IMAPEXT Working Group                                        A. Melnikov
Internet Draft                                                    Editor
Document: draft-ietf-imapext-acl-05.txt                        June draft-ietf-imapext-acl-06.txt                     October 2002

                          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.

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0.   Open issues

   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). Obsolete LISTRIGHTS?

Mark Crispin wrote:

  >    [...] If rights are tied in an
  >    implementation, the implementation must be conservative in granting
  >    rights in response to SETACL commands--unless all rights in a tied
  >    set are specified, none of that set should be included in the ACL
  >    entry for that identifier.  A client may discover the set of rights
  >    which may be granted to a given identifier in the ACL for a given
  >    mailbox by using the LISTRIGHTS command.
  >
  > This was in RFC 2086 as well.
  >
  > Do we really want these semantics?
  >
  > The effect of this is that it is mandatory to do a LISTRIGHTS prior to any
  > SETACL (and multiple LISTRIGHTS prior to any REPLACEACL), since otherwise
  > there is no way for a client to know how to grant a particular right to an
  > identifier.
  >
  > Presumably, the reason for these semantics is so a client can review the
  > impact of granting a particular right if it's tied to another.  But will
  > any client actually do this?  Or will clients just blindly do the
  > LISTRIGHTS to get the necessary incantation for SETACL?
  >
  > What I see is no savings of work for a client which cares about any side
  > effects due to tied-rights, but a lot of extra work for clients which do
  > not care.

Alexey Melnikov replied:

  I tend to agree. I would rather have a new response code LISTRIGHTS that is
  returned in NO response to SETACL command.
  Imagine that the client wants to grant "cp" rights to a user "smith". The
  client instead of doing:

                 C: A001 LISTRIGHTS ~/Mail/saved smith
                 S: * LISTRIGHTS ~/Mail/saved smith la r swi cdext
                 S: A001 OK Listrights completed
                 C: A002 SETACL ~/Mail/saved smith cdextp
                 S: A002 OK Setacl complete

  will do the following (this assumes that the server doesn't want to grant a
  right unless all tied rights are specified by the client):

                 C: A001 SETACL ~/Mail/saved smith cp
                 S: A001 NO [LISTRIGHTS la r swi cdext] No some rights are tied,
  specify both
                 C: A002 SETACL ~/Mail/saved smith cdextp
                 S: A002 OK Setacl complete
                 C: A002 LISTRIGHTS ~/Mail/saved smith

  and if the server sets all tied rights automatically if at least one of them
  is specified (not recommended by the document but still possible):

                 C: A001 SETACL ~/Mail/saved smith cp
                 S: * ACL ~/Mail/saved smith cdextp
                 S: A001 OK Setacl complete

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

1.   Abstract

   The ACL extension of the Internet Message Access Protocol [IMAP4]
   permits 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 extension is present in any IMAP4 implementation which
   returns a capability starting with "ACL2=" as one of the supported
   capabilities to the CAPABILITY command.

   An access control list is a set of <identifier,rights> pairs.

   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.
     e). "user=<xxx>" - refers to a user. Here "<xxx>" is a user name string
         accepted by the LOGIN or AUTHENTICATE commands.
     f). "group=<xxx>" - refers to a group.
     g). "-<identifier>", where <identifier> is one of a).-f). This is
         reserved for "negative rights", described below.

<<Reserve X- and V- prefixes?>>

   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
       a ACL GET command. If "x", "e" and "t" rights are not tied together,
       "d" right MUST NOT be returned in a ACL LIST 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.  A client may discover the set of rights
   which may be granted to a given identifier in the ACL for a given
   mailbox by using the ACL LIST command.

   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.

   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).

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

   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.

   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.

   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.

3.1. 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.

<<Allow if the user has "r" right?>>

    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.

<<Add an example here?>>

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

    CLOSE - "e" right on the selected mailbox. Server MUST NOT fail CLOSE operation if it is
            unable to expunge the mailbox.

   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. mailbox (*).

   Reading ACLs:
    ACL GET - "a" right on the mailbox. mailbox (*).
<<Several implementations allow GETACL (ACL GET) with "r" right. If this is allowed,
  ACL GET can disclose some identifiers existing on a mail system>>

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

    ACL LIST - same as for MYRIGHTS. MYRIGHTS(*).

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

4.   Commands

    All ACL commands (i.e. ACL STORE/DELETE/SET/GET/LIST, 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 bahaviour of ACL STORE/DELETE/SET/GET/LIST 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 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:    C: A002 ACL SET (INBOX Personal/*) user=Fred rwist
               S: * ACLFAILED Personal/ABC
               S: A002 OK acl set complete

<<Add some responses caused by side effects>>

   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: ACLINFO

   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 ACLINFO response to notify the
      client about the changes made.

4.2. ACL DELETE

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

   Data:       OPTIONAL untagged responses: ACLINFO

   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 ACLINFO response 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: ACLINFO

   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 ACLINFO response to notify the
      client about the changes made.

