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Versions: 00 01 02 03 04 05 06 07 08 09 10                              
IMAPEXT Working Group                                        A. Melnikov
Internet Draft                                                    Editor
Document: draft-ietf-imapext-acl-07.txt                    February 2003


                          IMAP4 ACL extension

Status of this Memo

   This document is an Internet Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   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
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     Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or
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   A revised version of this draft document will be submitted to the RFC
   editor as a Proposed Standard for the Internet Community.  Discussion
   and suggestions for improvement are requested.  Distribution of this
   draft is unlimited.


0.   Open issues and ToDo list

   This section will be removed when the draft will be published as RFC.
   It is intended to simplify discussion.

    1). Require support for special identifier "disabled" for
        "ACL2=MOST-SPECIFIC" model?

    2). "ACL2=MOST-SPECIFIC" model: If a user belongs to multiple groups,
        document how rights are calculated.

    3). IANA registry for <vendorname> prefix in identifiers?

    4). ACL2 interaction with QUOTA extension should be moved to "QUOTA="
        document?

    5). Do we need the following functionality: discover the set of rights
        which may be granted to a given identifier in the ACL for a given
        mailbox?

    6). There is a proposal to replace MYRIGHT with either ACL FETCH or
        ACL CHECK:

         ACL FETCH is slightly different than MYRIGHTS in that it takes
         an identifier as an argument and returns the rights for that
         identifier (?? is this desirable ??).

         Or perhaps there could be an ACL CHECK that, given an identifier
         and a set of right, returns whether or not that identifier has
         those rights. ACL CHECK is probably more useful.

        For now MYRIGHTS is left as is, because neither parameters, nor
        responses were affected by the change to identifiers. However,
        if there is a desire to accept multiple mailboxes or mailbox masks
        in MYRIGHT, it would be better to change it to ACL <something> as well.

    7). Cleanup appendix A before publication as RFC, as some changes don't
        apply to RFC 2086.

    8). Other editorial comments/questions are enclosed in << and >>.


                            Table of Contents
   1  Abstract .................................................. X
   2  Conventions Used in this Document ......................... X
   3  Introduction and Overview ................................. X
   3.1 Access Control ........................................... X
   3.2 Access calculation model ................................. X
   3.3 Rights required to perform different IMAP4rev1 commands .. X
   4  ACL manipulation commands ................................. X
   4.1 ACL STORE ................................................ X
   4.2 ACL DELETE ............................................... X
   4.3 ACL SET .................................................. X
   4.4 ACL GET .................................................. X
   4.5 MYRIGHTS ................................................. X
   5  ACL related responses ..................................... X
   5.1 Extended LIST response with ACL information .............. X
   5.2 RIGHTS-INFO .............................................. X
   5.3 ACLFAILED untagged response .............................. X
   5.4 MYRIGHTS untagged response ............................... X
   5.5 MYRIGHTS response code ................................... X
   6  Formal Syntax ............................................. X
   7  IANA considerations ....................................... X
   7.1 ACL access calculation rule Registration Template ........ X
   7.2 Initial Registrations .................................... X
   7.2.1 Registration: UNION access calculation rule ............ X
   7.2.2 Registration: MOST-SPECIFIC access calculation rule .... X
   8  Security Considerations ................................... X
   9  Other considerations ...................................... X
   9.1 Compatibility with RFC 2086 .............................. X
   9.2 ACL2 interaction with QUOTA extension .................... X
   9.3 Mapping of ACL rights to READ-WRITE and READ-ONLY
       response codes ........................................... X
   9.4 Additional requirements and Implementation notes ......... X
   10  Normative References ..................................... X
   11  Informative References ................................... X
   12  Aknowledgement ........................................... X
   13  Editor's Address ......................................... X
   14  Full Copyright Statement ................................. X


1.   Abstract

   The ACL (Access Control List) extension of the Internet Message Access
   Protocol [IMAP4] permits mailbox access control lists to be manipulated
   through the IMAP protocol.


2.   Conventions Used in this Document

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

   In all examples "/" character is used as hierarchy separator.

   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 RFC 2119 [KEYWORDS].


3.   Introduction and Overview

   The ACL (Access Control List) extension of the Internet Message Access
   Protocol [IMAP4] permits mailbox access control lists to be retrieved
   and manipulated through the IMAP protocol.

   The ACL extension is present in any IMAP4 implementation which
   returns a capability starting with "ACL2=" prefix as one of the
   supported capabilities to the CAPABILITY command. The prefix is
   followed by "rule identifier" as described in 7.1.

   The document contains the following parts: section 3.1 provides the
   definition of different classes of identifiers and defines standard
   rights; section 3.2 introduces access calculation model, i.e. it
   describes how to calculate from an ACL which rigths apply to a particular
   user; section 3.3 summarizes relationship of different access rights with
   IMAP commands; section 4 introduces new IMAP commands the client can use
   to manipulate ACLs; section 5 defines new ACL related responses; and
   section 9 lists important considerations for compatibility with [IMAP4],
   RFC 2086 and some IMAP extensions.


3.1. Access Control

   An access control list is a set of <identifier,rights> pairs.
   An ACL applies to a mailbox.

   Identifier is a UTF-8 string. An identifier may have one of the following
   forms:
     a). "anyone" - special identifier that refers to the universal identity
         (all authentications, including anonymous).
     b). "authuser" - special identifier that refer to all authenticated users,
         but not anonymous.
     c). "owner" - special identifier that refers to the owner of a mailbox
         (if any).
     d). "administrators" - special identifier that refers to all users with
         administrative rights for the server.
     e). "user=<username>" - refers to a user. Here "<username>" is a user name
         string accepted by the LOGIN or AUTHENTICATE commands.
     f). "group=<groupname>" - refers to a group. Here "<groupname>" is a group
         name.
     g). "vendor=<vendorname>.<xxx>" - refers to a vendor specific special
         identifier, not covered by a).-f).
     h). "-<identifier>", where <identifier> is one of a).-g). This is
         reserved for "negative rights", described below.

