-Disposition” and eventually the “Content-Transport-Encoding” header field to be present.

Submission and forwarding based on SMTP carries “from” and “receivers” information out-of-band, so that the “From” and “To” header fields are not strictly necessary. Nevertheless, “From”, “Date”, and at least one destination header field is mandatory as per [RFC5322]. They SHOULD be conserved for reliability.

The following header fields should contain a verbatim copy of the header fields of the inner message:

The entries with an asterisk mark (*) should only be set if also present in the original message.

Clients which follow pEp standards MUST set the header field value for “Subject” to “=?utf-8?Q?p=E2=89=A1p?=” or “pEp”. Clients which do not adhere to all pEp standards should set the header field value of “Subject” to a descriptive stub value. An example used in practice is

The following header fields MUST be initialized with proper values according to the MIME standards:

5. Processing Incoming Email under Progressive Header Disclosure

[[ TODO ]]

5.1. Processing of Signed-only Email

pEp either engages in a signed-and-encrypted communication or in an unsigned plaintext communication. Inbound signatures attached to plaintext messages are duly verified but cannot enhance the perceived quality of the message in the user interface (while an invalid signature degrades the perception) [I-D.birk-pep].

5.2. Incoming Filter Processing

The Mail User Agent may implement outgoing filtering of mails, which may veto, alter, redirect or replicate the messages. The functionality may be implemented on the mailbox server and be configurable through a well-known protocol, e.g., by means of The Sieve Mail-Filtering Language [RFC5490], or be implemented client-side, or in a combination of both.

A mailbox server, which is required to process the full range of possible filters, is requiring plaintext access to the header fields.

In an End-to-End-encryption context, which pEp enforces by default, upon first reception of the message the mailbox server is limited to see the transport- relevant headers of the outer wrapper message. A pEp client configured to trust the server (“trusted server” setting [I-D.marques-pep-email]) will later download the encrypted message, decrypt it and replace the copy on the server by the decrypted copy.

5.2.1. Staged Filtering of Inbound Messages

Inbound messages are expected to be delivered to the inbox while still being encrypted. At this point in time, server-side filtering can only evaluate the unprotected header fields in the wrapper message.

In an End-to-End-encryption context, which pEp enforces by default, the mailbox server is limited to see the transport-relevant headers of the outer wrapper message only upon first delivery. A pEp client configured to trust the server (“trusted server” setting [I-D.marques-pep-email]) will eventually download the encrypted message, decrypt it locally and replace the copy on the server by the decrypted copy. Server-side message filters SHOULD be able to detect such post-processed messages, and apply the pending filters. The client SHOULD easily reflect the post-filtered messages in the user interface.

5.3. Outgoing Filter Processing

The Mail User Agent may implement outgoing filtering of emails, which may veto, alter or replicate the email. They may also hint the MUA to store a copy in an alternate “Sent” folder.

Filters which veto the sending or do alter the mail or replicate it (e.g., mass-mail generators) SHOULD be completed prior to applying protection, and thus also prior to applying header protection. Redirection to alternate “Sent” folders MUST NOT be decided prior to applying protection, but MUAs MAY abide from this restriction if they implement the “trusted server” option and the option is selected for the specific mailbox server; in this case, MUAs MUST NOT allow to redirect a message to an untrusted server by these rules, to prevent information leakage to the untrusted server.

[[ TODO: Mention implicit filter for minimal color-rating for message replication. ]]

[[ TODO: How to produce key-export-mails manually this way? That is, what about non-pEp-mode? ]]

6. Security Considerations

[[ TODO ]]

7. Privacy Considerations

[[ TODO ]]

8. IANA Considerations

This document has no actions for IANA.

9. Implementation Status

9.1. 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.

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.”

9.2. Current software implementing pEp

The following software implementing the pEp protocols (to varying degrees) already exists:

pEp for Android, iOS and Outlook are provided by pEp Security, a commercial entity specializing in end-user software implementing pEp while Enigmail/pEp is pursued as community project, supported by the pEp Foundation.

All software is available as Free Software and published also in source form.

10. Acknowledgements

The authors would like to thank the following people who have provided significant contributions or feedback for the development of this document: Krista Bennett, Volker Birk, Bernie Hoeneisen and Hernani Marques.

