Jingle File Transfer (XEP-0234) [1] describes a very flexible and powerful method for peer-to-peer file transfer with interchangable transports. Unfortunatly only some of those transports can by encrypted (e.g. Jingle In-Band Bytestreams Transport Method (XEP-0261) [2]) and none integrate into existing end-to-end encryption schemes. This specification defines an approach to encrypt the actual file before transferring it by using the OMEMO encryption. Note that the encryption can and should happen on the fly.
Remeo and Juliet are already engaged in a private, OMEMO encrypted conversation and want to negotiate a file transfer. The session-initiate stanza of an OMEMO encrypted file tranfer differs from the session-initiate of a regular Jingle File Transfer in that the 'file' element is wrapped in an 'encrypted' element. This reuses the KeyTransportElement from the OMEMO specification.
<iq from='romeo@montague.lit/orchard'
id='nzu25s8'
to='juliet@capulet.lit/balcony'
type='set'>
<jingle xmlns='urn:xmpp:jingle:1'
action='session-initiate'
initiator='romeo@montague.lit/orchard'
sid='851ba2'>
<content creator='initiator' name='a-file-offer'>
<description xmlns='urn:xmpp:jingle:apps:encrypted:file-transfer:0'>
<encrypted xmlns='urn:xmpp:omemo:0'>
<header sid='27183'>
<key rid='31415'>BASE64ENCODED...</key>
<key rid='12321'>BASE64ENCODED...</key>
<iv>BASE64ENCODED...</iv>
</header>
<file>
<date>2015-08-30T23:24:05Z</date>
<desc>This is a test. If this were a real file...</desc>
<media-type>image/png</media-type>
<name>omemo.png</name>
<range/>
<size>51158</size>
<hash xmlns='urn:xmpp:hashes:1' algo='sha-1'/>
</file>
</encrypted>
</description>
<transport xmlns='urn:xmpp:jingle:transports:s5b:1'
mode='tcp'
sid='vj3hs98y'>
<candidate cid='hft54dqy'
host='192.168.4.1'
jid='romeo@montague.lit/orchard'
port='5086'
priority='8257636'
type='direct'/>
<candidate cid='hutr46fe'
host='24.24.24.1'
jid='romeo@montague.lit/orchard'
port='5087'
priority='8258636'
type='direct'/>
</transport>
</content>
</jingle>
</iq>
The rest of the negotiation is analogous to a regular file transfer. The file is then encrypted and decrypted respectively using the key/IV pair extracted from the encryption header in AES-GCM. The crypto operation SHOULD happen on the fly.
Even though AES-GCM comes with build-in integrity protection the hosting entity can—at any time during the lifetime of the session—communicate the checksum of the encrypted file.
<iq from='romeo@montague.lit/orchard'
id='kqh401b5'
to='juliet@capulet.lit/balcony'
type='set'>
<jingle xmlns='urn:xmpp:jingle:1'
action='session-info'
initiator='romeo@montague.lit/orchard'
sid='a73sjjvkla37jfea'>
<checksum xmlns='urn:xmpp:jingle:apps:encrypted:file-transfer:0'>
<file>
<hash xmlns='urn:xmpp:hashes:1' algo='sha-1'>552da749930852c69ae5d2141d3766b1</hash>
</file>
</checksum>
</jingle>
</iq>
To advertise its support for the OMEMO Encrypted Jingle File Transfer, when replying to service discovery information ("disco#info") requests an entity MUST return URNs for any version of this protocol that the entity supports -- e.g., 'urn:xmpp:jingle:apps:encrypted:file-transfer:0' for this version (see Namespace Versioning regarding the possibility of incrementing the version number).
<iq from='romeo@montague.lit/orchard'
id='uw72g176'
to='juliet@capulet.lit/balcony'
type='get'>
<query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
<iq from='juliet@capulet.lit/balcony'
id='uw72g176'
to='romeo@montague.lit/orchard'
type='result'>
<query xmlns='http://jabber.org/protocol/disco#info'>
<feature var='urn:xmpp:jingle:1'/>
<feature var='urn:xmpp:jingle:apps:encrypted:file-transfer:0'/>
<feature var='urn:xmpp:jingle:apps:file-transfer:4'/>
<feature var='urn:xmpp:jingle:transports:s5b:1'/>
<feature var='urn:xmpp:jingle:transports:ibb:1'/>
</query>
</iq>
In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic, presence-based profile of service discovery defined in Entity Capabilities (XEP-0115) [3]. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.
Since the OMEMO Encryption does not provide a direct relation between resources and Device ID’s the initiating entity might not the Device ID of the resource it is creating a session with. Thus it MAY include a key for every trusted device of the foreign entity. However it SHOULD omit the keys for its own devices.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA).
This specification defines the following XML namespace:
Upon advancement of this specification from a status of Experimental to a status of Draft, the XMPP Registrar [4] shall add the foregoing namespace to the registry located at <https://xmpp.org/registrar/namespaces.html>, as described in Section 4 of XMPP Registrar Function (XEP-0053) [5].
tbd
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1. XEP-0234: Jingle File Transfer <https://xmpp.org/extensions/xep-0234.html>.
2. XEP-0261: Jingle In-Band Bytestreams Transport Method <https://xmpp.org/extensions/xep-0261.html>.
3. XEP-0115: Entity Capabilities <https://xmpp.org/extensions/xep-0115.html>.
4. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.
5. XEP-0053: XMPP Registrar Function <https://xmpp.org/extensions/xep-0053.html>.
Note: Older versions of this specification might be available at https://xmpp.org/extensions/attic/
First draft.
@report{gultsch2015xepxxxx,
title = {OMEMO Encrypted Jingle File Transfer},
author = {Gultsch, Daniel},
type = {XEP},
number = {xxxx},
version = {0.0.1},
institution = {XMPP Standards Foundation},
url = {https://xmpp.org/extensions/xep-xxxx.html},
date = {2015-09-02/2015-09-02},
}END