The derivation is further described in 10/WAKU-USAGE
-The node’s pre-key bundle MUST be broadcast on a content topic derived from the node’s public key, so that the first message may be PFS-encrypted. +Each peer MUST publish their pre-key bundle periodically to this topic, otherwise they risk not being able to perform key-exchanges with other peers. +Each peer MAY publish to this topic when their metadata changes, so that the other peer can update their local record.
+If peer A wants to send a message to peer B, it MUST derive the topic from peer B’s public key, which has been shared out of band. +Partitioned topics have been used to balance privacy and efficiency of broadcasting pre-key bundles.
+The number of partitions that MUST be used is 5000.
+The topic MUST be derived as follows:
+var partitionsNum *big.Int = big.NewInt(5000)
+var partition *big.Int = big.NewInt(0).Mod(peerBPublicKey, partitionsNum)
+
+partitionTopic := "contact-discovery-" + strconv.FormatInt(partition.Int64(), 10)
+
+var hash []byte = keccak256(partitionTopic)
+var topicLen int = 4
+
+if len(hash) < topicLen {
+ topicLen = len(hash)
+}
+
+var contactCodeTopic [4]byte
+for i = 0; i < topicLen; i++ {
+ contactCodeTopic[i] = hash[i]
+}
+A node initializes a new session once a successful X3DH exchange has taken place. Subsequent messages will use the established session until re-keying is necessary.
+After the peers have performed the initial key exchange, they MUST derive a topic from their shared secret to send messages on. +To obtain this value, take the first four bytes of the keccak256 hash of the shared secret encoded in hexadecimal format.
+sharedKey, err := ecies.ImportECDSA(myPrivateKey).GenerateShared(
+ ecies.ImportECDSAPublic(theirPublicKey),
+ 16,
+ 16,
+)
+
+
+hexEncodedKey := hex.EncodeToString(sharedKey)
+
+var hash []byte = keccak256(hexEncodedKey)
+var topicLen int = 4
+
+if len(hash) < topicLen {
+ topicLen = len(hash)
+}
+
+var topic [4]byte
+for i = 0; i < topicLen; i++ {
+ topic[i] = hash[i]
+}
+To summarize, following is the process for peer B to establish a session with peer A:
+