   Note that a server is not required to implement any special identifier mentioned
   above. However if it allows a user to perform ACL operations on any one of them,
   server MUST use the semantic as described above.

   All other identifier names are reserved for future definition in an
   extension or revision to this specification (also known as ACL2).

   Rights is a string listing a (possibly empty) set of alphanumeric
   characters, each character listing a set of operations which is being
   controlled.  Letters are reserved for ''standard'' rights, listed
   below.  The set of standard rights may only be extended by a
   standards-track document.  Digits are reserved for implementation or
   site defined rights.  The currently defined standard rights are:

   l - lookup (mailbox is visible to LIST/LSUB commands, SUBSCRIBE mailbox)
   r - read (SELECT the mailbox, perform STATUS, CHECK, FETCH, SEARCH,
       COPY from mailbox)
   s - keep seen/unseen information across sessions (set or clear \SEEN flag
       via STORE, APPEND or COPY)
   w - write (set or clear flags other than \SEEN and \DELETED via STORE,
       APPEND or COPY)
   i - insert (perform APPEND, COPY into mailbox)
   p - post (send mail to submission address for mailbox,
       not enforced by IMAP4 itself)
   c - create mailboxes (CREATE new sub-mailboxes in any
       implementation-defined hierarchy, parent mailbox for the new
       mailbox name in RENAME).
       When a new mailbox is created it SHOULD inherit rights from
       the parent mailbox (if one exists) in the defined hierarchy.
   x - delete mailbox (DELETE mailbox, old mailbox name in RENAME)
   t - delete messages (set or clear \DELETED flag via STORE, set \DELETED flag
       during APPEND/COPY)
   e - perform EXPUNGE and expunge as a part of CLOSE
   d - This right is defined for backward compatibility with ACL
       extension (RFC 2086). If a client sets "d" right, the server MUST
       set "x", "e" and "t" rights. When the client clears the "d" right,
       the server MUST clear "x", "e" and "t" rights. When all three of "x",
       "e" and "t" are set, the server MUST return "d" right in response to
       an ACL GET command. If "x", "e" and "t" rights are not tied together,
       "d" right MUST NOT be returned in a RIGHTS-INFO response.
   a - administer (perform ACL STORE, ACL SET and ACL DELETE)

   An implementation may tie rights together or may force rights to
   always or never be granted to particular identifiers.  For example,
   in an implementation that uses unix mode bits, the rights "wisd" are
   tied, the "a" right is always granted to the owner of a mailbox and
   is never granted to another user.  If rights are tied in an
   implementation, the implementation must be conservative in granting
   rights in response to ACL STORE commands--unless all rights in a tied
   set are specified, none of that set should be included in the ACL
   entry for that identifier. If the server fails an ACL modification
   command (ACL STORE or ACL SET) because some rights are tied, it MUST
   return RIGHTS-INFO untagged response (see section 5.2).

   When an identifier in an ACL starts with a dash ("-"), that indicates
   that associated rights are to be removed from the identifier that is
   prefixed by the dash. This is referred to as a "negative right".
   This differs from ACL DELETE in that a negative right is added to the
   ACL, and is part of the calculation of the rights.

   For example, if the identifier "-user=fred" is granted the "w" right,
   that indicates that the "w" right is to be removed from users matching
   the identifier "user=fred", even though the user "fred" might have
   the "w" right as a consequence of some other identifier in the ACL.
   A ACL DELETE of "user=fred" simply deletes the identifier "user=fred"
   from the ACL; it does not affect any rights that the user "fred"
   may get from another ACL.

   Server implementations are not required to support "negative right"
   identifiers.


3.2. Access calculation model

   It is possible for multiple identifiers in an access control list to
   apply to a given user (or other authentication identity).  For
   example, an ACL may include rights to be granted to the identifier
   matching the user, one or more implementation-defined identifiers
   matching groups which include the user, and/or the identifier
   "anyone". How these rights are combined to determine the user's
   access is implementation-defined. The set of rules that describes
   how access is calculated is defined by a rule identifier (rule-ID).
   This document doesn't define any commands for manipulating a group
   membership.

   A client may determine the set of rights granted to the logged-in user
   for a given mailbox by using the MYRIGHTS command.

   This document defines two initial access calculation models: UNION and
   MOST-SPECIFIC.

   If a server implementing ACL2 uses the union of the rights granted to
   the applicable identifiers minus the union of the negative rights
   in order to calculate access, it MUST report "ACL2=UNION" in the server's
   capability list. See also section 7.2.1.

   An implementation may instead choose to only use those rights granted
   to the most specific identifier present in the ACL. In this case the
   server MUST report "ACL2=MOST-SPECIFIC" in the server's capability
   list. See also section 7.2.2.

   If the server implements any other policy for rights calculation,
   it MUST be either registered with IANA using the template provided in 7.1
   or start with "X-". The server MUST report one and only one "ACL2=<rule-ID>"
   capability in its CAPABILITY response.


3.3. Rights required to perform different IMAP4rev1 commands

   Before executing a command an ACL2 compliant server must check which rights
   are required to perform it. This section groups command by functions
   they perform and list the rights required. It also gives the detailed
   description of any special processing required.

   Listing and subscribing/unsubscribing mailboxes:
    LIST - "l" right is required.

