Network Working Group M. Crispin
INTERNET-DRAFT: IMAP URLAUTH University of Washington
June 2005
Document: internet-drafts/draft-ietf-lemonade-urlauth-07.txt
Internet Message Access Protocol (IMAP) - URLAUTH Extension
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Abstract
This document describes the URLAUTH extension to the Internet
Message Access Protocol (IMAP) (RFC 3501) and the IMAP URL Scheme
(IMAPURL) (RFC 2192). This extension provides a means by which an
IMAP client can use URLs carrying authorization to access limited
message data on the IMAP server.
An IMAP server which supports this extension indicates this with a
capability name of "URLAUTH".
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 [KEYWORDS].
The formal syntax uses the Augmented Backus-Naur Form (ABNF)
notation including the core rules defined in Appendix A of [ABNF].
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively. If a single "C:" or "S:" label applies to
multiple lines, then the line breaks between those lines are for
editorial clarity only and are not part of the actual protocol
exchange.
Introduction
In [IMAPURL], a URL of the form imap://fred@example.com/INBOX/;uid=20
requires authorization as userid "fred".
The URLAUTH extension provides a means by which an authorized user of
an IMAP server can create URLAUTH authorized IMAP URLs. A URLAUTH
authorized URL conveys authorization (not authentication) to the data
addressed by that URL, and can be used in another IMAP session to
access specific content on the IMAP server without otherwise
providing authorization to any other data owned by the authorizing
user (including other data in the mailbox specified in the URL).
A URLAUTH authorized URL can be used in the argument to the BURL
command in message composition, as described in [BURL], for such
purposes as a memory (or other resource) constrained client
submitting a message forward or resend from an IMAP mailbox without
requiring the client to fetch that message data.
The URLAUTH is generated using an authorization mechanism name and an
authorization token, which is generated using a secret mailbox
access key. An IMAP client can request the server to generate and
assign a new mailbox access key (thus effectively revoking all
current URLs using URLAUTH with the old mailbox access key) but can
not set the mailbox access key to a key of its own choosing.
1. Concepts
1.1. URLAUTH
The URLAUTH is a component, appended at the end of a URL, which
conveys authorization to access the data addressed by that URL. It
contains an authorized access identifier, an authorization
mechanism name, and an authorization token. The authorization
token is generated from the URL, the authorized access identifer,
authorization mechanism name, and a mailbox access key.
1.2. Mailbox Access Key
The mailbox access key is a random string with at least 128 bits of
entropy. It is generated by software (not by the human user), and
MUST be unpredictable.
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.
1.3. Authorized Access Identifier
The authorized access identifier restricts use of the URLAUTH
authorized URL to certain users authorized on the server, as
described in section 2.
1.4. Authorization Mechanism
The authorization mechanism is the algorithm by which the URLAUTH is
generated and subsequently verified, using the mailbox access key.
This specification defines the INTERNAL mechanism, which uses a token
generation algorithm of the server's choosing (a modern and
reasonably secure [HMAC] such as HMAC-SHA1 is recommended) and does
not involve disclosure of the mailbox access key to the client.
Note: If, for any reason, it is necesary to change the token
generation algorithm of the INTERNAL mechanism (e.g. because an
attack against the current algorithm has been discovered), all
currently existing URLAUTH authorized URLs are invalidated by the
change in algorithm. Since this would be an unpleasant surprise
to applications which depend upon the validity of a URLAUTH
authorized URL, and there is no good way to do a bulk update of
existing deployed URLs, it is best to avoid this situation by
using a secure algorithm as opposed to one that is "good enough".
Although this specification is extensible for other mechanisms, none
are defined in this document. In addition to the mechanism name
itself, other mechanisms may have mechanism specific data, which is
to be interpreted according to the definition of that mechanism.
1.5. Authorization Token
The authorization token is a deterministic string of at least 128
bits which an entity with knowledge of the secret mailbox access key
and URL authorization mechanism can use to verify the URL.
2. IMAP URL Extensions
[IMAPURL] is extended by allowing the addition of ;EXPIRE=<datetime>"
and ";URLAUTH=<access>:<mech>:<token>" to IMAP URLs which refer to
specific message or message parts.
";URLAUTH=<access>:<mech>:<token>" (the URLAUTH) MUST be at the end
of the URL.
URLAUTH does not apply to, and MUST NOT be used with, any IMAP URL
which refers to an entire IMAP server, list of mailboxes, an entire
IMAP mailbox, or IMAP search results.
