Internet DRAFT - draft-ietf-jmap-smime
draft-ietf-jmap-smime
Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Intended status: Standards Track 6 January 2022
Expires: 10 July 2022
S/MIME signature verification extension to JMAP
draft-ietf-jmap-smime-12
Abstract
This document specifies an extension to JMAP for Mail (RFC 8621) for
returning S/MIME signature verification status.
Status of This Memo
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This Internet-Draft will expire on 10 July 2022.
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document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 2
3. Addition to the capabilities object . . . . . . . . . . . . . 3
4. Extension for S/MIME signature verification . . . . . . . . . 3
4.1. Extension to Email/get . . . . . . . . . . . . . . . . . 3
4.1.1. "smimeStatus" response property extensibility . . . . 7
4.2. Extension to Email/query . . . . . . . . . . . . . . . . 8
4.3. Interaction with Email/changes . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5.1. JMAP capability registration for "smimeverify" . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
JMAP for Mail [RFC8621] is a JSON-based application protocol for
synchronising email data between a client and a server.
This document describes an extension to JMAP for returning S/MIME
[RFC8551] signature verification status, 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 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.
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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
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calculated according to [RFC8551] and [RFC8550]. 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: S/MIME message, but it was neither signed nor encrypted.
This can also be returned for a multipart/signed message which
contains an unrecognized signing protocol (for example OpenPGP).
signed: 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: 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 a
policy related decision (e.g. the message signer email address
doesn't match the From header field value), 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" allows clients
to compare the S/MIME signature verification status at delivery with
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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, but once 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. (I.e., 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 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.
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"smimeStatus" and "smimeErrors" values are calculated at the time the
corresponding JMAP request was processed (but see below about the
effect of result caching), not at the time when the message was
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 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": "joe@bloggs.example.net"}],
"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 was the S/MIME signature status last
verified. The response contains 2 S/MIME errors related to S/MIME
signature verification.
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["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": "jdoe@example.com"}],
"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 behaviour of properties such as smimeErrors response property
of Email/get (see Section 4.1) or hasVerifiedSmime property of Email/
query (see Section 4.2). In particular the new values can be treated
similar 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
and another one specifying that a message is both encrypted and
signed with an invalid S/MIME signature. The former value can be
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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
smimeStatus property value for each message. However caching of
smimeStatus values should ameliorate this cost somewhat.
(*) as well as "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 similar to
"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 "smimeVerifiedAt" response property value MUST NOT cause
the message to be included in the "updated" argument of Email/changes
response. However changes to "smimeStatus", "smimeStatusAtDelivery"
and/or "smimeErrors" response properties MUST result in message
inclusion in the "updated" argument of Email/changes response.
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5. IANA Considerations
5.1. JMAP capability registration for "smimeverify"
IANA is requested to register the "smimeverify" JMAP Capability as
follows:
Capability Name: "urn:ietf:params:jmap:smimeverify"
Specification document: this document
Intended use: common
Change Controller: IETF
Security and privacy considerations: this document, 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,
server configuration and its 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 "signed/failed" status instead of "signed/verified".) A
malicious or compromised server could return 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 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 JMAP specification (see
Section 8.1 of [RFC8620]).
Constant recalculation of 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.
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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>.
[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>.
Appendix A. 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
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Alexey Melnikov
Isode Ltd
14 Castle Mews
Hampton
TW12 2NP
United Kingdom
Email: Alexey.Melnikov@isode.com
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