Internet Engineering Task Force (IETF) A. Melnikov

Internet Engineering Task Force (IETF) A. Melnikov
Request for Comments: 9219 Isode Ltd
Category: Standards Track April 2022
ISSN: 2070-1721

S/MIME Signature Verification Extension to the JSON Meta Application
Protocol (JMAP)

Abstract

This document specifies an extension to "The JSON Meta Application
Protocol (JMAP) for Mail" (RFC 8621) for returning the S/MIME
signature verification status.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9219.

Copyright Notice

Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.

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Table of Contents

1. Introduction
2. Conventions Used in This Document
3. Addition to the Capabilities Object
4. Extension for S/MIME Signature Verification
4.1. Extension to Email/get
4.1.1. "smimeStatus" Response Property Extensibility
4.2. Extension to Email/query
4.3. Interaction with Email/changes
5. IANA Considerations
5.1. JMAP Capability Registration for "smimeverify"
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
Author's Address

1. Introduction

JMAP for Mail [RFC8621] is a JSON-based application protocol for
synchronizing email data between a client and a server.

This document describes an extension to JMAP for returning the S/MIME
signature verification status [RFC8551], without requiring a JMAP
client to download the signature body part and all signed body parts
(when the multipart/signed media type [RFC1847] is used) or to
download and decode the Cryptographic Message Syntax (CMS) (when the
application/pkcs7-mime media type (Section 3.2 of [RFC8551]) is
used). The use of the extension implies the client trusts the JMAP
server's S/MIME signature verification code and configuration. This
extension is suitable for cases where reduction in network bandwidth
and client-side code complexity outweigh security concerns about
trusting the JMAP server to perform S/MIME signature verifications.
One possible use case is when the same organization controls both the
JMAP server and the JMAP client.

2. Conventions Used in This Document

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.

Type signatures, examples, and property descriptions in this document
follow the conventions established in Section 1.1 of [RFC8620]. Data
types defined in the core specification are also used in this
document.

3. Addition to the Capabilities Object

The *capabilities* object is returned as part of the standard JMAP
Session object; see Section 2 of [RFC8620]. Servers supporting this
specification MUST add a property called
"urn:ietf:params:jmap:smimeverify" to the capabilities object.

The value of this property is an empty object in both the JMAP
Session _capabilities_ property and an account's
_accountCapabilities_ property.

4. Extension for S/MIME Signature Verification

4.1. Extension to Email/get

[RFC8621] defines the Email/get method for retrieving message-
specific information. This document defines the following pseudo
values in the _properties_ argument:

*smimeStatus*:
If "smimeStatus" is included in the list of requested properties,
it MUST be interpreted by the server as a request to return the
"smimeStatus" response property.

*smimeStatusAtDelivery*:
If "smimeStatusAtDelivery" is included in the list of requested
properties, it MUST be interpreted by the server as a request to
return the "smimeStatusAtDelivery" response property. (It is
effectively the same as the "smimeStatus" value calculated at the
date/time of delivery, as specified by "receivedAt".)

*smimeErrors*:
If "smimeErrors" is included in the list of requested properties,
it MUST be interpreted by the server as a request to return the
"smimeErrors" response property.

*smimeVerifiedAt*:
If "smimeVerifiedAt" is included in the list of requested
properties, it MUST be interpreted by the server as a request to
return the "smimeVerifiedAt" response property.

The "smimeStatus" response property is defined as follows:

*smimeStatus*:
"String|null" (server-set). null signifies that the message
doesn't contain any signature. Otherwise, this property contains
the S/MIME signature and certificate verification status
calculated according to [RFC8551], [RFC8550], and [RFC5280].
Possible string values of the property are listed below. Servers
MAY return other values not defined below, as defined in
extensions to this document. Clients MUST treat unrecognized
values as "unknown" or "signed/failed". Note that the value of
this property might change over time.

unknown:
An S/MIME message, but it was neither signed nor encrypted.
This can also be returned for a multipart/signed message that
contains an unrecognized signing protocol (for example,
OpenPGP).