    Note, that if the user has "l" right to a mailbox "A/B", but not to its parent
    mailbox "A", the LIST command should behave as if the mailbox "A" doesn't exist,
    for example:
               C: A777 LIST "" *
               S: * LIST (\NoInferiors) "/" "A/B"
               S: * LIST () "/" "C"
               S: * LIST (\NoInferiors) "/" "C/D"
               S: A777 OK LIST completed

    SUBSCRIBE - "l" right is required only if the server checks for mailbox existence
           when performing SUBSCRIBE.

    UNSUBSCRIBE - no rights required to perform this operation.

    LSUB - "l" right is required only if the server checks for mailbox existence when
           performing SUBSCRIBE.

   Mailbox management:
    CREATE - "c" right on a nearest existing parent mailbox. When a new mailbox
             is created it SHOULD inherit rights from the parent mailbox
             (if one exists) in the defined hierarchy.

    DELETE - "x" right on the mailbox.

    RENAME - Moving a mailbox from one parent to another
             requires "x" right on the mailbox itself and "c" right for the new parent.
             For example, if the user wants to rename mailbox named "A/B/C"
             to "D/E", the user must have "x" right
             for the mailbox "A/B/C" and "c" right for the mailbox "D".

   Copying or appending messages:

    Before performing a COPY/APPEND command the server MUST check if the user has "i" right
    for the target mailbox. If the user doesn't have "i" right, the operation fails.
    Otherwise for each copied/appended message the server MUST check if the user has
     "t" right - when the message has \Deleted flag set
     "s" right - when the message has \Seen flag set
     "w" right for all other message flags.
    Only when the user has a particular right the corresponding flags are stored for the
    newly created message. The server MUST NOT fail a COPY/APPEND if the user has no rights
    to set a particular flag.

   Example:    C: A003 MYRIGHTS TargetMailbox
               S: * MYRIGHTS INBOX rwis
               S: A003 OK Myrights complete

               C: A004 FETCH 1:3 (FLAGS)
               S: * 1 FETCH (FLAGS (\Draft \Deleted)
               S: * 2 FETCH (FLAGS (\Answered)
               S: * 3 FETCH (FLAGS ($Forwarded \Seen)
               S: A004 OK Fetch Completed

               C: A005 COPY 1:3 TargetMailbox
               S: A005 OK Copy completed

               C: A006 SELECT TargetMailbox
                  ...
               S: A006 Select Completed

       Let's assume that the copied messages received message numbers 77:79.

               C: A007 FETCH 77:79 (FLAGS)
               S: * 77 FETCH (FLAGS (\Draft)
               S: * 78 FETCH (FLAGS (\Answered)
               S: * 79 FETCH (FLAGS ($Forwarded \Seen)
               S: A007 OK Fetch Completed

       \Deleted flag was lost on COPY, as the user has no "t" right in the
       target mailbox.

       If the MYRIGHT command with the tag A003 would have returned:
               S: * MYRIGHTS INBOX rsti

       the response from the FETCH with the tag A007 would have been:

               C: A007 FETCH 77:79 (FLAGS)
               S: * 77 FETCH (FLAGS (\Deleted)
               S: * 78 FETCH (FLAGS ()
               S: * 79 FETCH (FLAGS (\Seen)
               S: A007 OK Fetch Completed

       In the latter case \Answered, $Forwarded and \Draft flags were lost
       on COPY, as the user has no "w" right in the target mailbox.

   Expunging the selected mailbox:
    EXPUNGE - "e" right on the selected mailbox.

    CLOSE - "e" right on the selected mailbox. If the server is unable to
            expunge the mailbox because the user doesn't have the "e" right,
            the server MUST ignore expunge request, close the mailbox
            and return tagged OK response.

   Fetch information about a mailbox and its messages:
    SELECT/EXAMINE/STATUS - "r" right on the mailbox.

    FETCH - A FETCH request that implies setting \Seen flag MUST NOT set it,
            if the current user doesn't have "s" right.

   Changing flags:
    STORE - the server MUST check if the user has
     "t" right - when the user modifies \Deleted flag
     "s" right - when the user modifies \Seen flag
     "w" right for all other message flags.
     STORE operation SHOULD NOT fail if the user has rights to modify at least
     one flag specified in the STORE.

   Changing ACLs:
    ACL STORE/DELETE/SET - "a" right on the mailbox (*).

   Reading ACLs:
    ACL GET - "a" right on the mailbox (*).

    MYRIGHTS - any of the following rights is required to perform the operation:
               "l", "r", "i", "c", "x", "e", "a".

   (*) Note, that when one or more mailbox pattern is specified,
   'l' right is required for each mailbox matching the mailbox pattern(s).


4.   ACL manipulation commands

    All ACL commands (i.e. ACL STORE/DELETE/SET/GET, except for MYRIGHTS)
    accept either a single mailbox name or several mailbox patterns as a
    parameter. Mailbox pattern is a mailbox with wildcards, wildcards are
    interpreted as described in [IMAP4] LIST command. In order to distinguish
    between a mailbox name (that is allowed to have wildcard characters '*'
    and '%') and a mailbox pattern, the latter is always represented as a
    parenthesized list.

    For simplicity the behaviour of ACL STORE/DELETE/SET/GET commands
    is described for a single mailbox case. When one or more mailbox pattern
    is specified, the server internally performs LIST command for all specified
    patterns and than it combines the results. Note, that only mailboxes for
    which the user has 'l' right are included in the combined result. If the
    combined result has no mailboxes, an ACL operation completes with success
    and the tagged OK response is sent. Otherwise the requested operation is
    performed for each mailbox in the combined result. If a particular mailbox
    causes the operation to fail (e.g. insufficient permissions), instead of
    failing the whole command, an untagged ACLFAILED or RIGHTS-INFO response
    is sent for this mailbox and the operation continues for the rest of the
    mailboxes. If the server knows that the operation will fail in the same
    manner for all matching mailboxes (e.g. user doesn't exist), it SHOULD
    return tagged NO response instead of sending several untagged ACLFAILED
    responses.

