Network Working Group P. Saint-Andre

Network Working Group P. Saint-Andre
Request for Comments: 3923 Jabber Software Foundation
Category: Standards Track October 2004

End-to-End Signing and Object Encryption for the
Extensible Messaging and Presence Protocol (XMPP)

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2004).

Abstract

This memo defines methods of end-to-end signing and object encryption
for the Extensible Messaging and Presence Protocol (XMPP).

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Securing Messages . . . . . . . . . . . . . . . . . . . . . 4
4. Securing Presence . . . . . . . . . . . . . . . . . . . . . 9
5. Securing Arbitrary XMPP Data . . . . . . . . . . . . . . . . 13
6. Rules for S/MIME Generation and Handling . . . . . . . . . . 15
7. Recipient Error Handling . . . . . . . . . . . . . . . . . . 18
8. Secure Communications Through a Gateway . . . . . . . . . . 20
9. urn:ietf:params:xml:xmpp-e2e Namespace . . . . . . . . . . . 21
10. application/xmpp+xml Media Type . . . . . . . . . . . . . . 21
11. Security Considerations . . . . . . . . . . . . . . . . . . 22
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . 22
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
A. Schema for urn:ietf:params:xml:ns:xmpp-e2e . . . . . . . . . 26
Author's Address. . . . . . . . . . . . . . . . . . . . . . . . . 26
Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . 27

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RFC 3923 XMPP E2E October 2004

1. Introduction

This memo defines methods of end-to-end signing and object encryption
for the Extensible Messaging and Presence Protocol (XMPP). (For
information about XMPP, see [XMPP-CORE] and [XMPP-IM].) The method
specified herein enables a sender to sign and/or encrypt an instant
message sent to a specific recipient, sign and/or encrypt presence
information that is directed to a specific user, and sign and/or
encrypt any arbitrary XMPP stanza directed to a specific user. This
memo thereby helps the XMPP specifications meet the requirements
specified in [IMP-REQS].

1.1. Terminology

This document inherits terminology defined in [CMS], [IMP-MODEL],
[SMIME], 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 BCP
14, RFC 2119 [TERMS].

2. Requirements

For the purposes of this memo, we stipulate the following
requirements:

1. The method defined MUST address signing and encryption
requirements for minimal instant messaging and presence, as those
are defined in [IMP-REQS]. In particular, the method MUST
address the following requirements, which are copied here
verbatim from [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)

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RFC 3923 XMPP E2E October 2004

* 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 published by the Instant
Messaging and Presence (IMPP) Working Group. Two corollaries of
this requirement are:

* Prior to signing and/or encrypting, the format of an instant
message MUST conform to the CPIM Message Format defined in
[MSGFMT].

* Prior to signing and/or encrypting, the format of presence
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:

* Signing MUST use [SMIME] signatures with [CMS] SignedData.

* Encryption MUST use [SMIME] encryption with [CMS]
EnvelopeData.

4. In order to enable interoperable implementations, sending and
receiving applications MUST implement the algorithms specified
under Mandatory-to-Implement Cryptographic Algorithms (Section
6.10).

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RFC 3923 XMPP E2E October 2004

We further stipulate that the following functionality is out of scope
for this memo:

o Discovery of support for this protocol. An entity could discover
whether another entity supports this protocol by (1) attempting to
send signed or encrypted stanzas and receiving an error stanza
("technical" discovery) or a textual message in reply ("social"
discovery) if the protocol is not supported, or (2) using a
dedicated service discovery protocol, such as [DISCO] or [CAPS].
However, the definition of a service discovery protocol is out of
scope for this memo.

o Signing or encryption of XMPP groupchat messages, which are
mentioned in [XMPP-IM] but not defined therein since they are not
required by [IMP-REQS]; such messages are best specified in [MUC].

o Signing or encryption of broadcasted presence as described in
[XMPP-IM] (the methods defined herein apply to directed presence
only).

o Signing or encryption of communications that occur within the
context of applications other than instant messaging and presence
as those are described in [IMP-MODEL] and [IMP-REQS].

3. Securing Messages

3.1. Process for Securing Messages

In order to sign and/or encrypt a message, a sending agent MUST use
the following procedure:

1. Generate a "Message/CPIM" object as defined in [MSGFMT].

2. Sign and/or encrypt both the headers and content of the
"Message/CPIM" object as specified in Requirement 3 of Section 2
above.