signed:
An S/MIME signed message, but the signature was not yet
verified. Some servers might not attempt to verify a signature
until a particular message is requested by the client. (This
is a useful optimization for a JMAP server to avoid doing work
until exact information is needed. A JMAP client that only
needs to display an icon that signifies presence of an S/MIME
signature can still use this value.) JMAP servers compliant
with this document SHOULD attempt signature verification and
return "signed/verified" or "signed/failed" instead of this
signature status.

signed/verified:
An S/MIME signed message, and the sender's signature was
successfully verified according to [RFC8551] and [RFC8550].
Additionally, the signer email address extracted from the S/
MIME certificate matches the From header field value, and the
signer certificate SHOULD be checked for revocation.

signed/failed:
S/MIME signed message, but the signature failed to verify
according to [RFC8551] and [RFC8550]. This might be because of
a policy-related decision (e.g., the message signer email
address doesn't match the From header field value), the message
was modified, the signer's certificate has expired or was
revoked, etc.

encrypted+signed/verified:
This value is reserved for future use. It is typically handled
in the same way as "signed/verified".

encrypted+signed/failed:
This value is reserved for future use. It is typically handled
in the same way as "signed/failed".

The "smimeStatusAtDelivery" response property has the same syntax as
"smimeStatus" but is calculated in relationship to the "receivedAt"
date/time. Unlike "smimeStatus", the "smimeStatusAtDelivery"
response property value doesn't change unless trust anchors are
added. (For example, addition of a trust anchor can change the value
of a message "smimeStatusAtDelivery" property from "signed/failed" to
"signed/verified". Note that trust anchor removal doesn't affect
this response property.) The "smimeStatusAtDelivery" response
property value allows clients to compare the S/MIME signature
verification status at delivery with the current status as returned
by "smimeStatus", for example, to help to answer questions like "was
the signature valid at the time of delivery?".

Note that the "smimeStatusAtDelivery" response property value doesn't
have to be calculated at delivery time. A JMAP server can defer its
calculation until it is explicitly requested; however, once it is
calculated, its value is remembered for later use.

The "smimeErrors" response property is defined as follows:

*smimeErrors*:
"String[]|null" (server-set). null signifies that the message
doesn't contain any signature or that there were no errors when
verifying the S/MIME signature. (That is, this property is non-
null only when the corresponding "smimeStatus" response property
value is "signed/failed" or "encrypted+signed/failed". Note that
future extensions to this document can specify other "smimeStatus"
values that can be used with "smimeErrors".) Each string in the
array is a human-readable description (in the language specified
in the Content-Language header field, if any) of a problem with
the signature, the signing certificate, or the signing certificate
chain. (See Section 3.8 of [RFC8620] in regards to how this is
affected by the language selection.) In one example, the signing
certificate might be expired and the message From email address
might not correspond to any of the email addresses in the signing
certificate. In another example, the certificate might be expired
and the JMAP server might be unable to retrieve a Certificate
Revocation List (CRL) for the certificate. In both of these
cases, there would be 2 elements in the array.

The "smimeVerifiedAt" response property is defined as follows:

*smimeVerifiedAt*:
"UTCDate|null" (server-set). null signifies that the message
doesn't contain any S/MIME signature or that there is a signature,
but there was no attempt to verify it. (Retrieval of the
"smimeStatus" value can be used to distinguish these 2 cases). In
all other cases, it is set to the date and time of when the S/MIME
signature was most recently verified. Note that a request to
fetch "smimeStatus", "smimeStatusAtDelivery", and/or "smimeErrors"
would force this response property to be set to a non-null value
if an S/MIME signature exists.

The "smimeStatus" and "smimeErrors" values are calculated at the time
the corresponding JMAP request is processed (but see below about the
effect of result caching), not at the time when the message is
generated (according to its Date header field value). In all cases,
"smimeVerifiedAt" is set to the time when "smimeStatus" and
"smimeErrors" were last updated. As recalculating these values is
expensive for the server, they MAY be cached for up to 24 hours from
the moment when they were calculated.

