INTERNET-DRAFT
Network Working Group                                   A. Melnikov (Ed.)
Document: draft-ietf-lemonade-rfc2192bis-09.txt Melnikov, Ed.
Request for Comments: 5092                                    Isode Ltd.
Expires: February 2008
Obsoletes: 2192                                                C. Newman
Intended status:
Updates: 4467                                            Sun Microsystes
Category: Standards Track                        Sun Microsystems
Obsoletes: RFC 2192 (if approved)                         August 8,                                   October 2007
Updates: RFC 4467

                            IMAP URL Scheme

Status of this This Memo

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

   Internet-Drafts are working documents of

   This document specifies an Internet standards track protocol for the
   Internet Engineering
   Task Force (IETF), its areas, community, and its working groups.  Note that
   other groups may also distribute working documents as Internet-Drafts.

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

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

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

   A revised version edition of this draft document will be submitted to the RFC
   editor as a Proposed Standard "Internet
   Official Protocol Standards" (STD 1) for the Internet Community.  Discussion
   and suggestions for improvement are requested, standardization state
   and should be sent to
   the IMAPEXT Mailing list <ietf-imapext@imc.org>. status of this protocol.  Distribution of this
   draft memo is unlimited.

Copyright Notice

     Copyright (C) The IETF Trust (2007).

Abstract

   IMAP (RFC 3501) is a rich protocol for accessing remote message
   stores.  It provides an ideal mechanism for accessing public mail-
     ing mailing
   list archives as well as private and shared message stores.  This
   document defines a URL scheme for referencing objects on an IMAP
   server.

   This document obsoletes RFC 2192.  It also updates RFC 4467.

Table of Contents

   1. Introduction ....................................................2
   2. Conventions used in this document

     The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
     "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" Used in
     this document are to be interpreted as described in RFC 2119
     [KEYWORDS]. This document references many productions from [URI-GEN]. When the
     document needs to emphasize Document ...............................3
   3. IMAP URI-specific differences from
     [URI-GEN] (i.e. for parts userinfo Component (iuserinfo) .............................4
      3.1. IMAP Mailbox Naming Scope ..................................4
      3.2. IMAP User Name and Authentication Mechanism ................4
      3.3. Limitations of enc-user ....................................6
   4. IMAP URIs which have more restricted
     syntax than generic URIs), it uses a non terminal i<foo> Server .....................................................7
   5. Lists of Messages ...............................................7
   6. A Specific Message or Message Part ..............................8
      6.1. URLAUTH Authorized URL .....................................9
           6.1.1. Concepts ............................................9
                  6.1.1.1. URLAUTH ....................................9
                  6.1.1.2. Mailbox Access Key .........................9
                  6.1.1.3. Authorized Access Identifier ...............9
                  6.1.1.4. Authorization Mechanism ...................10
                  6.1.1.5. Authorization Token .......................10
           6.1.2. URLAUTH Extensions to define
     an IMAP specific version of the non terminal <foo> from [URI-GEN].

     Note that the ABNF syntax shown in section 11 is normative.  Sec-
     tions 2-6 may use a less formal syntax that does not necessarily
     match the normative ABNF shown in section 11. If there are any dif-
     ferences between syntax shown in sections 2-6 and section 11, then
     the syntax shown in the Section 11 must be treated as authorita-
     tive.  Non-syntax requirements included in sections 2-6 are, URL .....................10
   7. Relative IMAP URLs .............................................11
      7.1. absolute-path References ..................................12
      7.2. relative-path References ..................................12
   8. Internationalization Considerations ............................13
   9. Examples .......................................................13
      9.1. Examples of
     course, normative.

2. Relative URLs .................................16
   10. Security Considerations .......................................16
      10.1. Security Considerations Specific to URLAUTH Authorized
            URL ......................................................17
   11. ABNF for IMAP URL Scheme ......................................17
   12. IANA Considerations ...........................................21
      12.1. IANA Registration of imap: URI Scheme ....................21
   13. References ....................................................22
      13.1. Normative References .....................................22
      13.2. Informative References ...................................23
   Appendix A. Sample Code............................................24
   Appendix B. List of Changes since RFC 2192.........................30
   Appendix C. List of Changes since RFC 4467.........................31
   Appendix D. Acknowledgments........................................31

1.  Introduction

   The IMAP URL scheme is used to designate IMAP servers, mailboxes,
   messages, MIME bodies [MIME], and search programs on Internet hosts
   accessible using the IMAP protocol over TCP.

   The IMAP URL follows the common Internet scheme syntax as defined in
   [URI-GEN].  If :<port> is omitted, the port defaults to 143 (as
   defined in Section 2.1 of [IMAP4]).

   An absolute IMAP URL takes one of the following forms:

      imap://<iserver>[/]

      imap://<iserver>/<enc-mailbox>[<uidvalidity>][?<enc-search>]

      imap://<iserver>/<enc-mailbox>[<uidvalidity>]<iuid>
       [<isection>][<ipartial>][<iurlauth>]

   The first form is used to refer to an IMAP server (see section Section 4),
   the second form refers to the contents of a mailbox or a set of
   messages resulting from a search (see section Section 5), and the final form
   refers to a specific message or message part, and possibly a byte
   range in that part (see section Section 6).  If [URLAUTH] extension is
   supported, then the final form can have the <iurlauth> component (see section
   Section 6.1 for more details).

   The <iserver> component common to all types of absolute IMAP URLs has
   the following syntax expressed in ABNF [ABNF]:

      [iuserinfo "@"] host [ ":" port ]

   The <iserver> component is the same as "authority" defined in [URI-
     GEN].
   [URI-GEN].  The syntax and uses of the <iuserinfo> ("IMAP userinfo com-
     ponent")
   component") are described in details detail in section Section 3. Syntax  The syntax of
   <host> and <port> is described in [URI-GEN].

2.  Conventions Used in This Document

   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].

   This document references many productions from [URI-GEN].  When the
   document needs to emphasize IMAP URI-specific differences from [URI-
   GEN] (i.e., for parts of IMAP URIs that have more restricted syntax
   than generic URIs), it uses a non-terminal i<foo> to define an IMAP-
   specific version of the non-terminal <foo> from [URI-GEN].

   Note that the ABNF syntax shown in Section 11 is normative.  Sections
   2-6 may use a less formal syntax that does not necessarily match the
   normative ABNF shown in Section 11.  If there are any differences
   between the syntax shown in Sections 2-6 and Section 11, then the
   syntax shown in Section 11 must be treated as authoritative.  Non-
   syntax requirements included in Sections 2-6 are, of course,
   normative.

3.  IMAP userinfo component Component (iuserinfo)

   The <iuserinfo> component conforms to the generic syntax of <user-
     info>
   <userinfo> defined in [URI-GEN].  It has the following syntax
   expressed in ABNF [ABNF]:

      enc-user [iauth] / [enc-user] iauth

   The meaning of the different parts is described in subsections of
   this section.

3.1.  IMAP mailbox naming scope Mailbox Naming Scope

   The "enc-user" part of the "iuserinfo" component, if present, denotes
   mailbox naming scope.  If it is absent, the IMAP URL can only
   reference mailboxes with globally unique names, i.e. i.e., mailboxes with
   names that don't change depending on the user the client
   authenticated as to the IMAP server. (Note,  Note that not all IMAP
   implementations support globally unique names.) names.

   For example, a personal mailbox described by the following URL
   <imap://michael@example.org/INBOX> is most likely be different from a
   personal mailbox described by <imap://bester@example.org/INBOX>, even
   though both URLs use the same mailbox name.

