Isode Ltd

14 Castle Mews Hampton, Middlesex TW12 2NP United Kingdom Alexey.Melnikov@isode.com

JMAP S/MIME 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 .

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Table of Contents

• .  Introduction
• .  Conventions Used in This Document
• .  Addition to the Capabilities Object
• .  Extension for S/MIME Signature Verification
  • .  Extension to Email/get
    • .  "smimeStatus" Response Property Extensibility
  • .  Extension to Email/query
  • .  Interaction with Email/changes
• .  IANA Considerations
  • .  JMAP Capability Registration for "smimeverify"
• .  Security Considerations
• .  References
  • .  Normative References
  • .  Informative References
• Acknowledgements
• Author's Address

Introduction JMAP for Mail 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 , without requiring a JMAP client to download the signature body part and all signed body parts (when the multipart/signed media type is used) or to download and decode the Cryptographic Message Syntax (CMS) (when the application/pkcs7-mime media type () 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.

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 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 . Data types defined in the core specification are also used in this document.

Addition to the Capabilities Object The capabilities object is returned as part of the standard JMAP Session object; see . 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.

Extension for S/MIME Signature Verification

Extension to Email/get 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 , , and . 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 and . 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 and . 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 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"]]

"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 ) or the "hasVerifiedSmime" property of Email/query (see ). 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").

Extension to Email/query 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.

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.

IANA Considerations

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,

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 ). 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.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document specifies conventions for X.509 certificate usage by Secure/Multipurpose Internet Mail Extensions (S/MIME) v4.0 agents. S/MIME provides a method to send and receive secure MIME messages, and certificates are an integral part of S/MIME agent processing. S/MIME agents validate certificates as described in RFC 5280 ("Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile"). S/MIME agents must meet the certificate-processing requirements in this document as well as those in RFC 5280. This document obsoletes RFC 5750. This document defines Secure/Multipurpose Internet Mail Extensions (S/MIME) version 4.0. S/MIME provides a consistent way to send and receive secure MIME data. Digital signatures provide authentication, message integrity, and non-repudiation with proof of origin. Encryption provides data confidentiality. Compression can be used to reduce data size. This document obsoletes RFC 5751. This document specifies a protocol for clients to efficiently query, fetch, and modify JSON-based data objects, with support for push notification of changes and fast resynchronisation and for out-of- band binary data upload/download. This document specifies a data model for synchronising email data with a server using the JSON Meta Application Protocol (JMAP). Clients can use this to efficiently search, access, organise, and send messages, and to get push notifications for fast resynchronisation when new messages are delivered or a change is made in another client. Informative References This document defines a framework within which security services may be applied to MIME body parts. [STANDARDS-TRACK] This memo defines a new Simple Mail Transfer Protocol (SMTP) [1] reply code, 521, which one may use to indicate that an Internet host does not accept incoming mail. This memo defines an Experimental Protocol for the Internet community. This memo defines an extension to the SMTP service whereby an interrupted SMTP transaction can be restarted at a later time without having to repeat all of the commands and message content sent prior to the interruption. This memo defines an Experimental Protocol for the Internet community.

Acknowledgements This document is a product of the JMAP Working Group. Special thank you to , , , , , , , , , and for suggestions, comments, and corrections to this document.

Author's Address Isode Ltd

14 Castle Mews Hampton, Middlesex TW12 2NP United Kingdom Alexey.Melnikov@isode.com