   Example:

    In the example below ACL SET command fails for 2 mailboxes
    "Personal/Jokes" and "Personal/Deaf and Blind". For the latter a human
    readable error description is returned. Also, the ACL for the mailbox
    "Personal/Secret" was updated to include the "r" right for a user
    "user=Boss" as a side effect of the ACL SET command (see also 4.3).

               C: A002 ACL SET (INBOX Personal/*) user=Fred rwist
               S: * ACLFAILED Personal/Jokes
               S: * ACLFAILED "Personal/Deaf and Blind" Temporary Error
               S: * LIST () "/" Personal/Secret (("ACL" (("user=Boss" "r"))))
               S: A002 OK acl set completed

   Example:    C: A002 ACL SET (Fruits/Apples/*) user=Zak lrs
               S: A002 NO User Zak doesn't exist


4.1. ACL STORE

   Arguments:  mailbox name or one or more mailbox masks
               authentication identifier
               access right modification

   Data:       OPTIONAL untagged responses: LIST with ACL information (see 5.1)

   Result:     OK - ACL STORE completed
               NO - ACL STORE failure: can't set acl
               BAD - command unknown or arguments invalid

      The ACL STORE command changes the access control list on the
      specified mailbox so that the specified identifier is granted
      permissions as specified in the third argument.

      The third argument is a string containing an optional plus ("+")
      or minus ("-") prefix, followed by zero or more rights characters.
      If the string starts with a plus, the following rights are added
      to any existing rights for the identifier.  If the string starts
      with a minus, the following rights are removed from any existing
      rights for the identifier.  If the string does not start with a
      plus or minus, the rights replace any existing rights for the
      identifier.

      Note, that for "ACL2=UNION" access calculation rule
      <ACL STORE mailbox identifier ""> MUST be treated as
      <ACL DELETE mailbox identifier>. Also note that these two commands
      don't have the same result for "ACL2=MOST-SPECIFIC".

      An ACL2 server MAY modify one or more ACL for one or more identifier
      as a side effect of modifying the ACL specified in ACL STORE. If the
      server does that it MUST send untagged LIST response with ACL information
      (see section 5.1) to notify the client about the changes made.

      If the server is unwilling to perform the command, because some rights
      for the identifier are tied, it MUST return RIGHTS-INFO untagged response
      (see section 5.2).

   Example:    C: A002 ACL STORE ~/Mail/saved user=smith cp
               S: * RIGHTS-INFO ~/Mail/saved user=smith la r swi cdext p
               S: A002 NO Acl store failed, some rights are tied

     Client decides to grant both rights to the identifier:

               C: A003 ACL STORE ~/Mail/saved smith cdextp
               S: A003 OK Setacl complete


4.2. ACL DELETE

   Arguments:  mailbox name or one or more mailbox masks
               authentication identifier

   Data:       OPTIONAL untagged responses: LIST with ACL information (see 5.1)

   Result:     OK - ACL DELETE completed
               NO - ACL DELETE failure: can't delete acl
               BAD - command unknown or arguments invalid

      The ACL DELETE command removes any <identifier,rights> pair for the
      specified identifier from the access control list for the
      specified mailbox.

      An ACL2 server MAY modify one or more ACL for one or more identifier
      as a side effect of modifying the ACL specified in ACL DELETE. If the
      server does that it MUST send untagged LIST response with ACL information
     (see section 5.1) to notify the client about the changes made.


4.3. ACL SET

   Arguments:  mailbox name or one or more mailbox masks
               list of (authentication identifier, access rights) pairs

   Data:       OPTIONAL untagged responses: LIST with ACL information (see 5.1)

   Result:     OK - replaceacl completed
               NO - replaceacl failure: can't replace acl
               BAD - command unknown or arguments invalid

      The ACL SET command replaces the access control list of the
      specified mailbox with the one provided as the second parameter to
      ACL SET. This command is semantically equivalent to the following
      sequence of commands:

      1). ACL GET <mailbox name>
      2). For each (authentication identifier AID, access rights RD) pair returned
          in the untagged ACL response that was caused by ACL GET, perform
           ACL DELETE <mailbox name> AID
      3). For each (authentication identifier AIA, access rights RA) pair from
          the second parameter of ACL SET perform
           ACL STORE <mailbox name> AIA RA

      An ACL2 server MAY modify one or more ACL for one or more identifier
      as a side effect of modifying the ACL specified in ACL SET. If the
      server does that it MUST send untagged LIST response with ACL information
      (see section 5.1) to notify the client about the changes made.

      If the server is unwilling to perform the command, because some rights
      for an identifier from the second list parameter are tied, it MUST
      return RIGHTS-INFO untagged response (see section 5.2).


4.4. ACL GET

   Arguments:  mailbox name or one or more mailbox masks

   Data:       REQUIRED untagged responses: LIST with ACL information (see 5.1)

   Result:     OK - ACL GET completed
               NO - ACL GET failure: can't get acl
               BAD - command unknown or arguments invalid

      The ACL GET command returns the access control list for mailbox in
      an untagged LIST response (see section 5.1).

   Example:    C: A002 ACL GET INBOX
               S: * LIST () "/" INBOX (("ACL" (("user=Fred" "rwipslextda"))))
               S: A002 OK acl get complete


4.5. MYRIGHTS

   Arguments:  mailbox name

   Data:       untagged responses: MYRIGHTS

   Result:     OK - myrights completed
               NO - myrights failure: can't get rights
               BAD - command unknown or arguments invalid

      The MYRIGHTS command returns the set of rights that the user has
      to mailbox in an untagged MYRIGHTS reply.

