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Versions: 00 01 02 03 04                                                
Network Working Group                                         A. Brotman
Internet-Draft                                              Comcast, Inc
Intended status: Best Current Practice                           T. Zink
Expires: February 6, 2020                      Zink Magical Contraptions
                                                          August 5, 2019

   Receivers Guidance for Implementing Branded Indicators for Message
                         Identification (BIMI)


   This document is meant to assist receivers or other mailbox providers
   by providing guidance to implementing Brand Indicators for Message
   Identification (BIMI).  This document is a companion to the main BIMI
   drafts which should first be consulted and reviewed.

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
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   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 February 6, 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
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Goals for BIMI  . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Should your site implement BIMI?  . . . . . . . . . . . . . .   3
     3.1.  If your site satisfies the requirements, this is likely a
           "yes".  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Site implementations  . . . . . . . . . . . . . . . . . . . .   4
   5.  Validation of a BIMI message  . . . . . . . . . . . . . . . .   4
     5.1.  BIMI Site Requirements  . . . . . . . . . . . . . . . . .   5
     5.2.  BIMI Certificate Validation . . . . . . . . . . . . . . .   6
   6.  Communicating BIMI results between the MTA and the MUA  . . .   6
     6.1.  Image Retrieval . . . . . . . . . . . . . . . . . . . . .   6
     6.2.  TTL of cached images  . . . . . . . . . . . . . . . . . .   7
     6.3.  Privacy Concerns  . . . . . . . . . . . . . . . . . . . .   7
     6.4.  Basic flow example  . . . . . . . . . . . . . . . . . . .   7
   7.  Domain Reputation . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Rolling up based upon domain vs organizational domain . .   9
   8.  Working with MVAs . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Resolving disputes  . . . . . . . . . . . . . . . . . . .  11
   9.  Troubleshooting BIMI  . . . . . . . . . . . . . . . . . . . .  11
   10. Public documentation  . . . . . . . . . . . . . . . . . . . .  12
     10.1.  For Brands:  . . . . . . . . . . . . . . . . . . . . . .  12
     10.2.  For users: . . . . . . . . . . . . . . . . . . . . . . .  12
   11. Appendix  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     11.1.  Glossary . . . . . . . . . . . . . . . . . . . . . . . .  13
   12. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  14
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
   14. Normative References  . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   The Brand Indicators for Message Identification (BIMI) specification
   introduces a method by which Mail User Agent (MUA, e.g, an email
   client) providers combine DMARC-based message authentication in
   addition to cryptographic methods to ensure the identity of a sender,
   and then to retrieve iconography that the sender has selected.  The
   iconography can then be displayed within the MUA.  The displayed
   iconography grants the sender brand impressions via the BIMI-capable
   MUA, and should be a driving factor for the adoption of authenticated

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1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

2.  Goals for BIMI

   As stated in other BIMI drafts, BIMI intends to advance email
   authentication by granting a sending party brand impressions as long
   as the message passes authentication mechanisms and and meets other
   receiver qualifications (reputation, encryption, whitelisting, et
   cetera).  DMARC currently has wide adoption by some of the
   InternetA[cents]a'[not]a"[cents]s larger brands, but there is still a
   long tail of small-to-medium size brands (and many large ones) that
   do not have it.  Because BIMI provides a visual presence in the
   inbox, and because visual impressions are desirable for brands, BIMI
   provides an incentive for marketers to spur DMARC adoption, whereas a
   concern purely from security may not.

3.  Should your site implement BIMI?

3.1.  If your site satisfies the requirements, this is likely a "yes".

   As email has evolved over the past three decades, it is no longer a
   medium of merely exchanging text, but of enabling people to build
   rich experiences on top of it.  BIMI provides an incentive for brands
   to send email more securely because the desired behavior - a visual
   imprint in the inbox - first requires DMARC adoption.