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 be revoked as discussed below.
";URLAUTH=<access>:<mech>:<token>" indicates the access identifiers
which are permitted to use this URL, the authorization mechanism, and
the authorization token.
The "submit+" access identifier, 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 contacting the IMAP server.
The "user+" access identifier, followed by a userid, indicates that
use of this URL is limited to IMAP sessions which are logged in as
the specified userid (that is, have authorization identity as that
userid).
Note: if a SASL mechanism which provides both authorization and
authentication identifiers is used to authenticate to the IMAP
server, the "user+" access identifier must match the authorization
identifier.
The "authuser" access identifier indicates that use of this URL is
limited to IMAP sessions which are logged in as an authorized user
(that is, have authorization identity as an authorized user) of
that IMAP server. Use of this URL is prohibited to anonymous IMAP
sessions.
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 [IMAP]),
including anonymous sessions, may issue a URLFETCH using this URL.
The authorization token is represented as an ASCII-encoded
hexadecimal string, which is used to authorize the URL. The length
and the calculation of the authorization token depends upon the
mechanism used; but, in all cases, the authorization token is at
least 128 bits (and therefore at least 32 hexadecimal digits).
3. Discussion of URLAUTH Authorization Issues
In [IMAPURL], the userid before the "@" in the URL has two purposes:
1) It provides context for user-specific mailbox paths such
as "INBOX".
2) It specifies that resolution of the URL requires logging in as
that user and limits use of that URL to only that user.
An obvious limitation of using the same field for both purposes is
that the URL can only be resolved by the mailbox owner.
URLAUTH overrides the second purpose of the userid in the IMAP URL
and by default permits the URL to be resolved by any user permitted
by the access identifier.
The "user+<userid>" access identifier limits resolution of that URL
to a particular userid, whereas the "submit+<userid>" access
identifier is more general and simply requires the session be
authorized by a user that has been granted a "submit" role within
the authentication system. Use of either of these access
identifiers makes it impossible for an attacker, spying on the
session, to use the same URL, either directly or by submission to a
message submission entity.
The "authuser" and "anonymous" access identifiers do not have this
level protection, and should be used with caution. 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. Refer to the Security Considerations for
more details.
4. Generation of URLAUTH authorized URLs
A URLAUTH authorized URL is generated from an initial URL as follows:
An initial URL is built, ending with ";URLAUTH=<access>" but
without the ":<mech>:<token>" components. An authorization
mechanism is selected and used to calculate the authorization
token, with the initial URL as the data and a secret known to the
IMAP server as the key. The URLAUTH authorized URL is generated by
taking the initial URL and appending ":", the URL authorization
mechanism name, ":", and the ASCII-encoded hexadecimal
representation of the authorization token.
Note: ASCII-encoded hexadecimal is used instead of BASE64
because a BASE64 representation may have "=" padding characters
which would be problematic in a URL.
In the INTERNAL mechanism, the mailbox access key for that mailbox is
the secret known to the IMAP server, and a server-selected algorithm
such as HMAC-SHA1 is used to to calculate the authorization token.
5. Validation of URLAUTH authorized URLs
A URLAUTH authorized URL is validated as follows:
The URL is split at the ":" which separates "<access>" from
"<mech>:<token>" in the ";URLAUTH=<access>:<mech>:<token>" portion
of the URL. The "<mech>:<token>" portion is first parsed and saved
as the authorization mechanism and the authorization token. The
URL is truncated, discarding the ":" described above, to create a
"rump URL" (the URL minus the ":" and the "<mech>:<token>"
portion). The rump URL is then analyzed to identify the mailbox.
If the mailbox cannot be identified, an authorization token is
calculated on the rump URL, using random "plausible" keys (selected
by the server) as needed, before returning a validation failure.
This prevents timing attacks aimed at identifying mailbox names.
If the mailbox can be identified, the authorization token is
calculated on the rump URL and a secret known to the IMAP server
using the given URL authorization mechanism. Validation is
successful if, and only if, the calculated authorization token for
that mechanism matches the authorization token supplied in
";URLAUTH=<access>:<mech>:<token>".
Removal of the ":<mech>:<token>" portion of the URL MUST be the
only operation applied to the URLAUTH authorized URL to get the
rump URL. In particular, URL percent escape decoding and
case-folding MUST NOT occur.