Example 1: Retrieval of minimal information about a message,
including its From, Subject, and Date header fields, as well as the
S/MIME signature verification status at delivery and date/time when
the message was received.

["Email/get", {
"ids": [ "fe123u457" ],
"properties": [ "mailboxIds", "from", "subject", "date",
"smimeStatusAtDelivery", "receivedAt" ]
}, "#1"]

This might result in the following response:

[["Email/get", {
"accountId": "abc",
"state": "51234123231",
"list": [
{
"id": "fe123u457",
"mailboxIds": { "f123": true },
"from": [{"name": "Joe Bloggs",
"email": "[email protected]"}],
"subject": "Dinner tonight?",
"date": "2020-07-07T14:12:00Z",
"smimeStatusAtDelivery": "signed/verified",
"receivedAt": "2020-07-07T14:15:18Z"
}
]
}, "#1"]]

Example 2: Retrieval of minimal information about a message,
including its From, Subject, and Date header fields, as well as the
latest S/MIME signature verification status, S/MIME verification
errors (if any), and when the S/MIME signature status was last
verified. The response contains 2 S/MIME errors related to S/MIME
signature verification.

["Email/get", {
"ids": [ "ag123u123" ],
"properties": [ "mailboxIds", "from", "subject", "date",
"smimeStatus", "smimeErrors", "smimeVerifiedAt" ]
}, "#1"]

This might result in the following response:

[["Email/get", {
"accountId": "abc",
"state": "47234123231",
"list": [
{
"id": "ag123u123",
"mailboxIds": { "f123": true },
"from": [{"name": "Jane Doe",
"email": "[email protected]"}],
"subject": "Company takeover",
"date": "2020-01-31T23:00:00Z",
"smimeStatus": "signed/failed",
"smimeErrors": [
"From email address doesn't match the certificate",
"Can't retrieve CRL from the CRL URL"],
"smimeVerifiedAt": "2020-03-01T12:11:19Z"
}
]
}, "#1"]]

4.1.1. "smimeStatus" Response Property Extensibility

Future extensions to this document can specify extra allowed values
for the "smimeStatus" response property. All values (defined in this
document or in extensions to this document) MUST be in ASCII. (Note
that this response property contains tokens; thus, it is not subject
to internationalization or localization).

New "smimeStatus" response property values defined in extensions may
affect the behavior of properties, such as the "smimeErrors" response
property of Email/get (see Section 4.1) or the "hasVerifiedSmime"
property of Email/query (see Section 4.2). In particular, the new
values can be treated similarly to values defined in this document.

For example, a putative JMAP extension for automatically decrypting
S/MIME messages can specify two additional values, one specifying
that a message is both encrypted and signed with a valid S/MIME
signature (e.g. "encrypted+signed/verified") and another one
specifying that a message is both encrypted and signed with an
invalid S/MIME signature (e.g. "encrypted+signed/failed"). The
former value can be treated as "signed/verified" (and would thus
affect "hasVerifiedSmime") and the latter can be treated as "signed/
failed" (and thus can be used with "smimeErrors").

4.2. Extension to Email/query

[RFC8621] defines the Email/query method for searching for messages
with specific properties. This document defines the following
properties of the *FilterCondition* object:

*hasSmime*:
"Boolean". If "hasSmime" has the value true, only messages with
"smimeStatus" other than null match the condition. If "hasSmime"
has the value false, only messages with "smimeStatus" equal to
null match the condition.

*hasVerifiedSmime*:
"Boolean". If "hasVerifiedSmime" has the value true, only
messages with "smimeStatus" equal to "signed/verified" or
"encrypted+signed/verified" (*) match the condition. If
"hasVerifiedSmime" has the value false, only messages with
"smimeStatus" not equal to "signed/verified" and not equal to
"encrypted+signed/verified" (*) (including the value null) match
the condition. Note that use of this attribute is potentially
expensive for a JMAP server, as it forces calculation of the
"smimeStatus" property value for each message. However, caching
of the "smimeStatus" values should ameliorate this cost somewhat.

