XEP-0241: Encryption of Archived Messages

Abstract: This specification defines methods for encrypting messages that are archived at an XMPP server.
Author: Ian Paterson
Copyright: © 2008 – 2008 XMPP Standards Foundation. SEE LEGAL NOTICES.
Status: Deferred
WARNING: This document has been automatically Deferred after 12 months of inactivity in its previous Experimental state. Implementation of the protocol described herein is not recommended for production systems. However, exploratory implementations are encouraged to resume the standards process.
Type: Standards Track
Version: 0.1 (2008-04-30)

Document Lifecycle

Experimental
Deferred
Proposed
Stable
Final

1. Introduction¶

Message Archiving (XEP-0136) [1] defines a technology for archiving messages at an XMPP server instead of locally on a client device. This specification defines methods for encrypting such messages.

2. Encryption of Manually-Archived Collections¶

Clients SHOULD encrypt manually-archived collections (although early implementations of this protocol MAY prefer to defer encryption and decryption to later releases). Servers MUST support the manual-archiving of encrypted collections.

Before uploading a sequence of messages to a collection, the client SHOULD select a symmetric data encryption algorithm, generate a suitable random encryption key, give the key a unique (for the user) name, encrypt the symmetric key with one of the user's public keys, and wrap the result inside one or more <EncryptedKey/> elements, as specified in XML Encryption [2].

To ensure that all its user's clients will be able to decrypt the collection, the client SHOULD create one <EncryptedKey/> element for each of its user's public keys that are being published using Public Key Publishing (XEP-0189) [3]. However, the client MUST NOT create an <EncryptedKey/> element for any public key until it has confirmed that it belongs to the user. Note: The fact that a public key is being published using Public Key Publishing (XEP-0189) [3] is not sufficient proof of ownership, since the user's server may have been compromised at some stage. The method of confirmation is beyond the scope of this document.

The client SHOULD use the symmetric key to encrypt the joined sequence of <to/>, <from/> and <note/> elements, base64 encode the resulting sequence of bytes, and wrap it inside an <EncryptedData/> element, as described in XML Encryption.

Clients may add one or more <EncryptedData/> or <EncryptedKey/> elements to a collection using exactly the same method as for <to/>, <from/> and <note/> elements (see Uploading Messages to a Collection). One collection may contain <EncryptedData/> elements encrypted with different symmetric keys.

When appending <EncryptedData/> elements to a collection, the client MAY reuse a symmetric Key that has already been uploaded to the collection. In this case the client SHOULD NOT resend <EncryptedKey/> elements.

Note: A collection that contains <EncryptedData/> or <EncryptedKey/> elements MUST NOT contain <to/> or <from/> or <note/> elements.

The <CipherData/> child of each <EncryptedData/> element contains the base64-encoded symmetric-encrypted messages. The <EncryptionMethod/> and <KeyInfo/> children specify the symmetric encryption algorithm and the name of the symmetric key used to encrypt the messages.

The <CarriedKeyName/> child of each <EncryptedKey/> element contains the name of the symmetric key it contains. The name is referenced by the <KeyName/> child of the <KeyInfo/> child of an <EncryptedData/> element. The <CipherData/> child of each <EncryptedKey/> element contains the base64-encoded public-key-encrypted symmetric key. The <EncryptionMethod/> and <KeyInfo/> children specify the public key encryption algorithm and the name of the public key used to encrypt the symmetric key. The name of the public key MAY refer to the name in the <KeyName/> child of one of the <KeyInfo/> elements that are being published using Public Key Publishing (XEP-0189) [3].

The x:data form MAY be removed from a collection simply by uploading an empty form. Note: The server SHOULD NOT return an error if it finds that the form to be deleted does not exist.

3. Enabling Auto-Archiving with Encryption¶

The client can enable auto-archiving with server-side encryption by setting the 'save' attribute to "true" or "1" and setting the 'encrypt' attribute to "true" or "1".

If the server does not support encryption but the client attempts to enable encryption, the server MUST return a <feature-not-implemented/> error.

If the server supports encryption but there is no public key available for the user (e.g., as published via Public Key Publishing (XEP-0189) [3], the server MUST return a <not-acceptable/> error.

If the server supports encryption (see Determining Server Support), it MUST encrypt all the messages that it archives automatically (including any message collections that are currently being recorded) by following exactly the same procedure as clients use when manually archiving collections (see Encryption).