4.4. ACL GET

   Arguments:  mailbox name or one or more mailbox masks

   Data:       REQUIRED untagged responses: ACLINFO

   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 ACLINFO reply.

   Example:    C: A002 ACL GET INBOX
               S: * ACLINFO INBOX user=Fred rwipslextda
               S: A002 OK acl get complete

4.5. ACL LIST

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

   Data:       untagged responses: RIGHTS-INFO

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

      The ACL LIST command takes a mailbox name and an identifier and
      returns information about what rights may be granted to the
      identifier in the ACL for the mailbox.

   Example:    C: a001 ACL LIST ~/Mail/saved user=smith
               S: * RIGHTS-INFO ~/Mail/saved user=smith la r swi cdext
               S: a001 OK ACL LIST completed

               C: a005 ACL LIST archive.imap anyone
               S: * RIGHTS-INFO archive.imap anyone "" l r s w i p c dtex a
               0 1 2 3 4 5 6 7 8 9
               S: a005 OK ACL LIST completed

4.6. 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.

<<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.>>

      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.   Responses

5.1. ACLINFO

   Data:       mailbox name
               zero or more identifier rights pairs

      The ACLINFO response occurs as a result of a ACL GET command. It MAY
      also occur as a result of ACL STORE/DELETE/SET. The first string
      is the mailbox name for which this ACL applies. This is 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.

<< There is a proposal to use LISTEXT syntax instead. This requires
   publishing a new revision of LISTEXT, that in its current form, is not
   flexible enough. >>

5.2. RIGHTS-INFO

   Data:       mailbox name
               identifier
               required rights
               list of optional rights

      The RIGHTS-INFO response occurs as a result of a ACL LIST
      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:   OPTIONAL response code (failure reason)
               mailbox name

    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).

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

<<Add human readable text?>>

<<Currently no existing response code can be used in ACLFAILED>>

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.

5.4.

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" SPACE acl2_subcommand

   acl2_subcommand = replaceacl | deleteacl | updateacl | getacl | listrights

   acl_data        = "ACLINFO" SPACE mailbox *(SPACE identifier SPACE rights)

   always_granted  = rights

   deleteacl       = "DELETE" SPACE mailbox mbox_or_pat SPACE identifier

   getacl          = "GET" SPACE mailbox mbox_or_pat

   identifier      = astring
                      ;; UTF-8

<< Specify more detailed syntax for identifiers? >>

   listrights      = "LIST" SPACE mailbox mbox_or_pat SPACE identifier

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

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

   myrights        = "MYRIGHTS" SPACE mailbox

   myrights_data   = "MYRIGHTS" SPACE mailbox SPACE rights

   myrights_rspcod = "MYRIGHTS" SPACE rights

   replaceacl      = "SET" SPACE mailbox mbox_or_pat *(SPACE identifier SPACE rights)

   resp-text-code  =/ myrights_data

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

   updateacl       = "STORE" SPACE mailbox mbox_or_pat SPACE identifier SPACE mod_rights

   mbox_or_pat     = mailbox / patterns

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

   partialfail_rsp = "ACLFAILED" [SPACE "[" resp-text-code "]" ] SPACE mailbox
                     ;; May contain optional failure reason

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.

8.   Initial Registrations

8.1.  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

8.2.  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

9.   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 to the special identifier
   "anyone".

10.  Other considerations

10.1. Compatibility with RFC 2086

   << This has to be written >>

10.2. ACL2 interaction with QUOTA extension

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

    GETQUOTAROOT - "r" right
<< Replace with/add "a" and "i"? >>

    GETQUOTA - no rights required

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

10.3. 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 3.1 ("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 3.1) 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).

<<Editor: I know this looks ugly, but this is required for interoperability.
  Please check whether this makes any sense!>>

   The server MUST include a READ-ONLY prefix in the tagged OK response to
   a SELECT command if all none of the following rights are not 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 either at least one of the "i", "e" or "shared flag rights" is granted to the
   current user.

<<Continue examples 1-3>>

   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.

10.4. 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.

11.  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.

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, Curtis King, Lyndon Nerenberg, Larry Greenfield,
   Vladimir Butenko, Dave Cridland and other members of IMAPEXT working group.

13.  Editor's Address

   Alexey Melnikov
   mailto: mel@messagingdirect.com

   ACI WorldWide/MessagingDirect
   22 The Quadrant, Richmond,
   Surrey,
   59 Clarendon Road, Watford,
   Hertfordshire, United Kingdom, TW9 1BP WD17 1FQ

   Phone: +44 1923 81 2877

14.  Full Copyright Statement

    Copyright (C) The Internet Society 2002.  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 note about compatibility with RFC 2086

  13. Added "Implementation Notes" section.

  14. Updated "References" section.

  15. Deleted "PARTIAL", this is a depricated 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             => ACLINFO
        LISTRIGHTS      => RIGHTS-INFO

        MYRIGHTS was left as is

      Any better suggestions for names are welcome.

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

To Do List:

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

  2). Editorial: Convert to nroff, Word or XML format.

  3). Update syntax to allow for multiple mailboxes and masks.
      Describe partial failures.