lamps D. K. Gillmor, Ed.
Internet-Draft ACLU
Intended status: Informational B. Hoeneisen, Ed.
Expires: 16 March 2024 pEp Foundation
A. Melnikov, Ed.
Isode Ltd
13 September 2023
Guidance on End-to-End E-mail Security
draft-ietf-lamps-e2e-mail-guidance-12
Abstract
End-to-end cryptographic protections for e-mail messages can provide
useful security. However, the standards for providing cryptographic
protection are extremely flexible. That flexibility can trap users
and cause surprising failures. This document offers guidance for
mail user agent implementers to help mitigate those risks, and to
make end-to-end e-mail simple and secure for the end user. It
provides a useful set of vocabulary as well as suggestions to avoid
common failures. It also identifies a number of currently unsolved
usability and interoperability problems.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://dkg.gitlab.io/e2e-mail-guidance/. Status information for
this document may be found at https://datatracker.ietf.org/doc/draft-
ietf-lamps-e2e-mail-guidance/.
Discussion of this document takes place on the LAMPS Working Group
mailing list (mailto:spasm@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/spasm/. Subscribe at
https://www.ietf.org/mailman/listinfo/spasm/.
Source for this draft and an issue tracker can be found at
https://gitlab.com/dkg/e2e-mail-guidance.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 16 March 2024.
Copyright Notice
Copyright (c) 2023 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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.1. Structural Headers . . . . . . . . . . . . . . . . . 6
1.1.2. User-Facing Headers . . . . . . . . . . . . . . . . . 7
2. Usability . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1. Simplicity . . . . . . . . . . . . . . . . . . . . . . . 8
2.2. E-mail Users Want a Familiar Experience . . . . . . . . . 9
2.3. Warning About Failure vs. Announcing Success . . . . . . 10
3. Types of Protection . . . . . . . . . . . . . . . . . . . . . 10
3.1. Simplified Mental Model . . . . . . . . . . . . . . . . . 11
3.2. One Cryptographic Status Per Message . . . . . . . . . . 12
4. Cryptographic MIME Message Structure . . . . . . . . . . . . 13
4.1. Cryptographic Layers . . . . . . . . . . . . . . . . . . 13
4.1.1. S/MIME Cryptographic Layers . . . . . . . . . . . . . 13
4.1.2. PGP/MIME Cryptographic Layers . . . . . . . . . . . . 14
4.2. Cryptographic Envelope . . . . . . . . . . . . . . . . . 15
4.3. Cryptographic Payload . . . . . . . . . . . . . . . . . . 15
4.4. Types of Cryptographic Envelope . . . . . . . . . . . . . 15
4.4.1. Simple Cryptographic Envelopes . . . . . . . . . . . 16
4.4.2. Multilayer Cryptographic Envelopes . . . . . . . . . 16
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4.5. Errant Cryptographic Layers . . . . . . . . . . . . . . . 16
4.5.1. Mailing List Wrapping . . . . . . . . . . . . . . . . 16
4.5.2. A Baroque Example . . . . . . . . . . . . . . . . . . 17
4.6. Cryptographic Summary . . . . . . . . . . . . . . . . . . 18
5. Message Composition . . . . . . . . . . . . . . . . . . . . . 18
5.1. Message Composition Algorithm . . . . . . . . . . . . . . 18
5.2. Encryption Outside, Signature Inside . . . . . . . . . . 19
5.3. Avoid Offering Encrypted-only Messages . . . . . . . . . 19
5.4. Composing a Reply Message . . . . . . . . . . . . . . . . 20
6. Message Interpretation . . . . . . . . . . . . . . . . . . . 21
6.1. Rendering Well-formed Messages . . . . . . . . . . . . . 21
6.2. Errant Cryptographic Layers . . . . . . . . . . . . . . . 21
6.2.1. Errant Signing Layer . . . . . . . . . . . . . . . . 21
6.2.2. Errant Encryption Layer . . . . . . . . . . . . . . . 23
6.2.3. Avoiding Non-MIME Cryptographic Mechanisms . . . . . 24
6.3. Forwarded Messages with Cryptographic Protection . . . . 25
6.4. Signature failures . . . . . . . . . . . . . . . . . . . 25
7. Reasoning about Message Parts . . . . . . . . . . . . . . . . 26
7.1. Main Body Part . . . . . . . . . . . . . . . . . . . . . 27
7.2. Attachments . . . . . . . . . . . . . . . . . . . . . . . 28
7.3. MIME Part Examples . . . . . . . . . . . . . . . . . . . 28
8. Certificate Management . . . . . . . . . . . . . . . . . . . 29
8.1. Peer Certificates . . . . . . . . . . . . . . . . . . . . 30
8.1.1. Peer Certificate Selection . . . . . . . . . . . . . 30
8.2. Local Certificates . . . . . . . . . . . . . . . . . . . 31
8.2.1. Getting Certificates for the User . . . . . . . . . . 31
8.2.2. Local Certificate Maintenance . . . . . . . . . . . . 33
8.2.3. Shipping Certificates in Outbound Messages . . . . . 34
9. Common Pitfalls and Guidelines . . . . . . . . . . . . . . . 35
9.1. Reading Sent Messages . . . . . . . . . . . . . . . . . . 35
9.2. Reading Encrypted Messages after Certificate
Expiration . . . . . . . . . . . . . . . . . . . . . . . 36
9.3. Unprotected Message Headers . . . . . . . . . . . . . . . 37
9.4. Composing an Encrypted Message with Bcc . . . . . . . . . 37
9.4.1. Simple Encryption with Bcc . . . . . . . . . . . . . 38
9.5. Draft Messages . . . . . . . . . . . . . . . . . . . . . 39
9.6. Composing a Message to Heterogeneous Recipients . . . . . 39
9.7. Message Transport Protocol Proxy: A Dangerous
Implementation Choice . . . . . . . . . . . . . . . . . . 41
9.7.1. Dangers of a Submission Proxy for Message
Composition . . . . . . . . . . . . . . . . . . . . . 42
9.7.2. Dangers of an IMAP Proxy for Message Rendering . . . 43
9.7.3. Who Controls the Proxy? . . . . . . . . . . . . . . . 44
9.8. Intervening MUAs Do Not Handle End-to-End Cryptographic
Protections . . . . . . . . . . . . . . . . . . . . . . . 44
9.9. External Subresources in MIME Parts Break Cryptographic
Protections . . . . . . . . . . . . . . . . . . . . . . . 45
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47
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11. Security Considerations . . . . . . . . . . . . . . . . . . . 47
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 47
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 47
13.1. Normative References . . . . . . . . . . . . . . . . . . 47
13.2. Informative References . . . . . . . . . . . . . . . . . 48
Appendix A. Future Work . . . . . . . . . . . . . . . . . . . . 52
A.1. Test Vectors . . . . . . . . . . . . . . . . . . . . . . 52
A.2. Further Guidance on Peer Certificates . . . . . . . . . . 52
A.2.1. Certificate Discovery from Incoming Messages . . . . 52
A.2.2. Certificate Directories . . . . . . . . . . . . . . . 53
A.2.3. Checking for Certificate Revocation . . . . . . . . . 53
A.2.4. Further Peer Certificate Selection . . . . . . . . . 53
A.3. Further Guidance on Local Certificates and Secret Keys . 53
A.3.1. Cross-MUA sharing of Local Certificates and Secret
Keys . . . . . . . . . . . . . . . . . . . . . . . . 53
A.3.2. Use of Smartcards or Other Portable Secret Key
Mechanisms . . . . . . . . . . . . . . . . . . . . . 54
A.3.3. Active Local Certificate Maintenance . . . . . . . . 54
A.4. Certification Authorities . . . . . . . . . . . . . . . . 54
A.5. Indexing and Search of Encrypted Messages . . . . . . . . 54
A.6. Cached Signature Validation . . . . . . . . . . . . . . . 55
A.7. Aggregate Cryptographic Status . . . . . . . . . . . . . 55
A.8. Expectations of Cryptographic Protection . . . . . . . . 55
A.9. Secure Deletion . . . . . . . . . . . . . . . . . . . . . 56
A.10. Interaction with Opportunistic Encryption . . . . . . . . 56
A.11. Split Attachments . . . . . . . . . . . . . . . . . . . . 56
A.12. Proxy Extensions . . . . . . . . . . . . . . . . . . . . 57
Appendix B. Document History . . . . . . . . . . . . . . . . . . 57
B.1. Substantive changes from draft-ietf-...-11 to
draft-ietf-...-12 . . . . . . . . . . . . . . . . . . . 57
B.2. Substantive changes from draft-ietf-...-10 to
draft-ietf-...-11 . . . . . . . . . . . . . . . . . . . 57
B.3. Substantive changes from draft-ietf-...-09 to
draft-ietf-...-10 . . . . . . . . . . . . . . . . . . . 57
B.4. Substantive changes from draft-ietf-...-08 to
draft-ietf-...-09 . . . . . . . . . . . . . . . . . . . 57
B.5. Substantive changes from draft-ietf-...-07 to
draft-ietf-...-08 . . . . . . . . . . . . . . . . . . . 58
B.6. Substantive changes from draft-ietf-...-06 to
draft-ietf-...-07 . . . . . . . . . . . . . . . . . . . 58
B.7. Substantive changes from draft-ietf-...-05 to
draft-ietf-...-06 . . . . . . . . . . . . . . . . . . . 58
B.8. Substantive changes from draft-ietf-...-04 to
draft-ietf-...-05 . . . . . . . . . . . . . . . . . . . 59
B.9. Substantive changes from draft-ietf-...-03 to
draft-ietf-...-04 . . . . . . . . . . . . . . . . . . . 59
B.10. Substantive changes from draft-ietf-...-02 to
draft-ietf-...-03 . . . . . . . . . . . . . . . . . . . 59
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B.11. Substantive changes from draft-ietf-...-01 to
draft-ietf-...-02 . . . . . . . . . . . . . . . . . . . 59
B.12. Substantive changes from draft-ietf-...-00 to
draft-ietf-...-01 . . . . . . . . . . . . . . . . . . . 59
B.13. Substantive changes from draft-dkg-...-01 to
draft-ietf-...-00 . . . . . . . . . . . . . . . . . . . 59
B.14. Substantive changes from draft-dkg-...-00 to
draft-dkg-...-01 . . . . . . . . . . . . . . . . . . . . 60
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 60
1. Introduction
E-mail end-to-end security using S/MIME ([RFC8551]) and PGP/MIME
([RFC3156]) cryptographic standards can provide integrity,
authentication and confidentiality to MIME ([RFC4289]) e-mail
messages.
However, there are many ways that a receiving mail user agent can
misinterpret or accidentally break these security guarantees. For
example, [EFAIL]'s "Direct Exfiltration" attacks leak cleartext due
to attacks that splice existing ciphertext into novel messages, which
then are handled optimistically (and wrongly) by many mail user
agents.
A mail user agent that interprets a message with end-to-end
cryptographic protections needs to do so defensively, staying alert
to different ways that these protections can be bypassed by mangling
(either malicious or accidental) or a failed user experience.
A mail user agent that generates a message with end-to-end
cryptographic protections should be aware of these defensive
interpretation strategies, and should compose any new outbound
message conservatively if they want the protections to remain intact.
This document offers guidance to the implementer of a mail user agent
that provides these cryptographic protections, whether for sending or
receiving mail. An implementation that follows this guidance will
provide its users with stronger and easier-to-understand security
properties, and will also offer more reliable interoperability for
messages exchanged with other implementations.
In Appendix A, this document also identifies a number of
interoperability and usability concerns for end-to-end cryptographic
e-mail which have no current broadly accepted technical standard for
resolution. One major area not covered in this document is the
acquisition and long-term maintenance of cryptographic identity
information and metadata across multiple mail user agents controlled
by the same user.
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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.
1.1. Terminology
For the purposes of this document, we define the following concepts:
* _MUA_ is short for Mail User Agent; an e-mail client.
* _Protection_ of message data refers to cryptographic encryption
and/or signatures, providing confidentiality, authenticity, and/or
integrity.