3.2.  IMAP User Name and Authentication Mechanism

   The userinfo component (see [URI-GEN]) of an IMAP URI may contain an
   IMAP user name (a.k.a. authorization identity [SASL], "enc-
     user") "enc-user")
   and/or an authentication mechanism. (Note that the "enc-
     user" "enc-user" also
   defines a mailbox naming scope as described in section Section 3.1).  The
   IMAP user name and the authentication mechanism are used in the
   "LOGIN" or "AUTHENTICATE" commands after making the connec-
     tion connection to the
   IMAP server.

   If no user name and no authentication mechanism is are supplied, the
   client MUST authenticate as anonymous to the server.  If the server
   advertises AUTH=ANONYMOUS IMAP capability, the client MUST use the
   AUTHENTICATE command with ANONYMOUS [ANONYMOUS] SASL mechanism.  If
   SASL ANONYMOUS is not available, the (case-insensitive) user name
   "anonymous" is used with the "LOGIN" command and the Internet e-
     mail email
   address of the end user accessing the resource is supplied as the
   password.  The latter option is given in order to provide for
   interoperability with deployed servers.

   Note that that, as described in RFC 3501, the LOGIN "LOGIN" command MUST NOT be
   used when the IMAP server advertises the LOGINDISABLED capability.

   An authentication mechanism (as used by the IMAP AUTHENTICATE com-
     mand)
   command) can be expressed by adding ";AUTH=<enc-auth-type>" to the
   end of the user name in an IMAP URL.  When such an <enc-auth-type> is
   indicated, the client SHOULD request appropriate credentials from
   that mechanism and use the "AUTHENTICATE" command instead of the
   "LOGIN" command.  If no user name is specified, one MUST be obtained
   from the mechanism or requested from the user/configura-
     tion user/configuration as
   appropriate.

   The string ";AUTH=*" indicates that the client SHOULD select an
   appropriate authentication mechanism.  (Though the '*' character in
   this usage is not strictly a delimiter, it is being treated like a
   sub-delim [URI-GEN] in this instance.  It MUST NOT be percent
     encoded percent-encoded
   in this usage, as ";AUTH=%2A" will not match this produc-
     tion.) production.)  It
   MAY use any mechanism listed in the response to the CAPABILITY
   command (or CAPABILITY response code) or use an out of
     band out-of-band security
   service resulting in a PREAUTH connection.  If no user name is
   specified and no appropriate authentication mechanisms are available,
   the client SHOULD fall back to anonymous login as described above.
   The behavior prescribed in this section allows a URL which that grants
   read-write access to authorized users, users and read-
     only read-only anonymous access
   to other users.

   If a user name is included with no authentication mechanism, then
   ";AUTH=*" is assumed.

   Clients must take care when resolving a URL which that requires or requests
   any sort of authentication, since URLs can easily come from untrusted
   sources.  Supplying authentication credentials to the wrong server
   may compromise the security of the user's account,
     therefore account; therefore, the
   program resolving the URL should meet at least one of the following
   criteria in this case:

     (1)

   1) The URL comes from a trusted source, such as a referral server
     which
      that the client has validated and trusts according to site policy.
      Note that user entry of the URL may or may not count as a trusted
      source, depending on the experience level of the user and site pol-
     icy.
     (2)
      policy.

   2) Explicit local site policy permits the client to connect to the
      server in the URL.  For example, a company example.com may have a
      site policy to trust all IMAP server names ending in example.com,
      whereas such a policy would be unwise for example.edu where random
      students can set up IMAP servers.
     (3)

   3) The user confirms that connecting to that domain name with the
      specified credentials and/or mechanism is permitted.  For example,
      when using LOGIN "LOGIN" or SASL PLAIN with TLS, Transport Layer Security
      (TLS), the IMAP URL client presents a dialog box "Is it OK to send
      your password to server "example.com"?  Please be aware the owners
      of example.com will be able to reuse your password to connect to
      other servers on your
     behalf."
     (4) behalf".

   4) A mechanism is used which that validates the server before passing
      potentially compromising client credentials.  For example, a site
      has a designated TLS certificate used to certify site-trusted IMAP
      server certificates certificates, and this has been configured explicitly into
      the IMAP URL client.  Another example is use of a SASL Simple
      Authentication and Security Layer (SASL) mechanism such as
      DIGEST-MD5 [DIGEST-MD5], which supports mutual authentica-
     tion.
     (5) authentication.

   5) An authentication mechanism is used which that will not reveal any
      information to the server which that could be used to compromise future
      connections. For example,  Examples are SASL ANONYMOUS [ANONYMOUS] or GSSAPI
      [GSSAPI].

   URLs which that do not include a user name, name but include an authentica-
     tion authentication
   mechanism (";AUTH=<mech>") must be treated with extra care, since for
   some <mech>s they are more likely to compromise the user's primary
   account.  A URL containing ";AUTH=*" must also be treated with extra
   care since it might fall back on a weaker secu-
     rity security mechanism.
   Finally, clients are discouraged from using a
     plain text plaintext password as a
   fallback with ";AUTH=*" unless the connec-
     tion connection has strong encryption.

   A program interpreting IMAP URLs MAY cache open connections to an
   IMAP server for later re-use. reuse.  If a URL contains a user name, only
   connections authenticated as that user may be re-used. reused.  If a URL does
   not contain a user name or authentication mechanism, then only an
   anonymous connection may be re-used. reused.

   Note that if unsafe or reserved characters such as " " (space) or ";"
   are present in the user name or authentication mechanism, they MUST
   be percent-encoded as described in [URI-GEN].

3.3.  Limitations of enc-user

   As per sections Sections 3.1 and 3.2 of this document document, the IMAP URI enc-user
   has two purposes:

      1) It provides context for user-specific mailbox paths such as
         "INBOX" (section (Section 3.1).

      2) It specifies that resolution of the URL requires logging in as
         that user and limits use of that URL to only that user (Section
         3.2).

   An obvious limitation of using the same field for both purposes is
   that the URL can be resolved only by the mailbox owner.  In order to
   avoid this restriction, implementations should use globally unique
   mailbox names (see Section 3.1) whenever possible.

      Note: There is currently no general way in IMAP of learning a
      globally unique name for a mailbox.  However, by looking at the
      NAMESPACE [NAMESPACE] command result, it is possible (*). to determine
      whether or not a mailbox name is globally unique.

   The URLAUTH component overrides the second purpose of the enc-user in
   the IMAP URI and by default permits the URI to be resolved by any
   user permitted by the <access> identifier.  URLAUTH and <access>
   identifier are described in section Section 6.1.

     (*) There is currently no general way in IMAP of learning a glob-
     ally unique name for a mailbox. However by looking at the NAMESPACE
     [NAMESPACE] command result it is possible to determine if a mailbox
     name is globally unique or not.

4.  IMAP server Server

   An IMAP URL referring to an IMAP server has the following form:

      imap://<iserver>[/]

   This URL type is frequently used to describe a location of an IMAP
   server, both in referrals and in configuration.  It may optionally
   contain the "iuserinfo" <iuserinfo> component (see Sections 3 and 11).  A pro-
     gram program
   interpreting this URL would issue the standard set of commands it
   uses to present a view of the content of the IMAP server, as visible
   to the user described by the "enc-user" part of the "iuser-
     info" <iuserinfo>
   component, if the "enc-user" part is specified.

5.  Lists of messages Messages

   An IMAP URL referring to a list of messages has the following form:

      imap://<iserver>/<enc-mailbox>[<uidvalidity>][?<enc-search>]

   The <enc-mailbox> field is used as the argument to the IMAP4 "SELECT"
   or "EXAMINE" command.  Note that if unsafe or reserved characters
   such as " " (space), ";", or "?" are present in <enc-
     mailbox> <enc-mailbox>, they
   MUST be percent-encoded as described in [URI-GEN].