3. Provide the resulting signed and/or encrypted object within an
XML CDATA section (see Section 2.7 of [XML]) contained in an
<e2e/> child of a <message/> stanza, where the <e2e/> element is
qualified by the 'urn:ietf:params:xml:ns:xmpp-e2e' namespace as
specified more fully in Section 9 below.

3.2. Example of a Signed Message

The following example illustrates the defined steps for signing a
message.

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RFC 3923 XMPP E2E October 2004

First, the sending agent generates a "Message/CPIM" object in
accordance with the rules and formats specified in [MSGFMT].

Example 1: Sender generates "Message/CPIM" object:

| Content-type: Message/CPIM
|
| From: Juliet Capulet <im:[email protected]>
| To: Romeo Montague <im:[email protected]>
| DateTime: 2003-12-09T11:45:36.66Z
| Subject: Imploring
|
| Content-type: text/plain; charset=utf-8
| Content-ID: <[email protected]>
|
| Wherefore art thou, Romeo?

Once the sending agent has generated the "Message/CPIM" object, the
sending agent may sign it. The result is a multipart [SMIME] object
(see [MULTI]) that has a Content-Type of "multipart/signed" and
includes two parts: one whose Content-Type is "Message/CPIM" and
another whose Content-Type is "application/pkcs7-signature".

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Example 2: Sender generates multipart/signed object:

| Content-Type: multipart/signed; boundary=next;
| micalg=sha1;
| protocol=application/pkcs7-signature
|
| --next
| Content-type: Message/CPIM
|
| From: Juliet Capulet <im:[email protected]>
| To: Romeo Montague <im:[email protected]>
| DateTime: 2003-12-09T23:45:36.66Z
| Subject: Imploring
|
| Content-type: text/plain; charset=utf-8
| Content-ID: <[email protected]>
|
| Wherefore art thou, Romeo?
| --next
| Content-Type: application/pkcs7-signature
| Content-Disposition: attachment;handling=required;\
| filename=smime.p7s
|
| [signed body part]
|
| --next--

The sending agent now wraps the "multipart/signed" object in an XML
CDATA section, which is contained in an <e2e/> element that is
included as a child element of the XMPP message stanza and that is
qualified by the 'urn:ietf:params:xml:ns:xmpp-e2e' namespace.

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Example 3: Sender generates XMPP message stanza:

| <message to='[email protected]/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:[email protected]>
| To: Romeo Montague <im:[email protected]>
| DateTime: 2003-12-09T23:45:36.66Z
| Subject: Imploring
|
| Content-type: text/plain; charset=utf-8
| Content-ID: <[email protected]>
|
| Wherefore art thou, Romeo?
| --next
| Content-Type: application/pkcs7-signature
| Content-Disposition: attachment;handling=required;\
| filename=smime.p7s
|
| [signed body part]
|
| --next--
| ]]>
| </e2e>
| </message>

3.3. Example of an Encrypted Message

The following example illustrates the defined steps for encrypting a
message.

First, the sending agent generates a "Message/CPIM" object in
accordance with the rules and formats specified in [MSGFMT].

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Example 4: Sender generates "Message/CPIM" object:

| Content-type: Message/CPIM
|
| From: Juliet Capulet <im:[email protected]>
| To: Romeo Montague <im:[email protected]>
| DateTime: 2003-12-09T11:45:36.66Z
| Subject: Imploring
|
| Content-type: text/plain; charset=utf-8
| Content-ID: <[email protected]>
|
| Wherefore art thou, Romeo?

Once the sending agent has generated the "Message/CPIM" object, the
sending agent may encrypt it.