* _Cryptographic Layer_, _Cryptographic Envelope_, _Cryptographic
Payload_, _Cryptographic Summary_, and _Errant Cryptographic
Layer_ are defined in Section 4
* A _well-formed_ e-mail message with cryptographic protection has
both a _Cryptographic Envelope_ and a _Cryptographic Payload_.
* _Structural Headers_ are documented in Section 1.1.1.
* _User-Facing Headers_ are documented in Section 1.1.2.
* _Main Body Part_ is the part (or parts) that are typically
rendered to the user as the message itself (not "as an
attachment"). See Section 7.1.
1.1.1. Structural Headers
A message header field named MIME-Version, or whose name begins with
Content- is referred to in this document as a "structural" header.
This is a less-ambiguous name for what [RFC2045] calls "MIME Header
Fields".
These headers indicate something about the specific MIME part they
are attached to, and cannot be transferred or copied to other parts
without endangering the readability of the message.
This includes:
* MIME-Version
* Content-Type
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* Content-Transfer-Encoding
* Content-Disposition
1.1.2. User-Facing Headers
Of all the headers that an e-mail message may contain, only a handful
are typically presented directly to the user. Typically, user-facing
headers are:
* Subject
* From
* To
* Cc
* Date
* Reply-To
* Followup-To
* Sender
* Resent-From
* Resent-To
* Resent-Cc
* Resent-Date
* Resent-Sender
The above list are the header fields most often presented directly to
the user who views a message, though a MUA may also decide to treat
any other header field as "user-facing". Of course, many of these
header fields are entirely absent from any given message, and an
absent header field is not presented to the user at all.
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Note that the resending header fields (those beginning with Resent-)
are typically only added by an intervening MUA (see Section 3.6.6 of
[RFC5322] and Section 9.8 of this document). As such, though they
may in some cases be presented to the user, they will typically not
bear any end-to-end cryptographic protection (even if the original
headers of a message are protected, see Section 9.3), because they
are unknown to the original sender.
Other header fields may affect the visible rendering of the message
(e.g., References and In-Reply-To may affect the placement of a
message in a threaded discussion; or the List-* and Archived-At
headers added by mailing lists may cause additional buttons to be
displayed during rendering), but they are not directly displayed to
the user and so are not considered "user-facing".
2. Usability
Any MUA that enables its user to transition from unprotected messages
to messages with end-to-end cryptographic protection needs to
consider how the user understands this transition. That said, the
primary goal of the user of an MUA is communication -- so interface
elements that get in the way of communication should be avoided where
possible.
Furthermore, it is a near certainty that the user will continue to
encounter unprotected messages, and may need to send unprotected
messages (for example, if a given recipient cannot handle
cryptographic protections). This means that the MUA needs to provide
the user with some guidance, so that they understand what protections
any given message or conversation has. But the user should not be
overwhelmed with choices or presented with unactionable information.
2.1. Simplicity
The end user (the operator of the MUA) is unlikely to understand
complex end-to-end cryptographic protections on any e-mail message,
so keep it simple.
For clarity to the user, any cryptographic protections should apply
to the message as a whole, not just to some subparts.
This is true for message composition: the standard message
composition user interface of an MUA should offer minimal controls
which indicate which types of protection to apply to the new message
as a whole.
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This is also true for message interpretation: the standard message
rendering user interface of an MUA should offer a minimal, clear
indicator about the end-to-end cryptographic status of the message as
a whole.
See Section 3 for more detail about mental models and cryptographic
status.
(It is of course possible that a message forwarded as a MIME
attachment could have its own cryptographic status while still being
a message subpart; but that status should be distinct from the status
of the enclosing message.)
2.2. E-mail Users Want a Familiar Experience
A person communicating over the Internet today often has many options
for reaching their desired correspondent, including web-based
bulletin boards, contact forms, and instant messaging services.
E-mail offers a few distinctions from these other systems, most
notably features like:
* Ubiquity: Most correspondents will have an e-mail address, while
not everyone is present on every alternate messaging service,
* Federation: interaction between users on distinct domains who have
not agreed on a common communications provider is still possible,
and
* User Control: the user can interact with the e-mail system using a
MUA of their choosing, including automation and other control over
their preferred and/or customized workflow.
Other systems (like some popular instant messaging applications, such
as WhatsApp and Signal Private Messenger) offer built-in end-to-end
cryptographic protections by default, which are simpler for the user
to understand. ("All the messages I see on Signal are confidential
and integrity-protected" is a clean user story)
A user of e-mail is likely using e-mail instead of other systems
because of the distinctions outlined above. When adding end-to-end
cryptographic protection to an e-mail endpoint, care should be taken
not to negate any of the distinct features of e-mail as a whole. If
these features are violated to provide end-to-end crypto, the user
may just as well choose one of the other systems that don't have the
drawbacks that e-mail has. Implmenters should try to provide end-to-
end protections that retain the familiar experience of e-mail itself.
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Furthermore, an e-mail user is likely to regularly interact with
other e-mail correspondents who _cannot_ handle or produce end-to-end
cryptographic protections. Care should be taken that enabling
cryptography in a MUA does not inadvertently limit the ability of the
user to interact with legacy correspondents.
2.3. Warning About Failure vs. Announcing Success
Moving the web from http to https offers useful historical
similarities to adding end-to-end encryption to e-mail.
In particular, the indicators of what is "secure" vs. "insecure" for
web browsers have changed over time. For example, years ago the
default experience was http, and https sites were flagged with
"secure" indicators like a lock icon. In 2018, some browsers
reversed that process by downplaying https, and instead visibly
marking http as "not secure" (see [chrome-indicators]).
By analogy, when the user of a MUA first enables end-to-end
cryptographic protection, it's likely that they will want to see
which messages _have_ protection. But a user whose private e-mail
communications with a given correspondent, or within a given domain
are known to be entirely end-to-end protected might instead want to
know which messages do _not_ have the expected protections.
Note also that some messages may be expected to be confidential, but
other messages are expected to be public -- the types of protection
(see Section 3) that apply to each particular message will be
different. And the types of protection that are _expected_ to be
present in any context might differ (for example, by sender, by
thread, or by date).
It is out of scope for this document to define expectations about
protections for any given message, but an implementer who cares about
usable experience should be deliberate and judicious about the
expectations their interface assumes that the user has in a given
context. See also Appendix A.8 for future work.
3. Types of Protection
A given message might be:
* signed,
* encrypted,
* both signed and encrypted, or
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* none of the above.
Given that many e-mail messages offer no cryptographic protections,
the user needs to be able to detect which protections are present for
any given message.
3.1. Simplified Mental Model
To the extent that an e-mail message actually does have end-to-end
cryptographic protections, those protections need to be visible and
comprehensible to the end user. If the user is unaware of the
protections, then they do not extend all the way to the "end".
However, most users do not have (or want to have) a sophisticated
mental model of what kinds of protections can be associated with a
given message. Even the four states above approach the limits of
complexity for an interface for normal users.
While Section 5.3 recommends avoiding deliberate creation of
encrypted-only messages, some messages may end up in the encrypted-
only state due to signature failure or certificate revocation.
A simple model for the user could be that a message is in one of
three normal states:
* Unprotected
* Verified (has a valid signature from the apparent sender of the
message)
* Confidential (meaning, encrypted, with a valid signature from the
apparent sender of the message)
And one error state:
* Encrypted But Unverified (meaning, encrypted without a valid
signature from the apparent sender of the message)
Note that this last state is not "Confidential" (a secret shared
exclusively between the participants in the communication) because
the recipient does not know for sure who sent it.
In an ecosystem where encrypted-only messages are never deliberately
sent (see Section 5.3), representing an Encrypted But Unverified
message as a type of user-visible error is not unreasonable.
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Alternately, a MUA may prefer to represent the state of a Encrypted
but Unverified message to the user as though it was Unprotected,
since no verification is possible. However the MUA represents the
message to the user, though, it MUST NOT leak cleartext of an
encrypted message (even an Encrypted but Unverified message) in
subsequent replies (see Section 5.4) or similar replications of the
message.
Note that a cleartext message with an invalid signature SHOULD NOT be
represented to the user as anything other than Unprotected (see
Section 6.4).
In a messy legacy ecosystem, a MUA may prefer instead to represent
"Signed" and "Encrypted" as orthogonal states of any given message,
at the cost of an increase in the complexity of the user's mental
model.
3.2. One Cryptographic Status Per Message
Some MUAs may attempt to generate multiple copies of a given e-mail
message, with different copies offering different types of protection
(for example, opportunistically encrypting on a per-recipient basis).
A message resulting from this approach will have a cryptographic
state that few users will understand. Even if the sender understands
the different statuses of the different copies, the recipients of the
messages may not understand (each recipient might not even know about
the other copies). See for example the discussion in Section 9.6 for
how this can go wrong.
For comprehensibility, a MUA SHOULD NOT create multiple copies of a
given message that differ in the types of end-to-end cryptographic
protections afforded.
For opportunistic cryptographic protections that are not surfaced to
the user (that is, that are not end-to-end), other mechanisms like
transport encryption ([RFC3207]) or domain-based signing ([RFC6376])
may be preferable due to ease of implementation and deployment.
These opportunistic transport protections are orthogonal to the end-
to-end protections described in this document.
To the extent that opportunistic message protections are made visible
to the user for a given copy of a message, a reasonable MUA will
distinguish that status from the message's end-to-end cryptographic
status. But the potential confusion caused by rendering this
complex, hybrid state may not be worth the value of additional
knowledge gained by the user. The benefits of opportunistic
protections accrue (or don't) even without visibility to the user
(see [RFC7435]).
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The user needs a single clear, simple, and correct indication about
the end-to-end cryptographic status of any given message. See
Section 4.6 for more details.
4. Cryptographic MIME Message Structure
Implementations use the structure of an e-mail message to establish
(when sending) and understand (when receiving) the cryptographic
status of the message. This section establishes some conventions
about how to think about message structure.
4.1. Cryptographic Layers
"Cryptographic Layer" refers to a MIME substructure that supplies
some cryptographic protections to an internal MIME subtree. The
internal subtree is known as the "protected part" though of course it
may itself be a multipart object.
In the diagrams below, "↧" (DOWNWARDS ARROW FROM BAR, U+21A7)
indicates "decrypts to", and "⇩" (DOWNWARDS WHITE ARROW, U+21E9)
indicates "unwraps to".
4.1.1. S/MIME Cryptographic Layers
For S/MIME [RFC8551], there are four forms of Cryptographic Layers:
multipart/signed, PKCS#7 signed-data, PKCS7 enveloped-data, PKCS7
authEnveloped-data.
4.1.1.1. S/MIME Multipart Signed Cryptographic Layer
└┬╴multipart/signed; protocol="application/pkcs7-signature"
├─╴[protected part]
└─╴application/pkcs7-signature
This MIME layer offers authentication and integrity.
4.1.1.2. S/MIME PKCS7 signed-data Cryptographic Layer
└─╴application/pkcs7-mime; smime-type="signed-data"
⇩ (unwraps to)
└─╴[protected part]
This MIME layer offers authentication and integrity.
4.1.1.3. S/MIME PKCS7 enveloped-data Cryptographic Layer
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└─╴application/pkcs7-mime; smime-type="enveloped-data"
↧ (decrypts to)
└─╴[protected part]
This MIME layer offers confidentiality.
4.1.1.4. S/MIME PKCS7 authEnveloped-data Cryptographic Layer
└─╴application/pkcs7-mime; smime-type="authEnveloped-data"
↧ (decrypts to)
└─╴[protected part]
This MIME layer offers confidentiality and integrity.
Note that enveloped-data (Section 4.1.1.3) and authEnveloped-data
(Section 4.1.1.4) have identical message structure and very similar
semantics. The only difference between the two is ciphertext
malleability.
The examples in this document only include enveloped-data, but the
implications for that layer apply to authEnveloped-data as well.
4.1.1.5. PKCS7 Compression is NOT a Cryptographic Layer
The Cryptographic Message Syntax (CMS) provides a MIME compression
layer (smime-type="compressed-data"), as defined in [RFC3274]. While
the compression layer is technically a part of CMS, it is not
considered a Cryptographic Layer for the purposes of this document.
