S/MIME signature verification extension to JMAP

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Table of Contents

4. Extension to Email/get for S/MIME signature verification

[RFC8621] defines 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 "smimeStatus" response property.¶
• *smimeErrors*: If "smimeErrors" is included in the list of requested properties, it MUST be interpreted by the server as a request to return "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 "smimeVerifiedAt" 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 "smimeStatusAtDelivery" response property. (It effectively the same as "smimeStatus" value calculated at the date/time of deliver, as specified by "receivedAt".)¶

The "smimeStatus" response property is defined as follows:¶

smimeStatus: "String|null". null signifies that the message doesn't contain any signature. This property contains the S/MIME signature and certificate verification status calculated according to [RFC8551] and [RFC8550]. Possible string values of the property are listed below. Servers MAY return other values not defined below. Client 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 is neither signed, nor encrypted. This can also be returned for a multipart/signed message which contains 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 signature until a particular message is requested by the client. JMAP servers compliant with this document SHOULD 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, 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 calculated at the "receivedAt" date/time. Unlike "smimeStatus", the "smimeStatusAtDelivery" response property value is immutable. "smimeStatusAtDelivery" allows clients to compare S/MIME signature verification status at delivery with the current status as returned by "smimeStatus", for example it helps 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 S/MIME signature. (I.e. this property is non null only when the corresponding "smimeStatus" response property value is "signed/failed".) Each string in the array is a human readable description (in the language specified in Content-Language header field, if any) of a problem with the signature or the signing certificate. (See Section 3.8 of [RFC8620] in regards to how this is affected by the language selection.) For 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. Or the certificate might be expired and the JMAP server might be unable to retrieve 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 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 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 it's Date header field value). In all cases "smimeVerifiedAt" is set to 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.¶

      ["Email/get", {
      "ids": [ "f123u987" ],
      "properties": [ "mailboxIds", "from", "subject", "date",
       "smimeStatusAtDelivery", "receivedAt" ]
      }, "#1"]

This will result in the following response:

      [["Email/get", {
         "accountId": "abc",
         "state": "41234123231",
         "list": [
           {
             "id": "f123u457",
             "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"]]

      ["Email/get", {
      "ids": [ "af123u123" ],
      "properties": [ "mailboxIds", "from", "subject", "date",
       "smimeStatus", "smimeErrors", "smimeVerifiedAt" ]
      }, "#1"]

This will result in the following response:

      [["Email/get", {
         "accountId": "abc",
         "state": "41234123231",
         "list": [
           {
             "id": "af123u123",
             "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"]]

[RFC8621] defines 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.¶

6. Security Considerations

Server side S/MIME signature verification requires the client to trust server verification code and configuration to perform S/MIME signature verification. For example, if the server is not configured with some Trust Anchors, some messages will have "signed/failed" status instead of "signed/verified".¶

Constant recalculation of S/MIME signature status can result in Denial-of-Service condition. For that reason it is RECOMMENDED to cache results of signature verification for 10 minutes.¶

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