JMAP extension for S/MIME signing and encryption

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Table of Contents

4. Extension to Email/set for S/MIME signing and/or encryption

[RFC8621] defines Email/set method for creating new email messages. This document defines the following additional request arguments that can be used to create S/MIME signed and/or encrypted messages:¶

• smimeSign: "Boolean" (default: false). If included and has the value "true", this requests the JMAP server to create an S/MIME signed message from the message constructed according to other specified arguments (the "original message"). This is done by encapsulating the original message either inside application/pkcs7-mime [RFC8551] or multipart/signed [RFC1847] container. (smimeSignOpaque argument (see below) controls which of the two mechanisms is used.) The signature's private key/certificate is associated with the email address in the Sender header field, if present; otherwise, it is associated with the email address in the From header field, if present.¶

  If multiple addresses are present in one of these header fields, or there is more than one Sender/From header field, the server SHOULD reject the Email/set as invalid with the "invalidEmail" error code; otherwise, it MUST take the first address in the last Sender/From header field. If JMAP account is not authorized to sign message as the selected sender (as above), it SHOULD return "signedSenderNotAllowed" error code.¶

• smimeEncrypt: "Boolean" (default: false). If included and has the value "true", this requests the JMAP server to create an S/MIME encrypted message from the constructed message. This is done by encapsulating the message inside application/pkcs7-mime [RFC8551] media type. The message MUST be encrypted to the sender and all To/Cc/Bcc recipients. This extension assumes that there is some kind of per user or organizational addressbook, that can be used to lookup public keys of recipients. If lookup of a particular public key fails, or results in an expired or revoked certificate, the Email/set operation MUST fail with the "validEncryptionKeyNotFound" error code.¶
• smimeHeaderProtect: "Boolean" (default: true). If has the value "true", this requests the JMAP server to use S/MIME header protection as specified in [draft-ietf-lamps-header-protection] when at least one of smimeEncrypt/smimeSign is true.¶
• smimeSignOpaque: "Boolean" (default: true). If has the value "true", this requests the JMAP server to use application/pkcs7-mime media type for S/MIME signing, otherwise multipart/signed media type.¶

If both "smimeSign" and "smimeEncrypt" are set to true, the message is first signed and then the signed version is encrypted (in that order).¶

(Note that this extension doesn't allow management of private keys/certificates. How private keys are managed or configured for a particular user is out of scope for this document.)¶

      [[ "Email/set", {
        "accountId": "ue150411c",
        "create": {
          "k192": {
            "mailboxIds": {
              "2ea1ca41b38e": true
            },
            "keywords": {
              "$seen": true,
              "$draft": true
            },
            "from": [{
              "name": "Joe Bloggs",
              "email": "joe@example.com"
            }],
            "subject": "World domination",
            "receivedAt": "2021-07-07T01:03:11Z",
            "sentAt": "2021-07-10T11:03:11+10:00",
            "smimeSign": true,
            "smimeEncrypt": true,
            "bodyStructure": {
              "type": "text/plain",
              "partId": "bd48",
              "header:Content-Language": "en"
            },
            "bodyValues": {
              "bd48": {
                "value": "I have the most brilliant plan.  Let me tell
                  you all about it.",
                "isTruncated": false
              }
            }
          }
        }
      }, "0" ]]

This will result in the following response:

      [[ "Email/set", {
        "accountId": "ue150411c",
        "oldState": "780823",
        "newState": "780839",
        "created": {
          "k192": {
            "id": "Mf40b5f831efa7233b9eb1c7f",
            "blobId": "Gf40b5f831efa7233b9eb1c7f8f97d84eeeee64f7",
            "threadId": "Td957e72e89f516dc",
            "size": 5096
          }
        },
        ...
      }, "0" ]]

5. Extension to Email/get for S/MIME decryption

[RFC8621] defines Email/get method for retrieving information about email messages. This document defines the following additional request arguments of the "bodyProperties" object that can be used to facilitate decryption of S/MIME encrypted messages:¶

• smimeBlobId: "Id|null". The id representing the raw octets of the decrypted contents of the part. When processing this request argument, the server first removes the Content-Transfer-Encoding (see then "blobId" request argument in RFC 8621). If the Content-Transfer-Encoding is supported by the server, the content is then S/MIME decrypted. The resulting Id can be used in Email/parse method to import the decrypted message. If the body part is not encrypted, the returned smimeBlobId attribute has the same value as the "blobId" attribute.¶

    [[ "Email/get", {
        "ids": [ "f123u986" ],
        "properties": [ "threadId", "mailboxIds", "from", "subject",
          "receivedAt", "htmlBody" ],
        "bodyProperties": [ "partId", "smimeBlobId", "size", "type" ]
      }, "#1" ]]

This will result in the following response:

    [[ "Email/get", {
     "accountId": "abc",
     "state": "41234123231",
     "list": [
       {
         "id": "f123u986",
         "threadId": "cb1314a",
         "mailboxIds": { "da123c": true },
         "from": [{ "name": "Mice Ace", "email": "mike@example.com" }],
         "subject": "Dinner on Saturday?",
         "receivedAt": "2023-03-09T14:12:00Z",

         "htmlBody": [{
           "partId": "1",
           "smimeBlobId": "B841623871",
           "size": 120537,
           "type": "text/html"
         }]
       }
     ]
   }, "#1" ]]

7. Security Considerations

This JMAP extension assumes trust between the user and the JMAP server for purposes of signing and encrypting messages on user's behalf.¶

This JMAP extension also relies on access to user's (or organization's) addressbook which contain up-to-date certificates for recipients.¶

This JMAP extension doesn't support management of user's private keys and corresponding certificates.¶

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