4.1.2. PGP/MIME Cryptographic Layers
For PGP/MIME [RFC3156] there are two forms of Cryptographic Layers,
signing and encryption.
4.1.2.1. PGP/MIME Signing Cryptographic Layer (multipart/signed)
└┬╴multipart/signed; protocol="application/pgp-signature"
├─╴[protected part]
└─╴application/pgp-signature
This MIME layer offers authenticity and integrity.
4.1.2.2. PGP/MIME Encryption Cryptographic Layer (multipart/encrypted)
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└┬╴multipart/encrypted
├─╴application/pgp-encrypted
└─╴application/octet-stream
↧ (decrypts to)
└─╴[protected part]
This MIME layer can offer any of:
* confidentiality (via a Symmetrically Encrypted Data Packet, see
Section 5.7 of [RFC4880]; a MUA MUST NOT generate this form due to
ciphertext malleability)
* confidentiality and integrity (via a Symmetrically Encrypted
Integrity Protected Data Packet (SEIPD), see Section 5.13 of
[RFC4880]), or
* confidentiality, integrity, and authenticity all together (by
including an OpenPGP Signature Packet within the SEIPD).
4.2. Cryptographic Envelope
The Cryptographic Envelope is the largest contiguous set of
Cryptographic Layers of an e-mail message starting with the outermost
MIME type (that is, with the Content-Type of the message itself).
If the Content-Type of the message itself is not a Cryptographic
Layer, then the message has no cryptographic envelope.
"Contiguous" in the definition above indicates that if a
Cryptographic Layer is the protected part of another Cryptographic
Layer, the layers together comprise a single Cryptographic Envelope.
Note that if a non-Cryptographic Layer intervenes, all Cryptographic
Layers within the non-Cryptographic Layer _are not_ part of the
Cryptographic Envelope. They are Errant Cryptographic Layers (see
Section 4.5).
Note also that the ordering of the Cryptographic Layers implies
different cryptographic properties. A signed-then-encrypted message
is different than an encrypted-then-signed message. See Section 5.2.
4.3. Cryptographic Payload
The Cryptographic Payload of a message is the first non-Cryptographic
Layer -- the "protected part" -- within the Cryptographic Envelope.
4.4. Types of Cryptographic Envelope
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4.4.1. Simple Cryptographic Envelopes
As described above, if the "protected part" identified in the section
above is not itself a Cryptographic Layer, that part _is_ the
Cryptographic Payload.
If the application wants to generate a message that is both encrypted
and signed, it MAY use the simple MIME structure from Section 4.1.2.2
by ensuring that the [RFC4880] Encrypted Message within the
application/octet-stream part contains an [RFC4880] Signed Message
(the final option described in Section 4.1.2.2).
4.4.2. Multilayer Cryptographic Envelopes
It is possible to construct a Cryptographic Envelope consisting of
multiple layers with either S/MIME or PGP/MIME , for example using
the following structure:
A └─╴application/pkcs7-mime; smime-type="enveloped-data"
B ↧ (decrypts to)
C └─╴application/pkcs7-mime; smime-type="signed-data"
D ⇩ (unwraps to)
E └─╴[protected part]
When handling such a message, the properties of the Cryptographic
Envelope are derived from the series A, C.
As noted in Section 4.4.1, PGP/MIME applications also have a simpler
MIME construction available with the same cryptographic properties.
4.5. Errant Cryptographic Layers
Due to confusion, malice, or well-intentioned tampering, a message
may contain a Cryptographic Layer that is not part of the
Cryptographic Envelope. Such a layer is an Errant Cryptographic
Layer.
An Errant Cryptographic Layer SHOULD NOT contribute to the message's
overall cryptographic state.
Guidance for dealing with Errant Cryptographic Layers can be found in
Section 6.2.
4.5.1. Mailing List Wrapping
Some mailing list software will re-wrap a well-formed signed message
before re-sending to add a footer, resulting in the following
structure seen by recipients of the e-mail:
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H └┬╴multipart/mixed
I ├┬╴multipart/signed
J │├─╴text/plain
K │└─╴application/pgp-signature
L └─╴text/plain
In this message, L is the footer added by the mailing list. I is now
an Errant Cryptographic Layer.
Note that this message has no Cryptographic Envelope at all.
It is NOT RECOMMENDED to produce e-mail messages with this structure,
because the data in part L may appear to the user as though it were
part of J, though they have different cryptographic properties. In
particular, if the user believes that the message is signed, but
cannot distinguish L from J then the author of L can effectively
tamper with content of the signed message, breaking the user's
expectation of integrity and authenticity.
4.5.2. A Baroque Example
Consider a message with the following overcomplicated structure:
M └┬╴multipart/encrypted
N ├─╴application/pgp-encrypted
O └─╴application/octet-stream
P ↧ (decrypts to)
Q └┬╴multipart/signed
R ├┬╴multipart/mixed
S │├┬╴multipart/signed
T ││├─╴text/plain
U ││└─╴application/pgp-signature
V │└─╴text/plain
W └─╴application/pgp-signature
The 3 Cryptographic Layers in such a message are rooted in parts M,
Q, and S. But the Cryptographic Envelope of the message consists
only of the properties derived from the series M, Q. The
Cryptographic Payload of the message is part R. Part S is an Errant
Cryptographic Layer.
Note that this message has both a Cryptographic Envelope _and_ an
Errant Cryptographic Layer.
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It is NOT RECOMMENDED to generate messages with such complicated
structures. Even if a receiving MUA can parse this structure
properly, it is nearly impossible to render in a way that the user
can reason about the cryptographic properties of part T compared to
part V.
4.6. Cryptographic Summary
The cryptographic status of an e-mail message with end-to-end
cryptographic protections is known as the Cryptographic Summary. A
reasonable, simple Cryptographic Summary is derived from the
aggregate properties of the layers in the Cryptographic Envelope.
This is a conceptual tool and a feature that a MUA can use to guide
behavior and user experience, but it is not necessarily always
directly exposed in any given user interface. See Section 6.1 for
guidance and considerations about rendering the Cryptographic Summary
to the user.
5. Message Composition
This section describes the ideal composition of an e-mail message
with end-to-end cryptographic protection. A message composed with
this form is most likely to achieve its end-to-end security goals.
5.1. Message Composition Algorithm
This section roughly describes the steps that a MUA should use to
compose a cryptographically-protected message that has a proper
cryptographic envelope and payload.
The message composition algorithm takes three parameters:
* origbody: the traditional unprotected message body as a well-
formed MIME tree (possibly just a single MIME leaf part). As a
well-formed MIME tree, origbody already has structural headers
present (see Section 1.1.1).
* origheaders: the intended non-structural headers for the message,
represented here as a list of (h,v) pairs, where h is a header
field name and v is the associated value.
* crypto: The series of cryptographic protections to apply (for
example, "sign with the secret key corresponding to X.509
certificate X, then encrypt to X.509 certificates X and Y"). This
is a routine that accepts a MIME tree as input (the Cryptographic
Payload), wraps the input in the appropriate Cryptographic
Envelope, and returns the resultant MIME tree as output.
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The algorithm returns a MIME object that is ready to be injected into
the mail system:
* Apply crypto to origbody, yielding MIME tree output
* For each header name and value (h,v) in origheaders:
- Add header h to output with value v
* Return output
5.2. Encryption Outside, Signature Inside
Users expect any message that is both signed and encrypted to be
signed _inside_ the encryption, and not the other way around. For
example, when crafting an encrypted and signed message using a simple
Cryptographic Envelope of a single layer (Section 4.4.1) with PGP/
MIME, the OpenPGP Encrypted Message object should contain an OpenPGP
Signed Message. Likewise, when using a multilayer Cryptographic
Envelope (Section 4.4.2), the outer layer should be an encryption
layer and the inner layer should be a signing layer.
Putting the signature inside the encryption has two advantages:
* The details of the signature remain confidential, visible only to
the parties capable of decryption.
* Any mail transport agent that modifies the message is unlikely to
be able to accidentally break the signature.
A MUA SHOULD NOT generate an encrypted and signed message where the
only signature is outside the encryption.
5.3. Avoid Offering Encrypted-only Messages
When generating an e-mail, the user has options about what forms of
end-to-end cryptographic protections to apply to it.
In some cases, offering any end-to-end cryptographic protection is
harmful: it may confuse the recipient and offer no benefit.
In other cases, signing a message is useful (authenticity and
integrity are desirable) but encryption is either impossible (for
example, if the sender does not know how to encrypt to all
recipients) or meaningless (for example, an e-mail message to a
mailing list that is intended to be be published to a public
archive).
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In other cases, full end-to-end confidentiality, authenticity, and
integrity are desirable.
It is unclear what the use case is for an e-mail message with end-to-
end confidentiality but without authenticity or integrity.
A reasonable MUA will keep its message composition interface simple,
so when presenting the user with a choice of cryptographic
protection, it SHOULD offer no more than three choices:
* no end-to-end cryptographic protection
* Verified (signed only)
* Confidential (signed and encrypted)
Note that these choices correspond to the simplified mental model in
Section 3.1.
5.4. Composing a Reply Message
When replying to a message, most MUAs compose an initial draft of the
reply that contains quoted text from the original message. A
responsible MUA will take precautions to avoid leaking the cleartext
of an encrypted message in such a reply.
If the original message was end-to-end encrypted, the replying MUA
MUST either:
* compose the reply with end-to-end encryption, or
* avoid including quoted text from the original message.
In general, MUAs SHOULD prefer the first option: to compose an
encrypted reply. This is what users expect.
However, in some circumstances, the replying MUA cannot compose an
encrypted reply. For example, the MUA might not have a valid,
unexpired, encryption-capable certificate for all recipients. This
can also happen during composition when a user adds a new recipient
into the reply, or manually toggles the cryptographic protections to
remove encryption.
In this circumstance, the composing MUA SHOULD strip the quoted text
from the original message.
Note additional nuance about replies to malformed messages that
contain encryption in Section 6.2.2.1.
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6. Message Interpretation
Despite the best efforts of well-intentioned senders to create e-mail
messages with well-formed end-to-end cryptographic protection,
receiving MUAs will inevitably encounter some messages with malformed
end-to-end cryptographic protection.
This section offers guidance on dealing with both well-formed and
malformed messages containing Cryptographic Layers.
6.1. Rendering Well-formed Messages
A message is well-formed when it has a Cryptographic Envelope, a
Cryptographic Payload, and no Errant Cryptographic Layers. Rendering
a well-formed message is straightforward.
The receiving MUA should evaluate and assemble the cryptographic
properties of the Cryptographic Envelope into a Cryptographic Summary
and display that status to the user in a secure, strictly-controlled
part of the UI. In particular, the part of the UI used to render the
Cryptographic Summary of the message MUST NOT be spoofable,
modifiable, or otherwise controllable by the received message itself.
Aside from this Cryptographic Summary, the message itself should be
rendered as though the Cryptographic Payload is the body of the
message. The Cryptographic Layers themselves SHOULD not be rendered
otherwise.
6.2. Errant Cryptographic Layers
If an incoming message has any Errant Cryptographic Layers, the
interpreting MUA SHOULD ignore those layers when rendering the
Cryptographic Summary of the message to the user.
6.2.1. Errant Signing Layer
When rendering a message with an Errant Cryptographic Layer that
provides authenticity and integrity (via signatures), the message
should be rendered by replacing the Cryptographic layer with the part
it encloses.
For example, a message with this structure:
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A └┬╴multipart/mixed
B ├╴text/plain
C ├┬╴multipart/signed
D │├─╴image/jpeg
E │└─╴application/pgp-signature
F └─╴text/plain
Should be rendered identically to this:
A └┬╴multipart/mixed
B ├─╴text/plain
D ├─╴image/jpeg
F └─╴text/plain
In such a situation, an MUA SHOULD NOT indicate in the Cryptographic
Summary that the message is signed.
6.2.1.1. Exception: Mailing List Footers
The use case described in Section 4.5.1 is common enough in some
contexts, that a MUA MAY decide to handle it as a special exception.
