Internet DRAFT - draft-melnikov-rfc2192bis
draft-melnikov-rfc2192bis
Network Working Group C. Newman
Document: draft-melnikov-rfc2192bis-00.txt Sun Microsystems
Expires: March 2006 A. Melnikov
Intended category: Standards Track Isode Ltd.
September 2005
IMAP URL Scheme
Status of this Memo
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A revised version of this draft document will be submitted to the RFC
editor as a Proposed Standard for the Internet Community. Discussion
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the IMAPEXT Mailing list <ietf-imapext@imc.org>. Distribution of this
draft is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
IMAP [IMAP4] is a rich protocol for accessing remote message
stores. It provides an ideal mechanism for accessing public mail-
ing list archives as well as private and shared message stores.
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This document defines a URL scheme for referencing objects on an
IMAP server.
1. Conventions used in this document
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
in this document are to be interpreted as defined in "Key words for
use in RFCs to Indicate Requirement Levels" [KEYWORDS].
2. IMAP scheme
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.
The IMAP URL follows the common Internet scheme syntax as defined
in [URI-GEN] <<except that clear text passwords are not permit-
ted>>. If :<port> is omitted, the port defaults to 143.
An IMAP URL takes one of the following forms:
imap://<iserver>/
imap://<iserver>/<enc-list-mailbox>;TYPE=<list-type>
imap://<iserver>/<enc-mailbox>[uidvalidity][?<enc-search>]
imap://<iserver>/<enc-mailbox>[uidvalidity]<iuid>[isection]
The first form is used to refer to an IMAP server, the second form
refers to a list of mailboxes, the third form refers to the con-
tents of a mailbox or a set of messages resulting from a search,
and the final form refers to a specific message or message part.
Note that the syntax here is informal. The authoritative formal
syntax for IMAP URLs is defined in section 11.
3. IMAP User Name and Authentication Mechanism
A user name (authorization identity) and/or authentication mecha-
nism may be supplied. They are used in the "LOGIN" or "AUTHENTI-
CATE" commands after making the connection to the IMAP server. If
no user name or authentication mechanism is supplied, the user name
"anonymous" is used with the "LOGIN" command and the password is
supplied as the Internet e-mail address of the end user accessing
the resource. If the URL doesn't supply a user name, the program
interpreting the IMAP URL SHOULD request one from the user if nec-
essary.
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<<SASL ANONYMOUS?>>
<<How LOGINDISABLES affects LOGIN>>
An authentication mechanism can be expressed by adding ";AUTH=<enc-
auth-type>" to the end of the user name. When such an <enc-auth-
type> is indicated, the client SHOULD request appropriate creden-
tials from that mechanism and use the "AUTHENTICATE" command
instead of the "LOGIN" command. If no user name is specified, one
SHOULD be obtained from the mechanism or requested from the user as
appropriate.
The string ";AUTH=*" indicates that the client SHOULD select an
appropriate authentication mechanism. It MAY use any mechanism
listed in the CAPABILITY command or use an out of band security
service resulting in a PREAUTH connection. If no user name is
specified and no appropriate authentication mechanisms are avail-
able, the client SHOULD fall back to anonymous login as described
above. This allows a URL which grants read-write access to autho-
rized users, and read-only anonymous access to other users.
If a user name is included with no authentication mechanism, then
";AUTH=*" is assumed.
Since URLs can easily come from untrusted sources, care must be
taken when resolving a URL which requires or requests any sort of
authentication. If authentication credentials are supplied to the
wrong server, it may compromise the security of the user's account.
The program resolving the URL should make sure it meets at least
one of the following criteria in this case:
(1) The URL comes from a trusted source, such as a referral server
which 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) Explicit local site policy permits the client to connect to the
server in the URL. For example, if the client knows the site
domain name, site policy may dictate that any hostname ending in
that domain is trusted.
(3) The user confirms that connecting to that domain name with the
specified credentials and/or mechanism is permitted.
(4) A mechanism is used which validates the server before passing
potentially compromising client credentials. <<mutual auth?>>
(5) An authentication mechanism is used which will not reveal
information to the server which could be used to compromise future
connections.
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URLs which do not include a user name must be treated with extra
care, since 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 security mechanism.
Finally, clients are discouraged from using a plain text password
as a fallback with ";AUTH=*" unless the connection has strong
encryption (<<e.g. a key length of greater than 56 bits>>).
A program interpreting IMAP URLs MAY cache open connections to an
IMAP server for later re-use. If a URL contains a user name, only
connections authenticated as that user may be re-used. If a URL
does not contain a user name or authentication mechanism, then only
an anonymous connection may be re-used. If a URL contains an
authentication mechanism without a user name, then any non-anony-
mous connection may be re-used.