Example 5: Sender generates encrypted object:

| U2FsdGVkX19okeKTlLxa/1n1FE/upwn1D20GhPWqhDWlexKMUKYJInTWzERP+vcQ
| /OxFs40uc9Fx81a5/62p/yPb/UWnuG6SR6o3Ed2zwcusDImyyz125HFERdDUMBC9
| Pt6Z4cTGKBmJzZBGyuc3Y+TMBTxqFFUAxeWaoxnZrrl+LP72vwbriYc3KCMxDbQL
| Igc1Vzs5/5JecegMieNY24SlNyX9HMFRNFpbI64vLxYEk55A+3IYbZsluCFT31+a
| +GeAvJkvH64LRV4mPbUhENTQ2wbAwnOTvbLIaQEQrii78xNEh+MK8Bx7TBTvi4yH
| Ddzf9Sim6mtWsXaCAvWSyp0X91d7xRJ4JIgKfPzkxNsWJFCLthQS1p734eDxXVd3
| i08lEHzyll6htuEr59ZDAw==

The sending agent now wraps the encrypted object in an XML CDATA
section, which is contained in an <e2e/> element that is included as
a child element of the XMPP message stanza and that is qualified by
the 'urn:ietf:params:xml:ns:xmpp-e2e' namespace.

Example 6: Sender generates XMPP message stanza:

| <message to='[email protected]/orchard' type='chat'>
| <e2e xmlns='urn:ietf:params:xml:ns:xmpp-e2e'>
| <![CDATA[
| U2FsdGVkX19okeKTlLxa/1n1FE/upwn1D20GhPWqhDWlexKMUKYJInTWzERP+vcQ
| /OxFs40uc9Fx81a5/62p/yPb/UWnuG6SR6o3Ed2zwcusDImyyz125HFERdDUMBC9
| Pt6Z4cTGKBmJzZBGyuc3Y+TMBTxqFFUAxeWaoxnZrrl+LP72vwbriYc3KCMxDbQL
| Igc1Vzs5/5JecegMieNY24SlNyX9HMFRNFpbI64vLxYEk55A+3IYbZsluCFT31+a
| +GeAvJkvH64LRV4mPbUhENTQ2wbAwnOTvbLIaQEQrii78xNEh+MK8Bx7TBTvi4yH
| Ddzf9Sim6mtWsXaCAvWSyp0X91d7xRJ4JIgKfPzkxNsWJFCLthQS1p734eDxXVd3
| i08lEHzyll6htuEr59ZDAw==
| ]]>
| </e2e>
| </message>

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RFC 3923 XMPP E2E October 2004

4. Securing Presence

4.1. Process for Securing Presence Information

In order to sign and/or encrypt presence information, a sending agent
MUST use the following procedure:

1. Generate an "application/pidf+xml" object as defined in [PIDF].
2. Sign and/or encrypt the "application/pidf+xml" object as
specified in Requirement 3 of Section 2 above.
3. Provide the resulting signed and/or encrypted object within an
XML CDATA section (see Section 2.7 of [XML]) contained in 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].

4.2. Example of Signed Presence Information

The following example illustrates the defined steps for signing
presence information.

First, the sending agent generates an "application/pidf+xml" object
in accordance with the rules and formats specified in [PIDF].

Example 7: Sender generates "application/pidf+xml" object:

| <?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:[email protected]">
| <tuple id="hr0zny"
| <status>
| <basic>open</basic>
| <im:im>away</im:im>
| </status>
| <note xml:lang="en">retired to the chamber</note>
| <timestamp>2003-12-09T23:53:11.31</timestamp>
| </tuple>
| </presence>

Once the sending agent has generated the "application/pidf+xml"
object, the sending agent may sign it. The result is a multipart
[SMIME] object (see [MULTI]) that has a Content-Type of
"multipart/signed" and includes two parts: one whose Content-Type is
"application/pidf+xml" and another whose Content-Type is
"application/pkcs7-signature".

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RFC 3923 XMPP E2E October 2004

Example 8: Sender generates multipart/signed object:

| Content-Type: multipart/signed; boundary=next;
| micalg=sha1;
| protocol=application/pkcs7-signature
|
| --next
| Content-type: application/pidf+xml
| Content-ID: <[email protected]>
|
| <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:[email protected]">
| <tuple id="hr0zny">
| <status>
| <basic>open</basic>
| <im:im>away</im:im>
| </status>
| <note xml:lang="en">retired to the chamber</note>
| <timestamp>2003-12-09T23:53:11.31Z</timestamp>
| </tuple>
| </presence>
| --next
| Content-Type: application/pkcs7-signature
| Content-Disposition: attachment;handling=required;\
| filename=smime.p7s
|
| [signed body part]
|
| --next--

The sending agent now wraps the "multipart/signed" object in an XML
CDATA section, which is contained in an <e2e/> element that is
included as a child element of the XMPP message stanza and that is
qualified by the 'urn:ietf:params:xml:ns:xmpp-e2e' namespace.