If the MUA determines that the message comes from a mailing list (for
example, it has a List-ID header), and it has a structure that
appends a footer to a signing-only Cryptographic Layer with a valid
signature, such as:
H └┬╴multipart/mixed
I ├┬╴multipart/signed
J │├─╴[protected part, may be arbitrary MIME subtree]
K │└─╴application/{pgp,pkcs7}-signature
L └─╴[footer, typically text/plain]
or:
H └┬╴multipart/mixed
I ├─╴application/pkcs7-mime; smime-type="signed-data"
│⇩ (unwraps to)
J │└─╴[protected part, may be an arbitrary MIME subtree]
L └─╴[footer, typically text/plain]
Then, the MUA MAY indicate to the user that this is a signed message
that has been wrapped by the mailing list.
In this case, the MUA MUST distinguish the footer (part L) from the
protected part (part J) when rendering any information about the
signature.
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One way to do this is to offer the user two different views of the
message: the "mailing list" view, which hides any Cryptographic
Summary but shows the footer:
Cryptographic Protections: none
H └┬╴multipart/mixed
J ├─╴[protected part, may be arbitrary MIME subtree]
L └─╴[footer, typically text/plain]
or the "sender's view", which shows the Cryptographic Summary but
hides the footer:
Cryptographic Protections: signed [details from part I]
J └─╴[protected part, may be arbitrary MIME subtree]
6.2.2. Errant Encryption Layer
An MUA may encounter a message with an Errant Cryptographic Layer
that offers confidentiality (encryption), and the MUA is capable of
decrypting it.
The user wants to be able to see the contents of any message that
they receive, so an MUA in this situation SHOULD decrypt the part.
In this case, though, the MUA MUST NOT indicate in the message's
Cryptographic Summary that the message itself was encrypted. Such an
indication could be taken to mean that other (non-encrypted) parts of
the message arrived with cryptographic confidentiality.
Furthermore, when decrypting an Errant Cryptographic Layer, the MUA
MUST treat the decrypted cleartext as a distinct MIME subtree, and
not attempt to merge or splice it together with any other part of the
message. This offers protection against the direct exfiltration
(also known as EFAIL-DE) attacks described in [EFAIL] and so-called
multipart/oracle attacks described in [ORACLE].
6.2.2.1. Replying to a Message with an Errant Encryption Layer
Note that there is an asymmetry here between rendering and replying
to a message with an Errant Encryption Layer.
When rendering, the MUA does not indicate that the message was
encrypted, even if some subpart of it was decrypted for rendering.
When composing a reply to a message that has any encryption layer,
even an errant one, the reply message SHOULD be marked for
encryption, as noted in Section 5.4.
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When composing a reply to a message with an errant cryptographic
layer, the MUA MUST NOT decrypt any errant cryptographic layers when
generating quoted or attributed text. This will typically mean
either leaving the ciphertext itself in the generated reply message,
or simply no generating any quoted or attributed text at all. This
offers protection against the reply-based attacks described in
[REPLY].
In all circumstances, if the reply message cannot be encrypted (or if
the user elects to not encrypt the reply), the composed reply MUST
NOT include any material from the decrypted subpart.
6.2.3. Avoiding Non-MIME Cryptographic Mechanisms
In some cases, there may be a cryptographic signature or encryption
that does not coincide with a MIME boundary. For example so-called
"PGP Inline" messages typically contain base64-encoded ("ASCII-
armored", see Section 6 of [RFC4880]) ciphertext, or within the
content of a MIME part.
6.2.3.1. Do Not Validate Non-MIME Signatures
When encountering cryptographic signatures in these positions, a MUA
MUST NOT attempt to validate any signature. It is challenging to
communicate to the user exactly which part of such a message is
covered by the signature, so it is better to leave the message marked
as unsigned. See [SPOOFING] for examples of spoofed message
signatures that rely on permissive clients willing to validate
signatures in poorly-structured messages.
6.2.3.2. Skip or Isolate Non-MIME Decryption When Rendering
When encountering what appears to be encrypted data not at a MIME
boundary, the MUA MAY decline to decrypt the data at all.
During message rendering, if the MUA attempts decryption of such a
non-MIME encrypted section of an e-mail, it MUST synthesize a
separate MIME part to contain only the decrypted data, and not
attempt to merge or splice that part together with any other part of
the message. Keeping such a section distinct and isolated from any
other part of the message offers protection against the direct
exfiltration attacks (also known as EFAIL-DE) described in [EFAIL].
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6.2.3.3. Do Not Decrypt Non-MIME Decryption when Replying
When composing a reply to a message with such a non-MIME encrypted
section, the MUA MUST NOT decrypt the any non-MIME encrypted section
when generating quoted or attributed text, similar to the guidance in
Section 6.2.2.1.
This offers protection against the reply-based attacks described in
[REPLY].
6.3. Forwarded Messages with Cryptographic Protection
An incoming e-mail message may include an attached forwarded message,
typically as a MIME subpart with Content-Type: message/rfc822
([RFC5322]) or Content-Type: message/global ([RFC5355]).
Regardless of the cryptographic protections and structure of the
incoming message, the internal forwarded message may have its own
Cryptographic Envelope.
The Cryptographic Layers that are part of the Cryptographic Envelope
of the forwarded message are not Errant Cryptographic Layers of the
surrounding message -- they are simply layers that apply to the
forwarded message itself.
The rendering MUA MUST NOT conflate the cryptographic protections of
the forwarded message with the cryptographic protections of the
incoming message.
The rendering MUA MAY render a Cryptographic Summary of the
protections afforded to the forwarded message by its own
Cryptographic Envelope, as long as that rendering is unambiguously
tied to the forwarded message itself, and cannot be spoofed either by
the enclosing message or by the forwarded message.
6.4. Signature failures
A cryptographic signature may fail in multiple ways. A receiving MUA
that discovers a failed signature should treat the message as though
the signature did not exist. This is similar to the standard
guidance for about failed DKIM signatures (see Section 6.1 of
[RFC6376]).
A MUA SHOULD NOT render a message with a failed signature as more
dangerous or more dubious than a comparable message without any
signature at all.
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A MUA that encounters an encrypted-and-signed message where the
signature is invalid SHOULD treat the message the same way that it
would treat a message that is encryption-only.
Some different ways that a signature may be invalid on a given
message:
* the signature is not cryptographically valid (the math fails).
* the signature relies on suspect cryptographic primitives (e.g.
over a legacy digest algorithm, or was made by a weak key, e.g.,
1024-bit RSA)
* the signature is made by a certificate which the receiving MUA
does not have access to.
* the certificate used to verify the signature was revoked.
* the certificate used to verify the signature was expired at the
time that the signature was made.
* the certificate used to verify the signature does not correspond
to the author of the message. (for X.509, there is no
subjectAltName of type RFC822Name whose value matches an e-mail
address found in From: or Sender:)
* the certificate used to verify the signature was not issued by an
authority that the MUA user is willing to rely on for certifying
the sender's e-mail address, and the user has no other reasonable
indication that the certificate belongs to the sender's e-mail
address.
* the signature indicates that it was made at a time much before or
much after from the date of the message itself.
* The signature covers a message that depends on an external
subresource that might change (see Section 9.9).
A valid signature must pass all these tests, but of course invalid
signatures may be invalid in more than one of the ways listed above.
7. Reasoning about Message Parts
When generating or rendering messages, it is useful to know what
parts of the message are likely to be displayed, and how. This
section introduces some common terms that can be applied to parts
within the Cryptographic Payload.
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7.1. Main Body Part
When an e-mail message is composed or rendered to the user there is
typically one main view that presents a (mostly textual) part of the
message.
While the message itself may be constructed of several distinct MIME
parts in a tree, the part that is rendered to the user is the "Main
Body Part".
When rendering a message, one of the primary jobs of the receiving
MUA is identifying which part (or parts) is the Main Body Part.
Typically, this is found by traversing the MIME tree of the message
looking for a leaf node that has a primary content type of text (e.g.
text/plain or text/html) and is not Content-Disposition: attachment.
MIME tree traversal follows the first child of every multipart node,
with the exception of multipart/alternative. When traversing a
multipart/alternative node, all children should be scanned, with
preference given to the last child node with a MIME type that the MUA
is capable of rendering directly.
A MUA MAY offer the user a mechanism to prefer a particular MIME type
within multipart/alternative instead of the last renderable child.
For example, a user may explicitly prefer a text/plain alternative
part over text/html.
Note that due to uncertainty about the capabilities and configuration
of the receiving MUA, the composing MUA SHOULD consider that multiple
parts might be rendered as the Main Body Part when the message is
ultimately viewed.
When composing a message, an originating MUA operating on behalf of
an active user can identify which part (or parts) are the "main"
parts: these are the parts the MUA generates from the user's editor.
Tooling that automatically generates e-mail messages should also have
a reasonable estimate of which part (or parts) are the "main" parts,
as they can be programmatically identified by the message author.
For a filtering program that attempts to transform an outbound
message without any special knowledge about which parts are Main Body
Parts, it can identify the likely parts by following the same routine
as a receiving MUA.
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7.2. Attachments
A message may contain one or more separated MIME parts that are
intended for download or extraction. Such a part is commonly called
an "attachment", and is commonly identified by having Content-
Disposition: attachment.
An MUA MAY identify a subpart as an attachment, or permit extraction
of a subpart even when the subpart does not have Content-Disposition:
attachment.
For a message with end-to-end cryptographic protection, any
attachment SHOULD be included within the Cryptographic Payload. If
an attachment is found outside the Cryptographic Payload, then the
message is not well-formed (see Section 6.1), and will not be handled
by other MUAs as intended.
Some MUAs have tried to compose messages where each attachment is
placed in its own cryptographic envelope. Such a message is
problematic for several reasons:
* The attachments can be stripped, replaced, or reordered without
breaking any cryptographic integrity mechanism.
* The resulting message may have a mix of cryptographic statuses
(e.g. if a signature on one part fails but another succeeds, or if
one part is encrypted and another is not). This mix of statuses
is difficult to represent to the user in a comprehensible way.
* The divisions between the different attachments are visible to
operators of any mail transport agent (MTA) that handles the
message, potentially resulting in a metadata leak. For example,
the MTA operator may learn the number of attachments, and the size
of each attachment.
These messages are unlikely to be usefully interoperable without
additional standardization work (see Appendix A.11).
7.3. MIME Part Examples
Consider a common message with the folloiwing MIME structure:
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M └─╴application/pkcs7-mime
↧ (decrypts to)
N └─╴application/pkcs7-mime
⇩ (unwraps to)
O └┬╴multipart/mixed
P ├┬╴multipart/alternative
Q │├─╴text/plain
R │└─╴text/html
S └─╴image/png
Parts M and N comprise the Cryptographic Envelope.
Parts Q and R are both Main Body Parts.
If part S is Content-Disposition: attachment, then it is an
attachment. If part S has no Content-Disposition header, it is
potentially ambiguous whether it is an attachment or not. If the
sender prefers a specific behavior, it should explicitly set the
Content-Disposition header on part S to either inline or attachment
as guidance to the receiving MUA.
Consider also this alternate structure:
M └─╴application/pkcs7-mime
↧ (decrypts to)
N └─╴application/pkcs7-mime
⇩ (unwraps to)
O └┬╴multipart/alternative
P ├─╴text/plain
Q └┬╴multipart/related
R ├─╴text/html
S └─╴image/png
In this case, parts M and N are still the Cryptographic Envelope.
Parts P and R (the first two leaf nodes within each subtree of part
O) are the Main Body Parts.
Part S is more likely not to be an attachment, as the subtree layout
suggests that it is only relevant for the HTML version of the
message. For example, it might be rendered as an image within the
HTML alternative.
8. Certificate Management
A cryptographically-capable MUA typically maintains knowledge about
certificates for the user's own account(s), as well as certificates
for the peers that it communicates with.
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8.1. Peer Certificates
Most certificates that a cryptographically-capable MUA will use will
be certificates belonging to the parties that the user communicates
with through the MUA. This section discusses how to manage the
certificates that belong to such a peer.
The MUA will need to be able to discover X.509 certificates for each
peer, cache them, and select among them when composing an encrypted
message.
