XMPP Working Group P. Saint-Andre
Internet-Draft Jabber Software Foundation
Expires: January 10, 2005 July 12, 2004
End-to-End Signing and Object Encryption in the Extensible Messaging
and Presence Protocol (XMPP)
draft-ietf-xmpp-e2e-09
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This memo defines a method for end-to-end signing and object
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 . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . 20
10. application/xmpp+xml Media Type . . . . . . . . . . . . . . 21
11. Security Considerations . . . . . . . . . . . . . . . . . . 21
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . 21
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
13.1 Normative References . . . . . . . . . . . . . . . . . . . 22
13.2 Informative References . . . . . . . . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . 25
A. Schema for urn:ietf:params:xml:ns:xmpp-e2e . . . . . . . . . 25
B. Revision History . . . . . . . . . . . . . . . . . . . . . . 25
Intellectual Property and Copyright Statements . . . . . . . 28
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1. Introduction
This memo define a method for end-to-end signing and object
encryption in 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
[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)
* 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
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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).
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
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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
child of a stanza, where the 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.
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
| To: Romeo Montague
| DateTime: 2003-12-09T11:45:36.66Z
| 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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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
| To: Romeo Montague
| DateTime: 2003-12-09T23:45:36.66Z
| 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 "mulipart/signed" object in an XML
CDATA section, which is contained in an 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:
|
|
|
| To: Romeo Montague
| DateTime: 2003-12-09T23:45:36.66Z
| 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--
| ]]>
|
|
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
| To: Romeo Montague
| DateTime: 2003-12-09T11:45:36.66Z
| 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 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:
|
|
|
|
|
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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
child of a stanza, where the element is
qualified by the 'urn:ietf:params:xml:ns:xmpp-e2e' namespace.
The 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:
|
|
|
| open
| away
|
| retired to the chamber
| 2003-12-09T23:53:11.31
|
|
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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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>
|
|
|
|
| open
| away
|
| retired to the chamber
| 2003-12-09T23:53:11.31Z
|
|
| --next
| Content-Type: application/pkcs7-signature
| Content-Disposition: attachment;handling=required;\
| filename=smime.p7s
|
| [signed body part]
|
| --next--
The sending agent now wraps the "mulipart/signed" object in an XML
CDATA section, which is contained in an 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:
|
|
|
|
|
|
|
|
| open
| away
|
| retired to the chamber
| 2003-12-09T23:53:11.31Z
|
|
| --next
| Content-Type: application/pkcs7-signature
| Content-Disposition: attachment;handling=required;\
| filename=smime.p7s
|
| [signed body part]
|
| --next--
| ]]>
|
|
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:
|
|
|
| open
| away
|
| retired to the chamber
| 2003-12-09T23:53:11.31
|
|
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 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:
|
|
|
|
|
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 () in addition to
and , 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:
|
|
|
|
| I told him what I thought, and told no more
| Than what he found himself was apt and true.
|
|
|
|
Example 14: Presence stanza with extended data contained in
"application/xmpp+xml" MIME type:
|
|
|
| dnd
| Fomenting dissension
|
|
|
Example 15: IQ stanza with extended data contained in "application/
xmpp+xml" MIME type:
|
|
|
|
| Stabber
| 666
| FiendOS
|
|
|
|
Just as with the "Message/CPIM" and "application/pidf+xml" objects,
the "application/xmpp+xml" object would be signed and/or encrypted,
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then encapsulated within an XML CDATA section (see Section 2.7 of
[XML]) contained in an child of a stanza, where
the 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 or , where
the "address" portion is an XMPP address (also referred to as a
Jabber Identifier or JID) as defined in [XMPP-CORE], prepended with
the 'im:' or 'pres:' URI scheme. The address SHOULD be of the form
(i.e., a "bare JID"), although any valid JID form MAY
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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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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.
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).
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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.
In Case #1, the receiving application MUST do one and only one of the
following: (1) ignore the extension, (2) ignore the entire
stanza, or (3) return a 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
error to the sender (as described in [XMPP-CORE]), optionally
supplemented by an application-specific error condition element
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as shown below:
Example 16: Recipient returns error:
[CDATA section here]
In Case #4, the receiving application SHOULD return a
error to the sender (as described in [XMPP-CORE]),
optionally supplemented by an application-specific error condition
element as shown below:
Example 17: Recipient returns error:
[CDATA section here]
In Case #5, the receiving application SHOULD return a
error to the sender (as described in [XMPP-CORE]), optionally
supplemented by an application-specific error condition element
as shown below:
Example 18: Recipient returns error:
[CDATA section here]
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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 and 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
and 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 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:
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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 element is simply a wrapper for a ,
, or 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
, 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].
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 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].)
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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: IESG,
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
[draft-ietf-xmpp-core-24], 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
[draft-ietf-xmpp-core-24], the encoding must be UTF-8.