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Example 9: Sender generates XMPP presence stanza:

| <presence to='[email protected]/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: <[email protected]>
|
| <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:[email protected]">
| <tuple id="hr0zny">
| <status>
| <basic>open</basic>
| <im:im>away</im:im>
| </status>
| <note xml:lang="en">retired to the chamber</note>
| <timestamp>2003-12-09T23:53:11.31Z</timestamp>
| </tuple>
| </presence>
| --next
| Content-Type: application/pkcs7-signature
| Content-Disposition: attachment;handling=required;\
| filename=smime.p7s
|
| [signed body part]
|
| --next--
| ]]>
| </e2e>
| </presence>

4.3. Example of Encrypted Presence Information

The following example illustrates the defined steps for encrypting
presence information.

First, the sending agent generates an "application/pidf+xml" object
in accordance with the rules and formats specified in [PIDF].

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Example 10: Sender generates "application/pidf+xml" object:

| <?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:[email protected]">
| <tuple id="hr0zny"
| <status>
| <basic>open</basic>
| <im:im>away</im:im>
| </status>
| <note xml:lang="en">retired to the chamber</note>
| <timestamp>2003-12-09T23:53:11.31</timestamp>
| </tuple>
| </presence>

Once the sending agent has generated the "application/pidf+xml"
object, the sending agent may encrypt it.

Example 11: Sender generates encrypted object:

| U2FsdGVkX18VJPbx5GMdFPTPZrHLC9QGiVP+ziczu6zWZLFQxae6O5PP6iqpr2No
| zOvBVMWvYeRAT0zd18hr6qsqKiGl/GZpAAbTvPtaBxeIykxsd1+CX+U+iw0nEGCr
| bjiQrk0qUKJ79bNxwRnqdidjhyTpKSbOJC0XZ8CTe7AE9KDM3Q+uk+O3jrqX4byL
| GBlKThbzKidxz32ObojPEEwfFiM/yUeqYUP1OcJpUmeQ8lcXhD6tcx+m2MAyYYLP
| boKQxpLknxRnbM8T/voedlnFLbbDu69mOlxDPbr1mHZd3hDsyFudb1fb4rI3Kw0K
| Nq+3udr2IkysviJDgQo+xGIQUG/5sED/mAaPRlj4f/JtTzvT4EaQTawv69ntXfKV
| MCr9KdIMMdjdJzOJkYLoAhNVrcZn5tw8WsJGwuKuhYb/SShy7InzOapPaPAl7/Mm
| PHj7zj3NZ6EEIweDOuAwWlIG/dT506tci27+EW7JnXwMPnFMkF+6a7tr/0Y+iiej
| woJxUIBqCOgX+U7srHpK2NYtNTZ7UQp2V0yEx1JV8+Y=

The sending agent now wraps the encrypted object in an XML CDATA
section, which is contained in an <e2e/> element that is included as
a child element of the XMPP message stanza and that is qualified by
the 'urn:ietf:params:xml:ns:xmpp-e2e' namespace.

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Example 12: Sender generates XMPP presence stanza:

| <presence to='[email protected]/orchard'>
| <e2e xmlns='urn:ietf:params:xml:ns:xmpp-e2e'>
| <![CDATA[
| U2FsdGVkX18VJPbx5GMdFPTPZrHLC9QGiVP+ziczu6zWZLFQxae6O5PP6iqpr2No
| zOvBVMWvYeRAT0zd18hr6qsqKiGl/GZpAAbTvPtaBxeIykxsd1+CX+U+iw0nEGCr
| bjiQrk0qUKJ79bNxwRnqdidjhyTpKSbOJC0XZ8CTe7AE9KDM3Q+uk+O3jrqX4byL
| GBlKThbzKidxz32ObojPEEwfFiM/yUeqYUP1OcJpUmeQ8lcXhD6tcx+m2MAyYYLP
| boKQxpLknxRnbM8T/voedlnFLbbDu69mOlxDPbr1mHZd3hDsyFudb1fb4rI3Kw0K
| Nq+3udr2IkysviJDgQo+xGIQUG/5sED/mAaPRlj4f/JtTzvT4EaQTawv69ntXfKV
| MCr9KdIMMdjdJzOJkYLoAhNVrcZn5tw8WsJGwuKuhYb/SShy7InzOapPaPAl7/Mm
| PHj7zj3NZ6EEIweDOuAwWlIG/dT506tci27+EW7JnXwMPnFMkF+6a7tr/0Y+iiej
| woJxUIBqCOgX+U7srHpK2NYtNTZ7UQp2V0yEx1JV8+Y=
| ]]>
| </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
an XML media type named "application/xmpp+xml" as specified more
fully in Section 10 below.

