There are two main end-to-end encryption schemes in common use in the XMPP ecosystem, Off-the-Record (OTR) messaging (Current Off-the-Record Messaging Usage (XEP-0364) ) and OpenPGP (Current Jabber OpenPGP Usage (XEP-0027) ). Older OTR versions have had significant usability drawbacks for inter-client mobility. As OTR sessions existed between exactly two clients, the chat history would not be synchronized across other clients of the involved parties. Furthermore, OTR chats were only possible if both participants were online at the same time, due to how the rolling key agreement scheme of OTR worked. Some of those problems have been addressed in OTRv4. OpenPGP, while not suffering from these mobility issues, does not provide any kind of forward secrecy and is vulnerable to replay attacks. Additionally, PGP over XMPP uses a custom wireformat which is defined by convention rather than standardization, and involves quite a bit of external complexity. The wire format issues were resolved with OpenPGP for XMPP (XEP-0373) .
This XEP defines a protocol that leverages the Double Ratchet encryption scheme to provide multi-end to multi-end encryption, allowing messages to be synchronized securely across multiple clients, even if some of them are offline. The Double Ratchet encryption scheme is based on work by Trevor Perrin and Moxie Marlinspike and was first published as the Axolotl protocol. The specification for the protocol is available in the public domain.
The general idea behind this protocol is to maintain separate, long-standing Double Ratchet-encrypted sessions with each device of each contact (as well as with each of our other devices), which are used as secure key transport channels. In this scheme, each message is encrypted with a fresh, randomly generated encryption key. An encrypted header is added to the message for each device that is supposed to receive it. These headers simply contain the key that the payload message is encrypted with, and they are separately encrypted using the session corresponding to the counterpart device. The encrypted payload is sent together with the headers as a <message> stanza. Individual recipient devices can decrypt the header item intended for them, and use the contained payload key to decrypt the payload message.
As the encrypted payload is common to all recipients, it only has to be included once, reducing overhead. Furthermore, the transparent handling by the Double Ratchet encryption scheme of messages that were lost or received out of order, as well as those sent while the recipient was offline, is maintained by this protocol. As a result, in combination with Message Carbons (XEP-0280)  and Message Archive Management (XEP-0313) , the desired property of inter-client history synchronization is achieved.
While in the future a dedicated key server component could be used to distribute key material for session creation, the current specification relies on Personal Eventing Protocol (XEP-0163)  to publish and acquire key bundles.
It is a result of XMPP's federated nature that a message may pass more than just one server. Therefore it is in the users' interest to secure their communication from any intermediate host. End-to-end encryption is an efficient way to protect any data exchanged between sender and receiver against passive and active attackers such as servers and network nodes.
OMEMO is an end-to-end encryption protocol based on the Double Ratchet specified in section Double Ratchet. It provides the following guarantees under the threat model described in the next section:
OMEMO is not intended to protect against the following use cases:
Trust management is a difficult topic, which is out of scope of this document.
The OMEMO protocol protects against passive and active attackers which are able to read, modify, replay, delay and delete messages.
This protocol uses the Double Ratchet encryption scheme in conjunction with the X3DH key exchange. The following section provides detailed technical information about the protocol that should be sufficient to build an implementation of the OMEMO Double Ratchet. Readers who do not intend to build an OMEMO-compatible library can safely skip this section, relevant details are repeated where needed.
The X3DH key exchange is specified here and placed under the public domain. OMEMO uses this key exchange mechanism with the following parameters/settings:
AD = Encode(IK_A) || Encode(IK_B)
The key exchange is done just-in-time when sending the first message to a device. Thus, each key exchange message always also contains encrypted content as produced by the Double Ratchet encryption scheme below.
OMEMOKeyExchange.proto refer to the protobuf structures as defined here.
The Double Ratchet encryption scheme is specified here and placed under the public domain. OMEMO uses this protocol with the following parameters/settings:
CONCAT(ad, header) = ad || OMEMOMessage.proto(header)NOTE: the
OMEMOMessage.protois initialized without the ciphertext, which is optional. NOTE: Implementations are not strictly required to return a parseable byte array here, as the unpacked/parsed data is required later in the protocol.
0x01as HMAC input to produce the next message key and a single byte constant
0x02as HMAC input to produce the next chain key.
CONCATinto the original ad and the
OMEMOMessage.protostructure into a parseable byte array by concatenating ad and the serialized protobuf structure.
