It is a result of XMPPs 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.
+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:
@@ -276,7 +276,7 @@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.
+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 RECOMMENDED to set the access model of the &nsdevices; 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.
@@ -485,7 +485,7 @@ The &content; element is encrypted as described in the section about Message Encryption.- Clients SHOULD only consider the devices on the &nsdevices; node of each recipient (i.e. including his own devices node, but excluding itself). + Clients SHOULD only consider the devices on the &nsdevices; node of each recipient (i.e. including their own devices node, but excluding itself).
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 an 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 revocering from broken sessions in the Business Rules.
+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.
@@ -591,11 +591,11 @@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.
+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 a OMEMOKeyExchange is received as part of a message catch-up mechanism (like &xep0313;) 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 then also send a normal OMEMO encrypted message with an empty SCE payload before sending any payloads using this session, to trigger re-keying. (as above) This practice can mitigate the previously mentioned race condition by preventing message loss.
As the asynchronous nature of OMEMO allows decryption at a later time to currently offline devices client SHOULD include a &xep0334; <store /> hint in their OMEMO messages. Otherwise, server implementations of &xep0313; will generally not retain OMEMO messages, since they do not contain a <body />
-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 they 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 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 the 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 device.
While it is RECOMMENDED that clients postpone private key deletion until after message catch-up, the standards mandates that clients MUST 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 document requires no interaction with the Internet Assigned Numbers Authority (IANA).