The following examples illustrate the structure of the
"application/xmpp+xml" MIME type. (Note: The
'http://jabber.org/protocol/evil' namespace used in these examples is
associated with an April Fool's protocol written to be the instant
messaging equivalent of RFC 3514; it is included only as an instance
of extended information included in an XML stanza and should not be
taken seriously as a functional XMPP extension.)

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Example 13: 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='[email protected]/pda'
| to='[email protected]/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 14: 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='[email protected]/pda'>
| <show>dnd</show>
| <status>Fomenting dissension</status>
| <evil xmlns='http://jabber.org/protocol/evil'/>
| </presence>
| </xmpp>

Example 15: 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='[email protected]/pda'
| to='[email protected]/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>

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Just as with the "Message/CPIM" and "application/pidf+xml" objects,
the "application/xmpp+xml" object would be signed and/or encrypted,
then encapsulated within an XML CDATA section (see Section 2.7 of
[XML]) contained in an <e2e/> child of a <presence/> stanza, where
the <e2e/> element is qualified by the
'urn:ietf:params:xml:ns:xmpp-e2e' namespace.

6. Rules for S/MIME Generation and Handling

6.1. Certificate Enrollment

[SMIME] 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 address 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].

However, at a minimum, for initial S/MIME deployment, a user agent
SHOULD automatically generate a message to an intended recipient
requesting that recipient's certificate in a signed return message.
Receiving 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 by XMPP entities in the context of this
memo SHOULD contain a valid instant messaging and presence address.
The address SHOULD be specified as both an 'im:' URI (for instant
messaging, as defined in [CPIM]) and a 'pres:' URI (for presence, as
defined in [CPP]); each of these URIs SHOULD be specified in a
separate GeneralName entry of type uniformResourceIdentifier inside
the subjectAltName (i.e., two separate entries). Information in the
subject distinguished name SHOULD be ignored.

Each URI MUST be of the form <im:address> or <pres:address>, where
the "address" portion is an XMPP address (also referred to as a
Jabber Identifier or JID) as defined in [XMPP-CORE], prepended with

Saint-Andre Standards Track [Page 15]

RFC 3923 XMPP E2E October 2004

the 'im:' or 'pres:' URI scheme. The address SHOULD be of the form
<node@domain> (i.e., a "bare JID"), although any valid JID form MAY
be used.

The value of the JID contained in the XMPP 'from' attribute MUST
match a JID provided in the signer's certificate, with the exception
that the resource identifier portion of the JID contained in the
'from' attribute SHOULD be ignored for matching purposes.

Receiving agents MUST check that the 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 SHOULD be ignored for matching purposes. A receiving agent
SHOULD provide some explicit alternate processing of the stanza if
this comparison fails, which may be to display a message informing
the recipient of 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 instant messaging and presence address
that corresponds to the entity for this certificate. Any XMPP
address present in the certificate MUST be encoded using the ASN.1
Object Identifier "id-on-xmppAddr" as specified in Section 5.1.1 of
[XMPP-CORE].

6.4. Transfer Encoding

Because it is expected that XMPP applications will not interface with
older 7-bit systems, the transfer encoding (as defined in Section
3.1.2 of [SMIME]) MUST be "binary".

6.5. Order of Signing and Encrypting

If a stanza is both signed and encrypted, it SHOULD be signed first,
then encrypted.

6.6. Inclusion of Certificates

If the sender and recipient are involved in an active messaging
session over a period of time, the sending agent SHOULD include the
sender's certificate along with at least one encrypted message stanza
every five minutes. Outside the context of an active messaging
session, the sending agent SHOULD include the sender's certificate
along with each encrypted message stanza. A sending agent MAY
include the sender's certificate along with each encrypted presence
stanza. However, a sending agent SHOULD NOT include a certificate
more than once every five minutes.