Note that if unsafe or reserved characters such as " " or ";" are
present in the user name or authentication mechanism, they MUST be
encoded as described in [URI-GEN].
4. IMAP server
An IMAP URL referring to an IMAP server has the following form:
imap://<iserver>/
A program interpreting this URL would issue the standard set of
commands it uses to present a view of the contents of an IMAP
server. This is likely to be semanticly equivalent to one of the
following URLs:
imap://<iserver>/;TYPE=LIST
imap://<iserver>/;TYPE=LSUB
<<The program interpreting this URL SHOULD use the LSUB form if it
supports mailbox subscriptions.>>
5. Lists of mailboxes
An IMAP URL referring to a list of mailboxes has the following
form:
imap://<iserver>/<enc-list-mailbox>;TYPE=<list-type>
The <list-type> may be either "LIST" or "LSUB", and is case insen-
sitive. The field ";TYPE=<list-type>" MUST be included.
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The <enc-list-mailbox> is any argument suitable for the list-mail-
box field of the IMAP [IMAP4] LIST or LSUB commands. The field
<enc-list-mailbox> may be omitted, in which case the program inter-
preting the IMAP URL may use "*" or "%" as the <enc-list-mailbox>.
The program SHOULD use "%" if it supports a hierarchical view, oth-
erwise it SHOULD use "*".
Note that if unsafe or reserved characters such as " " or "%" are
present in <enc-list-mailbox> they MUST be encoded as described in
[URI-GEN]. If the character "/" is present in enc-list-mailbox, it
SHOULD NOT be encoded.
6. Lists of 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" command. Note that if unsafe or reserved characters such
as " ", ";", or "?" are present in <enc-mailbox> they MUST be
encoded as described in [URI-GEN]. If the character "/" is present
in enc-mailbox, it SHOULD NOT be encoded.
The [uidvalidity] field is optional. If it is present, it MUST be
the argument to the IMAP4 UIDVALIDITY status response at the time
the URL was created. This SHOULD be used by the program interpret-
ing the IMAP URL to determine if the URL is stale.
The [?<enc-search>] field is optional. If it is not present, the
contents of the mailbox SHOULD be presented by the program inter-
preting the URL. If it is present, it SHOULD be used as the argu-
ments following an IMAP4 SEARCH command with unsafe characters such
as " " (which are likely to be present in the <enc-search>) encoded
as described in [URI-GEN].
7. A specific message or message part
An IMAP URL referring to a specific message or message part has the
following form:
imap://<iserver>/<enc-mailbox>[uidvalidity]<iuid>[isection]
The <enc-mailbox> and [uidvalidity] are as defined above.
If [uidvalidity] is present in this form, it SHOULD be used by the
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program interpreting the URL to determine if the URL is stale.
The <iuid> refers to an IMAP4 message UID, and 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 a "UID FETCH <uid> BODYS-
TRUCTURE" command and locating the appropriate part in the result-
ing BODYSTRUCTURE. Note that unsafe characters in [isection] MUST
be encoded as described in [URI-GEN].
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8. Relative IMAP URLs
Relative IMAP URLs are permitted and are resolved according to the
rules defined in [URI-GEN] with one exception. In IMAP URLs,
parameters are treated as part of the normal path with respect to
relative URL resolution. This is believed to be the behavior of
the installed base and is likely to be documented in a future revi-
sion of the relative URL specification.
The following observations are also important:
The <iauth> grammar element is considered part of the user name for
purposes of resolving relative IMAP URLs. This means that unless a
new login/server specification is included in the relative URL, the
authentication mechanism is inherited from a 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 delimiter. Relative URLs may be used on mailboxes which 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.
The base URL for a list of mailboxes or messages which was referred
to by an IMAP URL is always the referring IMAP URL itself. The
base URL for a message or message part which was referred to by an
IMAP URL may be more complicated to determine. The program inter-
preting the relative URL will have to check the headers of the MIME
entity and any enclosing MIME entities in order to locate the "Con-
tent-Base" and "Content-Location" headers. These headers are used
to determine the base URL as defined in <<[HTTP]>>. For example,
if the referring IMAP URL contains a "/;SECTION=1.2" parameter,
then the MIME headers for section 1.2, for section 1, and for the
enclosing message itself SHOULD be checked in that order for "Con-
tent-Base" or "Content-Location" headers.
9. Multinational Considerations
IMAP4 [IMAP4] section 5.1.3 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 [UTF8]. Since 8-bit characters are not per-
mitted in URLs, the UTF-8 characters are encoded as required by the
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URL specification [URI-GEN]. Sample code is included in Appendix A
to demonstrate this conversion.