In the INTERNAL mechanism, the mailbox access key for that mailbox
is used as the secret known to the IMAP server, and the same
server-selected algorithm used for generating URLs is used to
calculate the authorization token for verification.
6. Additional Commands
These commands are extension to the [IMAP] base protocol.
The section headings of these commands are intended to correspond
with where they would be located in the base protocol document if
they were part of that document.
BASE.6.3.RESETKEY. RESETKEY Command
Arguments: optional mailbox name
optional mechanism name(s)
Responses: none other than in result
Result: OK - RESETKEY completed, URLMECH containing new data
NO - RESETKEY error: can't change key of that mailbox
BAD - command unknown or arguments invalid
The RESETKEY command has two forms.
The first form accepts a mailbox name as an argument, and generates
a new mailbox access key for the given mailbox in the user's
mailbox access key table, replacing any previous mailbox access key
(and revoking any URLs that were authorized with a URLAUTH using
that key) in that table. By default, the mailbox access key is
generated for the INTERNAL mechanism; other mechanisms can be
specified with the optional mechanism argument.
The second form, with no arguments, removes all mailbox access keys
in the user's mailbox access key table, revoking all URLs currently
authorized using URLAUTH by the user.
Any current IMAP session logged in as the user which has the mailbox
selected will receive an untagged OK response with the URLMECH
status response code (see below for more details about the URLMECH
status response code).
Example:
C: a31 RESETKEY
S: a31 OK All keys removed
C: a32 RESETKEY INBOX
S: a32 OK [URLMECH INTERNAL] mechs
C: a33 RESETKEY INBOX XSAMPLE
S: a33 OK [URLMECH INTERNAL XSAMPLE=P34OKhO7VEkCbsiYY8rGEg==] done
BASE.6.3.GENURLAUTH. GENURLAUTH Command
Argument: one or more URL/mechanism pairs
Response: untagged response: GENURLAUTH
Result: OK - GENURLAUTH completed
NO - GENURLAUTH error: can't generate a URLAUTH
BAD - command unknown or arguments invalid
The GENURLAUTH command requests the server to generate a URLAUTH
authorized URL for each of the given URLs using the given URL
authorization mechanism.
The server MUST validate each supplied URL as follows:
(1) the mailbox component of the URL MUST refer to an existing
mailbox.
(2) the server component of the URL MUST contain a valid userid
that identifies the owner of the mailbox access key table
that will be used to generate the URLAUTH authorized URL.
As a consequence, the iserver rule of [IMAPURL] is modified
so that iuserauth is mandatory.
(3) there is a valid access identifier which, in the case of
"submit+" and "user+", will contain a valid userid. This
userid is not necessarily the same as the owner userid
described in (2).
(4) the server MAY also verify that the Message-ID and/or
second components (if present) are valid.
If any of the above checks fail, the server MUST return a tagged
BAD response with the following exception. In the case of an
invalid userid supplied as mailbox access key owner and/or as part
the access identifier, the server MAY issue a tagged OK response
with a generated mailbox key that always fails validation when used
with a URLFETCH command.
If there is currently no mailbox access key for the given mailbox
in the owner's mailbox access key table, one is automatically
generated. That is, it is not necessary to use RESETKEY prior to
first-time use of GENURLAUTH.
If the command is successful, a GENURLAUTH response code is returned
listing the requested URLs as URLAUTH authorized URLs.
Examples:
C: a775 GENURLAUTH "imap://joe@example.com/INBOX/;uid=20/
;section=1.2" INTERNAL
S: a775 BAD missing access identifier in supplied URL
C: a776 GENURLAUTH "imap://example.com/Shared/;uid=20/
;section=1.2;urlauth=submit+fred" INTERNAL
S: a776 BAD missing owner username in supplied URL
C: a777 GENURLAUTH "imap://joe@example.com/INBOX/;uid=20/
;section=1.2;urlauth=submit+fred" INTERNAL
S: * GENURLAUTH "imap://joe@example.com/INBOX/;uid=20/;section=1.2
;urlauth=submit+fred:internal:91354a473744909de610943775f92038"
S: a777 OK GENURLAUTH completed
BASE.6.3.URLFETCH. URLFETCH Command
Argument: one or more URLs
Response: untagged response: URLFETCH
Result: OK - urlfetch completed
NO - urlfetch failed due to server internal error
BAD - command unknown or arguments invalid
The URLFETCH command requests that the server return the text data
associated with the specified IMAP URLs, as described in [IMAPURL]
and extended by this document. The data is returned for all
validated URLs, regardless of whether or not the session would
otherwise be able to access the mailbox containing that data via
SELECT or EXAMINE.
