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feat(54/WAKU2-X3DH-SESSIONS): include topic usage (#587)

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* feat(54/WAKU2-X3DH-SESSIONS): include topic usage

* fix(54/WAKU2-X3DH-SESSIONS): wording

* fix(54/WAKU2-X3DH-SESSIONS): wording to include contactCodeTopic

* fix(54/WAKU2-X3DH-SESSIONS): better wording

* fix(54/WAKU2-X3DH-SESSIONS): typo

Co-authored-by: kaiserd <1684595+kaiserd@users.noreply.github.com>

* fix(54/WAKU2-X3DH-SESSIONS): better wording

---------

Co-authored-by: kaiserd <1684595+kaiserd@users.noreply.github.com>
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@ -28,14 +28,74 @@ A node identifies a peer by their `installation-id` which MAY be interpreted as
## Discovery of pre-key bundles
The node's pre-key bundle SHOULD be broadcasted on a content topic derived from the node's public key.
<!--- TODO: We need to move other specs from status to vac research, including STATUS-WAKU-USAGE -->
The derivation is further described in [10/WAKU-USAGE](https://specs.status.im/spec/10#contact-code-topic)
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]
}
```
## Initialization
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.
## Negotiated topic to be used for the session
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:
1. Listen to peer B's Contact Code Topic to retrieve their bundle information, including a list of active devices
2. Peer A sends their pre-key bundle on peer B's partitioned topic
3. Peer A and peer B perform the key-exchange using the shared pre-key bundles
3. The negotiated topic is derived from the shared secret
4. Peers A & B exchange messages on the negotiated topic
## Concurrent sessions
If a node creates two sessions concurrently between two peers, the one with the symmetric key first in byte order SHOULD be used, this marks that the other has expired.