   The <uidvalidity> field is optional.  If it is present, it MUST be
   the same as the value of IMAP4 UIDVALIDITY response code at the time
   the URL was created.  This MUST be used by the program inter-
     preting interpreting
   the IMAP URL to determine if the URL is stale.  If the IMAP URL is
   stale, then the program should behave as if the correspond-
     ing corresponding mailbox
   doesn't exist.

   Note that the <uidvalidity> field is a modifier to the <enc-mail-
     box>, i.e. <enc-mailbox>,
   i.e., it is considered a part of the last "component" (as used in
   [URI-GEN]) of the <enc-mailbox>.  This is significant during rel-
     ative relative
   URI resolution.

   The "?<enc-search>" field is optional.  If it is not present, the
   program interpreting the URL will present the entire content of the
   mailbox.

   If the "?<enc-search>" field is present, the program interpreting the
   URL should use the contents of this field as arguments follow-
     ing following an
   IMAP4 SEARCH command.  These arguments are likely to contain unsafe
   characters such as " " (space) (which are likely to be present in the
   <enc-search>).  If unsafe characters are present, they MUST be
   percent-encoded as described in [URI-GEN].

   Note that quoted strings and non-synchronizing literals [LITERAL+]
   are allowed in the <enc-search> content, however content; however, synchronizing lit-
     erals
   literals are not allowed, as their presence would effectively mean
   that the agent interpreting IMAP URLs needs to parse an <enc-
     search> <enc-search>
   content, find all synchronizing literals literals, and perform proper command
   continuation request handling (see sections Sections 4.3 and 7 of [IMAP4]).

6.  A specific message Specific Message or message part Message Part

   An IMAP URL referring to a specific message or message part has the
   following form:

      imap://<iserver>/<enc-mailbox>[<uidvalidity>]<iuid>
      [<isection>][<ipartial>][<iurlauth>]

   The <enc-mailbox> and [uidvalidity] are as defined in section Section 5
   above.

   If <uidvalidity> is present in this form, it SHOULD be used by the
   program interpreting the URL to determine if the URL is stale.

   The <iuid> refers to an IMAP4 message UID, Unique Identifier (UID), and it
   SHOULD be used as the <set> argument to the IMAP4 "UID FETCH"
   command.

   The <isection> field is optional.  If not present, the URL refers to
   the entire Internet message as returned by the IMAP command "UID
   FETCH <uid> BODY.PEEK[]".  If present, the URL refers to the object
   returned by a "UID FETCH <uid> BODY.PEEK[<section>]" command.  The
   type of the object may be determined with by using a "UID FETCH <uid> BODYS-
     TRUCTURE"
   BODYSTRUCTURE" command and locating the appropriate part in the result-
     ing
   resulting BODYSTRUCTURE.  Note that unsafe characters in [isection]
   MUST be percent-encoded as described in [URI-GEN].

   The <ipartial> field is optional.  If present, it effectively appends
   "<<partial-range>>" to the end of the UID FETCH BODY.PEEK[<section>]
   command constructed as described in the previ-
     ous previous paragraph.  In other words
   words, it allows the client to request a byte range of the
   message/message part.

   The <iurlauth> field is described in details detail in section Section 6.1.

6.1.

6.1  URLAUTH authorized Authorized URL

   URLAUTH authorized URLs are only supported by an IMAP server adver-
     tising
   advertising the URLAUTH IMAP capability [URLAUTH].

6.1.1.  Concepts

6.1.1.1.  URLAUTH

     The

   URLAUTH is a component, appended at the end of a URL, which that conveys
   authorization to access the data addressed by that URL.  It contains
   an authorized access identifier, an authorization mecha-
     nism mechanism name, and
   an authorization token.  The authorization token is generated from
   the URL, the authorized access identifer, authoriza-
     tion identifier, authorization mechanism
   name, and a mailbox access key.

     (Note that this

      Note: This specification only allows for the URLAUTH component in
      IMAP URLs describing a message or its part.) part.

6.1.1.2.  Mailbox Access Key

   The mailbox access key is a an unpredictable, random string.  To ensure
   unpredictability, the random string with at least 128 bits of
     entropy.  It entropy
   is generated by software or hardware (not by the human user), and
     MUST be unpredictable. user).

   Each user has a table of mailboxes and an associated mailbox access
   key for each mailbox.  Consequently, the mailbox access key is per-
   user and per-mailbox.  In other words, two users sharing the same
   mailbox each have a different mailbox access key for that mailbox,
   and each mailbox accessed by a single user also has a different
   mailbox access key.

6.1.1.3.  Authorized Access Identifier

   The authorized access <access> identifier restricts use of the URLAUTH
   authorized URL to certain users authorized on the server, as
   described in section Section 6.1.2.

6.1.1.4.  Authorization Mechanism

   The authorization mechanism is the algorithm by which the URLAUTH is
   generated and subsequently verified, using the mailbox access key.

6.1.1.5.  Authorization Token

   The authorization token is a deterministic string of at least 128
   bits which that an entity with knowledge of the secret mailbox access key
   and URL authorization mechanism can use to verify the URL.

6.1.2.  URLAUTH extensions Extensions to IMAP URL

   A specific message or message part IMAP URL can optionally contain
   ";EXPIRE=<datetime>" and/or ";URLAUTH=<access>:<mech>:<token>".

   When ";EXPIRE=<datetime>" is used, this indicates the latest date and
   time that the URL is valid.  After that date and time, the URL has
   expired and server implementations MUST reject the URL.  If
   ";EXPIRE=<datetime>" is not used, the URL has no expiration, but
     still can
   still be revoked using the RESETKEY command [URLAUTH].

   The URLAUTH takes the form ";URLAUTH=<access>:<mech>:<token>", and it
   MUST be at the end of the URL.  It is composed of three parts.  The
   <access> portion provides the authorized access identifiers which that may
   constrain the operations and users that are permitted to use this
   URL.  The <mech> portion provides the authorization mechanism used by
   the IMAP server to generate the authorization token that follows.
   The <token> portion provides the authorization token, which can be
   generated using the GENURLAUTH command [URLAUTH].

   The "submit+" <access> identifier prefix, followed by a userid,
   indicates that only a userid authorized as a message submission
   entity on behalf of the specified userid is permitted to use this
   URL.  The IMAP server does not validate the specified userid but does
   validate that the IMAP session has an authorization identity that is
   authorized as a message submission entity.  The authorized message
   submission entity MUST validate the userid prior to con-
     tacting contacting the
   IMAP server.

   The "user+" <access> identifier prefix, followed by a userid, indi-
     cates
   indicates that use of this URL is limited to IMAP sessions which that are
   logged in as the specified userid (that is, have authorization
   identity as that userid).

      Note: if If a SASL mechanism which that provides both authorization and
      authentication identifiers is used to authenticate to the IMAP
      server, the "user+" access <access> identifier MUST match the authoriza-
        tion
      authorization identifier.  If the SASL mechanism can't transport
      the authorization identifier, the "user+" access <access> identifier MUST
      match the authorization identifier derived from the authentica-
        tion authentication
      identifier (see [SASL]).

   The "authuser" <access> identifier indicates that use of this URL is
   limited to authenticated IMAP sessions which that are logged in as any
   non-anonymous user (that is, have authorization identity as a
     non-anonymous user non-
   anonymous user) of that IMAP server.  To restate this: use of this
   type of URL is prohibited to anonymous IMAP sessions, i.e. i.e., any
   URLFETCH command containing this type of URL issued in an anonymous
   session MUST return NIL in the URLFETCH response.

