Decrypted Content Attachment Flag for Internet Message Access Protocol (IMAP)

The "C:" and "S:" prefixes in examples indicate lines sent, respectively, by the client and server. 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 .

This document defines a new IMAP mailbox flag keyword that aids Mail User Agents (MUAs) in determining if the decrypted content of an encrypted message contains attachments. This document does not define any new command or response for IMAP. A conforming MUA, which after decrypting all or part of a message and finding that the decrypted content contains one or more attachments, MAY store $DECAF IMAP flag keyword for the message. Subsequently and without again repeating the decryption, conforming MUAs MAY act upon the presence of the $DECAF flag keyword; e.g., by indicating that the message has an attachment when presenting a list of messages. The $DECAF flag keyword put forth in this document does not indicate that the message itself has attachments. Rather, it indicates that one or more encrypted portions of the message, when decrypted, contain attachments. This document does not define encrypted content, places no requirements on the format or structure of decrypted content, and does not define what constitutes an "attachment". These details are left to MUA implementors to determine as suits their needs. That said, MUAs SHOULD use the $DECAF flag keyword for message encryption formats in which the structure of the encrypted content cannot be determined without first decrypting the encrypted content.

A MUA wishing to store $DECAF flag keyword should first verify that the IMAP mailbox in question can store the $DECAF flag keyword, either temporarily or permanently. This is done by opening the mailbox and examining the FLAGS and PERMANENTFLAGS responses for either the $DECAF keyword or an indication that arbitrary keywords may be created. Note that even if the $DECAF flag keyword cannot be stored permanently, MUAs may still derive benefit from storing it temporarily. For example, an MUA which maintains minimal state. Indeed, some MUAs may only want to store $DECAF flag keyword temporarily thereby avoiding long term disclosure of the implied information.

Upon receiving a message containing encrypted content, the MUA MAY store the $DECAF flag keyword for that message if it determines that, when decrypted, any of the encrypted content contains one or more attachments. The MUA SHOULD NOT clear the $DECAF flag keyword: it may have been stored by another MUA.

The client SHOULD verify that the $DECAF flag keyword is preserved on a COPY operation. Furthermore, when copying a message between servers with the APPEND command, the client SHOULD preserve the $DECAF flag keyword on the resulting copy of the message.

When saving a sent message to any folder, the client MAY store the $DECAF flag keyword provided that the saved message has one or more encrypted parts which, when decrypted, contain one or more attachments.

When saving an unfinished message to any folder, the client MAY store the $DECAF flag provided that the unfinished message has one or more encrypted parts which, when decrypted, contain one or more attachments.

Server implementors seeking to support this specification MUST insure that their server complies with either or . If the server also supports the IMAP ACL extension , it MUST also comply with .

If a server supports the creation of new flag keywords, then no changes are required to the server to make it compatible with the extension described in this document.

Servers that support only the $DECAF flag keyword SHOULD preserve it on the COPY operation. It is also expected that a server that supports SEARCH <flag> will also support SEARCH KEYWORD $DECAF.

Any server that conforms to either or and also supports the IMAP ACL extension , SHOULD preserve the $DECAF flag keyword on COPY even if the client does not have the "w" ACL right. Note that the server MUST still check if the client has rights to perform the COPY operation on a message according to .

MUA opens mailbox for the first time. The server supports storing of arbitrary keywords

The server supports storing of the $DECAF flag keyword

The MUA successfully stores the $DECAF flag keyword

The server refuses to store the $DECAF keyword

Credit is due Alexey Melnikov whose was emulated for this document.

This document requests that IANA create and maintain a First Come First Served registry for IMAP flag keywords. The name of the registry is "IMAP Flag Keywords". To register an IMAP flag keyword, the flag keyword name, a description of the flag keyword, and an owner or change controller MUST be specified. Optionally, a reference to a document describing the flag keyword MAY be provided. The review process for the registry is First Come First Served as per . The name of each registered flag keyword MUST begin with the character "$" and MUST satisfy the IMAP flag-keyword syntax . The initial assignments for the registry are as follows:

1. Name: $MDNSent Description: Indicator that no MDN needs to be sent for the message. Owner/Change Controller: Alexey Melnikov <Alexey.Melnikov@isode.com> Reference: RFC 3503 2. Name: $Forwarded Description: Indicator that the message has been resent to another email address, embedded within or attached to a new message. Owner/Change Controller: Stephen H. Maes <stephane.maes@oracle.com> Alexey Melnikov <Alexey.Melnikov@isode.com> Reference: RFC 4550 3. Name: $DECAF Description: Indicator that an encrypted message contains attachments within its encrypted content. Owner/Change Controller: Ned Freed <ned.freed@sun.com> Dan Newman <dan.newman@sun.com> Reference: &rfc.number;

Use of the $DECAF flag exposes information about the content of an encrypted message. Namely, when the $DECAF flag is stored, knowledge that the encrypted message contains one or more attachments becomes known to anyone with access to the mailbox regardless of whether or not they can decrypt the message itself. When the $DECAF flag is not stored, somewhat less information is revealed: it may not be stored because no conforming MUA has yet received and decrypted the message, or it may not be stored because the message indeed contains no attachments within its encrypted content.

The following syntax specification uses the augmented Backus-Naur Form (BNF) notation as specified in . Non-terminals referenced, but not defined below, are as defined by . Except as noted otherwise, all alphabetic characters are case insensitive. The use of upper or lower case characters to define token strings is for editorial clarity only. Implementations MUST accept these strings in a case insensitive fashion.