11. References

11.1. Normative References

[I-D.birk-pep] Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp): Privacy by Default", Internet-Draft draft-birk-pep-03, March 2019.
[I-D.marques-pep-email] Marques, H., "pretty Easy privacy (pEp): Email Formats and Protocols", Internet-Draft draft-marques-pep-email-02, October 2018.
[I-D.melnikov-lamps-header-protection] Melnikov, A., "Considerations for protecting Email header with S​/​MIME", Internet-Draft draft-melnikov-lamps-header-protection-00, October 2018.
[PGPMIME] Elkins, M., Del Torto, D., Levien, R. and T. Roessler, "MIME Security with OpenPGP", RFC 3156, DOI 10.17487/RFC3156, August 2001.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, DOI 10.17487/RFC2046, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D. and R. Thayer, "OpenPGP Message Format", RFC 4880, DOI 10.17487/RFC4880, November 2007.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007.
[RFC5322] Resnick, P., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008.
[RFC5490] Melnikov, A., "The Sieve Mail-Filtering Language -- Extensions for Checking Mailbox Status and Accessing Mailbox Metadata", RFC 5490, DOI 10.17487/RFC5490, March 2009.
[RFC6376] Crocker, D., Hansen, T. and M. Kucherawy, "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, September 2011.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, Version 1", RFC 7208, DOI 10.17487/RFC7208, April 2014.
[RFC7489] Kucherawy, M. and E. Zwicky, "Domain-based Message Authentication, Reporting, and Conformance (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015.
[RFC8551] Schaad, J., Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet Mail Extensions (S​/​MIME) Version 4.0 Message Specification", RFC 8551, DOI 10.17487/RFC8551, April 2019.

11.2. Informative References

[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.
[SRC.enigmailpep] "Source code for Enigmail/pEp", July 2019.
[SRC.pepforandroid] "Source code for pEp for Android", July 2019.
[SRC.pepforios] "Source code for pEp for iOS", July 2019.
[SRC.pepforoutlook] "Source code for pEp for Outlook", July 2019.

Appendix A. About pEp Design Principles

pretty Easy privacy (pEp) is working on bringing state-of-the-art automatic cryptography known from areas like TLS to electronic mail (email) communication. pEp is determined to evolve the existing standards as fundamentally and comprehensively as needed to gain easy implementation and integration, and for easy use for regular Internet users. pEp for email wants to attain to good security practice while still retaining backward compatibility for implementations widespread.

To provide the required stability as a foundation for good security practice, pEp for email defines a fixed MIME structure for its innermost message structure, so to remove most attack vectors which have permitted the numerous EFAIL vulnerabilities. (TBD: ref)

Security comes just next after privacy in pEp, for which reason the application of signatures without encryption to messages in transit is not considered purposeful. pEp for email herein referenced, and further described in [I-D.marques-pep-email], either expects to transfer messages in cleartext without signature or encryption, or transfer them encrypted and with enclosed signature and necessary public keys so that replies can be immediately upgraded to encrypted messages.

The pEp message format is equivalent to the S​/​MIME standard in ensuring header protection, in that the whole message is protected instead, by wrapping it and providing cryptographic services to the whole original message. The pEp message format is different compared to the S​/​MIME standard in that the pEp protocols propose opportunistic End-to-End security and signature, by allowing the transport of the necessary public key material along with the original messages.

For the purpose of allowing the insertion of such public keys, the root entity of the protected message is thus nested once more into an additional multipart/mixed MIME entity. The current pEp proposal is for PGP/MIME, while an extension to S​/​MIME is next.

pEp’s proposal is strict in that it requires that the cryptographic services applied to the protected message MUST include encryption. It also mandates a fixed MIME structure for the protected message, which always MUST include a plaintext and optionally an HTML representation (if HTML is used) of the same message, and requires that all other optional elements to be eventually presented as attachments. Alternatively the whole protected message could represent in turn a wrapped pEp wrapper, which makes the message structure fully recursive on purpose (e.g., for the purpose of anonymization through onion routing).

For the purpose of implementing mixnet routing for email, it is foreseen to nest pEp messages recursively. A protected message can in turn contain a protected message due for forwarding. This is for the purpose to increase privacy and counter the necessary leakage of plaintext addressing in the envelope of the email.

The recursive nature of the pEp message format allows for the implementation of progressive disclosure of the necessary transport relevant header fields just as-needed to the next mail transport agents along the transmission path.

Appendix B. Document Changelog

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

Appendix C. Open Issues

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

Author's Address

Claudio Luck pEp Foundation Oberer Graben 4 CH-8400 Winterthur, Switzerland EMail: claudio.luck@pep.foundation URI: https://pep.foundation/