      The user must have any of the following rights to perform this operation:
        "l", "r", "i", "c", "x", "e", "a".
      If the user doesn't have any right from the above list, the server
      MUST behave as if the mailbox doesn't exist.


   Example:    C: A003 MYRIGHTS INBOX
               S: * MYRIGHTS INBOX rwipsldexta
               S: A003 OK Myrights complete


5.   ACL related responses

5.1. Extended LIST response with ACL information

   Contents:   name attributes
               hierarchy delimiter
               mailbox name
               ACL information (zero or more identifier rights pairs)

      This version of LIST response occurs as a result of an ACL GET command.
      It MAY also occur as a result of ACL STORE/DELETE/SET.
      The proposed syntax conforms to the syntax of an extended LIST response
      as defined by mbox-list-extended ABNF element of [LISTEXT].

      The meaning of "name attributes" and "hierarchy delimiter" is described in
      section 7.2.2 of [IMAP4]. Hierarchy delimiter is followed by a mailbox
      name for which this ACL applies. This is followed by extention part that
      includes "ACL" tag followed by zero or more pairs of strings, each pair
      contains the identifier for which the entry applies followed by the set
      of rights that the identifier has.

<<Name attributes doesn't have to be precise for the purpose of this extention?>>

<<Do we need "ACL" option to the LIST command?>>

   Example:    S: * LIST () "/" INBOX (("ACL" (("user=Fred" "rwipslextda"))))

      The example above shows that a user Fred is granted "rwipslextda"
      rights to the mailbox "INBOX".

               S: * LIST () ":" Drafts (("ACL"
                  (("user=Fred" "rwipslextda") ("group=Devel" "r"))))

      The example above shows that a user Fred is granted "rwipslextda"
      rights and a member of a group "Devel" is granted "r" right to
      the mailbox "Drafts".

               S: * LIST () NIL Manson (("ACL" ()))

      The example above shows the mailbox "Manson" has an empty ACL.


5.2. RIGHTS-INFO

   Data:       mailbox name
               identifier
               required rights
               list of optional rights

      The RIGHTS-INFO response occurs as a result of a failed
      ACL STORE or ACL SET command. The first two strings are
      the mailbox name and identifier for which this rights list
      applies. Following the identifier is a string containing
      the (possibly empty) set of rights the identifier will always
      be granted in the mailbox.

      Following this are zero or more strings each containing a set of
      rights the identifier may be granted in the mailbox.  Rights
      mentioned in the same string are tied together--either all must be
      granted to the identifier in the mailbox or none may be granted.

      The same right may not be listed more than once in the RIGHTS-INFO
      response.

5.3. ACLFAILED untagged response

   Contents:   mailbox name
               OPTIONAL response code and human readable text for failure
                reason

    The ACLFAILED response containing a mailbox name indicates that
    the ACL operations failed for the specified mailbox. The reason for
    failure may be described by the response code (if included). If
    the command for the mailbox fails because some rights are tied,
    the server MUST return RIGHTS-INFO response instead of ACLFAILED.

   Example:    C: A002 ACL SET (INBOX Personal/*) user=Fred rwist
               S: * ACLFAILED Personal/ABC
               S: A002 OK acl set complete


5.4. MYRIGHTS untagged response

   Data:       mailbox name
               rights

      The MYRIGHTS response occurs as a result of a MYRIGHTS command.
      The first string is the mailbox name for which these rights apply.
      The second string is the set of rights that the client has.


5.5. MYRIGHTS response code

   Data:       rights

      The MYRIGHTS response code is sent in an untagged OK response
      that results from SELECT/EXAMINE. Also this response code can be
      sent at any time after opening a mailbox when the server detects
      that the set of rights allowed for the logged in user has changed.

      The MYRIGHT response code is equivalent to MYRIGHTS untagged response
      for the selected mailbox.


6.   Formal Syntax

   Formal syntax is defined using ABNF [ABNF] as modified by [IMAP4].
   Non-terminals referenced but not defined below are as defined by
   [IMAP4].

   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.


   acl2_command    = myrights | "ACL" SP acl2_subcommand

   acl2_subcommand = replaceacl | deleteacl | updateacl | getacl | listrights

   acl_list_data   = "(" acl_list_tag SP acl_data ")"
                     ;; acl_list_data conforms to the syntax of
                     ;; mbox-list-extended-item from [LISTEXT]

   acl_list_tag    = <"> "ACL" <">

   ace             = "(" identifier SP rights ")"

   acl_data        = "(" [ace *( SP ace )] ")"
                     ;; zero or more parenthesized identifier/rights pairs

<<Currently the empty list doesn't conform to mbox-list-extended-item syntax!>>

   always_granted  = rights

   deleteacl       = "DELETE" SP mbox_or_pat SP identifier

   getacl          = "GET" SP mbox_or_pat

   identifier      = astring
                      ;; UTF-8 string with syntax of ident_syntax

   ident_syntax    = ident | "-" ident

   ident           = ident_special | ident_user | ident_group |
                     ident_vendor

   ident_special   = "anyone" | "authuser" | "owner" | "administrators"

   ident_user      = "user=" ident_detail

   ident_group     = "group=" ident_detail

   ident_vendor    = "vendor=" ident_vname "." ident_detail

   ident_detail    = 1*UTF8-CHAR

   ident_vname     = 1*UTF8-CHAR
                      ;; MUST NOT contain "."