The client MAY also specify one or more public keys (in addition to any public keys that the user may be publishing using Public Key Publishing (XEP-0189) [3]). The server MUST use them all to encrypt all the symmetric keys it generates and add these to the collection wrapped in <EncryptedKey/> elements.

As soon as the server has finished archiving a collection, it MUST securely destroy all copies of the symmetric key it used to encrypt the messages. Note: If the security of the server is compromised, then only the collections being recorded during the attack will be revealed (i.e. only those messages that would have been compromised even if they had not been archived).

4. Retrieving a List of Encrypted Collections¶

If a collection contains <EncryptedData/> or <EncryptedKey/> elements then the 'crypt' attribute of the <chat/> element MUST be set to 'true':

5. Retrieving an Encrypted Collection¶

The items in encrypted collections are typically larger than the items in an unencrypted collection, since each <EncryptedData/> element typically contains many messages. So the client SHOULD take even more care not to request a page of <EncryptedData/> elements that is so big it might exceed rate limiting restrictions.

In addition to the requested <EncryptedData/> elements, the server MUST return all the <EncryptedKey/> elements that it possesses for the user whose symmetric key name (wrapped in its <CarriedKeyName/> child) is referenced by the <KeyName/> child of the <KeyInfo/> child of any of the <EncryptedData/> elements in the returned page.

The client MAY limit the number of <EncryptedKey/> elements that it receives by specifying the name of one or more public keys for which it holds the associated private keys. The name of each public key MUST be wrapped in a <KeyName/> element.

If the request includes one or more <KeyName/> elements then the server MUST only return those <EncryptedKey/> elements whose public key name (wrapped in the <KeyName/> child of the <KeyInfo/> child) is specified in the request.

6. Replacing EncryptedKey Elements¶

If a private key becomes obsolete or compromised then it may be necessary for a client to replace all <EncryptedKey/> elements that contain symmetric keys encrypted with the public key that is associated with the obsolete private key.

The client first requests a list of the affected <EncryptedKey/> elements from all collections by sending a <keys/> element to the server:

The server MUST return only <EncryptedKey/> elements whose symmetric encryption key is encrypted with the obsolete public key specified in the <KeyName/> child of the request:

The client decrypts each symmetric key with the obsolete private key and encrypts it again with the new public key. The client then wraps each symmetric key in an <EncryptedKey/> element and asks the server to archive it in its associated collection on the server (see Encryption):

Finally, the client asks the server to delete from each collection all <EncryptedKey/> elements whose symmetric encryption key is encrypted with the obsolete public key:

7. Determining Server Support¶

If the server supports the service-side encryption feature, it MUST return a <feature/> element with the 'var' attribute set to 'urn:xmpp:tmp:archive:encrypt' (see Protocol Namespaces regarding issuance of one or more permanent namespaces).

8. Security Considerations¶

Because the subject of each collection will not be encrypted even if its messages are encrypted, the client MUST warn its human user (if any) before including 'subject' attributes on encrypted collections.

9. IANA Considerations¶

10. XMPP Registrar Considerations¶

10.1 Service Discovery Features¶

The XMPP Registrar shall include the following features in its registry of service discovery features (see <https://xmpp.org/registrar/disco-features.html>), where the string "urn:xmpp:tmp:archive" shall be replaced with the URN issued by the XMPP Registrar:

urn:xmpp:tmp:archive:encrypt

11. XML Schema¶

If this specification is advanced to a status of Draft, the schema for the 'urn:xmpp:tmp:archive' namespace (see Protocol Namespaces regarding issuance of one or more permanent namespaces) shall be updated to add a boolean 'encrypt' attribute to the <auto/> element, as follows.

Appendices

Appendix A: Document Information¶

Appendix B: Author Information¶

Ian Paterson

Appendix C: Legal Notices¶

Copyright

Permissions

Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.

Disclaimer of Warranty

## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##

Limitation of Liability

In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.

IPR Conformance

This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).

Visual Presentation

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Appendix D: Relation to XMPP¶

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.

Appendix E: Discussion Venue¶

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.

Discussion on other xmpp.org discussion lists might also be appropriate; see <https://xmpp.org/community/> for a complete list.

Appendix F: Requirements Conformance¶

The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

Appendix G: Notes¶

1. XEP-0136: Message Archiving <https://xmpp.org/extensions/xep-0136.html>.

2. XML Encryption Syntax and Processing <http://www.w3.org/TR/2002/REC-xmlenc-core-20021210/>.

3. XEP-0189: Public Key Publishing <https://xmpp.org/extensions/xep-0189.html>.

4. XEP-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>.

5. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.