XMPP Working Group P. Saint-Andre
Internet-Draft Jabber Software Foundation
Expires: May 20, 2004 November 20, 2003
End-to-End Object Encryption in the Extensible Messaging and Presence
Protocol (XMPP)
draft-ietf-xmpp-e2e-06
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on May 20, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This memo defines a method for end-to-end object signing and
encryption in the Extensible Messaging and Presence Protocol (XMPP).
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Securing Messages . . . . . . . . . . . . . . . . . . . . . . 5
4. Securing Presence . . . . . . . . . . . . . . . . . . . . . . 6
5. Securing Arbitrary XMPP Data . . . . . . . . . . . . . . . . . 8
6. Rules for S/MIME Generation and Handling . . . . . . . . . . . 9
7. Secure Communications Through a Gateway . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
Normative References . . . . . . . . . . . . . . . . . . . . . 14
Informative References . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 15
A. Schema for urn:ietf:params:xml:ns:xmpp-e2e . . . . . . . . . . 15
B. Revision History . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . 18
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1. Introduction
This memo define a method for end-to-end signing and encryption in
the Extensible Messaging and Presence Protocol (XMPP). (For
information about XMPP, see [XMPP-CORE] and [XMPP-IM].) The method
defined herein enables a sender to encrypt and/or sign an instant
message sent to a specific recipient, encrypt and/or sign presence
information that is directed to a specific user, and sign presence
information that is broadcasted to a specific user. This memo
thereby helps the XMPP specifications meet the requirements defined
in [IMP-REQS].
1.1 Terminology
This document inherits terminology defined in [SMIME], [IMP-MODEL],
[CMS], and [XMPP-CORE].
The capitalized 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
[TERMS].
1.2 Discussion Venue
The authors welcome discussion and comments related to the topics
presented in this document. The preferred forum is the
<xmppwg@jabber.org> mailing list, for which archives and subscription
information are available at <http://www.jabber.org/cgi-bin/mailman/
listinfo/xmppwg/>.
1.3 Intellectual Property Notice
This document is in full compliance with all provisions of Section 10
of RFC 2026. Parts of this specification use the term "jabber" for
identifying namespaces and other protocol syntax. Jabber[tm] is a
registered trademark of Jabber, Inc. Jabber, Inc. grants permission
to the IETF for use of the Jabber trademark in association with this
specification and its successors, if any.
2. Requirements
For the purposes of this memo, we stipulate the following
requirements:
1. The method defined MUST address encryption and signing
requirements for minimal instant messaging and presence only, as
those are defined in [IMP-REQS]. The method is NOT REQUIRED to
support non-IM applications of XMPP, nor to support advanced
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instant messaging and presence functionality that is outside the
scope of [IMP-REQS]. In particular, the method MUST address the
following requirements defined in [IMP-REQS]:
* The protocol MUST provide means to ensure confidence that a
received message (NOTIFICATION or INSTANT MESSAGE) has not
been corrupted or tampered with. (Section 2.5.1)
* The protocol MUST provide means to ensure confidence that a
received message (NOTIFICATION or INSTANT MESSAGE) has not
been recorded and played back by an adversary. (Section
2.5.2)
* The protocol MUST provide means to ensure that a sent message
(NOTIFICATION or INSTANT MESSAGE) is only readable by ENTITIES
that the sender allows. (Section 2.5.3)
* The protocol MUST allow any client to use the means to ensure
non-corruption, non-playback, and privacy, but the protocol
MUST NOT require that all clients use these means at all
times. (Section 2.5.4)
* When A establishes a SUBSCRIPTION to B's PRESENCE INFORMATION,
the protocol MUST provide A means of verifying the accurate
receipt of the content B chooses to disclose to A. (Section
5.1.4)
* The protocol MUST provide A means of verifying that the
presence information is accurate, as sent by B. (Section
5.3.1)
* The protocol MUST provide A means of ensuring that no other
PRINCIPAL C can see the content of M. (Section 5.4.6)
* The protocol MUST provide A means of ensuring that no other
PRINCIPAL C can tamper with M, and B means to verify that no
tampering has occurred. (Section 5.4.7)
2. The method defined MUST enable interoperability with non-XMPP
messaging systems that support the Common Presence and Instant
Messaging (CPIM) specifications defined by the Instant Messaging
and Presence (IMPP) Working Group. Therefore:
* Prior to encrypting or signing, the format of an instant
message must conform to the CPIM Message Format defined in
[MSGFMT].
