Guidance on End-to-End E-mail Security

Internet-Draft	Guidance on End-to-End E-mail Security	January 2022
Gillmor	Expires 29 July 2022	[Page]

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 that need to compose or interpret e-mail messages with end-to-end cryptographic protection. It provides a useful set of vocabulary as well as suggestions to avoid common failures.¶

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 (e.g., [EFAIL]).¶

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.¶

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 ([RFC2119] and [RFC8174]) when, and only when, they appear in all capitals, as shown here.¶

1.2. 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, 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 1.2.1.¶
User-Facing Headers are documented in Section 1.2.2.¶
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.¶

1.2.1. Structural Headers

A message header whose name begins with Content- is referred to in this document as a "structural" header.¶

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 (but is not limited to):¶

Content-Type¶
Content-Transfer-Encoding¶
Content-Disposition¶

FIXME: are there any non-Content-* headers we should consider as structural?¶

1.2.2. User-Facing Headers

Of all the headers that an e-mail message may contain, only a handful are typically presented directly to the user. The user-facing headers are:¶

Subject¶
From¶
To¶
Cc¶
Date¶
Reply-To¶
Followup-To¶

The above is a complete list. No other headers are considered "user-facing".¶

Other headers 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), 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 likely is 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.¶

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.¶

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.¶

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 e-mail communications are entirely end-to-end protected might instead want to know which messages do not have the expected protections.¶

Note also that some messages are 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.¶

4. Cryptographic MIME Message Structure

Implementations use the structure of an e-mail message to protect the headers. 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

└─╴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 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)

└┬╴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

4.4.1. Simple Cryptographic Envelopes

As described above, if the "protected part" identified in the sction 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 Crytptographic 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:¶

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.¶

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.¶

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 1.2.1).¶
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.¶

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 of 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.¶

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).¶

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¶
signing-only¶
signed and encrypted¶

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.¶

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 summarize the cryptographic properties of the Cryptographic Envelope, 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:¶

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 (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.¶

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.¶

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.¶

But when composing a reply that contains quoted text from the decrypted subpart, the reply message SHOULD be marked for encryption, as noted in {#composing-reply}.¶

Alternately, 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.¶

6.3. 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 cryptograpic 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.¶

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.¶

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 R.SA)¶
the signature is made by a certificate which the receiving MUA does not have access to.¶
the certificate that made the signature was revoked.¶
the certificate that made the signature was expired at the time that the signature was made.¶
the certificate that made 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 that made 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.¶
the signature indicates that it was made at a time much before or much after from the date of the message itself.¶

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.¶

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, but the part that is rendered to the user is the "Main Body Part".¶

When rendering a message, one of the primary jobs of the recieving 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.¶

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, and is a subpart of a multipart/mixed or multipart/related container.¶

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 end-to-end encrypted messages, attachments MUST 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).¶

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.¶

7.3. MIME Part Examples

Consider a common message with the folloiwing MIME structure:¶

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.¶

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.¶

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.¶

8.1.1. Cert Discovery from Incoming Messages

TODO: incoming PKCS#7 messages tend to have a bundle of certificates in them. How should these certs be handled?¶

TODO: point to Autocrypt certificate discovery mechanism¶

TODO: point to OpenPGP embedded certificate subpacket proposal¶

TODO: compare mechanisms, explain where each case is useful.¶

8.1.2. Certificate Directories

Some MUAs may have the capability to look up peer certificates in a directory.¶

TODO: more information here about X.509 directories -- LDAP?¶

TODO: mention WKD for OpenPGP certificates?¶

8.1.3. Peer Certificate Selection

When composing an encrypted message, the MUA needs to select a certificate for each recipient that is capable of encryption.¶

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 exactly match the destination address.¶
The algorithm OID in the certificate's SPKI is known to the MUA and capable of encryption. Examples include (TODO: need OIDs)¶
- RSA, with keyUsage present and the "key encipherment" bit set¶
- EC Public Key, with keyUsage present and the "key agreement" bit set¶
- EC DH, with keyUsage present and the "key agreement" bit set¶
If extendedKeyUsage is present, it contains at least one of the following OIDs: e-mail protection, anyExtendedKeyUsage.¶

