End-to-end encrypted messaging is a popular feature within the community. Various protocols like OpenPGP for XMPP (XEP-0373) [1] or OMEMO Encryption (XEP-0384) [2] have been proposed to allow sending encrypted messages. Use of DTLS/SCTP in Jingle ICE-UDP (XEP-0343) [3] and Jingle Encrypted Transports (XEP-0391) [4] specify protocols for establishing an encrypted transport using Jingle to share files using Jingle File Transfer (XEP-0234) [5].
Stateless file sharing (XEP-xxxx) [6] describes a protocol that can be used to share files, previously uploaded using HTTP File Upload (XEP-0363) [7], but lacks means of encrypting files. This leaves files uploaded using HTTP File Upload (XEP-0363) [7] without any standardized means of encrypting them.
This XEP describes a protocol building on top of Stateless file sharing (XEP-xxxx) [6] to allow encrypting files.
This protocol is only meaningful for end-to-end encrypted file sharing when transported as end-to-end encrypted XML, like it's possible using Stanza Content Encryption (XEP-0420) [8]. However, usage without such end-to-end encryption still has its usecase, as it allows sharing files through untrusted intermediaries for as long as the intermediary XMPP servers, if any, are trusted.
Note: To make the examples in this document more readable, no end-to-end encryption is used.
Before sharing the file, the sending entity MUST create random symmetric private key and initialization vector (IV) as required by the selected encryption cipher (see Ciphers). The file is then encrypted using selected encryption cipher and the generated key and IV. After this it can be uploaded using HTTP File Upload (XEP-0363) [7] or prepared for any other means of file sharing.
The file is then shared using the protocol described in Stateless file sharing (XEP-xxxx) [6].
The <file/> metadata element still refers to the original file, i.e. it describes the original file name, size and hashes. The <size/> element and one or multiple <hash/> elements are REQUIRED when sending encrypted files.
For the encrypted file, a source is added as an <encrypted/> element to the <sources/>. It carries an attribute cipher with the namespace of the encryption cipher being used. The <encrypted/> element contains a <key/> and an <iv/> element, containing both values as Base64-encoded strings. The <encrypted/> element MAY also include <hash/> elements as described in Use of Cryptographic Hash Functions in XMPP (XEP-0300) [10], referring to the hash of the encrypted file. At last, the <encrypted/> element also includes another <sources/> element as described in Stateless file sharing (XEP-xxxx) [6], specifying sources to obtain the encrypted file.
The outer <sources/> may contain additional sources that directly allow for end-to-end encrypted file transfers, for example Jingle File Transfer (XEP-0234) [5] using Jingle Encrypted Transports (XEP-0391) [4].
<message to='juliet@shakespeare.lit' from='romeo@montague.lit/resource' id='sharing-a-file'>
<file-sharing xmlns='urn:xmpp:sfs:0'>
<file xmlns='urn:xmpp:file:metadata:0'>
<media-type>image/jpeg</media-type>
<name>summit.jpg</name>
<size>3032449</size>
<dimension>4096x2160</dimension>
<hash xmlns='urn:xmpp:hashes:2' algo='sha3-256'>2XarmwTlNxDAMkvymloX3S5+VbylNrJt/l5QyPa+YoU=</hash>
<hash xmlns='urn:xmpp:hashes:2' algo='id-blake2b256'>2AfMGH8O7UNPTvUVAM9aK13mpCY=</hash>
<desc>Photo from the summit.</desc>
<thumbnail xmlns='urn:xmpp:thumbs:1' uri='cid:sha1+ffd7c8d28e9c5e82afea41f97108c6b4@bob.xmpp.org' media-type='image/png' width='128' height='96'/>
</file>
<sources>
<encrypted xmlns='urn:xmpp:esfs:0' cipher='urn:xmpp:ciphers:aes-256-gcm-nopadding:0'>
<key>SuRJ2agVm/pQbJQlPq/B23Xt1YOOJCcEGJA5HrcYOGQ=</key>
<iv>T8RDMBaiqn6Ci4Nw</iv>
<hash xmlns='urn:xmpp:hashes:2' algo='sha3-256'>BgKI2gp2kNCRsARNvhFmw5kFf9BBo2pTbV2D8XHTMWI=</hash>
<hash xmlns='urn:xmpp:hashes:2' algo='id-blake2b256'>id4cnqqy9/ssfCkM4vYSkiXXrlE=</hash>
<sources xmlns='urn:xmpp:sfs:0'>
<url-data xmlns='http://jabber.org/protocol/url-data' target='https://download.montague.lit/4a771ac1-f0b2-4a4a-9700-f2a26fa2bb67/encrypted.jpg' />
</sources>
</encrypted>
<jinglepub xmlns='urn:xmpp:jinglepub:1' from='romeo@montague.lit/resource' id='9559976B-3FBF-4E7E-B457-2DAA225972BB'>
<description xmlns='urn:xmpp:jingle:apps:file-transfer:5' />
</jinglepub>
</sources>
</file-sharing>
</message>
On receive of a message including a <file-sharing/> element, that has an <encrypted/> element in its sources, normal processing as described in Stateless file sharing (XEP-xxxx) [6] applies.
