End-to-End Signing and Object Encryption for the Extensible Messaging and Presence Protocol (XMPP)
RFC 3923

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:juliet@example.com>
 | To: Romeo Montague <im:romeo@example.net>
 | DateTime: 2003年12月09日T11:45:36.66Z
 | Subject: Imploring
 |
 | Content-type: text/plain; charset=utf-8
 | Content-ID: <1234567890@example.com>
 |
 | 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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RFC 3923 XMPP E2E October 2004
 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:juliet@example.com>
 | To: Romeo Montague <im:romeo@example.net>
 | DateTime: 2003年12月09日T23:45:36.66Z
 | Subject: Imploring
 |
 | Content-type: text/plain; charset=utf-8
 | Content-ID: <1234567890@example.com>
 |
 | 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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RFC 3923 XMPP E2E October 2004
 Example 3: Sender generates XMPP message stanza:
 | <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年12月09日T23:45:36.66Z
 | Subject: Imploring
 |
 | Content-type: text/plain; charset=utf-8
 | Content-ID: <1234567890@example.com>
 |
 | 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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RFC 3923 XMPP E2E October 2004
 Example 4: 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年12月09日T11:45:36.66Z
 | Subject: Imploring
 |
 | Content-type: text/plain; charset=utf-8
 | Content-ID: <1234567890@example.com>
 |
 | 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='romeo@example.net/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: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年12月09日T23:53:11.31</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: <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&gt;
 | <basic>open</basic>
 | <im:im>away</im:im>
 | </status>
 | <note xml:lang="en">retired to the chamber</note>
 | <timestamp>2003年12月09日T23:53:11.31Z</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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RFC 3923 XMPP E2E October 2004
 Example 9: Sender generates XMPP presence stanza:
 | <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年12月09日T23:53:11.31Z</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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RFC 3923 XMPP E2E October 2004
 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: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年12月09日T23:53:11.31</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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RFC 3923 XMPP E2E October 2004
 Example 12: Sender generates XMPP presence stanza:
 | <presence to='romeo@example.net/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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RFC 3923 XMPP E2E October 2004
 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='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 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='iago@example.com/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='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>
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RFC 3923 XMPP E2E October 2004
 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='romeo@example.net/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='romeo@example.net/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='romeo@example.net/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).
Saint-Andre Standards Track [Page 20]
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].
Saint-Andre Standards Track [Page 21]
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, <iesg@ietf.org>
12.2. 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
 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,
 <iesg@ietf.org>
 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: stpeter@jabber.org
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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 REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
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Acknowledgement
 Funding for the RFC Editor function is currently provided by the
 Internet Society.
Saint-Andre Standards Track [Page 27]