Network Working Group M. Crispin
INTERNET-DRAFT: IMAP URLAUTH University of Washington
November 2004
Document: internet-drafts/draft-ietf-lemonade-urlauth-04.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) constrainted 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 which 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]) may
issue a URLFETCH using this URL including anonymous sessions.
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==] mechs
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.
If the command is successful, a GENURLAUTH response code is returned
listing the requested URLs as URLAUTH authorized URLs.
Example:
Note that lines which do not begin with "C:" or "S:" are
continuations of the previous line for typographical clarity and
are not separate lines in the protocol.
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 that lines which do not begin with "C:" or "S:" are
continuations of the previous line for typographical clarity and
are not separate lines in the protocol. 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 ready
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==] mechs
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.
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)]
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]
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*uchar
; 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
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 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]
References
The following references are normative:
[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.
The following references are informative:
[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 Addresses
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
Chris Newman
Sun Microsystems
1050 Lakes Drive
West Covina, CA 91790
EMail: chris.newman@sun.com
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patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
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