(*) as well as the "smimeStatus" values added by future extensions
to this document that are explicitly specified as having similar
effect to "signed/verified" as far as "hasVerifiedSmime"
calculation is concerned.

*hasVerifiedSmimeAtDelivery*:
"Boolean". The "hasVerifiedSmimeAtDelivery" property is handled
similarly to the "hasVerifiedSmime" property, but the value of
"smimeStatusAtDelivery" is used instead of "smimeStatus" to assess
whether a particular message matches the condition.

4.3. Interaction with Email/changes

Changes to the "smimeVerifiedAt" response property value MUST NOT
cause the message to be included in the "updated" argument of the
Email/changes response. However, changes to the "smimeStatus",
"smimeStatusAtDelivery", and/or "smimeErrors" response properties
MUST result in message inclusion in the "updated" argument of the
Email/changes response.

5. IANA Considerations

5.1. JMAP Capability Registration for "smimeverify"

IANA has registered the "smimeverify" JMAP capability as follows:

Capability Name: urn:ietf:params:jmap:smimeverify
Specification document: RFC 9219
Intended use: common
Change Controller: IETF
Security and privacy considerations: RFC 9219, Section 6

6. Security Considerations

Use of the server-side S/MIME signature verification JMAP extension
requires the client to trust the server signature verification code,
the server configuration, and the server's operational practices to
perform S/MIME signature verification, as well as to trust that the
channel between the client and the server is integrity protected.
(For example, if the server is not configured with some trust
anchors, some messages will have the "signed/failed" status instead
of "signed/verified".) A malicious or compromised server could
return a false verification status to a client. A successful
verification could be conveyed to a client for a forged or altered
message. A properly signed message could be signaled as having a
failed signature verification or no signature at all. In the case of
the latter attack, no new attack surface is presented with this
extension above what a malicious or compromised server could already
do by stripping or tampering with the S/MIME information in the
message. In the case of the former attack, client software capable
of performing S/MIME signature verification could detect this attack.
Local configuration of the client should determine if this client-
side verification should occur. For clients without local
verification capabilities, such an attack would be difficult to
detect.

Integrity protection of the channel between the client and the server
is provided by use of TLS, as required by the JMAP specification (see
Section 8.1 of [RFC8620]).

Constant recalculation of the S/MIME signature status can result in a
denial-of-service condition. For that reason, it is RECOMMENDED that
servers cache results of signature verification for up to 24 hours.

7. References

7.1. 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>.

[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.

[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>.

[RFC8550] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Certificate Handling", RFC 8550, DOI 10.17487/RFC8550,
April 2019, <https://www.rfc-editor.org/info/rfc8550>.

[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>.

[RFC8620] Jenkins, N. and C. Newman, "The JSON Meta Application
Protocol (JMAP)", RFC 8620, DOI 10.17487/RFC8620, July
2019, <https://www.rfc-editor.org/info/rfc8620>.

[RFC8621] Jenkins, N. and C. Newman, "The JSON Meta Application
Protocol (JMAP) for Mail", RFC 8621, DOI 10.17487/RFC8621,
August 2019, <https://www.rfc-editor.org/info/rfc8621>.

7.2. Informative References

[RFC1847] Galvin, J., Murphy, S., Crocker, S., and N. Freed,
"Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC 1847, DOI 10.17487/RFC1847,
October 1995, <https://www.rfc-editor.org/info/rfc1847>.

Acknowledgements

This document is a product of the JMAP Working Group. Special thank
you to Bron Gondwana, Neil Jenkins, Murray Kucherawy, Kirsty Paine,
Benjamin Kaduk, Roman Danyliw, Peter Yee, Robert Wilton, Erik Kline,
and Menachem Dodge for suggestions, comments, and corrections to this
document.

Author's Address

Alexey Melnikov
Isode Ltd
14 Castle Mews
Hampton, Middlesex
TW12 2NP
United Kingdom
Email: [email protected]