   The following terms are used throughout this document.

   o  MTA

   o  MUA

   o  DKIM

   o  SPF

   o  DMARC

   o  Alignment

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   o  BIMI Certificates

   o  IMAP

   o  Recipient Domain

   o  Sending Domain

   o  MVA

   o  FBL

   For definitions of these terms, see the Appendix.

4.  Site implementations

   In order for a site to correctly implement BIMI, the receiver must be
   able to perform the following:

   o  Validate SPF

   o  Validate DKIM signatures

   o  Validate DMARC

   o  Validate a BIMI Certificate (a new kind of Extended Validation
      (EV) certificate)

   o  Fetch an image located at an https location

   o  For some receivers, an additional requirement is a BIMI-capable
      IMAP daemon, or another method of a mail server signaling to an
      MUA that it is safe to load a BIMI image , as well as securely
      pointing to the BIMI location to pull it from.

   A site may wish to implement URI alteration and image caching for
   hosted recipients.  By implementing BIMI, a site agrees that through
   some combination of trust mechanisms, it will instruct a BIMI-capable
   MUA to display the image fetched from a URI within the message
   headers.  This URI is created after the MTA authenticates a message,
   and is also able to authenticate the BIMI certificate associated with
   the sending domain.

5.  Validation of a BIMI message

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5.1.  BIMI Site Requirements

   In the BIMI specification, a message MUST be authenticated via DMARC.
   As stated in the DMARC draft, this requires that only one of DKIM or
   SPF must successfully pass validation.  However, for additional local
   security measures, a receiving site may create additional
   requirements for senders in order to verify BIMI (that is, indicate
   to a downstream MUA that it is safe to load a BIMI logo in the email
   This may include, but is not limited to:

   o  Requiring both DKIM and SPF to validate and align with the
      organizational domain in the From: address (whereas DMARC only
      requires one of SPF or DKIM to align with the From: domain)

   o  A DMARC policy of quarantine or reject

   o  SPF "strength" requirements (e.g., requiring "-all", disallowing
      usage of "?all" or "+all", or not allowing inclusion of overly
      large address spaces)

   o  SMTP delivery via TLS

   o  Feedback Loop registration or other method of registration with
      the receiving site.

   These localized requirements are at the discretion of the receiving
   site.  In general, the stricter the criteria, the less chance there
   is of an MUA erroneously showing a logo and giving the wrong signal
   to a user.

   Upon receipt of an email, a receiver that implements BIMI should
   remove or rename any previously existing BIMI-* headers other than
   BIMI-Selector, as they may have come from an attacker (as long as the
   BIMI-Selector is covered by the DKIM signature; if not, it should be
   removed, renamed, or ignored).


   o  It may be useful to have messages exiting a site to have those
      BIMI-* headers removed as well.

   o  It is useful for a site that has not implemented BIMI to remove
      those headers so that an MUA that does make use of those headers
      would not accidentally display a BIMI image when the message has
      not been properly authenticated by the email receiver (even though
      an MUA should not make use of BIMI headers and instead rely upon

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      settings from the mailstore, it is possible that some MUAs will
      nevertheless use headers without taking appropriate precautions).

5.2.  BIMI Certificate Validation

   (Currently, see document in Reference below)

6.  Communicating BIMI results between the MTA and the MUA

   In order for a receiver that has implemented BIMI to notify an MUA
   that it should display the images:

   o  An MTA must verify BIMI and if successful, write to the mail store
      (where the messages are saved) that the message passed BIMI, and
      it is safe to load the logo.  For example, in an IMAP mailstore, a
      flag on the message could be set that indicates that the message
      passed BIMI, and a second flag that tells the MUA where to get the
      BIMI logo from.

   o  When displaying a message, the MUA does not look for any BIMI
      headers stamped by the MTA, but instead relies upon the mailstore
      flags or message properties that a message passed BIMI, and use
      that to decide show the logo.  The MUA then pulls the required
      image and displays it as appropriate.