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RFC 3923 XMPP E2E October 2004

6.7. Attachment and Checking of Signatures

Sending agents SHOULD attach a signature to each encrypted XML
stanza. If a signature is attached, a Content-Disposition header
field (as defined in [DISP]) SHOULD be included to specify how the
signature is to be handled by the receiving application.

If the receiving agent determines that the signature attached to an
encrypted XML stanza is invalid, it SHOULD NOT present the stanza to
the intended recipient (human or application), SHOULD provide some
explicit alternate processing of the stanza (which may be to display
a message informing the recipient that the attached signature is
invalid), and MAY return a stanza error to the sender as described
under Recipient Error Handling (Section 7).

6.8. Decryption

If the receiving agent is unable to decrypt the encrypted XML stanza,
it SHOULD NOT present the stanza to the intended recipient (human or
application), SHOULD provide some explicit alternate processing of
the stanza (which may be to display a message informing the recipient
that it has received a stanza that cannot be decrypted), and MAY
return a stanza error to the sender as described under Recipient
Error Handling (Section 7).

6.9. Inclusion and Checking of Timestamps

Timestamps are included in "Message/CPIM" and "application/pidf+xml"
objects to help prevent replay attacks. All timestamps MUST conform
to [DATETIME] and be presented as UTC with no offset, including
fractions of a second as appropriate. Absent a local adjustment to
the sending agent's perceived time or the underlying clock time, the
sending agent MUST ensure that the timestamps it sends to the
receiver increase monotonically (if necessary by incrementing the
seconds fraction in the timestamp if the clock returns the same time
for multiple requests). The following rules apply to the receiving
application:

o It MUST verify that the timestamp received is within five minutes
of the current time.

o It SHOULD verify that the timestamp received is greater than any
timestamp received in the last 10 minutes which passed the
previous check.

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RFC 3923 XMPP E2E October 2004

o If any of the foregoing checks fails, the timestamp SHOULD be
presented to the receiving entity (human or application) marked as
"old timestamp", "future timestamp", or "decreasing timestamp",
and the receiving entity MAY return a stanza error to the sender
as described under Recipient Error Handling (Section 7).

6.10. Mandatory-to-Implement Cryptographic Algorithms

All implementations MUST support the following algorithms.
Implementations MAY support other algorithms as well.

For CMS SignedData:

o The SHA-1 message digest as specified in [CMS-ALG] section 2.1.

o The RSA (PKCS #1 v1.5) with SHA-1 signature algorithm, as
specified in [CMS-ALG] section 3.2.

For CMS EnvelopedData:

o The RSA (PKCS #1 v1.5) key transport, as specified in [CMS-ALG]
section 4.2.1.

o The AES-128 encryption algorithm in CBC mode, as specified in
[CMS-AES].

7. Recipient Error Handling

When an XMPP entity receives an XML stanza containing data that is
signed and/or encrypted using the protocol described herein, several
scenarios are possible:

Case #1: The receiving application does not understand the protocol.

Case #2: The receiving application understands the protocol and is
able to decrypt the payload and verify the sender's signature.

Case #3: The receiving application understands the protocol and is
able to decrypt the payload and verify the sender's signature, but
the timestamps fail the checks specified above under Checking of
Timestamps (Section 6.9).

Case #4: The receiving application understands the protocol and is
able to decrypt the payload but is unable to verify the sender's
signature.

Case #5: The receiving application understands the protocol but is
unable to decrypt the payload.

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RFC 3923 XMPP E2E October 2004

In Case #1, the receiving application MUST do one and only one of the
following: (1) ignore the <e2e/> extension, (2) ignore the entire
stanza, or (3) return a <service-unavailable/> error to the sender,
as described in [XMPP-CORE].

In Case #2, the receiving application MUST NOT return a stanza error
to the sender, since this is the success case.