OMEMOMessage.protostructure and the HMAC into a new
If encrypting this message required a key exchange, the X3DH key exchange header data is placed into a new
OMEMOKeyExchange.proto structure together with the
To account for lost and out-of-order messages during the key exchange,
OMEMOKeyExchange.proto structures are sent until a response by the recipient confirms that the key exchange was successfully completed. To do so, the X3DH key exchange header data is stored and added on each subsequent message until a response is received. This looks roughly as follows:
OMEMOAuthenticatedMessage.protostructure. Both are packed into an
OMEMOKeyExchange.protostructure. The X3DH header is stored for following messages.
OMEMOAuthenticatedMessage.protostructure, as a new key exchange is not required. Together with the X3DH header that was stored in the previous step, an
OMEMOKeyExchange.protostructure is constructed and sent to the recipient.
The contents are encrypted and authenticated using a combination of AES-256-CBC and HMAC-SHA-256.
keyin the remainder of this algorithm.
The contents are decrypted by reversing the encryption steps.
To participate in OMEMO-encrypted chats, clients need to set up an OMEMO library and generate a device id, which is a randomly generated integer between 1 and 2^31 - 1. The device id must be unique for the account.
In order to determine whether a given contact has devices that support OMEMO, the devices node in PEP is consulted. Devices MUST subscribe to
urn:xmpp:omemo:1:devices via PEP, so that they are informed whenever their contacts add a new device. They MUST cache the most up-to-date version of the device list.
In order for other devices to be able to initiate a session with a given device, it first has to announce itself by adding its device id to the devices PEP node.
It is REQUIRED to set the access model of the
urn:xmpp:omemo:1:devices node to ‘open’ to give entities without presence subscription read access to the devices and allow them to establish an OMEMO session. Not having presence subscription is a common occurrence on the first few messages between two contacts and can also happen fairly frequently in group chats as not every participant had prior communication with every other participant.
The access model can be changed efficiently by using publish-options.
The device element MAY contain an attribute called label, which is a user defined string describing the device that published that bundle. It is RECOMMENDED to keep the length of the label under 53 Unicode code points.
NOTE: as per XEP-0060 §12.20, it is RECOMMENDED for the publisher to specify an ItemID of "current" to ensure that the publication of a new item will overwrite the existing item.
This step presents the risk of introducing a race condition: Two devices might simultaneously try to announce themselves, unaware of the other's existence. The second device would overwrite the first one. To mitigate this, devices MUST check that their own device id is contained in the list whenever they receive a PEP update from their own account. If they have been removed, they MUST reannounce themselves.
Furthermore, a device MUST publish its IdentityKey, a signed PreKey, and a list of PreKeys. This tuple is called a bundle and is provided by OMEMO libraries. Bundles are maintained as multiple items in a PEP node called
urn:xmpp:omemo:1:bundles. Each bundle MUST be stored in a seperate item. The item id MUST be set to the device id.
A bundle is an element called 'bundle' in the
urn:xmpp:omemo:1 namespace. It has a child element called ‘spk’ that contains the public part of the signed PreKey as base64 encoded data, a child element called ‘spks’ that contains the signed PreKey signature as base64 encoded data and a child element called ‘ik’ that contains the public part of the IdentityKey as base64 encoded data. PreKeys are multiple elements called ‘pk’ that each contain the public part of one PreKey as base64 encoded data. PreKeys are wrapped in an element called ‘prekeys’ which is a child of the bundle element. The ‘spk’ and the ‘pk’s are tagged with an ‘id’-attribute which is a positive integer that uniquely identifies the keys. The ‘spk’ and the ‘pk’s are considered separate, which means that an ‘spk’ can have the same ‘id’ as a ‘pk’. These ids are used to save bandwidth during key exchanges, which refer to the keys using their id instead of their full public parts.
When publishing bundles a client MUST make sure that the
urn:xmpp:omemo:1:bundles node is configured to store multiple items. This is not the default with Personal Eventing Protocol (XEP-0163) . If the node doesn’t exist yet it can be configured on the fly by using publish-options as described in XEP-0060 §7.1.5. The value for 'pubsub#max_items' in publish_options MUST be set to 'max'. If the node did exist and was configured differently the bundle publication will fail. Clients MUST then reconfigure the node as described in XEP-0060 §8.2.
As with the
urn:xmpp:omemo:1:devices node it is REQUIRED to set the access model of the
urn:xmpp:omemo:1:bundles to open.
The access model can be changed efficiently by using publish-options.
In order to build a session with a device, their bundle information is fetched.
A random pk entry is selected, and used to build an OMEMO session.
In order to send a message, extension elements that are deemed sensible first have to be encrypted. For this purpose, extensions that are only intended to be accessible to the recipient are placed inside a Stanza Content Encryption (XEP-0420)  <content/> element, which is then encrypted using a message key. For this reason OMEMO defines its own SCE profile.