   Alternatively, the MUA may also look for the flag in the mailstore
   and then attempt to extract the key/value pairs from the BIMI-
   Location headers.  In either case, the MUA must first check to see if
   a message passed BIMI before loading the BIMI image.

   While the MTA MAY stamp BIMI-related information in the message
   headers, they should not be relied upon by an MUA.

6.1.  Image Retrieval

   A core part of the BIMI specification is that the MUA will retrieve
   an image file to display for each BIMI-validated message.  There are
   multiple ways to accomplish this, for example:

   o  In its most basic setup, a BIMI-capable MUA could retrieve that
      image file directly from the site specified in the BIMI record.

   o  Other providers may choose to cache the associated images in a
      local store which could be used as the BIMI resource address in
      the headers of a BIMI-approved message in a sort of proxy

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6.2.  TTL of cached images

   In some circumstances it is necessary to cache the images that an MUA
   would want to load.  For example, if a domain owner has a short TTL
   time, it would force the MUA to look it up in an unreasonably short
   period of time.  In this case, a receiver may want to set its own

   One option is to set it to several hours, or a day; another option is
   to set the TTL to the same as the expiration period in the BIMI
   certificate that points to the BIMI image.  The downside is that the
   caching mechanism might need to check for certificate revocation, and
   then re-fetch images.

6.3.  Privacy Concerns

   There is some concern that the retrieval of the iconography could
   result in a privacy leak.
   As the images are retrieved, it's possible that the image provider
   could track the retrieving system in some way.  This has implications
   whether it be the sender or provider that is hosting the image.  For
   example, a sender could include a singular selector for a single
   recipient, or a provider could append a tracking string to the image
   URI in the header.

   An in-depth discussion of all the potential privacy leaks with
   respect to loading or embedding images is outside the scope of this

6.4.  Basic flow example

   One sample implementation of BIMI by a receiver, who does everything
   on-the-fly, is as following:

   o  An email receiver has established a relationship with several
      MVAs, trusts them, and has received their public keys for
      verifying BIMI certificates.  The email receiver makes these keys
      available to its mail servers (e.g., by distributing local copies
      to each server).  [NOTE: Use of MVA above per Thede]

   o  Upon receipt of a message, the receiver checks to see if the
      message passes aligned-SPF or DKIM, and DMARC, and ensures that
      the sending domain has a DMARC policy of "quarantine" or "reject"
      per local receiver policy, while properly applying the appropriate
      DMARC policy to the message.

   o  If the message passes prior checks, the receiver will then check
      to see if the domain in the From: address has a BIMI record (or,

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      if the message has a BIMI-Selector header that is covered by the
      DKIM-Signature, uses that to do the BIMI query in DNS).

   o  If a BIMI record is found, the receiver then retrieves the BIMI
      certificate from the location that the BIMI record points to, and
      attempts to verify the BIMI cert with each public key it has from
      the MVAs that it works with.

   o  Upon successful verification of the cert, the receiver checks to
      see if the signed image hash in the BIMI cert matches any of the
      hashes of the images that the BIMI record points to (the receiver,
      in this instance, is not storing any of the images locally, but
      instead is downloading them on-the-fly).  If a hash of a
      downloaded image from the BIMI record matches the hash in the BIMI
      cert, this is a successful BIMI verification.

   o  If the BIMI verification does not verify, then the MTA must not
      indicate to the MUA to show a BIMI image.  The MUA MAY show a
      default image such as a set of initials, or unidentified sender.

   o  The email receiver then does the rest of its anti-spam, anti-
      malware, and anti-phishing checks (these checks may be performed
      before BIMI is verified).  If a message fails a phishing or
      malware checks, the email receiver must not say the message passed
      BIMI.  If a message is neither malware nor phishing but is
      detected as spam (meaning that the message comes from a known
      brand, but contains spammy content), then the email receiver may
      optionally say that the message passed BIMI (and therefore a
      receiver should show the image) but it is up to the receiver.

   o  The email receiver then sets either the appropriate IMAP flags, or
      other mailstore flag, or other message property that signals to a
      downstream email client that the message passed BIMI and is safe
      to load the logo, along with a pointer to the logo (e.g., to the
      https location specified in the BIMI record).

   o  What eventually happens is the email client then looks at the
      flags set by the email receiver (MTA).  If the flags are set to
      show a BIMI logo, then the email client downloads the image and
      displays it in the sender photo (or however else it chooses to
      render the BIMI logo in conjunction with the message).