In Case #3, the receiving application MAY return a <not-acceptable/>
error to the sender (as described in [XMPP-CORE]), optionally
supplemented by an application-specific error condition element
<bad-timestamp/> as shown below:

Example 16: Recipient returns <not-acceptable/> error:

<message from='[email protected]/orchard' type='chat'>
<e2e xmlns='urn:ietf:params:xml:ns:xmpp-e2e'>
[CDATA section here]
</e2e>
<error type='modify'>
<not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<bad-timestamp xmlns='urn:ietf:params:xml:xmpp-e2e'/>
</error>
</message>

In Case #4, the receiving application SHOULD return a
<not-acceptable/> error to the sender (as described in [XMPP-CORE]),
optionally supplemented by an application-specific error condition
element <unverified-signature/> as shown below:

Example 17: Recipient returns <not-acceptable/> error:

<message from='[email protected]/orchard' type='chat'>
<e2e xmlns='urn:ietf:params:xml:ns:xmpp-e2e'>
[CDATA section here]
</e2e>
<error type='modify'>
<not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<unverified-signature xmlns='urn:ietf:params:xml:xmpp-e2e'/>
</error>
</message>

In Case #5, the receiving application SHOULD return a <bad-request/>
error to the sender (as described in [XMPP-CORE]), optionally
supplemented by an application-specific error condition element
<decryption-failed/> as shown below:

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RFC 3923 XMPP E2E October 2004

Example 18: Recipient returns <bad-request/> error:

<message from='[email protected]/orchard' type='chat'>
<e2e xmlns='urn:ietf:params:xml:ns:xmpp-e2e'>
[CDATA section here]
</e2e>
<error type='modify'>
<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<decryption-failed xmlns='urn:ietf:params:xml:xmpp-e2e'/>
</error>
</message>

8. 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
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).

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RFC 3923 XMPP E2E October 2004

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.

9. urn:ietf:params:xml:xmpp-e2e Namespace

The <e2e xmlns='urn:ietf:params:xml:ns:xmpp-e2e'/> element is a
wrapper for an XML CDATA section (see Section 2.7 of [XML]) that
contains a "Message/CPIM", "application/pidf+xml", or
"application/xmpp+xml" object. Thus the
'urn:ietf:params:xml:xmpp-e2e' namespace has no inherent semantics,
and the semantics of the encapsulated object are defined by one of
the following specifications:

o [MSGFMT] for "Message/CPIM"
o [PIDF] for "application/pidf+xml"
o [XMPP-CORE] for "application/xmpp+xml"

Although the "application/xmpp+xml" media type is specified in this
document, the <xmpp/> element is simply a wrapper for a <message/>,
<presence/>, or <iq/> stanza, where the semantics of those stanza
types are specified in [XMPP-CORE].

Given that the 'urn:ietf:params:xml:ns:xmpp-e2e' namespace has no
inherent semantics and specifies a using protocol only, versioning is
the responsibility of the protocols that define the encapsulated
objects ([MSGFMT], [PIDF], and [XMPP-CORE]).

10. application/xmpp+xml Media Type

The "application/xmpp+xml" media type adheres to the guidelines
specified in [XML-MEDIA]. 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 character encoding
for this XML media type MUST be UTF-8, in accordance with Section
11.5 of [XMPP-CORE].

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RFC 3923 XMPP E2E October 2004

11. 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). In addition,
all of the security considerations specified in [XML-MEDIA] apply to
the "application/xmpp+xml" media type.

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.

12. IANA Considerations

12.1. XML Namespace Name for e2e Data in XMPP

A URN sub-namespace of signed and encrypted content for 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: RFC 3923
Description: This is an XML namespace name of signed and encrypted
content for the Extensible Messaging and Presence Protocol as
defined by RFC 3923.
Registrant Contact: IESG, <[email protected]>

12.2. Content-type Registration for "application/xmpp+xml"

To: [email protected]

Subject: Registration of MIME media type application/xmpp+xml

MIME media type name: application
MIME subtype name: xmpp+xml
Required parameters: (none)
Optional parameters: (charset) Same as charset parameter of
application/xml as specified in RFC 3023; per Section 11.5 of
[XMPP-CORE], the charset must be UTF-8.
Encoding considerations: Same as encoding considerations of
application/xml as specified in RFC 3023; per Section 11.5 of
[XMPP-CORE], the encoding must be UTF-8.