   listrights      = "LIST" SP mbox_or_pat SP identifier

   listrights_data = "RIGHTS-INFO" SP mailbox SP identifier
                           SP always_granted *(SP rights)

   mod_rights      = quoted
                      ;; +rights to add, -rights to remove

   myrights        = "MYRIGHTS" SP mailbox

   myrights_data   = "MYRIGHTS" SP mailbox SP rights

   myrights_rspcod = "MYRIGHTS" SP rights

   replaceacl      = "SET" SP mbox_or_pat *(SP identifier SP rights)

   resp-text-code  =/ myrights_data

   rights          = quoted
                      ;; MUST NOT contain leading "+" or "-"

   updateacl       = "STORE" SP mbox_or_pat SP identifier SP mod_rights

   mbox_or_pat     = mailbox / patterns

   patterns        = "(" list-mailbox *(list-mailbox) ")"

   partialfail_rsp = "ACLFAILED" SP mailbox
                     [SP ["[" resp-text-code "]" SP] text]
                      ;; May contain optional failure reason followed by a
                      ;; human readable text

   UTF8-CHAR       = CHAR | UTF8-2 | UTF8-3 | UTF8-4 | UTF8-5 | UTF8-6

   CHAR            = %x01-7F

   UTF8-loworder   = %x80-BF

   UTF8-2          = %xC1-DF UTF8-loworder
                      ;; Disallow overlong sequences beginning with 0xC0.

   UTF8-3          = (%xE0 %xA0-BF UTF8-loworder) |
                     (%xE1-EC 2UTF8-loworder) |
                     (%xED %x80-9F UTF8-loworder) |
                     (%xEE 2UTF8-loworder) |
                     (%xEF %x80-BE UTF8-loworder) |
                     (%xEF %xBF %x80-BD)
                      ;; Disallow overlong sequences beginning with 0xE0,
                      ;; disallow encoded surrogate characters, and
                      ;; disallow U+FFFE, U+FFFF.

   UTF8-4          = (%xF0 %x90-BF 2UTF8-loworder) |
                     (%xF1-F7 3UTF8-loworder)
                      ;; Disallow overlong sequences beginning with 0xF0.

   UTF8-5             = %xF8-FB 4UTF8-1

   UTF8-6             = %xFC-FD 5UTF8-1


7.   IANA considerations

7.1.  ACL access calculation rule Registration Template

   When an access calculation rule for ACL2 extension is registered, the
   following information is supplied:

   Rule Identification: specify a string that identifies this
      rule.  Unless the rule is registered with the IANA, the
      rule's identification must start with "X-".
      The server supporting a particular rule <rule-ID> MUST report
      "ACL2=<rule-ID>" in the capability list.

   Rule Semantics: specify how access rights for a mailbox are calculated.

   Negative rights allowed: specify whether "negative right" identifiers are
      allowed.

   Groups allowed: specify whether group identifiers are allowed.

   Special Identifiers: describe whether any implementation defined
      aliases are allowed and define their meaning.

   Contact Information: specify the postal and electronic contact
      information for the author of the feature.


7.2.   Initial Registrations

7.2.1. Registration: UNION access calculation rule

   Rule Identification: UNION

   Rule Semantics: the union of the rights granted to
   the applicable identifiers minus the union of the negative rights.

   Negative rights allowed: Yes.

   Groups allowed: Yes, but not required.

   Special Identifiers: No.

   Contact Information: c.f., the "Editor's Address" section of this
      memo


7.2.2. Registration: MOST-SPECIFIC access calculation rule

   Rule Identification: MOST-SPECIFIC

   Rule Semantics: the rights granted to the most specific identifier
      present in the ACL are used, i.e. if the user identifier is present,
      its ACL is used. If no user identifier is present, but a group that
      includes this user as a member is present, the group ACL is used.
      If neither user, nor group identifier is present, but an ACL for
      a special group "anyone" is present, the ACL for "anyone" is used.

   Negative rights allowed: No.

   Groups allowed: Yes, but not required.

   Special Identifiers: No.

   Contact Information: c.f., the "Editor's Address" section of this
      memo


8.   Security Considerations

   An implementation must make sure the ACL commands themselves do not
   give information about mailboxes with appropriately restricted ACL's.
   For example, a ACL GET command on a mailbox for which the user has
   insufficient rights should not admit the mailbox exists, much less
   return the mailbox's ACL.

   IMAP clients implementing ACL2 that are able to modify ACLs SHOULD
   warn a user that wants to give full access (or even just "a" right)
   to the special identifier "anyone".


9.  Other considerations

9.1. Compatibility with RFC 2086

   This section gives guidelines how an existing RFC 2086 implementation
   may be modified to support ACL2.

   1). Special handling of new identifiers. Two approaches:
    a). prohibit the creation of users on the mail system with special
        names (ident_special token in the ABNF) and prohibit to use "=" in
        identifier names. Treat "user=<identif>" the same way as "<identif>"
        in both ACL and ACL2 version of commands.
    b). implement translation from "user=<identif>" to "<identif>"
        internally.

   2). "d" right mapping to "x", "t" and "e" and back.  Two approaches:
    a). Tie "x", "t" and "e" together - almost no changes
    b). Implement separate "x", "t" and "e". Return "d" right in a LIST
        response containing ACL information when all three of "x", "t"
        and "e" are granted.

   3). "rights" can be only sent as quoted strings, not as literals.

   4). Send untagged LIST response with ACL information when server changes
       other ACLs on ACL STORE/DELETE/SET (servers that don't do that don't
       have to care).

   Additional work required to implement ACL2 when RFC 2086 is already
   implemented:

   1). Recognize new command names

   2). Handle multiple mailboxes (a la LIST), return ACLFAILED on a failure.