7.  Domain Reputation

   Receivers are advised to consider incorporating local sources of
   domain trust intelligence into the processes which ultimately
   determine whether or not BIMI logos are displayed.  Simply because a
   sending domain passes BIMI requirements does not mean the images

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   should automatically be displayed in the MUA; a site may impose
   further restrictions based on domain reputation.

   One source of additional reputation intelligence could be a platform
   that the email provider has created to calculate domain trust based
   on historical traffic; another is an explicit list of trusted domains
   that has been curated by an individual provider; a third is a list
   that is purchased from a vendor that might be a pass/fail or a scored
   list; another option is some mix of any of the previous three.

7.1.  Rolling up based upon domain vs organizational domain

   BIMI is designed to be able to work on selectors, and so in theory a
   brand/domain could specify multiple BIMI logos and differentiate them
   on a per-domain (per-selector) basis.  The advantage for the brand is
   that they can choose the image they want the user to see depending
   upon various conditions (e.g., seasonal images, regional images,

   However, for an email receiver, it may be easier to roll up BIMI
   logos on an organizational domain basis.  One reason may be for the
   purposes of reputation, another may be for simplifying management of
   images.  In this case, it would need to be made clear to brands that
   this is how the loading of BIMI images works.  This documentation
   could live on a postmaster site, under technical documentation, or
   other official page maintained by the receiver.  It could then be
   referred to when sending organizations ask about how to on-board to
   BIMI at the receiver, and provide official guidance about the way it
   works at the site.

   If rolling up by organizational domain, then it may make sense to use
   a "lowest common denominator" approach.  That is, an organizational
   domain must meet all the requirements for BIMI, rather than only a
   subdomain.  The reason for this is that if sub.brand.com gets an
   image due to having strong authentication policies, but brand.com
   does not, then this may cause confusion because a user may learn to
   associate sub.brand.com and its image with brand.com; and if
   brand.com can be spoofed even though sub.brand.com cannot, that can
   lead to users becoming more susceptible to phishing from brand.com.

   To alleviate this, receivers may wish to show logos only for domains
   that have organizational domains with strong DMARC policies.  Or, if
   an organizational domain does not have a strong DMARC policy but a
   subdomain does, then it may treat the organizational domain as if it
   does have a strong DMARC policy so as to prevent a phisher or spammer
   from impersonating the brand or any of its subdomains.

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8.  Working with MVAs

   Email receivers need to know whether or not
   itA[cents]a'[not]a"[cents]s safe to download and display an image.
   That is, an attacker could go through the trouble of creating a BIMI
   logo and uploading it, but the logo may look visually similar to a
   real brand.  For example, a spammer or phisher could create a
   lookalike domain for a well-known brand such as Paypal, then copy/
   paste (or slightly modify) the logo.

   To prevent this, an email receiver could choose to verify logos of
   known brands by themselves (do it all in-house) and establish its own
   internal processes, or it could use a Mark Verifying Authority (MVA).
   The receiver could then outsource the maintenance of the list of
   trusted brands to the MVA, and simply download the list of brands and
   images from the MVA and display the logos in its email clients.

   However, even here a receiver would need to exercise caution.  It
   needs to ensure that MVAs follow best practices, respond to
   complaints, and do a good job of vetting brands.  If users ultimately
   end up getting phished because they trust signals in the email
   client, then it is the email receiver that will suffer the brunt of
   the complaints and loss of reputation, rather than the MVA.