Saint-Andre Standards Track [Page 22]

RFC 3923 XMPP E2E October 2004

Security considerations: All of the security considerations specified
in RFC 3023 and [XMPP-CORE] apply to this XML media type. Refer
to Section 11 of RFC 3923.
Interoperability considerations: (none)
Specification: RFC 3923
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: IESG,
<[email protected]>
Intended usage: COMMON
Author/Change controller: IETF, XMPP Working Group

13. References

13.1. Normative References

[CERT] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Certificate Handling",
RFC 3850, July 2004.

[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
3852, July 2004.

[CMS-AES] Schaad, J., "Use of the Advanced Encryption Standard
(AES) Encryption Algorithm in Cryptographic Message
Syntax (CMS)", RFC 3565, July 2003.

[CMS-ALG] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.

[CPIM] Peterson, J., "Common Profile for Instant Messaging
(CPIM)", RFC 3860, August 2004.

[CPP] Peterson, J., "Common Profile for Presence (CPP)", RFC
3859, August 2004.

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

[DISP] Troost, R., Dorner, S., and K. Moore, Ed.,
"Communicating Presentation Information in Internet
Messages: The Content-Disposition Header Field", RFC
2183, August 1997.

[IMP-MODEL] Day, M., Rosenberg, J., and H. Sugano, "A Model for
Presence and Instant Messaging", RFC 2778, February
2000.

Saint-Andre Standards Track [Page 23]

RFC 3923 XMPP E2E October 2004

[IMP-REQS] Day, M., Aggarwal, S., Mohr, G., and J. Vincent,
"Instant Messaging/Presence Protocol Requirements", RFC
2779, February 2000.

[MSGFMT] Klyne, G. and D. Atkins, "Common Presence and Instant
Messaging (CPIM): Message Format", RFC 3862, August
2004.

[MULTI] Galvin, J., Murphy, S., Crocker, S., and N. Freed,
"Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC 1847, October 1995.

[PIDF] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr,
W., and J. Peterson, "Presence Information Data Format
(PIDF)", RFC 3863, August 2004.

[SMIME] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.

[TERMS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[XML-MEDIA] Murata, M., St. Laurent, S. and D. Kohn, "XML Media
Types", RFC 3023, January 2001.

[XMPP-CORE] Saint-Andre, P., Ed., "Extensible Messaging and
Presence Protocol (XMPP): Core", RFC 3920, October
2004.

[XMPP-IM] Saint-Andre, P., Ed., "Extensible Messaging and
Presence Protocol (XMPP) Instant Messaging and
Presence", RFC 3921, October 2004.

Saint-Andre Standards Track [Page 24]

RFC 3923 XMPP E2E October 2004

13.2. Informative References

[CAPS] Hildebrand, J. and P. Saint-Andre, "Entity
Capabilities", JSF JEP 0115, August 2004.

[CMC] Myers, M., Liu, X., Schaad, J. and J. Weinstein,
"Certificate Management Messages over CMS", RFC 2797,
April 2000.

[CMP] Adams, C. and S. Farrell, "Internet X.509 Public Key
Infrastructure Certificate Management Protocols", RFC
2510, March 1999.

[DISCO] Hildebrand, J., Millard, P., Eatmon, R. and P. Saint-
Andre, "Service Discovery", JSF JEP 0030, July 2004.

[MUC] Saint-Andre, P., "Multi-User Chat", JSF JEP 0045, June
2004.

[XML] Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler,
"Extensible Markup Language (XML) 1.0 (3rd ed)", W3C
REC-xml, February 2004, <http://www.w3.org/TR/REC-xml>.

[XML-REG] Mealling, M., "The IETF XML Registry", BCP 81, RFC
3688, January 2004.

Saint-Andre Standards Track [Page 25]

RFC 3923 XMPP E2E October 2004

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:element name='decryption-failed' type='empty'/>
<xs:element name='signature-unverified' type='empty'/>
<xs:element name='bad-timestamp' type='empty'/>

<xs:simpleType name='empty'>
<xs:restriction base='xs:string'>
<xs:enumeration value=''/>
</xs:restriction>
</xs:simpleType>

</xs:schema>

Author's Address

Peter Saint-Andre
Jabber Software Foundation

EMail: [email protected]

Saint-Andre Standards Track [Page 26]

RFC 3923 XMPP E2E October 2004

Full Copyright Statement

Copyright (C) The Internet Society (2004).

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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Acknowledgement

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Internet Society.

Saint-Andre Standards Track [Page 27]