* Prior to encrypting or signing, the format of presence
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information must conform to the CPP Presence Information Data
Format defined in [PIDF].
3. The method MUST follow the required procedures (including the
specific algorithms) defined in [CPIM] and [CPP]. In particular,
these documents specify:
* Encryption MUST use [SMIME] encryption with [CMS]
EnvelopeData.
* Signing MUST use [SMIME] signatures with [CMS] SignedData.
4. In order to enable interoperable implementations, sending and
receiving applications MUST implement the algorithms defined
under Section 6.7.
3. Securing Messages
In order to encrypt a message, a sending entity MUST use the
following procedure:
1. Generate a "Message/CPIM" object as defined in [MSGFMT].
2. Encrypt and/or sign both the headers and content of the "Message/
CPIM" object as specified in Requirement 3 of Section 2 above.
3. Provide the resulting multipart S/MIME object (see [MULTI])
within a CDATA section of an <e2e/> child of a <message/> stanza,
where the <e2e/> element is qualified by the
'urn:ietf:params:xml:ns:xmpp-e2e' namespace.
Example 1: Sender generates "Message/CPIM" object:
Content-type: Message/CPIM
From: Juliet Capulet <im:juliet@example.com>
To: Romeo Montague <im:romeo@example.net>
DateTime: 2003-05-14T11:45:36Z
Subject: Imploring
Content-type: text/plain; charset=utf-8
Content-ID: <1234567890@example.com>
Wherefore art thou, Romeo?
Example 2: Sender generates signed message (the 'from' address on the
XMPP message stanza is stamped by sender's server):
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<message to='romeo@example.net/orchard' type='chat'>
<e2e xmlns='urn:ietf:params:xml:ns:xmpp-e2e'>
<![CDATA[
Content-Type: multipart/signed; boundary=next;
micalg=sha1;
protocol=application/pkcs7-signature
--next
Content-type: Message/CPIM
From: Juliet Capulet <im:juliet@example.com>
To: Romeo Montague <im:romeo@example.net>
DateTime: 2003-05-14T23:45:36Z
Subject: Imploring
Content-type: text/plain; charset=utf-8
Content-ID: <1234567890@example.com>
Wherefore art thou, Romeo?
--next
Content-Type: application/pkcs7-signature
[signed body part]
--next--
]]>
</e2e>
</message>
4. Securing Presence
In order to encrypt presence information, a sending entity MUST use
the following procedure:
1. Generate an "application/pidf+xml" object as defined in [PIDF].
2. Encrypt and/or sign the "application/pidf+xml" object as
specified in Requirement 3 of Section 2 above.
3. Provide the resulting S/MIME object within a CDATA section of an
<e2e/> child of a <presence/> stanza, where the <e2e/> element is
qualified by the 'urn:ietf:params:xml:ns:xmpp-e2e' namespace.
The <presence/> stanza MUST include a 'to' attribute, i.e., it
must be an instance of directed presence as defined in [XMPP-IM].