An OMEMO SCE <content/> element
The <content/> element is encrypted as described in the section about Message Encryption.
Clients MUST only consider the devices on the
urn:xmpp:omemo:1:devices node of each recipient (i.e. including their own devices node, but excluding itself).
An OMEMO encrypted message is specified to include an <encrypted> element in the 'urn:xmpp:omemo:1' namespace. It always contains two child nodes, the <header> and the <payload/> element. The <header> element has an attribute named 'sid' referencing the device id of the sending device and contains one or multiple <keys> elements, each with an attribute 'jid' of one of the recipients bare JIDs as well as one or multiple <key> elements. A <key> element has an attribute named 'rid' referencing the device id of the recipient device, and an attribute named 'kex' which defaults to 'false' and indicates if the enclosed encrypted message includes a key exchange. The ciphertext that is the key and HMAC encrypted using the long-standing OMEMO session for that recipient device is encoded using base64 and placed as text content into the <key> element. The ciphertext that is the encrypted <content/> element is encoded using base64 and placed as text content into the <payload/> element.
When an OMEMO element is received, the client MUST check whether there is a <keys> element with a jid attribute matching its own bare jid and an inner <key> element with a rid attribute matching its own device id. If this is not the case the message was not encrypted for this particular device and a warning message SHOULD be displayed instead. If such an element exists, the client checks whether the element's contents are an OMEMOKeyExchange.
If this is the case, a new session is built from this received element. The client MUST then republish their bundle information, replacing the used PreKey, such that it won't be used again by a different client. If the client already has a session with the sender's device, it MUST replace this session with the newly built session. The client MUST eventually delete the private key belonging to the PreKey after use (this is subject to the Business rules).
If the element's contents are a OMEMOAuthenticatedMessage, and the client has a session with the sender's device, it tries to decrypt the OMEMOAuthenticatedMessage using this session. If the decryption fails or there is no session with the sending device, a warning message SHOULD be displayed instead. Also refer to the section about recovering from broken sessions in the Business Rules.
After either the OMEMOKeyExchange or the OMEMOAuthenticatedMessage is decrypted, the content is decrypted as described in the section about Message Decryption.
An account can signal to a peer that it wants to stop communicating using
OMEMO encrypted messages and would like to proceed in plain text instead. To do
that any of that account’s devices sends an <opt-out/> element qualified
urn:xmpp:omemo:1 namespace to all intended recipient devices
inside an encrypted stanza. The element MAY contain a child element <reason>.
If a device is receiving an encrypted stanza containing an <opt-out/> element,
it SHOULD display the information, that the peer would like to receive plain text messages.
To prevent that the user is accidentally sending plaintext messages, the client MUST
block all outgoing message until the user has confirmed the switch to plaintext.
Any existing double ratchet sessions SHOULD remain intact. At any point any party MAY
revert their decision and go back to sending OMEMO encrypted messages again.
Note: OMEMO encrypted group chats are currently specified to work with Multi-User Chat (XEP-0045) . This XEP might be updated in the future to specify the usage of OMEMO in conjunction with Mediated Information eXchange (MIX) (XEP-0369) .
A Multi-User Chat room that supports OMEMO MUST be configured non-anonymous and SHOULD be configured members-only.
A participant wanting to send a message to a group chat MUST first retrieve the members list and then fetch the device list for each member (via pubsub and to their real JIDs) and then subsequently fetch all bundles referenced by the device lists.
On join a participant MUST request the member list, the admin list and the owner list as described in XEP-0045 §9.5, XEP-0045 §10.8, and XEP-0045 §10.5 respectively. The real JIDs from those three lists MUST be combined as the recipients of OMEMO encrypted messages. This includes recipients who are currently offline. Once joined a participant MUST keep track of affiliation changes that occur in the room. This is both for removals (users getting banned or have their affiliation set to none) and users becoming members, admins or owners.
Before sending a message a participant MUST explicitly fetch device lists (if not already cached) for each of the members.
Sending a message to a group chat is similiar to sending a message in a 1:1 conversation. Instead of the <header> element having two <keys> elements (one for the recipient and one for other devices of the sender) it will contain multiple <keys> elements. One for each participant of the room; including, again, other devices of the sender.
Before publishing a freshly generated device id for the first time, a device MUST check whether that device id already exists, and if so, generate a new one.