10. 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>
Results in the following client commands:
<connect to minbari.example.org, port 143>
C: A001 LOGIN ANONYMOUS sheridan@babylon5.example.org
C: A002 SELECT gray-council
<client verifies the UIDVALIDITY matches>
C: A003 UID FETCH 20 BODY.PEEK[]
The URL:
<imap://michael@minbari.example.org/users.*;type=list>
Results in the following client commands:
<client requests password from user>
<connect to minbari.example.org imap server, activate strong encryption>
C: A001 LOGIN MICHAEL zipper
C: A002 LIST "" users.*
The URL:
<imap://psicorp.example.org/~peter/%E6%97%A5%E6%9C%AC%E8%AA%9E/
%E5%8F%B0%E5%8C%97>
Results in the following client commands:
<connect to psicorp.example.org, port 143>
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 a mailbox>
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The URL:
<imap://;AUTH=GSSAPI@minbari.example.org/gray-council/;uid=20/
;section=1.2>
Results in the following client commands:
<connect to minbari.example.org, port 143>
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 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-Base Content-Location)])
<Client looks for Content-Base or 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 result in the following:
<connect to minbari.example.org, port 143>
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
...
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NOTE: In this final example, the client has implementation depen-
dent choices. The authentication mechanism could be anything,
including PREAUTH. And the final FETCH command could fetch more or
less information about the messages, depending on what it wishes to
display to the user.
11. Security Considerations
Security considerations discussed in the IMAP specification [IMAP4]
and the URL specification <<[BASIC-URL]>> are relevant. Security
considerations related to authenticated URLs are discussed in sec-
tion 3 of this document.
Many email clients store the plain text 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 from the user to supply that password to the specified host
name.
12. ABNF for IMAP URL scheme
Formal syntax is defined using ABNF [ABNF], extending the ABNF
rules in section 9 of [IMAP4]. Elements not defined here can be
found in the [ABNF], [IMAP4], [IMAPABNF] or [URI-GEN]. Strings are
not case sensitive and free insertion of linear-white-space is not
permitted.
uchar = unreserved | pct-encoded
<<needed?>>
achar = uchar / "&" / "="
bchar = achar / ":" / "@" / "/"
<<sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "="
pchar = unreserved / pct-encoded / sub-delims / ":" / "@">>
enc-auth-type = 1*achar
; encoded version of [IMAP4] "auth-type"
enc-list-mailbox = 1*bchar
; encoded version of [IMAP4] "list-mailbox"
enc-mailbox = 1*bchar
; encoded version of [IMAP4] "mailbox"
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enc-search = 1*bchar
; encoded version of [IMAPABNF] "search-program".
; Note that IMAP4 literals may not be used
; in a "search-program".
enc-section = 1*bchar
; encoded version of [IMAP4] "section-spec"
enc-user = 1*achar
; encoded version of [IMAP4] "userid"
imapurl = "imap://" iserver "/" [ icommand ]
iauth = ";AUTH=" ( "*" / enc-auth-type )
icommand = imailboxlist / imessagelist / imessagepart
imailboxlist = [enc-list-mailbox] ";TYPE=" list-type
imailbox-ref = enc-mailbox [uidvalidity]
; <<new production in this draft>>
imessagelist = imailbox-ref [ "?" enc-search ]
; "enc-search" is [URI-GEN] "query".
imessagepart = imailbox-ref iuid [isection]
isection = "/;SECTION=" enc-section
iserver = [iuserauth "@"] host [ ":" port ]
; This is the same as "authority" defined in
; [URI-GEN]. See [URI-GEN] for "host" and
; "port" definitions.
<<Just use [URI-GEN] "authority"?>>
iuid = "/;UID=" nz-number
; See [IMAP4] for "nz-number" definition
iuserauth = enc-user [iauth] / [enc-user] iauth
; conforms to the generic syntax of "userinfo"
; as defined in [URI-GEN].
list-type = "LIST" / "LSUB"
uidvalidity = ";UIDVALIDITY=" nz-number
; See [IMAP4] for "nz-number" definition
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13. Normative References
<<[BASIC-URL] Berners-Lee, Masinter, McCahill, "Uniform Resource
Locators (URL)", RFC 1738, CERN, Xerox Corporation, University of
Minnesota, December 1994.>>
[KEYWORDS] Bradner, "Key words for use in RFCs to Indicate Require-
ment Levels", RFC 2119, Harvard University, March 1997.
[IMAP4] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 3501, University of Washington, March 2003.
[IMAPABNF] Melnikov, A., and C. Daboo, "Collected extensions to
IMAP4 ABNF", work in progress, draft-melnikov-imap-ext-abnf-XX.txt.