   The "anonymous" <access> identifier indicates that use of this URL is
   not restricted by session authorization identity; that is, any IMAP
   session in authenticated or selected state (as defined in [IMAP4]),
   including anonymous sessions, may issue a URLFETCH [URLAUTH] using
   this URL.

   The authorization token is represented as an ASCII-encoded hexadec-
     imal
   hexadecimal string, which is used to authorize the URL.  The length
   and the calculation of the authorization token depends depend upon the mecha-
     nism used; but,
   mechanism used, but in all cases, the authorization token is at least
   128 bits (and therefore at least 32 hexadecimal digits).

   Example:

      <imap://joe@example.com/INBOX/;uid=20/;section=1.2;urlauth=
      submit+fred:internal:91354a473744909de610943775f92038>

7.  Relative IMAP URLs

   Relative IMAP URLs are permitted and are resolved according to the
   rules defined in [URI-GEN].  In particular particular, in IMAP URLs, URLs parameters
   (such as ";uid=" or ";section=") are treated as part of the normal
   path with respect to relative URL resolution.

   [URI-GEN] defines 4 four forms of relative URLs: <inetwork-path>, <iab-
     solute-path>,
   <iabsolute-path>, <irelative-path>, and <ipath-empty>.  Their syntax
   is defined in section Section 11.

   A relative reference that begins with two slash characters is termed
   a network-path reference (<inetwork-path>); such references are
   rarely used, because in most cases they can be replaced with an
   equivalent absolute URL.  A relative reference that begins with a
   single slash character is termed an absolute-path reference (<iab-
     solute-path>,
   (<iabsolute-path>; see also section Section 7.1).  A relative reference that
   does not begin with a slash character is termed a relative-path
   reference (<irelative-path>, (<irelative-path>; see also section Section 7.2).  The final form
   is <ipath-empty>, which is "same-document reference" (see section Section 4.4
   of [URI-GEN]).

   The following observations about relative URLs are important:

   The <iauth> grammar element (which is a part of <iuserinfo>, which
     is
   is, in its turn turn, a part of <iserver>; see section Section 3) is considered part
   of the user name for purposes of resolving relative IMAP URLs.  This
   means that unless a new user name/server specification is included in
   the relative URL, the authentication mechanism is inherited from the
   base IMAP URL.

   URLs always use "/" as the hierarchy delimiter for the purpose of
   resolving paths in relative URLs.  IMAP4 permits the use of any
   hierarchy delimiter in mailbox names.  For this reason, relative
   mailbox paths will only work if the mailbox uses "/" as the hierar-
     chy hierarchy
   delimiter.  Relative URLs may be used on mailboxes which that use other
   delimiters, but in that case, the entire mailbox name MUST be
   specified in the relative URL or inherited as a whole from the base
   URL.

   If an IMAP server allows for mailbox names starting with "./" or
   "../", ending with "/." or "/..", or containing sequences "/../" or
   "/./", then such mailbox names MUST be percent-encoded as described
   in [URI-GEN]. Otherwise  Otherwise, they would be misinterpreted as dot-seg-
     ments dot-
   segments (see Section 3.3 of [URI-GEN]), which are processed
   specially during the relative path resolution process.

7.1.  absolute-path References

   A relative reference that begins with a single slash character is
   termed an absolute-path reference (see section Section 4.2 of [URI-GEN]).  If
   an IMAP server permits mailbox names with a leading "/", then the
   leading "/" MUST be percent-encoded as described in [URI-GEN].
     Otherwise
   Otherwise, the produced absolute-path reference URI will be misin-
     terpreted
   misinterpreted as a network-path reference [URI-GEN] described by <inet-
     work-path> the
   <inetwork-path> non-terminal.

7.2.  relative-path References

   A relative reference that does not begin with a slash character is
   termed a relative-path reference [URI-GEN].  Implementations MUST NOT
   generate or accept relative-path IMAP references.

   See also section Section 4.2 of [URI-GEN] for restrictions on relative-path
   references.

8.  Internationalization Considerations
     IMAP4 [IMAP4] section

   IMAP4, Section 5.1.3 [IMAP4] includes a convention for encoding non-
   US-ASCII characters in IMAP mailbox names.  Because this convention
   is private to IMAP, it is necessary to convert IMAP's encoding to one
   that can be more easily interpreted by a URL display program.  For
   this reason, IMAP's modified UTF-7 encoding for mailboxes MUST be
   converted to UTF-8 [UTF-8].  Since 8-bit octets are not permit-
     ted permitted in
   URLs, the UTF-8 octets are percent-encoded as required by the URL
   specification [URI-GEN], section Section 2.1.  Sample code is included in
   Appendix A to demonstrate this conversion.

   IMAP usernames user names are UTF-8 strings and MUST be percent-encoded as
   required by the URL specification [URI-GEN], section Section 2.1.

   Also note that IMAP SEARCH criteria can contain non US-ASCII char-
     acters. non-US-ASCII
   characters.  8-bit octets in those strings MUST be percent-encoded as
   required by the URL specification [URI-GEN], section Section 2.1.

9.  Examples

   The following examples demonstrate how an IMAP4 client program might
   translate various IMAP4 URLs into a series of IMAP4 commands.
   Commands sent from the client to the server are prefixed with "C:",
   and responses sent from the server to the client are prefixed with
   "S:".

   The URL:

      <imap://minbari.example.org/gray-council;UIDVALIDITY=385759045/;
      UID=20/;PARTIAL=0.1024>

   may result in in the following client commands and server responses:

      <connect to minbari.example.org, port 143>
      S: * OK [CAPABILITY IMAP4rev1 STARTTLS AUTH=ANONYMOUS] Welcome
      C: A001 AUTHENTICATE ANONYMOUS
      S: +
      C: c2hlcmlkYW5AYmFieWxvbjUuZXhhbXBsZS5vcmc=
      S: A001 OK Welcome sheridan@babylon5.example.org
      C: A002 SELECT gray-council
      <client verifies the UIDVALIDITY matches>
      C: A003 UID FETCH 20 BODY.PEEK[]<0.1024>

   The URL:

      <imap://psicorp.example.org/~peter/%E6%97%A5%E6%9C%AC%E8%AA%9E/
      %E5%8F%B0%E5%8C%97>

     May results
   may result in the following client commands:

      <connect to psicorp.example.org, port 143>
      S: * OK [CAPABILITY IMAP4rev1 STARTTLS AUTH=CRAM-MD5] Welcome
      C: A001 LOGIN ANONYMOUS bester@psycop.psicorp.example.org
      C: A002 SELECT ~peter/&ZeVnLIqe-/&U,BTFw-
      <commands the client uses for viewing the contents of
       the mailbox>

   The URL:

      <imap://;AUTH=GSSAPI@minbari.example.org/gray-council/;uid=20/
      ;section=1.2>

     May

   may result in the following client commands:

      <connect to minbari.example.org, port 143>
      S: * OK Greetings
      C: A000 CAPABILITY
      S: * CAPABILITY IMAP4rev1 STARTTLS AUTH=GSSAPI
      S: A000 OK
      C: A001 AUTHENTICATE GSSAPI
      <authentication exchange>
      C: A002 SELECT gray-council
      C: A003 UID FETCH 20 BODY.PEEK[1.2]

   If the following relative URL is located in that body part:

      <;section=1.4>

     This

   this could result in the following client commands:

      C: A004 UID FETCH 20 (BODY.PEEK[1.2.MIME]
            BODY.PEEK[1.MIME]
            BODY.PEEK[HEADER.FIELDS (Content-Location)])
      <Client looks for Content-Location headers in
       result.  If no such headers, then it does the following>
      C: A005 UID FETCH 20 BODY.PEEK[1.4]