Detailed guidance about how to do this is beyond the scope of this
document, but future revisions may bring it into scope (see
Appendix A.2).
8.1.1. Peer Certificate Selection
When composing an encrypted message, the MUA needs to select an
encryption-capable certificate for each recipient.
To select such a certificate for a given destination e-mail address,
the MUA should look through all of its known certificates and verify
that _all_ of the conditions below are met:
* The certificate must be valid, not expired or revoked.
* It must have a subjectAltName of type rFC822Name whose contents
match the destination address. In particular, the local-part of
the two addresses should be an exact bytewise match, and the
domain parts of the two addresses should be matched by ensuring
label equivalence across the full domain name, as described in
Section 2.3.2.4 of [RFC5890].
* The algorithm OID in the certificate's SPKI is known to the MUA
and capable of encryption. Examples include:
- rsaEncryption (OID 1.2.840.113549.1.1.1), with keyUsage (OID
2.5.29.15) extension present and the "key encipherment" bit
(value 32) set.
- curveX25519 (OID 1.3.101.110) with keyUsage extension present
and the "key agreement" bit (value 8) set.
* If extendedKeyUsage (OID 2.5.29.37) is present, it contains at
least one of the following OIDs: e-mail protection (OID
1.3.6.1.5.5.7.3.4), anyExtendedKeyUsage(OID 2.5.29.37.0).
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A reasonable MUA may include more considerations when selecting a
peer certificate as well, see Appendix A.2.4 for examples.
8.2. Local Certificates
The MUA also needs to know about one or more certificates associated
with the user's e-mail account. It is typically expected to have
access to the secret key material associated with the public keys in
those certificates.
While some basic guidance is offered here, it is beyond the scope of
this document to describe all possible relevant guidance for local
certificate and key material handling. See Appendix A.3 for
suggestions of guidance that a future version might bring into scope.
8.2.1. Getting Certificates for the User
If the MUA does not have any certificate or secret key for the user,
it SHOULD help the user to generate, acquire, or import them with a
minimum of difficulty.
8.2.1.1. User Certificates for S/MIME
For S/MIME, the user SHOULD have both a signing-capable certificate
and an encryption-capable certificate (and the corresponding secret
keys). Using the same cryptographic key material for multiple
algorithms (i.e., for both encryption and signing) has been the
source of vulnerabilities in other (non-e-mail) contexts (e.g.,
[DROWN] and [IKE]). The simplest way to avoid any comparable risk is
to use distinct key material for each cryptographic algorithm. The
MUA SHOULD NOT encourage the use of a single S/MIME certificate for
both encryption and signing, to avoid possible cross-protocol key
misuse.
The simplest option for an S/MIME-capable MUA is for the MUA to
permit the user to import a PKCS #12 ([RFC7292]) object that is
expected to contain secret key material, end entity certificates for
the user, and intermediate certification authority certificates that
permit chaining from the end entity certs to widely-accepted trust
anchors.
An S/MIME-capable MUA that has access to user certificates and their
corresponding secret key material should also offer the ability to
export those objects into a well-formed PKCS #12 object that could be
imported into another MUA operated by the same user.
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Manual handling of PKCS #12 objects is challenging for most users.
Producing the initial PKCS #12 object typically can only be done with
the aid of a certification authority via non-standardized, labor-
intensive, and error-prone procedures that most users do not
understand. Furthermore, manual export and import incurs ongoing
labor (for example, before certificate expiration) by the user which
most users are unprepared to do (see Section 8.2.2).
A better approach is for the MUA to integrate some form of automated
certificate issuance procedure, for example, by using the ACME
protocol for end user S/MIME certificates ([RFC8823]).
See also [I-D.woodhouse-cert-best-practice] for more recommendations
about managing user certificates.
8.2.1.2. User Certificates for PGP/MIME
As distinct from S/MIME, OpenPGP ([RFC4880]) has a different set of
common practices. For one thing, a single OpenPGP certificate can
contain both a signing-capable key and a distinct encryption-capable
key, so only one certificate is needed for an e-mail user of OpenPGP,
as long as the certificate has distinct key material for the
different purposes.
Furthermore, a single OpenPGP certificate MAY be only self-signed, so
the MUA can generate such a certificate entirely on its own.
An OpenPGP-capable MUA should have the ability to import and export
OpenPGP Tranferable Secret Keys (see Section 11.2 of [RFC4880]), to
enable manual transfer of user certificates and secret key material
between multiple MUAs controlled by the user.
Since an OpenPGP certificate MAY be certified by third parties
(whether formal certification authorities or merely other well-
connected peers) the MUA SHOULD offer affordances to help the user
acquire and merge third party certifications on their certificate.
When doing this, the MUA should prioritize third-party certifications
from entities that the user's peers are likely to know about and be
willing to rely on.
Since an OpenPGP certificate can grow arbitrarily large with third-
party certifications, the MUA should assist the user in pruning it to
ensure that it remains a reasonable size when transmitting it to
other parties.
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8.2.1.3. Generate Secret Key Material Locally
Regardless of protocol used (S/MIME or PGP), when producing
certificates for the end user, the MUA SHOULD ensure that it has
generated secret key material locally, and MUST NOT accept secret key
material from an untrusted external party as the basis for the user's
certificate.
An MUA that accepts secret key material from a third party cannot
prevent that third party from retaining this material. A third party
with this level of access could decrypt messages intended to be
confidential for the user, or could forge messages that would appear
to come from the user.
8.2.2. Local Certificate Maintenance
In the context of a single e-mail account managed by a MUA, where
that e-mail account is expected to be able to use end-to-end
cryptographic protections, the MUA SHOULD warn the user (or
proactively fix the problem) when/if:
* The user's own certificate set for the account does not include a
valid, unexpired encryption-capable X.509 certificate, and a
valid, unexpired signature-capable X.509 certificate.
* Any of the user's own certificates for the account:
- is due to expire in the next month (or at some other reasonable
cadence).
- not match the e-mail address associated with the account in
question.
* Any of the user's own S/MIME certificates for the account:
- does not have a keyUsage extension.
- does not contain an extendedKeyUsage extension.
- would be considered invalid by the MUA for any other reason if
it were a peer certificate.
An MUA that takes active steps to fix any of these problems before
they arise is even more usable than just warning, but guidance on how
to do active certificate maintenance is beyond scope for this current
document (see Appendix A.3.3).
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If the MUA does find any of these issues and chooses to warn the
user, it should use one aggregate warning with simple language that
the certificates might not be acceptable for other people, and
recommend a course of action that the user can take to remedy the
problem.
8.2.3. Shipping Certificates in Outbound Messages
When sending mail, an MUA SHOULD include copies of the user's own
certificates (and potentially other certificates) in each message to
facilitate future communication.
The mechanism for including these certificates, and which
certificates to include in the message, are protocol specific.
8.2.3.1. Shipping Certificates in S/MIME Messages
In any S/MIME SignedData object, certificates can be shipped in the
"certificates" member. In S/MIME EnvelopedData object, certificates
can be shipped in the "originatorInfo.certs" member.
When a single S/MIME-protected e-mail message is encrypted and
signed, it is usually sufficient to ship all the relevant
certificates in the inner SignedData object's "certificates" member.
The S/MIME certificates shipped in such a message SHOULD include:
* The user's own S/MIME signing certificate, so that signature on
the current message can be validated.
* The user's own S/MIME encryption-capable certificate, so that the
recipient can reply in encrypted form.
* On an encrypted message to multiple recipients, the encryption-
capable peer certificates of the other recipients, so that any
recipient can easily "reply all" without needing to search for
certificates.
* Any intermediate certificates needed to chain all of the above to
a widely-trusted set of root authorities.
8.2.3.2. Shipping Certificates in PGP/MIME Messages
PGP/MIME does not have a single specific standard location for
shipping certificates.
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Some MUAs ship relevant OpenPGP certificates in a single MIME leaf of
Content-Type "application/pgp-keys". When such a message has
cryptographic protections, to ensure that the message is well-formed,
this kind of MIME part SHOULD be a leaf of the Cryptographic Payload,
and not outside of it. One problem with this approach is that it
appears to recipients with legacy MUAs as an "attachment", which can
lead to confusion if the user does not know how to use it.
Other implementations ship relevant OpenPGP certificates in
"Autocrypt" or "Autocrypt-Gossip" message headers (see [AUTOCRYPT]).
To ensure that those headers receive the same cryptographic
authenticity as the rest of the message, these headers SHOULD be
protected as described in [I-D.ietf-lamps-header-protection].
The OpenPGP certificates shipped in such a message SHOULD include:
* The user's own OpenPGP certificate, capable of both signing and
encryption, so that the user can validate the message's signature
and can encrypt future messages
* On an encrypted message to multiple recipients, the OpenPGP
certificates of the other recipients, so that any recipient can
easily "reply all" without needing to search for certificates.
9. Common Pitfalls and Guidelines
This section highlights a few "pitfalls" and guidelines based on
these discussions and lessons learned.
9.1. Reading Sent Messages
When composing a message, a typical MUA will store a copy of the
message sent in sender's Sent mail folder so that the sender can read
it later. If the message is an encrypted message, storing it
encrypted requires some forethought to ensure that the sender can
read it in the future.
It is a common and simple practice to encrypt the message not only to
the recipients of the message, but also to the sender. One advantage
of doing this is that the message that is sent on the wire can be
identical to the message stored in the sender's Sent mail folder.
This allows the sender to review and re-read the message even though
it was encrypted.
There are at least three other approaches that are possible to ensure
future readability by the sender of the message, but with different
tradeoffs:
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* Encrypt two versions of the message: one to the recipients (this
version is sent on the wire), and one to the sender only (this
version is stored in the sender's Sent folder). This approach
means that the message stored in the Sent folder is not byte-for-
byte identical to the message sent to the recipients. In the
event that message delivery has a transient failure, the MUA
cannot simply re-submit the stored message into the SMTP system
and expect it to be readable by the recipient.
* Store a cleartext version of the message in the Sent folder. This
presents a risk of information leakage: anyone with access to the
Sent folder can read the contents of the message. Furthermore,
any attempt to re-send the message needs to also re-apply the
cryptographic transformation before sending, or else the message
contents will leak upon re-send.
* A final option is that the MUA can store a copy of the message's
encryption session key. Standard e-mail encryption mechanisms
(e.g. S/MIME and PGP/MIME) are hybrid mechanisms: the asymmetric
encryption steps simply encrypt a symmetric "session key", which
is used to encrypt the message itself. If the MUA stores the
session key itself, it can use the session key to decrypt the Sent
message without needing the Sent message to be decryptable by the
user's own asymmetric key. An MUA doing this must take care to
store (and backup) its stash of session keys, because if it loses
them it will not be able to read the sent messages; and if someone
else gains access to them, they can decrypt the sent messages.
This has the additional consequence that any other MUA accessing
the same Sent folder cannot decrypt the message unless it also has
access to the stashed session key.
9.2. Reading Encrypted Messages after Certificate Expiration
When encrypting a message, the sending MUA should decline to encrypt
to an expired certificate (see Section 8.1.1). But when decrypting a
message, as long as the viewing MUA has access to the appropriate
secret key material, it should permit decryption of the message, even
if the associated certificate is expired. That is, the viewing MUA
should not prevent the user from reading a message that they have
already received.
The viewing MUA may warn the user when decrypting a message that
appears to have been encrypted to an encryption-capable certificate
that was expired at the time of encryption (e.g., based on the Date:
header field of the message, or the timestamp in the cryptographic
signature), but otherwise should not complain.
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The primary goal of certificate expiration is to facilitate rotation
of secret key material, so that secret key material does not need to
be retained indefinitely. Certificate expiration permits the user to
destroy an older secret key if access to the messages received under
it is no longer necessary (see also Appendix A.9).
9.3. Unprotected Message Headers
Many legacy cryptographically-aware MUAs only apply cryptographic
protections to the body of the message, but leave the headers
unprotected. This gives rise to vulnerabilities like information
leakage (e.g., the Subject line is visible to a passive intermediary)
or message tampering (e.g., the Subject line is replaced, effectively
changing the semantics of a signed message). These are not only
security vulnerabilities, but usability problems, because the
distinction between what is a header and what is the body of a
message is unclear to many end users, and requires a more complex
mental model than is necessary. Useful security comes from alignment
between simple mental models and tooling.
