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 Email/set argument that can be used to create S/MIME signed and/or encrypted messages:¶

smimeOperations: SmimeOperation[]|null (default: null). The list of S/MIME operations to perform, in the order specified. If none are specified, or if this property is omitted, no S/MIME operation is performed. If multiple messages are specified in the "create" argument, these operations apply to each of them.¶

headersToObscure: "String[String]"|"none"|null (default: null). If smimeOperations contains an "encrypt" operation, this specifies the list of header fields to obscure, as well as their obscured representation (see [draft-ietf-lamps-header-protection]). If the value is the string "none", no header field should be obscured. If the value is null, or if this property is omitted, the set of header fields to be protected is left up to the JMAP server.¶

headersToProtect: "String[]"|"all"|"none"|null (default: null). The list of header fields which should be protected by the smimeOperations specified. If the value is "all", all header fields of the original message should be protected. If the value is "none", no header fields should be protected. If the value is null, or if this property is omitted, the set of header fields to be protected is left up to the JMAP server.¶

The SmimeOperation object contains the following attributes:¶

• operation: "String". S/MIME operation to perform. This document only defines 2 possible values: "sign" and "encrypt". Future documents might add further values.¶

• options: "SignOptions"|"EncryptOptions"|null (default: null). Options controlling aspects of S/MIME signing or S/MIME encryption. If this attributes includes SignOptions when the operation attribute has the value "encrypt" or if this attributes includes EncryptOptions when the operation attribute has the value "sign", then JMAP server MUST return the invalidArguments error code.¶

The SignOptions object contains the following attributes:¶

• algorithm: "String"|null (default: null). The value represents the signature algorithm that the JMAP server should use to sign this message. If the algorithm is unspecified (the value "null") or this property is omitted, the JMAP server chooses the most appropriate algorithm to use. If the algorithm name specified is unrecognized by the server, it MUST return the unknownSignatureAlgo error code. [[Register unknownSignatureAlgo error code with IANA.]] Currently specified algorithms are "ecdsa_secp256r1_sha256", "ed25519", "rsa_pkcs1_sha256", "rsa_pss_pss_sha256". [[Alexey: probably need to define a new IANA registry or reference an existing one.]]¶

• opaque: "Boolean" (default: true). If the value is "true", this requests the JMAP server to use application/pkcs7-mime [RFC8551] media type for S/MIME signing, otherwise multipart/signed [RFC1847] media type.¶

The EncryptOptions object contains the following attributes:¶

• keyEncryptionAlgorithm: "String"|null (default: null). The value represents the key encryption algorithm that the JMAP server should use when encrypting this message. If the algorithm is unspecified, this property is omitted, or an unrecognized algorithm string is specified, the JMAP server chooses the most appropriate algorithm to use. [[Need a new IANA registry or a link to an existing one here.]]¶

• contentEncryptionAlgorithm: "String"|null (default: null). The value represents the data encryption algorithm that the JMAP server should use when encrypting this message. If the algorithm is unspecified, this property is omitted, or an unrecognized algorithm string is specified, the JMAP server chooses the most appropriate algorithm to use. [[Need a new IANA registry or a link to an existing one here.]]¶

When the operation attribute value is "sign", JMAP server constructs the S/MIME signed message from the original message (if this is the first operation) or the result of the previous operation, taking into considerations the SignOptions object (if present) or its defaults. 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.¶

When the operation is "encrypt", JMAP server constructs the S/MIME encrypted message from the original message (if this is the first operation) or the result of the previous operation, taking into considerations the EncryptOptions object (if present) or its defaults. 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.¶

(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",
            "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
              }
            }
          }
        },
        "smimeOperations": [{
            "operation": "sign",
            "options": {
              "opaque": false,
              "headersToProtect": "all"
            }
          },
          {
            "operation": "encrypt"
          }
        ]
      }, "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. Email/smimeDecrypt method for S/MIME decryption

The new method defined in this section is loosely based on Blob/upload method defined in [RFC9404]. The Email/smimeDecrypt method creates new blob(s) with decrypted content of existing blob(s). The resulting Id can be used in Email/parse method to import the decrypted message. If the specified blob doesn't specify encrypted content, the returned Id attribute has the same value as the "blobId" attribute.¶

*Parameters*¶

• accountId: "Id". The id of the account in which the blobs will be created.¶

• create: "Id[DecryptObjects]". A map of creation id to DecryptObjects¶

*Result*¶

The result is the same as for Foo/set in [RFC8620], with created and notCreated objects mapping from the creation id.¶

The created objects contain:¶

• id: "Id". The blobId that was created.¶

• type: "String|null". The media type as given in the creation (if any). If not provided, the server MAY perform content analysis and return one of the following: the calculated value, "application/octet- string", or null.¶

• size: "UnsignedInt". As per [RFC8620], the size of the created blob in octets.¶

The DecryptObjects objects contains:¶

• data: "DecryptDataSourceObject[]". An array of zero or more octet sources in order (zero to create an empty blob). The result of each of these sources is concatenated in order to create the blob.¶

• type: "String|null" (default: null). A hint for media type of the data.¶

The DecryptDataSourceObject objects contains exactly one of:¶

• data:asBase64: "String|null" (base64 representation of octets)¶

• or a blobId source:¶

• blobId: "Id"¶

• offset: "UnsignedInt|null" (MAY be zero). If null, then offset is assumed to be zero.¶

• length: "UnsignedInt|null" (MAY be zero). If null, then length is the remaining octets in the blob.¶

The created objects will also contain any other properties identical to those that would be returned in the JSON response of the upload endpoint described in [RFC8620]. This may be extended in the future; in this document, it is anticipated that implementations will extend both the upload endpoint and the Email/smimeDecrypt responses in the same way.¶

If there is a problem with a creation, then the server will return a notCreated response with a map from the failed creation id to a SetError object.¶

For each successful decryption, servers MUST add an entry to the createdIds map ([RFC8620], Section 3.3) for the request; even if the caller did not explicitly pass a createdIds, the value must be available to later methods defined in the same Request Object. This allows the blobId to be used via back-reference in subsequent method calls.¶

The created blob will have the same lifetime and same expiry semantics as any other binary object created via the mechanism specified in [RFC8620], Section 6.¶

First, find bodyparts that need decrypting:

    [[ "Email/get", {
        "ids": [ "f123u986" ],
        "properties": [ "threadId", "mailboxIds", "from", "subject",
          "receivedAt", "bodyStructure" ],
        "bodyProperties": [ "partId", "blobId", "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",
         "bodyStructure": [{
           "partId": "1",
           "blobId": "B841623871",
           "size": 120537,
           "type": "application/pkcs7-mime"
         }]
       }
     ]
   }, "#1" ]]

Then decrypt the specific body part using the retrieved blobId.

   [
    "Email/smimeDecrypt",
    {
     "accountId": "account1",
     "create": {
      "1": {
       "data" : [
       {
        "blobId": "B841623871"
       }
      ]
      }
     }
    },
    "R1"
   ]

Response:

   [
     "Email/smimeDecrypt",
     {
       "accountId" : "account1",
       "created" : {
         "1": {
           "id" : "G4c6751edf9dd6903ff54b792e432fba781271beb",
           "type" : "multipart/mixed",
           "size" : 1295
         }
       }
     },
     "R1"
   ]

8. 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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