   Therefore, an email receiver still needs to track complaints from its
   users, especially with respect to phishing and impersonation, and
   then send the feedback back to the MVA.  If an MVA still generates
   too many complaints, this could be indicative of a rogue MVA (one
   that intentionally signs up malicious accounts), or a
   A[cents]a'[not]Ae"sloppyA[cents]a'[not]A MVA (one with internal
   processes that not rigorous enough, or are designed to maximize
   revenue at the cost of lax security).

   An email receiver should use multiple MVAs to reduce the risk of
   becoming too reliant upon a single MVA in case they have to stop
   using it, and therefore lose many dozens, hundreds, or thousands of
   images with no replacement and thereby contributing to user
   dissatisfaction confusion.  Furthermore, because MVAs may be revoked,
   brands may wish to diversify their own risk by getting certified by
   at least two MVAs.  The reason for doing this is that if the MVA they
   use ever gets revoked by an email receiver because of its bad
   practices, then their own brand will suffer penalties (not having a
   logo displayed) despite never having done anything wrong.  By
   researching multiple MVAs, a brand can reduce the chances that losing
   one by a receiver affects their brand.

   For this reason, brands are encouraged to get certified at multiple
   MVAs, and receivers are encouraged to use multiple MVAs.

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8.1.  Resolving disputes

   From time to time, disputes may arise between brands where one brand
   says that another is infringing on its logo.

   A brand owner would want to have all email receivers stop showing
   logos for the infringing brand because it could damage its own
   brandA[cents]a'[not]a"[cents]s reputation.  However, an email
   receiver is not necessarily in a good position to determine what
   constitutes legitimate usage of a logo, nor determine ownership of a
   logo, nor may want the legal risk associated with making this

   Therefore, email receivers are strongly encouraged to partner with
   Dispute Resolution Agencies.  These agencies specialize in copyright
   infringement resolution.  An affected party would then contact the
   Dispute Resolution Agency, rather than the email receiver, who would
   then make the decision about if use of the logo were legitimate.
   Then, they would publish the result of the dispute publicly where it
   could be viewed by anyone.

   MVAs should respect the decision of the courts and any brand found to
   be infringing ought to be removed from their list of domains for
   which they load BIMI logos for.  The issuing MVA of the infringing
   brandA[cents]a'[not]a"[cents]s BIMI Certificate should formally
   revoke it.  However, this is not guaranteed in the case of a rogue
   MVA or a sloppy MVA.  Therefore, email receivers should also pay
   attention to the Dispute Resolution Agencies, and any results that
   they say are infringing should be prevented from loading in their
   email clients.  The email receiver should also keep track of how
   often disputes occur and are found against various MVAs, as an MVA
   with too many disputes ruled against it could be evidence of a sloppy
   MVA or a rogue MVA.

9.  Troubleshooting BIMI

   There are several factors to consider for email receivers on things
   that can go wrong, below are a handful of considerations:

   o  Failing to verify BIMI certs when they otherwise should be.  This
      can be caused by:

   o  Not having the key to a corresponding MVA

   o  Not having access to the key when required

   o  The wrong key is associated with the wrong MVA

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   o  Failing to load a logo in the email client

   o  Failing to access the logo (e.g., permissions errors)

   o  Connectivity problems to the logo

   o  Failing to display a correct logo in the email client

   o  Having the wrong logo stored for a brand (i.e., uploading it to a
      local store but associating it with the wrong brand)

   o  Caching a logo for too long after it has updated

   There are many reasons why a logo may fail to load; having tools to
   investigate (logs, headers in messages, internal documentation that
   is clearly written, having the knowledge pushed out to multiple
   escalation channels) are important for investigation.