   The URL:

      <imap://;AUTH=*@minbari.example.org/gray%20council?
      SUBJECT%20shadows>

     Could
   could result in the following:

      <connect to minbari.example.org, port 143>
      S: * OK Welcome
      C: A001 CAPABILITY
      S: * CAPABILITY IMAP4rev1 AUTH=DIGEST-MD5
      S: A001 OK
      C: A002 AUTHENTICATE DIGEST-MD5
      <authentication exchange>
      S: A002 OK user lennier authenticated
      C: A003 SELECT "gray council"
      ...
      C: A004 SEARCH SUBJECT shadows
      S: * SEARCH 8 10 13 14 15 16
      S: A004 OK SEARCH completed
      C: A005 FETCH 8,10,13:16 ALL
      ...
     NOTE:

   In this final example, the example above, the client has implementation depen-
     dent implementation-dependent
   choices.  The authentication mechanism could be anything, including
   PREAUTH.  And the  The final FETCH command could fetch more or less
   information about the messages, depending on what it wishes to
   display to the user.

   The URL:

      <imap://john;AUTH=*@minbari.example.org/babylon5/personel?
      charset%20UTF-8%20SUBJECT%20%7B14+%7D%0D%0A%D0%98%D0%B2%
      D0%B0%D0%BD%D0%BE%D0%B2%D0%B0>

   shows that 8-bit data can be sent using non-synchronizing literals
   [LITERAL+].  This could result in the following:

      <connect to minbari.example.org, port 143>
      S: * OK Hi there
      C: A001 CAPABILITY
      S: * CAPABILITY IMAP4rev1 LITERAL+ AUTH=DIGEST-MD5
      S: A001 OK
      C: A002 AUTHENTICATE DIGEST-MD5
      <authentication exchange>
      S: A002 OK user john authenticated
      C: A003 SELECT babylon5/personel
      ...
      C: A004 SEARCH CHARSET UTF-8 SUBJECT {14+}
      C: XXXXXXXXXXXXXX
      S: * SEARCH 7 10 12
      S: A004 OK SEARCH completed
      C: A005 FETCH 7,10,12 ALL
      ...

     Where

   where XXXXXXXXXXXXXX is 14 bytes of UTF-8 encoded data as specified
   in the URL above.

9.1.  Examples of relative Relative URLs

   The following absolute-path reference

      </foo/;UID=20/..>

   is the same as

      </foo>

     I.e.

   That is, both of them reference the mailbox "foo" located on the IMAP
   server described by the corresponding Base URI.

   The following relative-path reference

      <;UID=20>

   references a message with UID in the mailbox specified by the Base
   URI.

   The following edge case example demostrates demonstrates that the ;UIDVALIDITY=
   modifier is a part of the mailbox name as far as relative URI reso-
     lution
   resolution is concerned:

      <..;UIDVALIDITY=385759045/;UID=20>

   In this example example, ".." is not a dot-segment [URI-GEN].

10.  Security Considerations

   Security considerations discussed in the IMAP specification [IMAP4]
   and the URI specification [URI-GEN] are relevant.  Security consid-
     erations
   considerations related to authenticated URLs are discussed in section Section
   3.2 of this document.

   Many email clients store the plain text plaintext password for later use after
   logging into an IMAP server.  Such clients MUST NOT use a stored
   password in response to an IMAP URL without explicit permis-
     sion permission from
   the user to supply that password to the specified host name.

   Clients resolving IMAP URLs that wish to achieve data confidential-
     ity confidentiality
   and/or integrity SHOULD use the STARTTLS command (if supported by the
   server) before starting authentication, or use a SASL mecha-
     nism, mechanism, such
   as GSSAPI, that provides a confidentiality security layer.

10.1.  Security Consideration specific Specific to URLAUTH authorized Authorized URL

   The "user+<userid>" access <access> identifier limits resolution of that URL
   to a particular userid, whereas the "submit+<userid>" access iden-
     tifier <access>
   identifier is more general and simply requires that the session be
   authorized by a user that has been granted a "submit" role within the
   authentication system.  Use of either of these access identi-
     fiers makes it impossible for an attacker, spying on mechanisms limits the session,
     to
   scope of the URL.  An attacker who cannot authenticate using the
   appropriate credentials cannot make use of the same URL, either directly or by submission to a message
     submission entity. URL.

   The "authuser" and "anonymous" access <access> identifiers do not have this
   level of protection.  These access identifiers are primarily useful
   for public export of data from an IMAP server, without requiring that
   it be copied to a web or anonymous FTP server.

   The decision to use the "authuser" access <access> identifier should be made
   with caution.  An "authuser" access <access> identifier can be used by any
   authorized user of the IMAP server; and therefore therefore, use of this access
   identifier should be limited to content which that may be dis-
     closed disclosed to any
   authorized user of the IMAP server.

   The decision to use the "anonymous" access <access> identifier should be
   made with extreme caution.  An "anonymous" access <access> identifier can be
   used by anyone; and therefore therefore, use of this access identifier should be
   limited to content which that may be disclosed to anyone.

11.  ABNF for IMAP URL scheme Scheme

   Formal syntax is defined using ABNF [ABNF], extending the ABNF rules
   in section Section 9 of [IMAP4].  Elements not defined here can be found in the
   [ABNF], [IMAP4], [IMAPABNF] [IMAPABNF], or [URI-GEN].  Strings are not case sensitive
   sensitive, and free insertion of linear-white-space linear white space is not permitted.

   sub-delims-sh = "!" / "$" / "'" / "(" / ")" /
                   "*" / "+" / ","
                      ;; Same as [URI-GEN] sub-delims,
                      ;; but without ";", "&" and "=".

   uchar            = unreserved / sub-delims-sh / pct-encoded

   achar            = uchar / "&" / "="
                      ;; Same as [URI-GEN] 'unreserved / sub-delims /
                      ;; pct-encoded', but ";" is disallowed.

   bchar            = achar / ":" / "@" / "/"
   enc-auth-type    = 1*achar
                   ; %-encoded version of [IMAP4] "auth-type"

   enc-mailbox      = 1*bchar
                  ; %-encoded version of [IMAP4] "mailbox"

   enc-search       = 1*bchar
                           ; %-encoded version of [IMAPABNF]
                           ; "search-program".  Note that IMAP4
                           ; literals may not be used in
                           ; a "search-program", i.e. i.e., only
                           ; quoted or non-synchronizing
                           ; literals (if the server supports
                           ; LITERAL+ [LITERAL+]) are allowed.

   enc-section      = 1*bchar
                  ; %-encoded version of [IMAP4] "section-spec"

   enc-user         = 1*achar
                  ; %-encoded version of [IMAP4] authorization
                  ; identity or "userid".

   imapurl          = "imap://" iserver ipath-query
               ; Defines an absolute IMAP URL

   ipath-query      = ["/" [ icommand ]]
                    ; Corresponds to "path-abempty [ "?" query ]"
                    ; in [URI-GEN]

   Generic syntax for relative URLs is defined in Section 4.2 of
   [URI-GEN].  For ease of implementation, the relative IMAP URL syntax
   is defined below:

   imapurl-rel     = inetwork-path

                     / iabsolute-path
                     / irelative-path
                     / ipath-empty

   inetwork-path   = "//" iserver ipath-query
               ; Corresponds to '"//" authority path-abempty
               ; [ "?" query ]' in [URI-GEN]

   iabsolute-path  = "/" [ icommand ]
               ; icommand, if present, MUST NOT start with '/'.
               ;
               ; Corresponds to 'path-absolute [ "?" query ]'
               ; in [URI-GEN]
   irelative-path  = imessagelist /
                     imsg-or-part
               ; Corresponds to 'path-noscheme [ "?" query ]'
               ; in [URI-GEN]

   imsg-or-part    = ( imailbox-ref "/" iuid-only ["/" isection-only]
                       ["/" ipartial-only] ) /
                     ( iuid-only ["/" isection-only]
                       ["/" ipartial-only] ) /
                     ( isection-only ["/" ipartial-only] ) /
                     ipartial-only

   ipath-empty     = 0<pchar>
                    ; Zero characters.
                    ; The same-document reference.

