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Code Issues Releases Wiki Activity

Implementation notes about deactivating OMEMO, switched to 32 byte keys

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Tim Henkes 2020-03-08 18:27:33 +01:00
parent c7a29c7462
commit 5f1fb48220
1 changed files with 3 additions and 2 deletions
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xep-0384.xml
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@ -256,7 +256,7 @@
<ol>
<li>Use HKDF-SHA-256 to generate 80 bytes of output from the message key by providing mk as HKDF input, 256 zero-bits as HKDF salt and &quot;OMEMO Message Key Material&quot; as HKDF info.</li>
<li>Divide the HKDF output into a 32-byte encryption key, a 32-byte authentication key and a 16 byte IV.</li>
<li>Encrypt the plaintext (which is a 16 bytes key as specified <link url="#protocol-message_encryption">here</link>) using AES-256-CBC with PKCS#7 padding, using the encryption key and IV derived in the previous step.</li>
<li>Encrypt the plaintext (which is a 32 bytes key as specified <link url="#protocol-message_encryption">here</link>) using AES-256-CBC with PKCS#7 padding, using the encryption key and IV derived in the previous step.</li>
<li>Split the associated data as returned by <tt>CONCAT</tt> into the original ad and the <tt>OMEMOMessage.proto</tt> structure.</li>
<li>Add the ciphertext to the <tt>OMEMOMessage.proto</tt> structure.</li>
<li>Serialize the ad and the <tt>OMEMOMessage.proto</tt> structure into a parseable byte array by concatenating ad and the serialized protobuf structure.</li>
@ -281,7 +281,7 @@
The contents are encrypted and authenticated using a combination of AES-256-CBC and HMAC-SHA-256.
</p>
<ol>
<li>Generate 16 bytes of cryptographically secure random data, called <tt>key</tt> in the remainder of this algorithm.</li>
<li>Generate 32 bytes of cryptographically secure random data, called <tt>key</tt> in the remainder of this algorithm.</li>
<li>Use HKDF-SHA-256 to generate 80 bytes of output from the key by providing the key as HKDF input, 256 zero-bits as HKDF salt and &quot;OMEMO Payload&quot; as HKDF info.</li>
<li>Divide the HKDF output into a 32-byte encryption key, a 32-byte authentication key and a 16 byte IV.</li>
<li>Encrypt the plaintext using AES-256-CBC with PKCS#7 padding, using the encryption key and IV derived in the previous step.</li>
@ -594,6 +594,7 @@
<p>As the asynchronous nature of OMEMO allows decryption at a later time to currently offline devices client SHOULD include a &xep0334; &lt;store /&gt; hint in their OMEMO messages. Otherwise, server implementations of &xep0313; will generally not retain OMEMO messages, since they do not contain a &lt;body /&gt;</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
</section1>
<section1 topic='Implementation Notes' anchor='impl'>
<section2 topic='Server side requirements' anchor='server-side'>