Example 3: Sender generates "application/pidf+xml" object:
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<?xml version="1.0" encoding="UTF-8"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:im="urn:ietf:params:xml:ns:pidf:im"
entity="pres:juliet@example.com">
<tuple id="h40zny"
<status>
<basic>open</basic>
<im:im>away</im:im>
</status>
<note xml:lang="en">retired to the chamber</note>
<timestamp>2003-05-14T23:53:11Z</timestamp>
</tuple>
</presence>
Example 4: Sender generates signed presence (the 'from' address on
the XMPP presence stanza is stamped by sender's server):
<presence to='romeo@example.net/orchard'>
<e2e xmlns='urn:ietf:params:xml:ns:xmpp-e2e'>
<![CDATA[
Content-Type: multipart/signed; boundary=next;
micalg=sha1;
protocol=application/pkcs7-signature
--next
Content-type: application/pidf+xml
Content-ID: <2345678901@example.com>
<xml version="1.0" encoding="UTF-8"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:im="urn:ietf:params:xml:ns:pidf:im"
entity="pres:juliet@example.com">
<tuple id="hr0zny">
<status>
<basic>open</basic>
<im:im>away</im:im>
</status>
<note xml:lang="en">retired to the chamber</note>
<timestamp>2003-05-14T23:53:11Z</timestamp>
</tuple>
</presence>
--next
Content-Type: application/pkcs7-signature
[signed body part]
--next--
]]>
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</e2e>
</presence>
5. Securing Arbitrary XMPP Data
The foregoing sections of this memo describe how to secure "least
common denominator" messaging and presence data of the kind that can
be directly translated into the MSGFMT or PIDF formats. However,
XMPP possesses a third base-level stanza type (<iq/>) in addition to
<message/> and <presence/>, as well as the ability to include
extended XML data within arbitrary child elements of the three core
stanza types. Therefore it would be desirable to secure such data if
possible.
Because [MSGFMT] specifies the ability to encapsulate any MIME type,
the approach taken in this memo is to include arbitrary XMPP data in
a new MIME type, "application/xmpp+xml". The root element for this
MIME type is <xmpp/>, and the root element MUST contain one and only
one child element, corresponding to one of the XMPP stanza types
(i.e., message, presence, or iq) if the default namespace is
'jabber:client' or 'jabber:server' as defined in [XMPP-CORE].
The following examples illustrate the structure of the "application/
xmpp+xml" MIME type.
Example 5: Message stanza with extended data contained in
"application/xmpp+xml" MIME type:
<?xml version='1.0' encoding='UTF-8'?>
<xmpp xmlns='jabber:client'>
<message
from='iago@example.com/pda'
to='emilia@example.com/cell'>
<body>
I told him what I thought, and told no more
Than what he found himself was apt and true.
</body>
<evil xmlns='http://jabber.org/protocol/evil'/>
</message>
</xmpp>
Example 6: Presence stanza with extended data contained in
"application/xmpp+xml" MIME type:
<?xml version='1.0' encoding='UTF-8'?>
<xmpp xmlns='jabber:client'>
<presence from='iago@example.com/pda'>
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<show>dnd</show>
<status>Fomenting dissension</status>
<evil xmlns='http://jabber.org/protocol/evil'/>
</presence>
</xmpp>
Example 7: IQ stanza with extended data contained in "application/
xmpp+xml" MIME type:
<?xml version='1.0' encoding='UTF-8'?>
<xmpp xmlns='jabber:client'>
<iq type='result'
from='iago@example.com/pda'
to='emilia@example.com/cell'
id='evil1'>
<query xmlns='jabber:iq:version'>
<name>Stabber</name>
<version>666</version>
<os>FiendOS</os>
</query>
<evil xmlns='http://jabber.org/protocol/evil'/>
</iq>
</xmpp>
6. Rules for S/MIME Generation and Handling
6.1 Certificate Enrollment
S/MIME v3 does not specify how to obtain a certificate from a
certificate authority, but instead mandates that every sending agent
must already have a certificate. The PKIX Working Group has, at the
time of this writing, produced two separate standards for certificate
enrollment: [CMP] and [CMC]. Which method to use for certificate
enrollment is outside the scope of this memo.