   The following 3 three rules are only used in the presence of the IMAP
   [URLAUTH] extension:

   authimapurl     = "imap://" iserver "/" imessagepart
                     ; Same as "imapurl" when "[icommand]" is
                     ; "imessagepart"

   authimapurlfull = authimapurl iurlauth
                     ; Same as "imapurl" when "[icommand]" is
                     ; "imessagepart iurlauth"

   authimapurlrump = authimapurl iurlauth-rump

   enc-urlauth     = 32*HEXDIG

   iurlauth        = iurlauth-rump iua-verifier

   iua-verifier    = ":" uauth-mechanism ":" enc-urlauth

   iurlauth-rump   = [expire] ";URLAUTH=" access

   access          = ("submit+" enc-user) / ("user+" enc-user) /
                       "authuser" / "anonymous"

   expire          = ";EXPIRE=" date-time
                         ; date-time is defined in [DATETIME]

   uauth-mechanism = "INTERNAL" / 1*(ALPHA / DIGIT / "-" / ".")
                        ; Case-insensitive.
                        ; New mechanisms MUST be registered with IANA.

   iauth            = ";AUTH=" ( "*" / enc-auth-type )

   icommand         = imessagelist /
                      imessagepart [iurlauth]

   imailbox-ref     = enc-mailbox [uidvalidity]

   imessagelist     = imailbox-ref [ "?" enc-search ]
                  ; "enc-search" is [URI-GEN] "query".

   imessagepart     = imailbox-ref iuid [isection] [ipartial]

   ipartial         = "/" ipartial-only

   ipartial-only    = ";PARTIAL=" partial-range

   isection         = "/" isection-only

   isection-only    = ";SECTION=" enc-section

   iserver          = [iuserinfo "@"] host [ ":" port ]
                           ; This is the same as "authority" defined
                           ; in [URI-GEN].  See [URI-GEN] for "host"
                           ; and "port" definitions.

   iuid             = "/" iuid-only

   iuid-only        = ";UID=" nz-number
                  ; See [IMAP4] for "nz-number" definition

   iuserinfo        = enc-user [iauth] / [enc-user] iauth
                                ; conforms to the generic syntax of
                                ; "userinfo" as defined in [URI-GEN].

   partial-range    = number ["." nz-number]
                  ; partial FETCH.  The first number is
                           ; the offset of the first byte,
                           ; the second number is the length of
                           ; the fragment.

   uidvalidity      = ";UIDVALIDITY=" nz-number
                       ; See [IMAP4] for "nz-number" definition

12.  IANA Considerations

   IANA is requested to update has updated the "imap" definition in the "Uniform Resource
   Identifier scheme registry" to point to this document.

   The registration template (as per [URI-REG]) is specified in sec-
     tion Section
   12.1 of this document.

12.1.  IANA Registration of imap: URI Scheme

   This section provides the information required to register the imap:
   URI scheme.

   URI scheme name: imap

   Status: permanent

   URI scheme syntax:

      See section Section 11 of [RFCXXXX]. [RFC5092].

   URI scheme semantics:

      The imap: URI scheme is used to designate IMAP servers, mail-
     boxes, mailboxes,
      messages, MIME bodies [MIME] and their parts, and search programs
      on Internet hosts accessible using the IMAP protocol.

      There is no MIME type associated with this URI.

   Encoding considerations:

      See Section 8 of [RFCXXXX]. [RFC5092].

   Applications/protocols that use this URI scheme name:

      The imap: URI is intended to be used by applications that might
      need access to an IMAP mailstore.  Such applications may include
      (but are not limited to) IMAP-capable web browsers; IMAP clients
      that wish to access a mailbox, message, or edit a message on the
      server using [CATENATE]; [SUBMIT] clients and servers that are
      requested to assemble a complete message on submission using
      [BURL].

   Interoperability considerations:

      A widely deployed IMAP client Netscape Mail (and possibly
     Mozilla/ Thubderbird/Seamonkey) use
      Mozilla/Thunderbird/Seamonkey) uses a different imap: scheme inter-
     nally.
      internally.

   Security considerations:

      See Security Considerations (Section 10) of [RFCXXXX]. [RFC5092].

   Contact:

      Alexey Melnikov <alexey.melnikov@isode.com>

   Author/Change controller:

      IESG

   References:

        [RFCXXXX]

      [RFC5092] and [IMAP4].

13. References

13.1.  Normative References

   [KEYWORDS]   Bradner, S., "Key words for use in RFCs to Indicate Require-
     ment
                Requirement Levels", BCP 14, RFC 2119, Harvard University, March 1997.

   [IMAP4]      Crispin, M., "Internet Message Access Protocol "INTERNET MESSAGE ACCESS PROTOCOL - Version VERSION
                4rev1", RFC 3501, University of Washington, March 2003.

   [IMAPABNF]   Melnikov, A., A. and C. Daboo, "Collected extensions Extensions to
                IMAP4 ABNF", RFC 4466, April 2006.

   [ABNF]       Crocker, D., Ed., and P. Overell, "Augmented BNF for
                Syntax Specifications: ABNF", RFC 4234, October 2005.

   [MIME]       Freed, N., N. and N. Borenstein, N., "Multipurpose Internet Mail
     Extensions",
                Extensions (MIME) Part One: Format of Internet Message
                Bodies", RFC 2045, November 1996.

   [URI-GEN]    Berners-Lee, T., Fielding, R. R., and L. Masinter, "Uniform
                Resource Identifier (URI): Generic Syntax", STD 66, RFC
                3986, January 2005.

   [UTF-8]      Yergeau, F., "UTF-8, a transformation format of ISO
                10646", STD 63, RFC 3629, November 2003.

   [NAMESPACE]  Gahrns, M. and C. Newman, "IMAP4 Namespace", RFC 2342,
                May 1998.

   [LITERAL+]   Myers, J., "IMAP4 non-synchronizing literals", RFC 2088,
                January 1997.

   [ANONYMOUS]  Zeilenga, K. (Ed.), K., "Anonymous Simple Authentication and
                Security Layer (SASL) Mechanism", RFC 4505, June 2006.

   [DATETIME]   Klyne, G., G. and C. Newman, C., "Date and Time on the Inter-
     net: Internet:
                Timestamps", RFC 3339, July 2002.

   [URLAUTH]    Crispin, M., "Internet Message Access Protocol (IMAP) -
                URLAUTH Extension", RFC 4467, May 2006.

13.2.  Informative References

   [SUBMIT]     Gellens, R. and J. Klensin, "Message Submission for
                Mail", RFC 4409, April 2006.

   [BURL]       Newman, C. C., "Message Submission BURL Extension", RFC
                4468, May 2006.

   [CATENATE]   Resnick, P., "Internet Message Access Protocol (IMAP)
                CATENATE Extension", RFC 4469, April 2006.

   [SASL]       Melnikov, A. A., Ed., and K. Zeilenga, Ed., "Simple
                Authentication and Security Layer (SASL)", RFC 4422,
                June 2006.