<<[HTTP] Fielding, Gettys, Mogul, Frystyk, Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2068, UC Irvine, DEC, MIT/LCS,
January 1997.>>
<<[ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifica-
tions: ABNF", RFC 2234, Internet Mail Consortium, Demon Internet
Ltd, November 1997.>>
[MIME] Freed, N., Borenstein, N., "Multipurpose Internet Mail
Extensions", RFC 2045, Innosoft, First Virtual, November 1996.
[URI-GEN] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", RFC 3986, January 2005.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
14. Author's Address
Chris Newman
Sun Microsystems
1050 Lakes Drive
West Covina, CA 91790 USA
EMail: chris.newman@sun.com
Alexey Melnikov
Isode Limited
5 Castle Business Village
36 Station Road
Hampton, Middlesex
TW12 2BX, UK
Email: Alexey.Melnikov@isode.com
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URI: http://www.melnikov.ca/
Appendix A. Sample code
Here is sample C source code to convert between URL paths and IMAP mail-
box names, taking into account mapping between IMAP's modified UTF-7
[IMAP4] and hex-encoded UTF-8 which is more appropriate for URLs. This
code has not been rigorously tested nor does it necessarily behave rea-
sonably 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 or server components of the URL.
<<Does the code comply with new URI rules?>>
#include <stdio.h>
#include <string.h>
/* hexadecimal lookup table */
static char hex[] = "0123456789ABCDEF";
/* URL unsafe printable characters */
static char urlunsafe[] = " \"#%&+:;<=>?@[\\]^`{|}";
/* UTF7 modified base64 alphabet */
static 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;
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unsigned char base64[256], utf8[6];
/* initialize modified base64 decoding table */
memset(base64, UNDEFINED, sizeof (base64));
for (i = 0; i < sizeof (base64chars); ++i) {
base64[base64chars[i]] = i;
}
/* loop until end of string */
while (*src != '\0') {
c = *src++;
/* deal with literal characters and &- */
if (c != '&' || *src == '-') {
if (c < ' ' || c > '~' || strchr(urlunsafe, 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 {
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ucs4 = utf16;
}
/* convert UTF-16 range of UCS4 to UTF-8 */
if (ucs4 <= 0x7fUL) {
utf8[0] = ucs4;
i = 1;
} else if (ucs4 <= 0x7ffUL) {
utf8[0] = 0xc0 | (ucs4 >> 6);
utf8[1] = 0x80 | (ucs4 & 0x3f);
i = 2;
} else if (ucs4 <= 0xffffUL) {
utf8[0] = 0xe0 | (ucs4 >> 12);
utf8[1] = 0x80 | ((ucs4 >> 6) & 0x3f);
utf8[2] = 0x80 | (ucs4 & 0x3f);
i = 3;
} else {
utf8[0] = 0xf0 | (ucs4 >> 18);
utf8[1] = 0x80 | ((ucs4 >> 12) & 0x3f);
utf8[2] = 0x80 | ((ucs4 >> 6) & 0x3f);
utf8[3] = 0x80 | (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
*/
void URLtoMailbox(char *dst, char *src)
{
unsigned int utf8pos, utf8total, i, c, utf7mode, bitstogo, utf16flag;
unsigned long ucs4, bitbuf;
unsigned char hextab[256];
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/* initialize hex lookup table */
memset(hextab, 0, sizeof (hextab));
for (i = 0; i < sizeof (hex); ++i) {
hextab[hex[i]] = i;
if (isupper(hex[i])) hextab[tolower(hex[i])] = i;
}
utf7mode = 0;
utf8total = 0;
bitstogo = 0;
while ((c = *src) != '\0') {
++src;
/* undo hex-encoding */
if (c == '%' && src[0] != '\0' && src[1] != '\0') {
c = (hextab[src[0]] << 4) | hextab[src[1]];
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;
}
*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) {
/* save UTF8 bits into UCS4 */
ucs4 = (ucs4 << 6) | (c & 0x3FUL);
if (++utf8pos < utf8total) {
continue;
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}
} else {
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;
}
/* 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';
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}
Appendix B. List of changes since RFC 2192
Updated boilerplate, list of editor's, etc.
Updated references.
Updated ABNF not to use _, to use SP instead of SPACE.
Updated example domains to use example.org.
Fixed ABNF error in "imessagelist" non-terminal.
Updated ABNF, due to changes in RFC 3501, IMAPABNF and RFC 3986.
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described in this document or the extent to which any license
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to implement this standard. Please address the information to the
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Full Copyright Statement
Copyright (C) The Internet Society (2005).
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
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ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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