Clients SHOULD NOT immediately fetch the bundle and build a session as soon as a new device is announced. Before the first message is exchanged, the contact does not know which PreKey has been used (or, in fact, that any PreKey was used at all). As they have not had a chance to remove the used PreKey from their bundle announcement, this could lead to collisions where both Alice and Bob pick the same PreKey to build a session with a specific device. As each PreKey SHOULD only be used once, the party that sends their initial OMEMOKeyExchange later loses this race condition. This means that they think they have a valid session with the contact, when in reality their messages MAY be ignored by the other end. By postponing building sessions, the chance of such issues occurring can be drastically reduced. It is RECOMMENDED to construct sessions only immediately before sending a message.
There are various reasons why decryption of an OMEMOKeyExchange or an OMEMOAuthenticatedMessage could fail. One reason is if the message was received twice and already decrypted once, in this case the client MUST ignore the decryption failure and not show any warnings/errors. In all other cases of decryption failure, clients SHOULD respond by forcibly doing a new key exchange and sending a new OMEMOKeyExchange with a potentially empty SCE payload. By building a new session with the original sender this way, the invalid session of the original sender will get overwritten with this newly created, valid session. This does NOT apply to the actual SCE content. If decrypting the SCE content fails, e.g. because the HMAC does not verify, this is not a reason to forcibly initiate a new key exchange.
If an OMEMOKeyExchange is received as part of a message catch-up mechanism (like Message Archive Management (XEP-0313) ) and used to establish a new session with the sender, the client SHOULD postpone deletion of the private key corresponding to the used PreKey until after the catch-up is completed. If this is done, the client MUST send an OMEMO encrypted message with empty SCE payload right after the key exchange is completed, to forward the ratchet and to move away from the possibly double-used PreKey. This practice can mitigate the previously mentioned race condition by preventing message loss.
When a client receives the first message for a given ratchet key with a counter of 53 or higher, it MUST send a heartbeat message. Heartbeat messages are normal OMEMO encrypted messages where the SCE payload does not include any elements. These heartbeat messages cause the ratchet to forward, thus consequent messages will have the counter restarted from 0.
When a client receives a message from a device id that is not on the device list, it SHOULD try to retrieve that user's devices node directly to ensure their local cached version of the devices list is up-to-date.
When the user of a client deactivates OMEMO for an account or globally, the client SHOULD delete the corresponding bundles and device ids from the PEP nodes. That way other clients should stop encrypting for that account.
While OMEMO uses a Pubsub Service (Publish-Subscribe (XEP-0060) ) on the user’s account it has more requirments than those defined in Personal Eventing Protocol (XEP-0163) . The requirements are:
Clients MUST NOT use a newly built session to transmit data without user intervention. If a client were to opportunistically start using sessions for sending without asking the user whether to trust a device first, an attacker could publish a fake device for this user, which would then receive copies of all messages sent by/to this user. A client MAY use such "not (yet) trusted" sessions for decryption of received messages, but in that case it SHOULD indicate the untrusted nature of such messages to the user.
When prompting the user for a trust decision regarding a key, the client SHOULD present the user with a fingerprint in the form of a hex string, QR code, or other unique representation, such that it can be compared by the user. The fingerprint is often chosen to be the public part of the device's IdentityKey, but could also be a different combination data that guarantees absence of a man-in-the-middle when verified. When displaying the fingerprint as a hex string, one way to make it easier to compare the fingerprint is to split the hex string into 8 substrings of 8 chars each, then coloring each 8-char group using Consistent Color Generation (XEP-0392) . Lowercase letters are recommended when displaying the fingerprint as a hex string.
While it is RECOMMENDED that clients postpone private key deletion until after message catch-up, the X3DH standard mandates that clients should not use duplicate-PreKey sessions for sending, so clients MAY delete such keys immediately for security reasons. For additional information on potential security impacts of this decision, refer to .
This document requires no interaction with the Internet Assigned Numbers Authority (IANA).
This specification defines the following XMPP namespaces:
If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.
Big thanks to Daniel Gultsch for mentoring me during the development of this protocol. Thanks to Thijs Alkemade and Cornelius Aschermann for talking through some of the finer points of the protocol with me. And lastly I would also like to thank Sam Whited, Holger Weiss, and Florian Schmaus for their input on the standard.
The authors would like to thank the Chaosdorf for hosting them during the development of version 0.4.0 of this specification.
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12. Menezes, Alfred, and Berkant Ustaoglu. "On reusing ephemeral keys in Diffie-Hellman key agreement protocols." International Journal of Applied Cryptography 2, no. 2 (2010): 154-158.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Make examples show items published to the id "current", as per XEP-0060 §12.20.
Depend on SignalProtocol instead of Olm.
Changed to eu.siacs.conversations.axolotl Namespace which is currently used in the wild
Initial version approved by the council.
Depend on Olm instead of Axolotl.