6.2 Certificate Retrieval
A receiving agent MUST provide some certificate retrieval mechanism
in order to gain access to certificates for recipients of digital
envelopes. This memo does not cover how S/MIME agents handle
certificates, only what they do after a certificate has been
validated or rejected. S/MIME certification issues are covered in
[CERT].
At a minimum, for initial S/MIME deployment, a user agent could
automatically generate a message to an intended recipient requesting
that recipient's certificate in a signed return message. Receiving
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and sending agents SHOULD also provide a mechanism to allow a user to
"store and protect" certificates for correspondents in such a way so
as to guarantee their later retrieval.
6.3 Certificate Names
End-entity certificates used in the context of this memo SHOULD a
valid instant messaging address. The address SHOULD be one of the
following:
1. An XMPP address (also referred to as a Jabber Identifier or JID)
as defined in [XMPP-CORE]; the address should be of the form
<node@domain> (i.e., a "bare JID"), although any valid JID form
MAY be used. The JID SHOULD be contained in the subjectAltName
extension, and SHOULD NOT be in the subject distinguished name.
2. An "instant inbox address" as defined in [IMP-MODEL] and
[MSGFMT]. As explained in [XMPP-CPIM], an instant inbox address
maps to a "bare JID" (<node@domain>) once the 'im:' URI scheme
has been removed. The appropriate container for instant inbox
address shall be defined in [MSGFMT].
The value of the JID contained in the XMPP 'from' attribute SHOULD
match the JID provided in the signer's certificate, with the
exception that the resource identifier portion of the JID contained
in the 'from' attribute MAY be ignored for matching purposes.
Receiving agents MUST recognize XMPP addresses (JIDs) in the
subjectAltName field.
Receiving agents SHOULD check that sending JID matches a JID provided
in the signer's certificate, with the exception that the resource
identifier portion of the JID contained in the 'from' attribute MAY
be ignored for matching purposes. A receiving agent SHOULD provide
some explicit alternate processing of the message if this comparison
fails, which may be to display a message that shows the recipient the
addresses in the certificate or other certificate details.
The subject alternative name extension is used in S/MIME as the
preferred means to convey the JID that corresponds to the entity for
this certificate. Any JIDs present SHOULD be encoded using the
otherName CHOICE of the subjectAltName type, where the type-id is
"xmpp" and the value is the bare JID of the entity.
6.4 Transfer Encoding
According to various S/MIME specifications for message wrapping,
[CMS] objects MAY optionally be wrapped in MIME to dynamically
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support 7-bit transport. Because it is expected that XMPP will not
be used to interface with older 7-bit systems, this outer wrapping is
NOT REQUIRED for XMPP transport, and generally SHOULD NOT be applied
in a homogeneous XMPP environment or in an environment that supports
XMPP-CPIM gateways.
6.5 Attachment of Signatures
Sending agents SHOULD attach a signature to each encrypted message or
presence stanza, but are NOT REQUIRED to do so.
6.6 Inclusion of Certificates
Sending agents are NOT REQUIRED to include the sender's certificate
along with each encrypted message or presence stanza.
6.7 Mandatory to Implement Technologies
At a minimum, all implementations MUST support the following CMS
algorithms as defined in [CMS-ALG]:
for digest: DIGEST-MD5
for signing: RSA
for content encryption: Triple-DES CBC
7. Secure Communications Through a Gateway
A common method for achieving interoperability between two disparate
services is through the use of a "gateway" that interprets the
protocols of each service and translates them into the protocols of
the other. The CPIM specifications (specifically [MSGFMT] and [PIDF]
define the common profiles to be used for interoperability between
instant messaging and presence services that comply with [IMP-REQS].