When the receiving entity tries to obtain the file from the source described by the <encrypted/> element, it will try to obtain any of its inner sources instead.
On success, it decrypts the obtained file using the encryption cipher, private key and IV provided.
If the resulting file is larger than the number of bytes specified in the <size/> metadata element, the additional bytes are cut off.
The protocol to attach a source described in Stateless file sharing (XEP-xxxx) [6] can also be used to attach encrypted sources. After receiving a file using encrypted means, it is RECOMMENDED to only attach additional sources that support encryption.
Note The following table was copied from Jingle Encrypted Transports (XEP-0391) [4].
In order to encrypt the file, the sending entity must transmit a cipher key to the responder. There are multiple options available:
| Namespace | Type | Length (bits) | Parameters |
|---|---|---|---|
| urn:xmpp:ciphers:aes-128-gcm-nopadding:0 | AES | Key: 128, IV: 96 | GCM/NoPadding |
| urn:xmpp:ciphers:aes-256-gcm-nopadding:0 | AES | Key: 256, IV: 96 | GCM/NoPadding |
For compatibility reasons, it is RECOMMENDED to append the GCM authentication tag to the uploaded file when using any AES cipher with GCM. The GCM authentication tag is not needed when using the protocol described in this document as a hash of the resulting file is transported independently.
Yes.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [11].
The XMPP Registrar [12] includes 'urn:xmpp:esfs:0' in its registry of protocol namespaces (see <https://xmpp.org/registrar/namespaces.html>).
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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.
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 <http://xmpp.org/about/discuss.shtml> for a complete list.
Errata can be sent to <editor@xmpp.org>.
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".
1. XEP-0373: OpenPGP for XMPP <https://xmpp.org/extensions/xep-0373.html>.
2. XEP-0384: OMEMO Encryption <https://xmpp.org/extensions/xep-0384.html>.
3. XEP-0343: Use of DTLS/SCTP in Jingle ICE-UDP <https://xmpp.org/extensions/xep-0343.html>.
4. XEP-0391: Jingle Encrypted Transports <https://xmpp.org/extensions/xep-0391.html>.
5. XEP-0234: Jingle File Transfer <https://xmpp.org/extensions/xep-0234.html>.
6. XEP-xxxx: Stateless file sharing <https://xmpp.org/extensions/inbox/sfs.html>.
7. XEP-0363: HTTP File Upload <https://xmpp.org/extensions/xep-0363.html>.
8. XEP-0420: Stanza Content Encryption <https://xmpp.org/extensions/xep-0420.html>.
9. There is a widely-deployed protocol for encrypted file sharing known as "OMEMO media sharing" or "aesgcm-links" that was never accepted as a XEP. While backwards compatibility with such non-standard is not a maxime of the XSF, it was still considered during the design of this protocol.
10. XEP-0300: Use of Cryptographic Hash Functions in XMPP <https://xmpp.org/extensions/xep-0300.html>.
11. 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/>.
12. 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/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
First draft.
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