To avoid these concerns, a MUA SHOULD implement header protection as
described in [I-D.ietf-lamps-header-protection].
Note that some message header fields, like List-*, Archived-At, and
Resent-* are typically added by an intervening MUA (see Section 9.8),
not by one of the traditional "ends" of an end-to-end e-mail
exchange. A receiving MUA may choose to consider the contents of
these header fields on an end-to-end protected message as markers
added during message transit, even if they are not covered by the
end-to-end cryptographic protection.
9.4. Composing an Encrypted Message with Bcc
When composing an encrypted message containing at least one recipient
address in the Bcc header field, there is a risk that the encrypted
message itself could leak information about the actual recipients,
even if the Bcc header field does not mention the recipient. For
example, if the message clearly indicates which certificates it is
encrypted to, the set of certificates can identify the recipients
even if they are not named in the message headers.
Because of these complexities, there are several interacting factors
that need to be taken into account when composing an encrypted
message with Bcc'ed recipients.
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* Section 3.6.3 of [RFC5322] describes a set of choices about
whether (and how) to populate the Bcc field explicitly on Bcc'ed
copies of the message, and in the copy stored in the sender's Sent
folder.
* When separate copies are made for Bcced recipients, should each
separate copy _also_ be encrypted to the named recipients, or just
to the designated Bcc recipient?
* When a copy is stored in the Sent folder, should that copy also be
encrypted to Bcced recipients? (see also Section 9.1)
* When a message is encrypted, if there is a mechanism to include
the certificates of the recipients, whose certificates should be
included?
9.4.1. Simple Encryption with Bcc
Here is a simple approach that tries to minimize the total number of
variants of the message created while leaving a coherent view of the
message itself:
* No cryptographic payload contains any Bcc header field.
* The main copy of the message is signed and encrypted to all named
recipients and to the sender. A copy of this message is also
stored in the sender's Sent folder.
* Each Bcc recipient receives a distinct copy of the message, with
an identical cryptographic payload, and the message is signed and
encrypted to that specific recipient and all the named recipients.
These copies are not stored in the sender's Sent folder.
* To the extent that spare certificates are included in the message,
each generated copy of the message should include certificates for
the sender and for each named recipient. Certificates for Bcc'ed
recipients are not included in any message.
9.4.1.1. Rationale
The approach described in Section 9.4.1 aligns the list of
cryptographic recipients as closely as possible with the set of named
recipients, while still allowing a Bcced recipient to read their own
copy, and to "Reply All" should they want to.
This should reduce user confusion on the receiving side: a recipient
of such a message who naively looks at the user-facing headers from
their own mailbox will have a good sense of what cryptographic
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treatments have been applied to the message. It also simplifies
message composition and user experience: the message composer sees
fields that match their expectations about what will happen to the
message. Additionally, it may preserve the ability for a Bcc'ed
recipient to retain their anonymity, should they need to offer the
signed cryptographic payload to an outside party as proof of the
original sender's intent without revealing their own identity.
9.5. Draft Messages
When composing a message, most MUAs offer the opportunity to save a
copy of the as-yet-unsent message to a "Drafts" folder. If that
folder is itself stored somewhere not under the user's control (e.g.
and IMAP mailbox), it would be a mistake to store the draft message
in the clear, because its contents could leak.
This is the case even if the message is ultimately sent deliberately
in the clear. During message composition, the MUA does not know
whether the message is intended to be sent encrypted or not. For
example, just before sending, the sender could decide to encrypt the
message, and the MUA would have had no way of knowing.
Furthermore, when encrypting a draft message, the message SHOULD only
be encrypted to the user's own certificate, or to some equivalent
secret key that only the user possesses. A draft message encrypted
in this way can be decrypted when the user wants to resume composing
the message, but cannot be read by anyone else, including a potential
intended recipient. Note that a draft message encrypted in this way
will only be resumable by another MUA attached to the same mailbox if
that other MUA has access to the user's decryption-capable secret
key. This shared access to key material is also likely necessary for
useful interoperability, but is beyond the scope of this document
(see Appendix A.3.1).
The message should only be encrypted to its recipients upon actually
sending the message. No reasonable user expects their message's
intended recipients to be able to read a message that is not yet
complete.
9.6. Composing a Message to Heterogeneous Recipients
When sending a message that the user intends to be encrypted, it's
possible that some recipients will be unable to receive an encrypted
copy. For example, when Carol composes a message to Alice and Bob,
Carol's MUA may be able to find a valid encryption-capable
certificate for Alice, but none for Bob.
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In this situation, there are four possible strategies, each of which
has a negative impact on the experience of using encrypted mail.
Carol's MUA can:
1. send encrypted to Alice and Bob, knowing that Bob will be unable
to read the message.
2. send encrypted to Alice only, dropping Bob from the message
recipient list.
3. send the message in the clear to both Alice and Bob.
4. send an encrypted copy of the message to Alice, and a cleartext
copy to Bob.
Each of these strategies has different drawbacks.
The problem with approach 1 is that Bob will receive unreadable mail.
The problem with approach 2 is that Carol's MUA will not send the
message to Bob, despite Carol asking it to.
The problem with approach 3 is that Carol's MUA will not encrypt the
message, despite Carol asking it to.
Approach 4 has two problems:
* Carol's MUA will release a cleartext copy of the message, despite
Carol asking it to send the message encrypted.
* If Alice wants to "reply all" to the message, she may not be able
to find an encryption-capable certificate for Bob either. This
puts Alice in an awkward and confusing position, one that users
are unlikely to understand. In particular, if Alice's MUA is
following the guidance about replies to encrypted messages in
Section 5.4, having received an encrypted copy will make Alice's
Reply buffer behave in an unusual fashion.
This is particularly problematic when the second recipient is not
"Bob" but in fact a public mailing list or other visible archive,
where messages are simply never encrypted.
Carol is unlikely to understand the subtleties and negative
downstream interactions involved with approaches 1 and 4, so
presenting the user with those choices is not advised.
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The most understandable approach for a MUA with an active user is to
ask the user (when they hit "send") to choose between approach 2 and
approach 3. If the user declines to choose between 2 and 3, the MUA
can drop them back to their message composition window and let them
make alternate adjustments.
See also further discussion of these scenarios in
[I-D.dkg-mail-cleartext-copy].
9.7. Message Transport Protocol Proxy: A Dangerous Implementation
Choice
An implementor of end-to-end cryptographic protections may be tempted
by a simple software design that piggybacks off of a mail protocol
like SMTP Submission ([RFC6409]), IMAP ([RFC3501]), or JMAP
([RFC8621]) to handle message assembly and interpretation. In such
an architecture, a naive MUA speaks something like a "standard"
protocol like SMTP, IMAP, or JMAP to a local proxy, and the proxy
handles signing and encryption (outbound), and decryption and
verification (inbound) internally on behalf of the user. While such
a "pluggable" architecture has the advantage that it is likely to be
easy to apply to any mail user agent, it is problematic for the goals
of end-to-end communication, especially in an existing cleartext
ecosystem like e-mail, where any given message might be unsigned or
signed, cleartext or encrypted. In particular:
* the user cannot easily and safely identify what protections any
particular message has (including messages currently being
composed), and
* the proxy itself is unaware of subtle nuances about the message
that the MUA actually knows.
With a trustworthy and well-synchronized sidechannel or protocol
extension between the MUA and the proxy, it is possible to deploy
such an implementation safely, but the requirement for the
sidechannel or extension eliminates the universal-deployability
advantage of the scheme.
Similar concerns apply to any implementation bound by an API which
operates on message objects alone, without any additional contextual
parameters.
This section attempts to document some of the inherent risks involved
with such an architecture.
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9.7.1. Dangers of a Submission Proxy for Message Composition
When composing and sending a message, the act of applying
cryptographic protections has subtleties that cannot be directly
expressed in the SMTP protocol used by Submission [RFC6409], or in
any other simple protocol that hands off a cleartext message for
further processing.
For example, the sender cannot indicate via SMTP whether or not a
given message _should_ be encrypted (some messages, like those sent
to a publicly archived mailing list, are pointless to encrypt), or
select among multiple certificates for a recipient, if they exist
(see Section 8.1.1).
Likewise, because such a proxy only interacts with the message when
it is ready to be sent, it cannot indicate back to the user _during
message composition_ whether or not the message is able to be
encrypted (that is, whether a valid certificate is available for each
intended recipient). A message author may write an entirely
different message if they know that it will be protected end-to-end;
but without this knowledge, the author is obliged either to write
text that they presume will be intercepted, or to risk revealing
sensitive content.
Even without encryption, deciding whether to sign or not (and which
certificate to sign with, if more than one exists) is another choice
that the proxy is ill-equipped to make. The common message-signing
techniques either render a message unreadable by any client that does
not support cryptographic mail (i.e., PKCS7 signed-data), or appear
as an attachment that can cause confusion to a naive recipient using
a legacy client (i.e., multipart/signed). If the sender knows that
the recipient will not check signatures, they may prefer to leave a
cleartext message without a cryptographic signature at all.
Furthermore, handling encryption properly depends on the context of
any given message, which cannot be expressed by the MUA to the
Submission proxy. For example, decisions about how to handle
encryption and quoted or attributed text may depend on the
cryptographic status of the message that is being replied to (see
Section 5.4).
Additionally, such a proxy would need to be capable of managing the
user's own key and certificate (see Section 8.2). How will the
implementation indicate to the user when their own certificate is
near expiry, for example? How will any other error conditions be
handled when communication with the user is needed?
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While an extension to SMTP might be able to express all the necessary
semantics that would allow a generic MUA to compose messages with
standard cryptographic protections via a proxy, such an extension is
beyond the scope of this document. See
[I-D.ietf-jmap-smime-sender-extensions] for an example of how a MUA
using a proxy protocol might indicate signing and encryption
instructions to its proxy.
9.7.2. Dangers of an IMAP Proxy for Message Rendering
When receiving and rendering a message, the process of indicating to
the user the cryptographic status of a message requires subtleties
that are difficult to offer from a straightforwad IMAP (or POP
[RFC1939], or JMAP) proxy.
One approach such a proxy could take is to remove all the
Cryptographic Layers from a well-formed message, and to package a
description of those layers into a special header field that the MUA
can read. But this merely raises the question: what semantics need
to be represented? For example:
* Was the message signed? If so, by whom? When?
* Should the details of the cryptographic algorithms used in any
signatures found be indicated as well?
* Was the message encrypted? if so, to whom? What key was used to
decrypt it?
* If both signed and encrypted, was the signing outside the
encryption or inside?
* How should errant Cryptographic Layers (see Section 4.5) be dealt
with?
* What cryptographic protections do the headers of the message have?
(see [I-D.ietf-lamps-header-protection])
* How are any errors or surprises communicated to the user?
If the proxy passes any of this cryptographic status to the client in
an added header field, it must also ensure that no such header field
is present on the messages it receives before processing it. If it
were to allow such a header field through unmodified to any client
that is willing to trust its contents, an attacker could spoof the
field to make the user believe lies about the cryptographic status of
the message. In order for a MUA to be confident in such a header
field, then, it needs a guarantee from the proxy that any header it
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produces will be safe. How does the MUA reliably negogiate this
guarantee with the proxy? If the proxy can no longer offer this
guarantee, how will the MUA know that things have changed?
If such a proxy handles certificate discovery in inbound messages
(see Appendix A.2.1), it will also need to communicate the results of
that discovery process to its corresponding proxy for message
composition (see Section 9.7.1).
While an extension to IMAP (or POP, or JMAP) might be able to express
all the necessary semantics that would allow a generic MUA to
indicate standardized cryptographic message status, such an extension
is beyond the scope of this document. [RFC9219] describes S/MIME
signature verification status over JMAP, which is a subset of the
cryptographic status information described here.
9.7.3. Who Controls the Proxy?
Finally, consider that the naive proxy deployment approach is risky
precisely because of its opacity to the end user. Such a deployment
could be placed anywhere in the stack, including on a machine that is
not ultimately controlled by the end user, making it effectively a
form of transport protection, rather than end-to-end protection.
