While in the future a dedicated key server component could be used to distribute - key material for session creation, the current specification relies on &xep0163; to publish + key material for session creation, the current specification relies on &xep0060; and &xep0163; 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 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:
- The X3DH key exchange is specified here and placed under the public domain. OMEMO uses this key exchange mechanism with the following parameters/settings: + The X3DH key exchange is specified here and placed under the public domain. OMEMO uses this key exchange mechanism with the following parameters/settings:
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.
NOTE: OMEMOMessage.proto, OMEMOAuthenticatedMessage.proto and OMEMOKeyExchange.proto refer to the protobuf structures as defined here.
+NOTE: OMEMOMessage.proto, OMEMOAuthenticatedMessage.proto and 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: + The Double Ratchet encryption scheme is specified here and placed under the public domain. OMEMO uses this protocol with the following parameters/settings:
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 &nsbundles;. 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 &ns; 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 &nsbundles; node is configured to store multiple items. This is not the default with &xep0163;. 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.
+When publishing bundles a client MUST make sure that the &nsbundles; node is configured to store multiple items. This is not the default with &xep0163;. 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.
In order to build a session with a device, their bundle information is fetched.
-- The &content; element is encrypted as described in the section about Message Encryption. + The &content; element is encrypted as described in the section about Message Encryption.
Clients MUST only consider the devices on the &nsdevices; node of each recipient (i.e. including their own devices node, but excluding itself). @@ -481,10 +481,10 @@
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. + 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 key and HMAC encrypted using the long-standing OMEMO session for that recipient device are encoded using base64 and placed as text content into the <key> element. + 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.
+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 an 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. + After either the OMEMOKeyExchange or the OMEMOAuthenticatedMessage is decrypted, the content is decrypted as described in the section about Message Decryption.
Before sending a message a participant MUST explicitly fetch device lists (if not already cached) for each of the members.
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