In the case of communications between an XMPP service and a non-XMPP
service, we can visualize this relationship as follows:
+-------------+ +-------------+ +------------+
| | | | | |
| XMPP | | XMPP-CPIM | | Non-XMPP |
| Service | <----> | Gateway | <----> | Service |
| | | | | |
+-------------+ +-------------+ +------------+
The end-to-end encryption method defined herein enables the exchange
of encrypted and/or signed instant messages and presence through an
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XMPP-CPIM gateway. In particular:
o When a gateway receives a secured XMPP message or presence stanza
from the XMPP service that is addressed to a user on the non-XMPP
service, it MUST remove the XMPP "wrapper" (everything down to and
including the <e2e> and </e2e> tags) in order to reveal the
multipart S/MIME object, then route the object to the non-XMPP
service (first wrapping it in the protocol used by the non-XMPP
service if necessary).
o When a gateway receives a secured non-XMPP instant message or
presence document from the non-XMPP service that is addressed to a
user on the XMPP service, it MUST remove the non-XMPP "wrapper"
(if any) in order to reveal the multipart S/MIME object, wrap the
object in an XMPP message or presence "wrapper" (including the
<e2e> and </e2e> tags), and then route the XMPP stanza to the XMPP
service.
The wrapped S/MIME object MUST be immutable and MUST NOT be modified
by an XMPP-CPIM gateway.
8. Security Considerations
This entire memo discusses security. Detailed security
considerations for instant messaging and presence protocols are given
in [IMP-REQS] (Sections 5.1 through 5.4), and for XMPP in particular
are given in [XMPP-CORE] (Sections 12.1 through 12.6).
The end-to-end security method defined here MAY result in exchanging
secured instant messages and presence information through a gateway
that implements the CPIM specifications. Such a gateway MUST be
compliant with the minimum security requirements of the instant
messaging and presence protocols with which it interfaces.
9. IANA Considerations
9.1 Content-type Registration for "application/xmpp+xml"
To: ietf-types@iana.org
Subject: Registration of MIME media type application/xmpp+xml
MIME media type name: application
MIME subtype name: xmpp+xml
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Required parameters: (none)
Optional parameters: charset Indicates the character encoding of the
enclosed XML; the default encoding is UTF-8.
Encoding considerations: Contains XML, which can employ 8-bit
characters, depending on the character encoding used.
Security considerations: Contains a message, presence information, or
IQ (request-response) data in XMPP, which may be considered
private. Appropriate precautions should be adopted to limit
disclosure of this information.
Interoperability considerations: (none)
Specification: XXXX
Applications which use this media type: XMPP-compliant instant
messaging and presence systems.
Additional information: (none)
Person and email address to contact for further information: IETF,
XMPP Working Group, <xmppwg@jabber.org>
Intended usage: COMMON
Author/Change controller: IETF, XMPP Working Group
9.2 XML Namespace Name for e2e Data in XMPP
A URN sub-namespace for signed and encrypted content in the
Extensible Messaging and Presence Protocol (XMPP) is defined as
follows. (This namespace name adheres to the format defined in
[XML-REG].)
URI: urn:ietf:params:xml:ns:xmpp-e2e
Specification: XXXX
Description: This is the XML namespace name for signed and encrypted
content in the Extensible Messaging and Presence Protocol as
defined by XXXX.
Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>
Normative References
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[CERT] Ramsdell, B., "S/MIME Version 3 Certificate Handling", RFC
2632, June 1999.
[CMC] Blaze, M., Ioannidis, J. and A. Keromytis, "DSA and RSA
Key and Signature Encoding for the KeyNote Trust
Management System", RFC 2792, March 2000.
[CMP] Adams, C. and S. Farrell, "Internet X.509 Public Key
Infrastructure Certificate Management Protocols", RFC
2510, March 1999.
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
3369, August 2002.
[CMS-ALG] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
[CPIM] Crocker, D. and J. Peterson, "Common Profile for Instant
Messaging (CPIM)", draft-ietf-impp-im-03 (work in
progress), May 2003.
[CPP] Crocker, D. and J. Peterson, "Common Profile for Presence
(CPP)", draft-ietf-impp-pres-03 (work in progress), May
2003.
[IMP-MODEL]
Day, M., Rosenberg, J. and H. Sugano, "A Model for
Presence and Instant Messaging", RFC 2778, February 2000,
<http://www.ietf.org/rfc/rfc2778.txt>.
