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

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

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   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": "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 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": "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 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: Alexey.Melnikov@isode.com