   3). Implement ACL SET

   4). Optional: send MYRIGHTS untagged response on SELECT/EXAMINE

   5). LISTRIGHTS command is deprecated. LISTRIGHTS response is replaced
       with RIGHTS-INFO response code.

   6). Report "ACL2=UNION" capability.

   7). Implement new special identifiers or groups if desired.


9.2. ACL2 interaction with QUOTA extension

   Server that implement both ACL2 and QUOTA extensions MUST use the following
   table to determine if a quota operation should be allowed for the user:

    GETQUOTAROOT - any of the following rights is required to perform the
                   operation: "r", "i", "a".

<<"r" allows to calculate usage, "i" allows to put a mailbox overquota and get
  mailbox usage with certain implementations, that check message size
  before receiving the message in APPEND>>

    GETQUOTA     - no rights required

    SETQUOTA     - implementation defined, as SETQUOTA operates on a quotaroot,
                   not on a mailbox.


9.3. Mapping of ACL rights to READ-WRITE and READ-ONLY response codes

   A particular ACL2 server implementation may allow "shared multiuser
   access" to some mailboxes. "Shared multiuser access" to a mailbox means
   that multiple different users are able to access the same mailbox,
   if they have proper access rights. "Shared multiuser access" to the
   mailbox doesn't mean that the ACL for the mailbox is currently set
   to allow access by multiple users. Let's denote a "shared multiuser
   write access" as a "shared multiuser access" when a user may be
   granted flag modification rights (any of "w", "s" or "t").

   Section 3.3 ("Rights required to perform different IMAP4rev1 commands")
   describes which rights are required for modifying different flags.

   If the ACL2 server implements some flags as shared for a mailbox (i.e.,
   the ACL for the mailbox may be set up so that changes to those flags are
   visible to another user), let's call the set of rights associated with these
   flags (as described in 3.3) for that mailbox collectively as "shared flag
   rights". Note, that "shared flag rights" set MAY be different for different
   mailboxes.

   If the server doesn't support "shared multiuser write access" to a
   mailbox or doesn't implement shared flags on the mailbox, "shared flag
   rights" for the mailbox is defined to be the empty set.


   Example 1: Mailbox "banan" allows "shared multiuser write access" and
              implements flags \Deleted, \Answered and $MDNSent as
              shared flags. "Shared flag rights" for the mailbox "banan"
              is a set containing flags "t" (because system flag \Deleted
              requires "t" right) and "w" (because both \Answered and
              $MDNSent require "w" right).

   Example 2: Mailbox "apple" allows "shared multiuser write access" and
              implements \Seen system flag as shared flag. "Shared flag
              rights" for the mailbox "apple" contains "s" right,
              because system flag \Seen requires "s" right.

   Example 3: Mailbox "pear" allows "shared multiuser write access" and
              implements flags \Seen, \Draft as shared flags. "Shared flag
              rights" for the mailbox "apple" is a set containing flags "s"
              (because system flag \Seen requires "s" right) and "w"
              (because system flag \Draft requires "w" right).

   The server MUST include a READ-ONLY prefix in the tagged OK response to
   a SELECT command if none of the following rights is granted to the
   current user:
    "i", "e" and "shared flag rights".
   The server SHOULD include a READ-WRITE prefix in the tagged OK response
   if at least one of the "i", "e" or "shared flag rights" is granted to the
   current user.

   Example 1 (continue): The user that has "lrs" rights for the mailbox
                         "banan". The server returns READ-ONLY response
                         code on SELECT, as none of "iewt" rights is
                         granted to the user.

   Example 2 (continue): The user that has "rit" rights for the mailbox
                         "apple". The server returns READ-WRITE response
                         code on SELECT, as the user has "i" right.

   Example 3 (continue): The user that has "rset" rights for the mailbox
                         "pear". The server returns READ-WRITE response
                         code on SELECT, as the user has "e" and "s" rights.


9.4. Additional requirements and Implementation notes

   Any server implementing an ACL2 extension MUST accurately reflect the current
   user's rights in FLAGS and PERMANENTFLAGS responses. The server SHOULD issue
   a MYRIGHTS response code in an untagged OK response as a result of a SELECT
   or EXAMINE command.

   Example:    C: A142 SELECT INBOX
               S: * 172 EXISTS
               S: * 1 RECENT
               S: * OK [UNSEEN 12] Message 12 is first unseen
               S: * OK [UIDVALIDITY 3857529045] UIDs valid
               S: * OK [UIDNEXT 4392] Predicted next UID
               S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
               S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited
               S: * OK [MYRIGHTS rwiste] Allowed rights
               S: A142 OK [READ-WRITE] SELECT completed

   An ACL2 server MAY modify one or more ACL for one or more identifier as a
   side effect of modifying the ACL specified in ACL STORE/DELETE/SET.
   If the server does that it MUST send untagged ACL response to notify the
   client about the changes made.


10.  Normative References

   [KEYWORDS] Bradner, "Key words for use in RFCs to Indicate
   Requirement Levels", RFC 2119, Harvard University, March 1997.

   [ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications:
   ABNF", RFC 2234, Internet Mail Consortium, Demon Internet Ltd,
   November 1997.

   [IMAP4] Crispin, M., "Internet Message Access Protocol - Version
   4rev1", RFC 2060, University of Washington, December 1996.

   [UTF-8] Yergeau, F., "UTF-8, a transformation format of IS0 10646",
   RFC 2279, Alis Technologies, January 1998.

   [QUOTA] Myers, J., "IMAP4 QUOTA extension", RFC 2087, Carnegie Mellon
   University, January 1997.


11.  Informative References

   [LISTEXT] Leiba, B., "IMAP4 LIST Command Extensions", work in progress,
   draft-ietf-imapext-list-extensions-02.txt, IBM T.J. Watson Research
   Center.