[IMP-REQS]
Day, M., Aggarwal, S. and J. Vincent, "Instant Messaging /
Presence Protocol Requirements", RFC 2779, February 2000.
[MSGFMT] Atkins, D. and G. Klyne, "Common Presence and Instant
Messaging: Message Format", draft-ietf-impp-cpim-msgfmt-08
(work in progress), January 2003.
[MULTI] Galvin, J., Murphy, S., Crocker, S. and N. Freed,
"Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC 1847, October 1995.
[PIDF] Fujimoto, S., Sugano, H., Klyne, G., Bateman, A., Carr, W.
and J. Peterson, "Presence Information Data Format",
draft-ietf-impp-cpim-pidf-08 (work in progress), May 2003.
[SMIME] Ramsdell, B., "S/MIME Version 3 Message Specification",
RFC 2633, June 1999.
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[TERMS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[XMPP-CORE]
Saint-Andre, P., "XMPP Core", draft-ietf-xmpp-core-20
(work in progress), November 2003.
[XMPP-IM] Saint-Andre, P., "XMPP Instant Messaging",
draft-ietf-xmpp-im-19 (work in progress), November 2003.
Informative References
[XML-REG] Mealling, M., "The IANA XML Registry",
draft-mealling-iana-xmlns-registry-05 (work in progress),
June 2003.
[XMPP-CPIM]
Saint-Andre, P., "Mapping the Extensible Messaging and
Presence Protocol (XMPP) to Common Presence and Instant
Messaging (CPIM)", draft-ietf-xmpp-cpim-03 (work in
progress), November 2003.
Author's Address
Peter Saint-Andre
Jabber Software Foundation
EMail: stpeter@jabber.org
Appendix A. Schema for urn:ietf:params:xml:ns:xmpp-e2e
The following XML schema is descriptive, not normative.
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:ietf:params:xml:ns:xmpp-e2e'
xmlns='urn:ietf:params:xml:ns:xmpp-e2e'
elementFormDefault='qualified'>
<xs:element name='e2e' type='xs:string'/>
</xs:schema>
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Appendix B. Revision History
Note to RFC Editor: please remove this entire appendix, and the
corresponding entries in the table of contents, prior to publication.
B.1 Changes from draft-ietf-xmpp-e2e-05
o Addressed I-D nits and RFC Editor formatting.
B.2 Changes from draft-ietf-xmpp-e2e-04
o Added text about instant inbox addresses.
B.3 Changes from draft-ietf-xmpp-e2e-03
o Specified that S/MIME multipart objects are enclosed in a CDATA
section.
o Changed "text/xml" to "text/plain" for message examples.
o Specified must-implement technologies, transfer encodings,
certificate enrollment, certificate retrieval, and certificate
names (including subjectAltName for JIDs).
o Specified requirements regarding attachment of signatures and
inclusion of certificates.
o Fixed some small terminological errors.
B.4 Changes from draft-ietf-xmpp-e2e-02
o Completely revised to use formats defined in the CPIM
specifications.
B.5 Changes from draft-ietf-xmpp-e2e-01
o Removed old Section 6 (Signalling Support via Presence) -- the
ability to sign broadcasted presence made it redundant.
o Made small editorial changes to address RFC Editor requirements.
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B.6 Changes from draft-ietf-xmpp-e2e-00
o Added support for all stanza types.
o Specified that the full stanza is encrypted.
o Added support for S/MIME in addition to OpenPGP.
o Specified that encrypted presence must be directed to a specific
recipient.
o Specified order of encrypting and signing.
o Added support for signing broadcasted presence.
o Added IANA considerations.
o Changed namespace to 'urn:ietf:params:xml:ns:xmpp-e2e'.
o Added XML schema.
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Internet-Draft XMPP E2E November 2003
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Saint-Andre Expires May 20, 2004 [Page 19]