Note: This command does not require that the URL refer to the
selected mailbox; nor does it require that any mailbox be
selected. It also does not in any way interfere with any selected
mailbox.
The URLFETCH command MUST return an untagged URLFETCH response and
a tagged OK response to any URLFETCH command that is syntactically
valid. A NO response indicates a server internal failure which may
be resolved on later retry.
Note: the possibility of a NO response is to accommodate
implementations which would otherwise have to issue an
untagged BYE with a fatal error due to an inability to
respond to a valid request. In an ideal world, a server
SHOULD NOT issue a NO response.
The server MUST return NIL for any IMAP URL which references an
entire IMAP server, list of mailboxes, an entire IMAP mailbox, or
IMAP search results.
Example
Note: for clarity, this example uses the LOGIN command which
SHOULD NOT be used over a non-encrypted communication path.
This example is of a submit server, obtaining a message segment
for a message that it has already validated was submitted by
"fred".
S: * OK [CAPABILITY IMAP4REV1 URLAUTH] example.com IMAP server
C: a001 LOGIN submitserver secret
S: a001 OK submitserver logged in
C: a002 URLFETCH "imap://joe@example.com/INBOX/;uid=20/
;section=1.2;urlauth=submit+fred:internal
:91354a473744909de610943775f92038"
S: * URLFETCH "imap://joe@example.com/INBOX/;uid=20/;section=1.2
;urlauth=submit+fred:internal
:91354a473744909de610943775f92038" {28}
S: Si vis pacem, para bellum.
S:
S: a002 OK URLFETCH completed
7. Additional Responses
These responses are extensions to the [IMAP] base protocol.
The section headings of these responses are intended to correspond
with where they would be located in the base protocol document if
they were part of that document.
BASE.7.1.URLMECH. URLMECH Status Response Code
The URLMECH status response code is followed by a list of URL
authorization mechanism names. Mechanism names other than INTERNAL
may be appended with an "=" and BASE64 encoded form of mechanism
specific data.
This status response code is returned in an untagged OK response in
response to a RESETKEY, SELECT, or EXAMINE command.
Example:
C: a33 RESETKEY INBOX XSAMPLE
S: a33 OK [URLMECH INTERNAL XSAMPLE=P34OKhO7VEkCbsiYY8rGEg==] done
In this example, the server supports the INTERNAL mechanism and an
experimental mechanism called XSAMPLE which also holds some mechanism
specific data (the name "XSAMPLE" is for illustrative purposes only).
BASE.7.4.GENURLAUTH. GENURLAUTH Response
Contents: One or more URLs
The GENURLAUTH response returns the URLAUTH authorized URL(s)
requested by a GENURLAUTH command.
Example:
C: a777 GENURLAUTH "imap://joe@example.com/INBOX/;uid=20/
;section=1.2;urlauth=submit+fred" INTERNAL
S: * GENURLAUTH "imap://joe@example.com/INBOX/;uid=20/;section=1.2
;urlauth=submit+fred:internal:91354a473744909de610943775f92038"
S: a777 OK GENURLAUTH completed
BASE.7.4.URLFETCH. URLFETCH Response
Contents: URL/nstring pair
The URLFETCH response returns the message text data associated with
an IMAP URLs, as described in [IMAPURL] and extended by this
document. This response occurs as the result of a URLFETCH
command.
The returned data string is NIL if the URL is invalid for any reason
(including validation failure). If the URL is valid, but the IMAP
fetch of the body part returned NIL (this should not happen), the
returned data string should be the empty string ("") and not NIL.
Note: This command does not require that the URL refer to the
selected mailbox; nor does it require that any mailbox be
selected. It also does not in any way interfere with any selected
mailbox.
Example:
C: a002 URLFETCH "imap://joe@example.com/INBOX/;uid=20/
;section=1.2;urlauth=submit+fred:internal
:91354a473744909de610943775f92038"
S: * URLFETCH "imap://joe@example.com/INBOX/;uid=20/;section=1.2
;urlauth=submit+fred:internal
:91354a473744909de610943775f92038" {28}
S: Si vis pacem, para bellum.
S:
S: a002 OK URLFETCH completed
8. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [ABNF].