12.  Aknowledgement

   This document is a revision of the RFC 2086 written by John G. Myers.

   Editor appreciates comments received from Mark Crispin, Chris Newman,
   Cyrus Daboo, John G. Myers, Steve Hole, Curtis King, Lyndon Nerenberg,
   Larry Greenfield, Vladimir Butenko, Dave Cridland, Harrie Hazewinkel
   and other participants of IMAPEXT working group.


13.  Editor's Address

   Alexey Melnikov
   mailto: mel@messagingdirect.com

   ACI WorldWide/MessagingDirect
   59 Clarendon Road, Watford,
   Hertfordshire, United Kingdom, WD17 1FQ

   Phone: +44 1923 81 2877


14.  Full Copyright Statement

    Copyright (C) The Internet Society 2003.  All Rights Reserved.

    This document and translations of it may be copied and furnished to
    others, and derivative works that comment on or otherwise explain it
    or assist in its implementation may be prepared, copied, published
    and distributed, in whole or in part, without restriction of any
    kind, provided that the above copyright notice and this paragraph
    are included on all such copies and derivative works.  However, this
    document itself may not be modified in any way, such as by removing
    the copyright notice or references to the Internet Society or other
    Internet organizations, except as needed for the purpose of
    developing Internet standards in which case the procedures for
    copyrights defined in the Internet Standards process must be
    followed, or as required to translate it into languages other than
    English.

    The limited permissions granted above are perpetual and will not be
    revoked by the Internet Society or its successors or assigns.

    This document and the information contained herein is provided on an
    "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
    TASK FORCE DISCLAIMS 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.


Appendix A. Changes since RFC 2086

   1. Changed the charset of "identifier" from US-ASCII to UTF-8.

   2. Changed identifier syntax. Now all users must start with "user="
      prefix. Specified that identifiers starting with a "group=" prefix are
      reserved for groups and implementation defined aliases.

   3. Specified that mailbox deletion is controled by the "x" right and
      EXPUNGE is controlled by "e" right.

   4. Clarified that RENAME requires "c" right for the new parent and "x"
      right for the old name.

   5. Changed capability name from "ACL" to "ACL2" because changes 2 and 3
      are not backward compatible with ACL RFC.

   6. Added "t" right that controls STORE \Deleted. Redefined "d" to be a
      macro for "e", "x" and "t".

   7. Added "ACL2=UNION" and "ACL2=MOST-SPECIFIC" capabilities and IANA
      registration template.

   8. Specified that "a" right also controls DELETEACL

   9. Specified that "r" right also controls STATUS

  10. Specified that "w" controls setting flags other than \Seen and
      \Deleted on APPEND. Same for "s" and "t" flags.

  11. Specified that "l" controls SUBSCRIBE/UNSUBSCRIBE.

  12. Added "Compatibility with RFC 2086" section.

  13. Added "Implementation Notes" section.

  14. Updated "References" section.

  15. Deleted "PARTIAL", this is a deprecated feature of RFC1730.

  16. Added MYRIGHT response code as per Cyrus suggestion.

  17. Added REPLACEACL (ACL SET) as per Mark and Cyrus request.

  18. Added new section that describes which rights are required and/or
      checked when performing various IMAP commands.

  19. Added section about interaction of ACL2 with the QUOTA extension.

  20. Added special identifiers "authuser", "owner" and "administrators".

  21. Added mail client security considerations when dealing with special
      identifier "anyone".

  22. Clarified that negative rights are not the same as DELETEACL (ACL DELETE).

  23. Removed the requirement to check the "r" right for CHECK, SEARCH and
      FETCH, as this is required for SELECT/EXAMINE to be successful.

  24. Added note that a server can modify an ACL for any identifier(s) as a
      side effect of performing SETACL/DELETEACL/REPLACEACL
      (ACL STORE/DELETE/SET). Also specified that the server MUST send
      untagged ACL response if it does that. Updated command definition
      to include optional ACL untagged response.

  25. Fixed typo in 10.1. (Was "ACL=...", not "ACL2=...")

  26. Cleaned up section 10.2 a bit.

  27. LISTRIGHTS (ACL LIST) requires same rights as MYRIGHTS.

  28. Added section about mapping of ACL rights to READ-WRITE and READ-ONLY
      response codes.

  29. Changed ABNF for rigths/mod_rights to be quoted instead of astring.

  30. Changed syntax of ACL2 commands according to the following table:
        SETACL          => ACL STORE
        DELETEACL       => ACL DELETE
        REPLACEACL      => ACL SET
        GETACL          => ACL GET
        LISTRIGHTS      => ACL LIST

        MYRIGHTS was left as is

      Changed syntax of ACL2 responses:
        ACL             => Extended LIST response with ACL information
        LISTRIGHTS      => RIGHTS-INFO

        MYRIGHTS was left as is

      Any better suggestions for names are welcome.

  31. Added mailbox patterns and partial failures. Updated ABNF.

  32. SUBSCRIBE is NOT allowed with "r" right.

  33. GETACL (ACL GET) is NOT allowed with "r" right.

  34. Added human readable text to ACLFAILED untagged response.

  35. GETQUOTAROOT requires any one of "r", "i" or "a".

  36. Removed ACL LIST (LISTRIGHTS) command.

  37. Added "vendor=" identifier prefix for vendor specific identifiers.

  38. Added ABNF for identifier syntax.

  39. Text and document structure change based on comments from Harrie
      Hazewinkel.

  40. Replaced "ACLINFO" response with a special variant of LIST.
      Updated text and ABNF.

  41. Replaced SPACE with SP in the ABNF section.