10.  Public documentation

10.1.  For Brands:

   It is ideal to publish the criteria that is used by your site to
   determine when BIMI will be displayed.  It is fine to say that you
   use some internal domain reputation metrics as additional criteria to
   determine whether or not a logo should be displayed, and it
   isnA[cents]a'[not]a"[cents]t necessary to give away the exact nature
   of the algorithm other than to say "You must maintain good sending

   If you use an explicit whitelist, a site may want to list the minimum
   requirements, and the method of applying to be whitelisted.
   Similarly, a provider may wish to state what type of activity will
   revoke the decision to display logos previously approved.

10.2.  For users:

   BIMI is not meant to instill additional trust in messages, and it is
   important to make this known to your users.  All messages, even those
   with logos, should still be treated with (mild) skepticism, and any
   action regarding the message should still be individually evaluated.
   ItA[cents]a'[not]a"[cents]s possible for a site that has a high trust
   value to become compromised and send fraudulent messages that could
   compromise a userA[cents]a'[not]a"[cents]s system.  Ensure your
   customers have a place that documents BIMI and demonstrates how to
   check messages for fraud.

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11.  Appendix

11.1.  Glossary

   o  MUA - Mail User Agent - The application used to read messages by
      the end user.  This could be a thick client or a web-based

   o  MTA - Mail Transfer Agent - Software used to transfer messages
      between two systems, typically between two sites, using SMTP as
      the protocol.

   o  SPF - SPF is a framework that designates which systems should be
      sending for a given domain.  This can be a list of IPs, CIDRs, or
      references to DNS records.  As the sender should be controlling
      their DNS, they should understand which IPs should be sending as
      their domain.

   o  DKIM - DKIM is a system by which a chosen set of headers, combined
      with the message contents, are cryptographically signed, and then
      validated by the receiving system.  Using DNS, the receiving
      system can retrieve a public key, and then validate the signature
      within the headers of a message.  When implemented properly, the
      systems responsible for sending the messages for a given domain
      name should be the only ones capable of creating messages that
      correctly validates.  Provided that certain restrictions are met,
      DKIM is one possible technology a receiver could utilize to
      authenticate messages in the context of BIMI.

   o  DMARC - DMARC is a message authentication mechanism that works
      with SPF and DKIM.  The BIMI specification requires that a message
      passes DMARC.  In order for a message to pass DMARC, one of SPF or
      DKIM must successfully validate, and the domain in the From:
      address must align with the domain that passed SPF or DKIM.

   o  MVA - Mark Verifying Authority - An entity that a receiver uses to
      certify that the iconography that they intend to use with BIMI is
      properly/legally licensed for their use.

   o  DRA - Dispute Resolution Authority - This organization will
      moderate between two entities that believe they are both entitled
      to use a logo.  Receivers should then abide by the decision of the
      DRA as it pertains to logo usage in the MUA.

   o  Alignment - Alignment refers to the organizational domain, as
      defined by DMARC, of the domain in the From: address being the
      same as the organizational domain that passed SPF or DKIM.  For
      example, foo.example.com has an organizational domain of

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      example.com; bar.foo.example.com also has an organizational domain
      of example.com.  It aligns with org.example.com, because both have
      the same organizational domain.

   o  BIMI Certificates - An Extended Validation Certificate is used in
      conjunction with BIMI to create a place where information
      pertaining to iconography for a sending domain can be securely
      verified.  In the case of BIMI, hashes for an MVA-approved set of
      iconography will be stored in a field within the certificate.
      This should allow a receiver site to validate the retrieved
      imagery before putting the BIMI image URI into the message

   o  Terry Zink - Alex BrotmanA[cents]a'[not]a"[cents]s best friend.

12.  Contributors


13.  References

   The full BIMI verification spec can be found at:

   Verified Mark Certificates Usage: <https://docs.google.com/document/

14.  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,

   [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>.

Authors' Addresses

   Alex Brotman
   Comcast, Inc

   Email: alex_brotman@comcast.com

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   Terry Zink
   Zink Magical Contraptions

   Email: tzink@terryzink.com

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