A MUA explicitly under the control of the end user with thoughtful
integration can offer UI/UX and security guarantees that a simple
proxy cannot provide. See also Appendix A.12 for suggestions of
future work that might augment a proxy to make it safer.
9.8. Intervening MUAs Do Not Handle End-to-End Cryptographic
Protections
Some Mail User Agents (MUAs) will resend a message in identical form
(or very similar form) to the way that they received it. For
example, consider the following use cases:
* A mail expander or mailing list that receives a message and re-
sends it to all subscribers.
* An individual user who "bounces" a message they received to a
different e-mail address (see Section 3.6.6 of [RFC5322]).
* An automated e-mail intake system that forwards a report to the
mailboxes of responsible staffers.
These MUAs intervene in message transport by receiving and then re-
injecting messages into the mail transport system. In some cases,
the original sender or final recipient of a message that has passed
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through such an MUA may be unaware of the intervention. (Note that
an MUA that forwards a received message as a attachment (MIME
subpart) of type message/rfc822 or message/global or "inline" in the
body of a message is _not_ acting as an intervening MUA in this
sense, because the forwarded message is encapsulated within a visible
outer message that is clearly from the MUA itself.)
An intervening MUA should be aware of end-to-end cryptographic
protections that might already exist on messages that they re-send.
In particular, it is unclear what the "end-to-end" properties are of
a message that has been handled by an intervening MUA. For signed-
only messages, if the intervening MUA makes any substantive
modifications to the the message as it passes it along, it may break
the signature from the original sender. In many cases, breaking the
original signature is the appropriate result, since the original
message has been modified, and the original sender has no control
over the modifications made by the intervening MUA. For encrypted-
and-signed messages, if the intervening MUA is capable of decrypting
the message, it must be careful when re-transmitting the message.
Will the new recipient be able to decrypt it? If not, will the
message be useful to the recipient? If not, it may not make sense to
re-send the message.
Beyond the act of re-sending, an intervening MUA should not itself
try to apply end-to-end cryptographic protections on a message that
it is resending unless directed otherwise by some future
specification. Additional layers of cryptographic protection added
in an ad-hoc way by an intervening MUA are more likely to confuse the
recipient and will not be interpretable as end-to-end protections as
they do not originate with the original sender of the message.
9.9. External Subresources in MIME Parts Break Cryptographic
Protections
A MIME part with a content type that can refer to external resources
(like text/html) may itself have some sort of end-to-end
cryptographic protections. However, retrieving or rendering these
external resources may violate the properties that users expect from
cryptographic protection.
As a baseline, retrieving the external resource at the time a message
is read can be used as a "web bug", leaking the activity and network
location of the receiving user to the server hosting the external
resource. This privacy risk is present, of course, even for messages
with no cryptographic protections, but may be even more surprising to
users who are shown some level of security indicator about a given
message.
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Other problems with external resources are more specifically bound to
cryptographic protections.
For example, an signed e-mail message with at text/html part that
refers to an external image (i.e. via 
) may render differently if the hosting webserver decides to
serve different content at the source URL for the image. This
effectively breaks the goals of integrity and authenticity that the
user should be able to rely on for signed messages, unless the
external subresource has strict integrity guarantees (e.g. via
[SRI]).
Likewise, fetching an external subresource for an encrypted-and-
signed message effectively breaks goals of privacy and
confidentiality for the user.
This is loosely analogous to security indicator problems that arose
for web browsers as described in [mixed-content]. However, while
fetching the external subresource over https is sufficient to avoid a
"mixed content" warning from most browsers, it is insufficicent for a
MUA that wants to offer its users true end-to-end guarantees for
e-mail messages.
A sending MUA that applies signing-only cryptographic protection to a
new e-mail message with an external subresource should take one of
the following options:
* pre-fetch the external subresource and include it in the message
itself,
* use a strong integrity mechanism like Subresource Integrity
([SRI]) to guarantee the content of the subresource (though this
does not fix the "web bug" privacy risk described above), or
* prompt the composing user to remove the subresource from the
message.
A sending MUA that applies encryption to a new e-mail message with an
external resource cannot depend on subresource integrity to protect
the privacy and confidentiality of the message, so it should either
pre-fetch the external resource to include it in the message, or
prompt the composing user to remove it before sending.
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A receiving MUA that encounters a message with end-to-end
cryptographic protections that contain a subresource should either
refuse to retrieve and render the external subresource, or it should
decline to treat the message as having cryptographic protections.
For example, it could decline to indicate that the message is signed,
or it could treat the message as not properly encrypted.
Note that when composing a message reply with quoted text from the
original message, if the original message did contain an external
resource, the composing MUA SHOULD NOT fetch the external resource
solely to include it in the reply message, as doing so would trigger
the "web bug" at reply composition time. Instead, the safest way to
deal with quoted text that contains an external resource in an end-
to-end encrypted reply is to strip any reference to the external
resource during initial composition of the reply.
10. IANA Considerations
This document does not require anything from IANA.
11. Security Considerations
This entire document addresses security considerations about end-to-
end cryptographic protections for e-mail messages.
12. Acknowledgements
The set of constructs and recommendations in this document are
derived from discussions with many different implementers, including
Bjarni Rúnar Einarsson, Daniel Huigens, David Bremner, Deb Cooley,
Eliot Lear, Fabian Ising, Holger Krekel, Jameson Rollins, John
Levine, Jonathan Hammell, juga, Patrick Brunschwig, Pete Resnick,
Santosh Chokhani, Stephen Farrell, and Vincent Breitmoser.
13. References
13.1. Normative References
[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, .
[RFC3156] Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
"MIME Security with OpenPGP", RFC 3156,
DOI 10.17487/RFC3156, August 2001,
.
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[RFC4289] Freed, N. and J. Klensin, "Multipurpose Internet Mail
Extensions (MIME) Part Four: Registration Procedures",
BCP 13, RFC 4289, DOI 10.17487/RFC4289, December 2005,
.
[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, .
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
.
[I-D.ietf-lamps-header-protection]
Gillmor, D. K., Hoeneisen, B., and A. Melnikov, "Header
Protection for Cryptographically Protected E-mail", Work
in Progress, Internet-Draft, draft-ietf-lamps-header-
protection-15, 26 April 2023,
.
13.2. Informative References
[AUTOCRYPT]
Breitmoser, V., Krekel, H., and D. K. Gillmor, "Autocrypt
- Convenient End-to-End Encryption for E-Mail", July 2018,
.
[chrome-indicators]
Schechter, E., "Evolving Chrome's security indicators",
May 2018, .
[EFAIL] Poddebniak, D., Dresen, C., Müller, J., Ising, F.,
Schinzel, S., Friedberger, S., Somorovsky, J., and J.
Schwenk, "Efail: breaking S/MIME and OpenPGP email
encryption using exfiltration channels", August 2018,
.
[mixed-content]
"Mixed Content", February 2023,
.
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[ORACLE] Ising, F., Poddebniak, D., Kappert, T., Saatjohann, C.,
and S. Schinzel, "Content-Type: multipart/oracle Tapping
into Format Oracles in Email End-to-End Encryption",
August 2023,
.
[SRI] "Subresource Integrity", June 2016,
.
[DROWN] Aviram, N., Schinzel, S., Somorovsky, J., Heninger, N.,
Dankel, M., Steube, J., Valenta, L., Adrian, D.,
Halderman, J. A., Dukhovni, V., Käsper, E., Cohney, S.,
Engels, S., Paar, C., and Y. Shavitt, "DROWN: Breaking TLS
using SSLv2", March 2016, .
[IKE] Felsch, D., Grothe, M., Schwenk, J., Czubak, A., and M.
Szymane, "The Dangers of Key Reuse: Practical Attacks on
IPsec IKE", August 2018,
.
[REPLY] Müller, J., Brinkmann, M., Poddebniak, D., Schinzel, S.,
and J. Schwenk, "Re: What’s Up Johnny?: Covert Content
Attacks on Email End-to-End Encryption", Applied
Cryptography and Network Security pp. 24-42,
DOI 10.1007/978-3-030-21568-2_2, 2019,
.
[SPOOFING] Müller, J., Brinkmann, M., Poddebniak, D., Böck, H.,
Schinzel, S., Somorovsky, J., and J. Schwenk, "“Johnny,
you are fired!” – Spoofing OpenPGP and S/MIME Signatures
in Emails", August 2019,
.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, .
[RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, DOI 10.17487/RFC6376, September 2011,
.
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[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, .
[RFC3274] Gutmann, P., "Compressed Data Content Type for
Cryptographic Message Syntax (CMS)", RFC 3274,
DOI 10.17487/RFC3274, June 2002,
.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007,
.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
.
[RFC5355] Stillman, M., Ed., Gopal, R., Guttman, E., Sengodan, S.,
and M. Holdrege, "Threats Introduced by Reliable Server
Pooling (RSerPool) and Requirements for Security in
Response to Threats", RFC 5355, DOI 10.17487/RFC5355,
September 2008, .
[RFC7292] Moriarty, K., Ed., Nystrom, M., Parkinson, S., Rusch, A.,
and M. Scott, "PKCS #12: Personal Information Exchange
Syntax v1.1", RFC 7292, DOI 10.17487/RFC7292, July 2014,
.
[RFC8823] Melnikov, A., "Extensions to Automatic Certificate
Management Environment for End-User S/MIME Certificates",
RFC 8823, DOI 10.17487/RFC8823, April 2021,
.
[I-D.woodhouse-cert-best-practice]
Woodhouse, D. and N. Mavrogiannopoulos, "Recommendations
for applications using X.509 client certificates", Work in
Progress, Internet-Draft, draft-woodhouse-cert-best-
practice-01, 25 July 2023,
.
[I-D.dkg-mail-cleartext-copy]
Gillmor, D. K., "Encrypted E-mail with Cleartext Copies",
Work in Progress, Internet-Draft, draft-dkg-mail-
cleartext-copy-01, 21 February 2023,
.
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[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
.
[RFC8621] Jenkins, N. and C. Newman, "The JSON Meta Application
Protocol (JMAP) for Mail", RFC 8621, DOI 10.17487/RFC8621,
August 2019, .
[I-D.ietf-jmap-smime-sender-extensions]
Melnikov, A., "JMAP extension for S/MIME signing and
encryption", Work in Progress, Internet-Draft, draft-ietf-
jmap-smime-sender-extensions-04, 3 August 2023,
.
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
.
[RFC9219] Melnikov, A., "S/MIME Signature Verification Extension to
the JSON Meta Application Protocol (JMAP)", RFC 9219,
DOI 10.17487/RFC9219, April 2022,
.
[I-D.ietf-openpgp-crypto-refresh]
Wouters, P., Huigens, D., Winter, J., and N. Yutaka,
"OpenPGP", Work in Progress, Internet-Draft, draft-ietf-
openpgp-crypto-refresh-10, 21 June 2023,
.
[RFC9216] Gillmor, D. K., Ed., "S/MIME Example Keys and
Certificates", RFC 9216, DOI 10.17487/RFC9216, April 2022,
.
[RFC4511] Sermersheim, J., Ed., "Lightweight Directory Access
Protocol (LDAP): The Protocol", RFC 4511,
DOI 10.17487/RFC4511, June 2006,
.
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[I-D.koch-openpgp-webkey-service]
Koch, W., "OpenPGP Web Key Directory", Work in Progress,
Internet-Draft, draft-koch-openpgp-webkey-service-16, 22
May 2023, .
[RFC8162] Hoffman, P. and J. Schlyter, "Using Secure DNS to
Associate Certificates with Domain Names for S/MIME",
RFC 8162, DOI 10.17487/RFC8162, May 2017,
.
[RFC7929] Wouters, P., "DNS-Based Authentication of Named Entities
(DANE) Bindings for OpenPGP", RFC 7929,
DOI 10.17487/RFC7929, August 2016,
.
Appendix A. Future Work
This document contains useful guidance for MUA implementers, but it
cannot contain all possible guidance. Future revisions to this
document may want to further explore the following topics, which are
out of scope for this version.