   [GSSAPI]     Melnikov, A., Ed., "The Kerberos V5 ("GSSAPI") Simple Authenti-
     cation
                Authentication and Security Layer (SASL) Mechanism", RFC
                4752, November 2006.

   [DIGEST-MD5] Leach, P. and C. Newman, "Using Digest Authentication as
                a SASL Mechanism", RFC 2831, May 2000.

   [URI-REG]    Hansen, T., Hardie, T. T., and L. Masinter, "Guidelines and
                Registration Procedures for New URI Schemes", BCP 115,
                RFC 4395, February 2006.

14. Author's Address

     Chris Newman (Author/Editor)
     Sun Microsystems
     3401 Centrelake Dr., Suite 410
     Ontario, CA  91761
     EMail: chris.newman@sun.com

     Alexey Melnikov (Editor)
     Isode Limited
     5 Castle Business Village
     36 Station Road
     Hampton, Middlesex
     TW12 2BX, UK
     Email: Alexey.Melnikov@isode.com
     URI:   http://www.melnikov.ca/

Appendix A.  Sample code Code

   Here is sample C source code to convert between URL paths and IMAP mail-
box
   mailbox names, taking into account mapping between IMAP's modified
   UTF-7 [IMAP4] and hex-encoded UTF-8 UTF-8, which is more appropriate for
   URLs.  This code has not been rigorously tested nor does it
   necessarily behave rea-
sonably reasonably with invalid input, but it should serve
   as a useful example.  This code just converts the mailbox portion of
   the URL and does not deal with parameters, query query, or server
   components of the URL.

/* Copyright (C) The IETF Trust (2007).  This version of
   sample C code is part of RFC XXXX; see the RFC itself
   for full legal notices.

   Regarding this sample C code (or any portion of it), the authors
   make no guarantees and are not responsible for any damage
   resulting from its use.  The authors grant irrevocable permission
   to anyone to use, modify, and distribute it in any way that does
   not diminish the rights of anyone else to use, modify, and
   distribute it, provided that redistributed derivative works do
   not contain misleading author or version information.

   Derivative works need not be licensed under similar terms.
 */

#include <stdio.h>
#include <string.h>

/* hexadecimal lookup table */
static const char hex[] = "0123456789ABCDEF";

#define XX 127
/*
 * Table for decoding hexadecimal in %encoding
 */
static const char index_hex[256] = {
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
     0, 1, 2, 3,  4, 5, 6, 7,  8, 9,XX,XX, XX,XX,XX,XX,
    XX,10,11,12, 13,14,15,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,10,11,12, 13,14,15,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
};
#define HEXCHAR(c)  (index_hex[(unsigned char)(c)])

/* "gen-delims" excluding "/" but including "%" */
#define GENERAL_DELIMS_NO_SLASH     ":?#[]@" "%"

/* "gen-delims" (excluding "/", but including "%")
   plus subset of "sub-delims" */
#define GENERAL_UNSAFE_NO_SLASH     GENERAL_DELIMS_NO_SLASH ";&=+"
#define OTHER_UNSAFE                " \"<>\\^`{|}"

/* URL unsafe printable characters */
static const char mailbox_url_unsafe[] = GENERAL_UNSAFE_NO_SLASH
                                         OTHER_UNSAFE;

/* UTF7 modified base64 alphabet */
static const char base64chars[] =
  "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+,";

#define UNDEFINED 64

/* UTF16 definitions */
#define UTF16MASK   0x03FFUL
#define UTF16SHIFT  10
#define UTF16BASE   0x10000UL
#define UTF16HIGHSTART   0xD800UL
#define UTF16HIGHEND     0xDBFFUL
#define UTF16LOSTART     0xDC00UL
#define UTF16LOEND  0xDFFFUL

/* Convert an IMAP mailbox to a URL path
 *  dst needs to have roughly 4 times the storage space of src
 *    Hex encoding can triple the size of the input
 *    UTF-7 can be slightly denser than UTF-8
 *     (worst case: 8 octets UTF-7 becomes 9 octets UTF-8)
 */
void MailboxToURL(char *dst, char *src)
{
    unsigned char c, i, bitcount;
    unsigned long ucs4, utf16, bitbuf;
    unsigned char base64[256], utf8[6];

    /* initialize modified base64 decoding table */
    memset(base64, UNDEFINED, sizeof (base64));
    for (i = 0; i < sizeof (base64chars); ++i) {
     base64[(int) base64chars[i]] = i;
    }

    /* loop until end of string */
    while (*src != '\0') {
     c = *src++;
     /* deal with literal characters and &- */
     if (c != '&' || *src == '-') {
         /* NB: There are no "URL safe" characters after the '~' */
         if (c < ' ' || c > '~' ||
             strchr(mailbox_url_unsafe, c) != NULL) {
          /* hex encode if necessary */
          dst[0] = '%';
          dst[1] = hex[c >> 4];
          dst[2] = hex[c & 0x0f];
          dst += 3;
         } else {
          /* encode literally */
          *dst++ = c;
         }
         /* skip over the '-' if this is an &- sequence */
         if (c == '&') ++src;

     } else {
        /* convert modified UTF-7 -> UTF-16 -> UCS-4 -> UTF-8 -> HEX */
         bitbuf = 0;
         bitcount = 0;
         ucs4 = 0;
         while ((c = base64[(unsigned char) *src]) != UNDEFINED) {
          ++src;
          bitbuf = (bitbuf << 6) | c;
          bitcount += 6;
          /* enough bits for a UTF-16 character? */
          if (bitcount >= 16) {
              bitcount -= 16;
              utf16 = (bitcount ? bitbuf >> bitcount
                             : bitbuf) & 0xffff;
              /* convert UTF16 to UCS4 */
              if
                    (utf16 >= UTF16HIGHSTART && utf16 <= UTF16HIGHEND) {
               ucs4 = (utf16 - UTF16HIGHSTART) << UTF16SHIFT;
               continue;
              } else if
                    (utf16 >= UTF16LOSTART && utf16 <= UTF16LOEND) {
               ucs4 += utf16 - UTF16LOSTART + UTF16BASE;
              } else {
               ucs4 = utf16;
              }
              /* convert UTF-16 range of UCS4 to UTF-8 */
              if (ucs4 <= 0x7fUL) {
               utf8[0] = (unsigned char) ucs4;
               i = 1;
              } else if (ucs4 <= 0x7ffUL) {
               utf8[0] = 0xc0 | (unsigned char) (ucs4 >> 6);
               utf8[1] = 0x80 | (unsigned char) (ucs4 & 0x3f);
               i = 2;
              } else if (ucs4 <= 0xffffUL) {
               utf8[0] = 0xe0 | (unsigned char) (ucs4 >> 12);
               utf8[1] = 0x80 | (unsigned char) ((ucs4 >> 6) & 0x3f);
               utf8[2] = 0x80 | (unsigned char) (ucs4 & 0x3f);
               i = 3;
              } else {
               utf8[0] = 0xf0 | (unsigned char) (ucs4 >> 18);
               utf8[1] = 0x80 | (unsigned char) ((ucs4 >> 12) & 0x3f);
               utf8[2] = 0x80 | (unsigned char) ((ucs4 >> 6) & 0x3f);
               utf8[3] = 0x80 | (unsigned char) (ucs4 & 0x3f);
               i = 4;
              }
              /* convert utf8 to hex */
              for (c = 0; c < i; ++c) {
               dst[0] = '%';
               dst[1] = hex[utf8[c] >> 4];
               dst[2] = hex[utf8[c] & 0x0f];
               dst += 3;
              }
          }
         }
         /* skip over trailing '-' in modified UTF-7 encoding */
         if (*src == '-') ++src;
     }
    }
    /* terminate destination string */
    *dst = '\0';
}