Security considerations: All of the security considerations specified
in RFC 3023 and [draft-ietf-xmpp-core-24] apply to this XML media
type. Refer to Section 11 of XXXX.
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: IESG,
Intended usage: COMMON
Author/Change controller: IETF, XMPP Working Group
13. References
13.1 Normative References
[CERT] Ramsdell, B., "S/MIME Version 3.1 Certificate Handling",
draft-ietf-smime-rfc2632bis-07 (work in progress), June
2004.
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)",
draft-ietf-smime-rfc3369bis-04 (work in progress), May
2004.
[CMS-AES] Schaad, J., "Use of the Advanced Encryption Standard (AES)
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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)", draft-ietf-impp-im-04 (work in progress),
February 2004.
[CPP] Peterson, J., "Common Profile for Presence (CPP)",
draft-ietf-impp-pres-04 (work in progress), February 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, "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,
.
[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 3.1 Message Specification",
draft-ietf-smime-rfc2633bis-09 (work in progress), April
2004.
[TERMS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[X509] Housley, R., Polk, W., Ford, W. and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[XML-MEDIA]
Murata, M., St. Laurent, S. and D. Kohn, "XML Media
Types", RFC 3023, January 2001.
[XMPP-CORE]
Saint-Andre, P., "XMPP Core", draft-ietf-xmpp-core-24
(work in progress), May 2004.
[XMPP-IM] Saint-Andre, P., "XMPP Instant Messaging",
draft-ietf-xmpp-im-22 (work in progress), May 2004.
13.2 Informative References
[CAPS] Hildebrand, J. and P. Saint-Andre, "JEP-0115: Entity
Capabilities", April 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, "JEP-0030: Service Discovery", June 2004,
.
[MUC] Saint-Andre, P., "JEP-0045: Multi-User Chat", 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, .
[XML-REG] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.
[XMPP-CPIM]
Saint-Andre, P., "Mapping the Extensible Messaging and
Presence Protocol (XMPP) to Common Presence and Instant
Messaging (CPIM)", draft-ietf-xmpp-cpim-05 (work in
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progress), May 2004.
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.
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-08
o Changed title to mention signing in addition to encryption.
o Specified several functionality areas that are out of scope for
this memo.
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o Added clarifying text to the examples, split several examples in
order to correspond to the defined process steps, and added
examples for encrypting both messages and presence information.
o Changed SHOULD to MUST regarding checking of XMPP addresses.
o Changed "could" to SHOULD regarding sending of initial message
requesting certificate retrieval.
o Specified that 'from' attributes MUST (rather than SHOULD) match a
JID contained in the sender's certificate, and that resource
identifiers SHOULD be ignored for matching purposes.
o More fully defined S/MIME handling, including correct processing
of XML stanzas with invalid signatures, stanzas that cannot be
decrypted, and messages that appear to be replayed.
o Defined recipient generation of XMPP error stanzas.
o Incorporated text for "Mandatory-to-Implement Cryptographic
Algorithms" suggested by Russ Housley.
o Referred to Section 5.1.1 of XMPP-CORE regarding ASN.1 Object
Identifier to be used when an XMPP address is represented in a
certificate.
o Added reference to RFC 3565.
o Fixed CMC reference to point to RFC 2797.
o Updated references to point to rfc2632bis, rfc2633bis, and
rfc3369bis.
o Updated boilerplate to refer to RFC 3668.
o Removed Discussion Venue subsection.
B.2 Changes from draft-ietf-xmpp-e2e-07
o Clarified relationship of the 'urn:ietf:params:xml:ns:xmpp-e2e'
namespace to its encapsulated objects, including versioning, and
placed this content in its own section.
o Clarified nature of "application/xmpp+xml" media type and placed
this content in its own section.
o Added reference to RFC 3023 and modified XML media type
registration to adhere to the guidelines specified therein.
o Changed XML params registrant to IESG, per RFC 3688.
o Updated other references.
B.3 Changes from draft-ietf-xmpp-e2e-06
o Specified use of SHA-1 for digest and AES for content encryption.
o Specified order of signing then encrypting.
o Specified format and checking of timestamps.
o Clarified use of subjectAltName field, where the GeneralName
content is a URI of the form im:user@host and pres:user@host.
o Clarified circumstances under which certificates should be
attached.
o Added Content-Disposition header field to examples.
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B.4 Changes from draft-ietf-xmpp-e2e-05
o Addressed I-D nits and RFC Editor formatting.
B.5 Changes from draft-ietf-xmpp-e2e-04
o Added text about instant inbox addresses.
B.6 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.7 Changes from draft-ietf-xmpp-e2e-02
o Completely revised to use formats defined in the CPIM
specifications, S/MIME only, etc.
B.8 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.
B.9 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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