S/MIME signature verification extension to JMAP
Isode Ltd14 Castle MewsHamptonMiddlesexTW12 2NPUKAlexey.Melnikov@isode.comJMAPS/MIME
This document specifies an extension to JMAP for returning S/MIME signature verification status.
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 signature verification status,
without requiring a JMAP client to download the signature body part and all signed body parts (when multipart/signed media type is used)
or to download and decode CMS (when application/pkcs7-mime media type is used).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in
.
The capabilities object is returned as part of the standard JMAP
Session object; see the JMAP spec. Servers supporting _this_
specification MUST add a property called
"urn:ietf:params:jmap:smime" 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.
defines Email/get method for retrieving message specific information.
This document defines the following pseudo values in the _properties_ argument:
o *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.
o *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.
o *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.
o *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
and .
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.
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).
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.
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.
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 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 verified the last time.
"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.
IANA is requested to register the "smime" JMAP Capability as follows:
Capability Name: "urn:ietf:params:jmap:smime"
Specification document: this document
Intended use: common
Change Controller: IETF
Security and privacy considerations: this document, 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.