The following modifications are made to the Formal Syntax in [IMAP]:
resetkey = "RESETKEY" [SP mailbox *(SP mechanism)]
capability =/ "URLAUTH"
command-auth =/ resetkey / genurlauth / urlfetch
resp-text-code =/ "URLMECH" SP "INTERNAL" *(SP mechanism ["=" base64])
genurlauth = "GENURLAUTH" 1*(SP url SP mechanism)
genurlauth-data = "*" SP "GENURLAUTH" 1*(SP url)
url = astring
; contains imapurl as defined below
urlfetch = "URLFETCH" 1*(SP url)
urlfetch-data = "*" SP "URLFETCH" 1*(SP url SP nstring)
The following modifications are made to the Formal Syntax in
[IMAPURL]:
enc-urlauth = 32*HEXDIG
imessagepart = enc-mailbox [uidvalidity] iuid [isection] [iurlauth]
; enc-mailbox is called "enc_mailbox" in [IMAPURL]
iserver = iuserauth "@" hostport
; replaces iserver rule in [IMAPURL], as iuserauth
; is mandatory with URLAUTH authorized URLs
iurlauth = [expire] ";URLAUTH=" access ":" mechanism ":"
enc-urlauth
access = ("submit+" iuserauth) / ("user+" iuserauth) /
"authuser" / "anonymous"
expire = ";EXPIRE=" date-time
; date-time defined in [DATETIME]
mechanism = "INTERNAL" / 1*(alpha / digit / "-" / ".")
; case-insensitive
; new mechanisms MUST be registered with IANA
9. Security Considerations
Security considerations are discussed throughout this memo.
The mailbox access key SHOULD have at least 128 bits of entropy
(refer to [RANDOM] for more details) and MUST be unpredictable.
The URLMECH status response code may expose sensitive data in the
mechanism specific data for mechanisms other than INTERNAL. A server
implementation MUST implement a configuration that will not return
a URLMECH status response code unless some mechanism is provided
that protects the session from snooping, such as a TLS or SASL
security layer that provides confidentiality protection.
The calculation of a authorization token with a "plausible" key if
the mailbox can not be identified is necessary to avoid attacks in
which the server is probed to see if a particular mailbox exists on
the server by measuring the amount of time taken to reject a known
bad name vs. some other name.
To protect against a computational denial-of-service attack, a server
MAY impose progressively longer delays on multiple URL requests that
fail validation.
The decision to use the "authuser" access identifier should be made
with caution. An "authuser" access identifier can be used by any
authorized user of the IMAP server; and therefore use of this access
identifier should be limited to content which may be disclosed to any
authorized user of the IMAP server.
The decision to use the "anonymous" access identifier should be
made with extreme caution. An "anonymous" access identifier can be
used by anyone; and therefore use of this access identifier should
be limited to content which may be disclosed to anonymous. Many
IMAP servers do not permit anonymous access; in the case of such
servers the "anonymous" access identifer is equivalent to
"authuser", but this MUST NOT be relied upon.
IANA Considerations
This document consitutes registration of the URLAUTH capability in
the imap4-capabilities registry.
URLAUTH authorization mechanisms are registered by publishing a
standards track or IESG approved experimental RFC. The registry is
currently located at:
[to be defined by IANA]
This registry is case-insensitive.
This document consitutes registration of the INTERNAL URLAUTH
authorization mechanism.
IMAP URLAUTH Authorization Mechanism Registry
Mechanism Name Reference
-------------- ---------
INTERNAL [this document, to be filled in by IANA]
Normative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[BURL] Newman, C., "Message Submission BURL Extension",
draft-newman-lemonade-burl-00.txt (work in progress),
March 2004.
[DATETIME] Klyne, G., and Newman, C., "Date and Time on the Internet:
Timestamps", RFC 3339, July 2002.
[IMAP] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 3501, March 2003.
[IMAPURL] Newman, C., "IMAP URL Scheme", RFC 2192, September 1997.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Informative References:
[HMAC] Krawczyk, H., Bellare, M., and Canetti, R., "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104,
February 1997.
[RANDOM] Eastlake, D., Crocker, S., and Schiller, J., "Randomness
Recommendations for Security", RFC 1750, December 1994.
Author's Address
Mark R. Crispin
Networks and Distributed Computing
University of Washington
4545 15th Avenue NE
Seattle, WA 98105-4527
Phone: (206) 543-5762
EMail: MRC@CAC.Washington.EDU
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draft-ietf-lemonade-urlauth-07
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