/* Convert hex coded UTF-8 URL path to modified UTF-7 IMAP mailbox
 *  dst should be about twice the length of src to deal with non-hex
 *  coded URLs
 */
int URLtoMailbox(char *dst, char *src)
{
    unsigned int utf8pos = 0;
    unsigned int utf8total, i, c, utf7mode, bitstogo, utf16flag;
    unsigned long ucs4 = 0, bitbuf = 0;
    utf7mode = 0; /* is the output UTF7 currently in base64 mode? */
    utf8total = 0; /* how many octets is the current input UTF-8 char;
                      0 == between characters */
    bitstogo = 0; /* bits that need to be encoded into base64; if
                     bitstogo != 0 then utf7mode == 1 */
    while ((c = (unsigned char)*src) != '\0') {
     ++src;
     /* undo hex-encoding */
     if (c == '%' && src[0] != '\0' && src[1] != '\0') {
         c = HEXCHAR(src[0]);
         i = HEXCHAR(src[1]);
         if (c == XX || i == XX) {
             return 0;
         } else {
             c = (char)((c << 4) | i);
         }
         src += 2;
     }
     /* normal character? */
     if (c >= ' ' && c <= '~') {
         /* switch out of UTF-7 mode */
         if (utf7mode) {
          if (bitstogo) {
          *dst++ = base64chars[(bitbuf << (6 - bitstogo)) & 0x3F];
          }
          *dst++ = '-';
          utf7mode = 0;
          bitstogo = bitbuf = 0;
         }
         *dst++ = c;
         /* encode '&' as '&-' */
         if (c == '&') {
          *dst++ = '-';
         }
         continue;
     }
     /* switch to UTF-7 mode */
     if (!utf7mode) {
         *dst++ = '&';
         utf7mode = 1;
     }
     /* Encode US-ASCII characters as themselves */
     if (c < 0x80) {
         ucs4 = c;
         utf8total = 1;
     } else if (utf8total) {
         /* this is a subsequent octet of a multi-octet character */
         /* save UTF8 bits into UCS4 */
         ucs4 = (ucs4 << 6) | (c & 0x3FUL);
         if (++utf8pos < utf8total) {
          continue;
         }
     } else {
         /* this is the first octet of a multi-octet character */
         utf8pos = 1;
         if (c < 0xE0) {
          utf8total = 2;
          ucs4 = c & 0x1F;
         } else if (c < 0xF0) {
          utf8total = 3;
          ucs4 = c & 0x0F;
         } else {
          /* NOTE: can't convert UTF8 sequences longer than 4 */
          utf8total = 4;
          ucs4 = c & 0x03;
         }
         continue;
     }
     /* Finished with UTF-8 character.  Make sure it isn't an
        overlong sequence.  If it is, return failure. */
     if ((ucs4 < 0x80 && utf8total > 1) ||
         (ucs4 < 0x0800 && utf8total > 2) ||
         (ucs4 < 0x00010000 && utf8total > 3) ||
         (ucs4 < 0x00200000 && utf8total > 4) ||
         (ucs4 < 0x04000000 && utf8total > 5) ||
         (ucs4 < 0x80000000 && utf8total > 6)) {
         return 0;
     }
     /* loop to split ucs4 into two utf16 chars if necessary */
     utf8total = 0;
     do {
         if (ucs4 >= UTF16BASE) {
                ucs4 -= UTF16BASE;
          bitbuf = (bitbuf << 16) | ((ucs4 >> UTF16SHIFT)
                            + UTF16HIGHSTART);
          ucs4 = (ucs4 & UTF16MASK) + UTF16LOSTART;
          utf16flag = 1;
         } else {
          bitbuf = (bitbuf << 16) | ucs4;
          utf16flag = 0;
         }
         bitstogo += 16;
         /* spew out base64 */
         while (bitstogo >= 6) {
          bitstogo -= 6;
          *dst++ = base64chars[(bitstogo ? (bitbuf >> bitstogo)
                               : bitbuf)
                         & 0x3F];
         }
     } while (utf16flag);
    }
    /* if in UTF-7 mode, finish in ASCII */
    if (utf7mode) {
     if (bitstogo) {
         *dst++ = base64chars[(bitbuf << (6 - bitstogo)) & 0x3F];
     }
     *dst++ = '-';
    }
    /* tie off string */
    *dst = '\0';
    return 1;
}

Appendix B.  List of changes Changes since RFC 2192

   Updated boilerplate, list of editor's, etc.
   Updated references.
   Updated ABNF not to use _, to use SP instead of SPACE, etc.
   Updated example domains to use example.org.
   Fixed ABNF error in "imessagelist" non-terminal.
   Updated ABNF, due to changes in RFC 3501, RFC 4466 4466, and RFC 3986.
   Renamed "iuserauth" non-terminal to "iuserinfo". <iuserinfo>.
   Clarified that the userinfo component describes both authorization
   identity and mailbox naming scope.
   Allow for non-synchronizing literals in "enc-search".
   Added "ipartial" specifier that denotes a partial FETCH.
   Moved URLAUTH text from RFC 4467 to this document.
   Updated ABNF for the whole server to allow missing trailing "/"
      (e.g.
   (e.g., "imap://imap.example.com" is now valid and is the same as
       "imap://imap.example.com/")
   "imap://imap.example.com/").
   Clarified how relative-path references are constructed.
   Added more examples demonstrating relative-path references.
   Added rules for relative URLs and restructured ABNF as the result.
   Removed text on use of relative URLs in MHTML.
   Added examples demonstrating security considerations when resolving
   URLs.
   Recommend usage of STARTTLS/SASL security layer to protect
   confidential data.
   Removed some advices about connection reuse, which reuse that were incorrect.
   Removed URLs referencing a list of mailboxes, as this feature
      haven't
   hasn't seen any deployments.
   Clarified that user name "anonymous" is case-insensitive.

Appendix C.  List of changes Changes since RFC 4467

   Renamed <mechanism> to <uauth-mechanism>.  Restructured ABNF.

Appendix D.  Acknowledgments

   Text describing URLAUTH was lifted from [URLAUTH] by Mark Crispin.

   Stephane H. Maes contributed some ideas to this document, document; he also
   co-edited early versions of this document.

     Editors

   The editors would like to thank Mark Crispin, Ken Murchison, Ted
     Hardie, Zoltan Ordogh, Dave Cridland, Kjetil Torgrim Homme, Lisa
     Dusseault, Spencer Dawkins, Filip Navara thank Mark Crispin, Ken Murchison, Ted
   Hardie, Zoltan Ordogh, Dave Cridland, Kjetil Torgrim Homme, Lisa
   Dusseault, Spencer Dawkins, Filip Navara, Shawn M. Emery, Sam
   Hartman, Russ Housley, and Lars Eggert for the time they devoted to
   reviewing this document and/or for the comments received.

Authors' Addresses

   Chris Newman (Author/Editor)
   Sun Microsystems
   3401 Centrelake Dr., Suite 410
   Ontario, CA 91761
   EMail: chris.newman@sun.com

   Alexey Melnikov (Editor)
   Isode Limited
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex
   TW12 2BX, UK
   EMail: Alexey.Melnikov@isode.com
   URI:   http://www.melnikov.ca/

Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and Shawn M. Emery for restrictions
   contained in BCP 78, and except as set forth therein, the
     time they devoted to reviewing of this authors
   retain all their rights.

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

Intellectual Property

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

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

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

Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

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

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

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.