A.1. Test Vectors
A future version of this document (or a companion document) could
contain examples of well-formed and malformed messages using
cryptographic key material and certificates from
[I-D.ietf-openpgp-crypto-refresh] and [RFC9216].
It may also include example renderings of these messages.
A.2. Further Guidance on Peer Certificates
A.2.1. Certificate Discovery from Incoming Messages
As described in Section 8.2.3, an incoming e-mail message may have
one or more certificates embedded in it. This document currently
acknowledges that receiving MUA should assemble a cache of
certificates for future use, but providing more detailed guidance for
how to assemble and manage that cache is currently out of scope.
Existing recommendations like [AUTOCRYPT] provide some guidance for
handling incoming certificates about peers, but only in certain
contexts. A future version of this document may describe in more
detail how these incoming certs should be handled.
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A.2.2. Certificate Directories
Some MUAs may have the capability to look up peer certificates in a
directory, for example via LDAP [RFC4511], WKD
[I-D.koch-openpgp-webkey-service], or the DNS (e.g. SMIMEA [RFC8162]
or OPENPGPKEY [RFC7929] resource records).
A future version of this document may describe in more detail what
sources a MUA should consider when searching for a peer's
certificates, and what to do with the certificates found by various
methods.
A.2.3. Checking for Certificate Revocation
A future version of this document could discuss how/when to check for
revocation of peer certificates, or of the user's own certificate.
Such discussion should address privacy concerns: what information
leaks to whom when checking peer cert revocations?
A.2.4. Further Peer Certificate Selection
A future version of this document may describe more prescriptions for
deciding whether a peer certificate is acceptable for encrypting a
message. For example, if the SPKI is an EC Public Key and the
keyUsage extension is absent, what should the encrypting MUA do?
A future version of this document might also provide guidance on what
to do if multiple certificates are all acceptable for encrypting to a
given recipient. For example, the sender could select among them in
some deterministic way; it could encrypt to all of them; or it could
present them to the user to let the user select any or all of them.
A.3. Further Guidance on Local Certificates and Secret Keys
A.3.1. Cross-MUA sharing of Local Certificates and Secret Keys
Many users today use more than one MUA to access the same mailbox
(for example, one MUA on a mobile device, and another MUA on a
desktop computer).
A future version of this document might offer guidance on how
multiple MUAs attached to the same mailbox can efficiently and
securely share the user's own secret key material and certificates
between each other. This guidance should include suggestions on how
to maintain the user's keys (e.g. avoiding certificate expiration)
and safe secret key transmission.
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A.3.2. Use of Smartcards or Other Portable Secret Key Mechanisms
Rather than having to transfer secret key material between clients,
some users may prefer to rely on portable hardware-backed secret keys
in the form of smartcards, USB tokens or other comparable form
factors. These secret keys sometimes require direct user interaction
for each use, which can complicate the usability of any MUA that uses
them to decrypt a large number of messages.
Guidance on the use of this kind of secret key management are beyond
the scope of this document, but future revisions may bring them into
scope.
A.3.3. Active Local Certificate Maintenance
Section 8.2.2 describes conditions where the MUA SHOULD warn the user
that something is going wrong with their certificate.
A future version of this document might outline how a MUA could
actively avoid these warning situations, for example, by
automatically updating the certificate or prompting the user to take
specific action.
A.4. Certification Authorities
A future document could offer guidance on how an MUA should select
and manage root certification authorities (CAs).
For example:
* Should the MUA cache intermediate CAs?
* Should the MUA share such a cache with other PKI clients (e.g.,
web browsers)?
* What distinctions are there between a CA for S/MIME and a CA for
the web?
A.5. Indexing and Search of Encrypted Messages
A common use case for MUAs is search of existing messages by keyword
or topic. This is done most efficiently for large mailboxes by
assembling an index of message content, rather than by a linear scan
of all message content.
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When message contents and headers are encrypted, search by index is
complicated. If the cleartext is not indexed, then messages cannot
be found by search. On the other hand, if the cleartext is indexed,
then the index effectively contains the sensitive contents of the
message, and needs to be protected.
Detailed guidance on the tradeoff here, including choices about
remote search vs local search, are beyond the scope of this document,
but a future version of the document may bring them into scope.
A.6. Cached Signature Validation
Asymmetric signature validation can be computationally expensive, and
the results can also potentially vary over time (e.g. if a signing
certificate is discovered to be revoked). In some cases, the user
may care about the signature validation that they saw when they first
read or received the message, not only about the status of the
signature verification at the current time.
So, for both performance reasons and historical perspective, it may
be useful for an MUA to cache signature validation results in a way
that they can be easily retrieved and compared. Documenting how and
when to cache signature validation, as well as how to indicate it to
the user, is beyond the scope of this document, but a future version
of the document may bring these topics into scope.
A.7. Aggregate Cryptographic Status
This document limits itself to consideration of the cryptographic
status of single messages as a baseline concept that can be clearly
and simply communicated to the user. However, some users and some
MUAs may find it useful to contemplate even higher-level views of
cryptographic status which are not considered directly here.
For example, a future version of the document may also consider how
to indicate a simple cryptographic status of message threads (groups
of explicitly related messages), conversations (groups of messages
with shared sets of participants), peers, or other perspectives that
a MUA can provide.
A.8. Expectations of Cryptographic Protection
As mentioned in Section 2.3, the types of security indicators
displayed to the user may well vary based on the expectations of the
user for a given communication. At present, there is no widely
shared method for the MUA to establish and maintain reasonable
expectations about whether a specific received message should have
cryptographic protections.
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If such a standard is developed, a future version of this document
should reference it and encourage the deployment of clearer and
simpler security indicators.
A.9. Secure Deletion
One feature many users desire from a secure communications medium is
the ability to reliably destroy a message such that it cannot be
recovered even by a determined adversary. Doing this with standard
e-mail mechanisms like S/MIME and PGP/MIME is challenging because of
two interrelated factors:
* A copy of of an e-mail message may be retained by any of the mail
transport agents that handle it during delivery, and
* The secret key used to decrypt an encrypted e-mail message is
typically retained indefinitely.
This means that an adversary aiming to recover the cleartext contents
of a deleted message can do so by getting access to a copy of the
encrypted message and the long-term secret key material.
Some mitigation measures may be available to make it possible to
delete some encrypted messages securely, but this document considers
this use case out of scope. A future version of the document may
elaborate on secure message deleton in more detail.
A.10. Interaction with Opportunistic Encryption
This document focuses on guidance for strong, user-comprehensible
end-to-end cryptographic protections for e-mail. Other approaches
are possible, including various forms of opportunistic and transport
encryption, which are out of scope for this document.
A future version of this document could describe the interaction
between this guidance and more opportunistic forms of encryption, for
example some of the scenarios contemplated in
[I-D.dkg-mail-cleartext-copy].
A.11. Split Attachments
As noted in Section 7.2, the standard form for encrypted e-mail
messages is a single cryptographic envelope. In a scenario where
multiple user agents are drafting a single encrypted message over
low-bandwidth links, this can create a poor user experience, as each
MUA has to retrieve the full message, including attachments, to
modify the draft. Similarly, when retrieving a message with a large
attachment, the receiving MUA might want to only render the main body
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part, and will have a significant delay in doing so if required to
process the full message before handling.
Future work might include an attempt to standardize a mechanism that
eases this use case, potentially at the risk of additional metadata
leakage about the message (e.g., the size and number of message
parts). Any such work should explicitly try to minimize the risks
and concerns described in Section 7.2.
A.12. Proxy Extensions
As noted in Section 9.7, a proxy-based implementation can be a
tempting approach. But its naive form is likely to be insufficient
to provide safe end-to-end encrypted e-mail.
A future version of this document, or a separate but related
document, could try to outline the specific additional information,
state, and network API surface that would be needed to allow an MUA
to be safely integrated with an encryption provider. Any such work
should try to address the potential problems described in
Section 9.7.
Appendix B. Document History
B.1. Substantive changes from draft-ietf-...-11 to draft-ietf-...-12
* More nuance (and future work) around split attachments
* More nuance (and future work) around proxy-style design
* Clearer caveats about external resources in text/html parts
B.2. Substantive changes from draft-ietf-...-10 to draft-ietf-...-11
* Mention List-* and Archived-At message header fields
* Add additional references to papers that describe flaws in e2e
e-mail
B.3. Substantive changes from draft-ietf-...-09 to draft-ietf-...-10
* Introduction: describe one major theme of Future Work
B.4. Substantive changes from draft-ietf-...-08 to draft-ietf-...-09
* Clarify goals of document
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* Identify cross-protocol attacks in more detail to justify
prescription against key reuse
* Add informative references to Opportunistic Encryption RFC and
Certificate Best Practice draft
* Add "Reading Encrypted Messages after Certificate Expiration"
section to Common Pitfalls and Guidelines
* Clean up nits identified by Stephen Farrell and Eliot Lear
B.5. Substantive changes from draft-ietf-...-07 to draft-ietf-...-08
* Add guidance about importing and exporting secret key material
* More explanation about "encryption outside, signature inside"
* Guidance about Intervening (resending) MUAs
* Include Sender and Resent-* as user-facing headers
* Guidance about external subresources for cryptographically
protected messages
* Relax "user-facing" definition to be more advisory
B.6. Substantive changes from draft-ietf-...-06 to draft-ietf-...-07
* Add Bernie Hoeneisen and Alexey Melnikov as editors
* Explicitly avoid requiring anything from IANA
* Simplify description of "attachments"
* Add concrete detail on how to compare e-mail addresses
* Explicitly define "Cryptographic Summary"
B.7. Substantive changes from draft-ietf-...-05 to draft-ietf-...-06
* Expand proxy implementation warning to comparable APIs
* Move many marked TODO and FIXME items to "Future Work"
* More detailed guidance on local certificates
* Provide guidance on encrypting draft messages
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* More detailed guidanc eon shipping certificates in outbound
messages
* Recommend implementing header protection
* Added "Future Work" subsection about interactions with
opportunistic approaches
B.8. Substantive changes from draft-ietf-...-04 to draft-ietf-...-05
* Adopt and update text about Bcc from draft-ietf-lamps-header-
protection
* Add section about the dangers of an implementation based on a
network protocol proxy
B.9. Substantive changes from draft-ietf-...-03 to draft-ietf-...-04
* Added reference to multipart/oracle attacks
* Clarified that "Structural Headers" are the same as RFC2045's
"MIME Headers"
B.10. Substantive changes from draft-ietf-...-02 to draft-ietf-...-03
* Added section about mixed recipients
* Noted SMIMEA and OPENPGPKEY DNS RR cert discovery mechanisms
* Added more notes about simplified mental models
* More clarification on one-status-per-message
* Added guidance to defend against EFAIL
B.11. Substantive changes from draft-ietf-...-01 to draft-ietf-...-02
* Added definition of "user-facing" headers
B.12. Substantive changes from draft-ietf-...-00 to draft-ietf-...-01
* Added section about distinguishing Main Body Parts and Attachments
* Updated document considerations section, including reference to
auto-built editor's copy
B.13. Substantive changes from draft-dkg-...-01 to draft-ietf-...-00
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* WG adopted draft
* moved Document History and Document Considerations sections to end
of appendix, to avoid section renumbering when removed
B.14. Substantive changes from draft-dkg-...-00 to draft-dkg-...-01
* consideration of success/failure indicators for usability
* clarify extendedKeyUsage and keyUsage algorithm-specific details
* initial section on certificate management
* added more TODO items
Authors' Addresses
Daniel Kahn Gillmor (editor)
American Civil Liberties Union
125 Broad St.
New York, NY, 10004
United States of America
Email: dkg@fifthhorseman.net
Bernie Hoeneisen (editor)
pEp Foundation
Oberer Graben 4
CH- CH-8400 Winterthur
Switzerland
Email: bernie.hoeneisen@pep.foundation
URI: https://pep.foundation/
Alexey Melnikov (editor)
Isode Ltd
14 Castle Mews
Hampton, Middlesex
TW12 2NP
United Kingdom
Email: alexey.melnikov@isode.com
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