TODO: If OID is EC Public Key and keyUsage is absent, what should happen?¶

TODO: what if multiple certificates meet all of these criteria for a given recipient?¶

8.1.4. Checking for Revocation

TODO: discuss how/when to check for peer certificate revocation¶

TODO: privacy concerns: what information leaks to whom when checking peer cert revocations?¶

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.¶

8.2.1. Getting a Certificate for the User

TODO: mention ACME SMIME?¶

TODO: mention automatic self-signed certs e.g. OpenPGP?¶

TODO: SHOULD generate secret key material locally, and MUST NOT accept secret key material from an untrusted third party as the basis for the user's certificate.¶

8.2.2. Local Certificate Maintenance

The MUA should warn the user when/if:¶

The user's own certificate set 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 is due to expire soon (TODO: what is "soon"?)¶
Any of the user's own certificates does not match the e-mail address associated with the user's account.¶
Any of the user's own certificates does not have a keyUsage section¶
Any of the user's own certificates does not contain an extendedKeyUsage extension¶

TODO: how does the MUA do better than warning in the cases above? What can the MUA actually do here to fix problems before they happen?¶

TODO: discuss how/when to check for own certificate revocation, and what to do if it (or any intermediate certificate authority) is found to be revoked.¶

8.2.3. Shipping Certificates in Outbound Messages

TODO: What certificates should the MUA include in an outbound message so that peers can discover them?¶

local signing certificate so that signature can be validated¶
local encryption-capable certificate(s) so that incoming messages can be encypted.¶
On an encrypted message to multiple recipients, the encryption-capable peer certs of the other recipients (to enable "reply all")?¶
intermediate certificates to chain all of the above to some set of root authorities?¶

8.3. Certificate Authorities

TODO: how should the MUA select root certificate authorities?¶

TODO: should the MUA cache intermediate CAs?¶

TODO: should the MUA share such a cache with other PKI clients (e.g., web browsers)? Are there distinctions between a CA for S/MIME and for the web?¶

13. References

13.1. Normative References

[RFC2119]: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC3156]: Elkins, M., Del Torto, D., Levien, R., and T. Roessler, "MIME Security with OpenPGP", RFC 3156, DOI 10.17487/RFC3156, August 2001, <https://www.rfc-editor.org/info/rfc3156>.
[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, <https://www.rfc-editor.org/info/rfc4289>.
[RFC8174]: Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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, <https://www.rfc-editor.org/info/rfc8551>.

13.2. Informative References

[chrome-indicators]: Schechter, E., "Evolving Chrome's security indicators", May 2018, <https://blog.chromium.org/2018/05/evolving-chromes-security-indicators.html>.
[EFAIL]: "EFAIL", n.d., <https://efail.de>.
[I-D.draft-bre-openpgp-samples-01]: Einarsson, B. R., "juga", and D. K. Gillmor, "OpenPGP Example Keys and Certificates", Work in Progress, Internet-Draft, draft-bre-openpgp-samples-01, 20 December 2019, <https://www.ietf.org/archive/id/draft-bre-openpgp-samples-01.txt>.
[I-D.draft-ietf-lamps-samples-05]: Gillmor, D. K., "S/MIME Example Keys and Certificates", Work in Progress, Internet-Draft, draft-ietf-lamps-samples-05, 5 August 2021, <https://www.ietf.org/archive/id/draft-ietf-lamps-samples-05.txt>.
[RFC3274]: Gutmann, P., "Compressed Data Content Type for Cryptographic Message Syntax (CMS)", RFC 3274, DOI 10.17487/RFC3274, June 2002, <https://www.rfc-editor.org/info/rfc3274>.
[RFC4880]: Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, "OpenPGP Message Format", RFC 4880, DOI 10.17487/RFC4880, November 2007, <https://www.rfc-editor.org/info/rfc4880>.
[RFC5322]: Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008, <https://www.rfc-editor.org/info/rfc5322>.
[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, <https://www.rfc-editor.org/info/rfc5355>.
[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, <https://www.rfc-editor.org/info/rfc6376>.