S/MIME signature verification extension to JMAP
draft-ietf-jmap-smime-10

Network Working Group                                        A. Melnikov
Internet-Draft                                                 Isode Ltd
Intended status: Standards Track                         22 October 2021
Expires: 25 April 2022

            S/MIME signature verification extension to JMAP
                        draft-ietf-jmap-smime-10

Abstract

   This document specifies an extension to JMAP for Mail (RFC 8621) for
   returning S/MIME signature verification status.

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   This Internet-Draft will expire on 25 April 2022.

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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
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  JMAP capability registration for "smimeverify"  . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     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". 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

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   [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.  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, the sender matches the From header
      field, and the sender's certificate (and the certificate chain) is
      trusted for signing.

   signed/failed:  S/MIME signed message, but the signature failed to
      verify.  This might be a policy related decision (message signer
      doesn't match the From header field), message was modified, the
      signer's certificate has expired or was revoked, etc.

   The "smimeStatusAtDelivery" response property has the same syntax as
   "smimeStatus" but is calculated at the "receivedAt" date/time.
   Unlike "smimeStatus", the "smimeStatusAtDelivery" response property
   value is immutable. "smimeStatusAtDelivery" 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?".

   The "smimeErrors" response property is defined as follows:

   smimeErrors: "String[]|null". 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".  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

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   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.  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" and/or
   "smimeErrors" would force this response property to be set to a non
   null value, if an S/MIME signature exists.

   "smimeStatus" and "smimeErrors" values are calculated at the time the
   corresponding JMAP request was processed (but see below about 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 10 minutes 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.

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      ["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" (*), match the condition.  If "hasVerifiedSmime" has the
      value false, only messages with "smimeStatus" not equal to
      "signed/verified" (*) (including the value null) match the
      condition.

      (*) 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.

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

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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 to
   cache results of signature verification for 10 minutes.

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>.

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   [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 JMAP Working Group.  Special thank you
   to Bron Gondwana, Neil Jenkins, Murray Kucherawy, Kirsty Paine, Roman
   Danyliw and Peter Yee for suggestions, comments and corrections to
   this document.

Author's Address

   Alexey Melnikov
   Isode Ltd
   14 Castle Mews
   Hampton
   TW12 2NP
   United Kingdom

   Email: Alexey.Melnikov@isode.com

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