Decrypted Content Attachment Flag for Internet Message Access Protocol (IMAP)
draft-newman-imap-decaf-00.txt

Status of this Memo

By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”

The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html.

This Internet-Draft will expire on March 21, 2009.

Abstract

This document promulgates an Internet Message Access Protocol (IMAP) flag keyword which Mail User Agents (MUAs) may use to indicate that the decrypted content of an encrypted message contains one or more attachments.

This document intentionally leaves undefined the definition of an "attachment" and is neutral as regards the message encryption system.

This document also defines an IANA registry for IMAP flag keyword conventions.

Table of Contents

1.  Conventions Used in this Document
2.  Introduction and Overview
3.  Client behavior
    3.1.  Client behavior when receiving a message
    3.2.  Client behavior when copying a message
    3.3.  Client behavior when sending a message
    3.4.  Client behavior when saving a temporary message
4.  Server Behavior
    4.1.  Server that supports the creation of new flag keywords
    4.2.  Server that supports only the $DECAF flag keyword
    4.3.  Interaction with the IMAP ACL extension
5.  Examples
6.  Acknowledgements
7.  IANA Considerations
8.  Security Considerations
9.  Formal Syntax
10.  References
    10.1.  Normative References
    10.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Conventions Used in this Document

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 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

2.  Introduction and Overview

This document defines a new IMAP mailbox flag keyword [RFC3501] (Crispin, M., “INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1,” March 2003.) 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.

3.  Client behavior

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.

3.1.  Client behavior when receiving a message

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.

3.2.  Client behavior when copying a message

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.

3.3.  Client behavior when sending a 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.

3.4.  Client behavior when saving a temporary message

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.

4.  Server Behavior

Server implementors seeking to support this specification MUST insure that their server complies with either Section 4.1 (Server that supports the creation of new flag keywords) or Section 4.2 (Server that supports only the $DECAF flag keyword). If the server also supports the IMAP ACL extension [RFC2086] (Myers, J., “IMAP4 ACL extension,” January 1997.), it MUST also comply with Section 4.3 (Interaction with the IMAP ACL extension).

4.1.  Server that supports the creation of new flag keywords

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.

4.2.  Server that supports only the $DECAF flag keyword

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.

4.3.  Interaction with the IMAP ACL extension

Any server that conforms to either Section 4.1 (Server that supports the creation of new flag keywords) or Section 4.2 (Server that supports only the $DECAF flag keyword) and also supports the IMAP ACL extension [RFC2086] (Myers, J., “IMAP4 ACL extension,” January 1997.), 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 [RFC2086] (Myers, J., “IMAP4 ACL extension,” January 1997.).

5.  Examples

(1)

MUA opens mailbox for the first time.

(a)

The server supports storing of arbitrary keywords

C: A01 SELECT INBOX
S: * FLAGS (\Flagged \Draft \Deleted \Seen)
S: * OK [PERMANENTFLAGS (\Flagged \Draft \Deleted \Seen \*)]
S: * 56 EXISTS
S: * 0 RECENT
S: * OK [UNSEEN 55]
S: * OK [UIDVALIDITY 1078037321]
S: * OK [UIDNEXT 198245]
S: A01 OK [READ-WRITE] Completed

(b)

The server supports storing of the $DECAF flag keyword

C: A01 SELECT INBOX
S: * FLAGS (\Flagged \Draft \Deleted \Seen $DECAF)
S: * OK [PERMANENTFLAGS (\Flagged \Draft \Deleted \Seen $DECAF)]
S: * 56 EXISTS
S: * 0 RECENT
S: * OK [UNSEEN 55]
S: * OK [UIDVALIDITY 1078037321]
S: * OK [UIDNEXT 198245]
S: A01 OK [READ-WRITE] Completed

(2)

The MUA successfully stores the $DECAF flag keyword

C: A02 STORE 4 +FLAGS ($DECAF)
S: * 4 FETCH (FLAGS (\Seen $DECAF))
S: A02 OK Completed

(3)

The server refuses to store the $DECAF keyword

C: A02 STORE 4 +FLAGS ($DECAF)
S: A02 NO STORE failed : Too many keywords in mailbox

6.  Acknowledgements

Credit is due Alexey Melnikov whose [RFC3503] (Melnikov, A., “Message Disposition Notification (MDN) profile for Internet Message Access Protocol (IMAP),” March 2003.) was emulated for this document.

7.  IANA Considerations

This document requests that IANA create and maintain a First Come First Served [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.) registry for IMAP flag keywords.

(1)

The name of the registry is "IMAP Flag Keywords".

(2)

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.

(3)

The review process for the registry is First Come First Served as per [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

(4)

The name of each registered flag keyword MUST begin with the character "$" and MUST satisfy the IMAP flag-keyword syntax [RFC3501] (Crispin, M., “INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1,” March 2003.).

(5)

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: XXXX

8.  Security Considerations

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.

9.  Formal Syntax

The following syntax specification uses the augmented Backus-Naur Form (BNF) notation as specified in [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.). Non-terminals referenced, but not defined below, are as defined by [RFC3501] (Crispin, M., “INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1,” March 2003.).

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.

flag-keyword   = "$DECAF" / other_keywords

other-keywords = atom

10.  References

10.1. Normative References

10.2. Informative References

Authors' Addresses

Full Copyright Statement

Copyright © The IETF Trust (2008).

This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an “AS IS” basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org.