Merge pull request #3111 from massalabs/documentation_endorsements

Editing documentation endorsments

docs/technical-doc/Endorsements.rst

@@ -111,12 +111,34 @@ Note that this split also massively increases the frequency at which stakers rec
 
 Choosing the value of `E`
 =========================
+The value of `E` has implication both in the safety and in the performance of the protocol. In terms of performance, the greater the value of `E` is, 
+the more ressources (bandwith, memory, computing power) is needed to generate, broadcast and include endorsements, which could induce latency. 
+One the other hand, the value of `E` has implications in the safety of the protocol. The threat we are looking at here is the finality fork attack.
+
+Finality fork attacks
+---------------------
+A finality fork attack is when an attacker that controls a portion `beta` of the total stake, decides not to broadcast the blocks he has been selected to produce, in order to construct an alternative and undisclosed clique.
+The attacker's goal is to produce an attack clique that has a fitness greater than the honnest clique. If he manages to do so, the attacker can wait until the finalization time of a block `b` belonging to the honest clique, to broadcast its attack clique. 
+Because the fitness of the attack clique is greater than the honest one, a portion of the network will consider the attacker's clique to be the good version of the chain, while some other portion will have finalized block `b`. This results in a network partion, 
+where two or more portion of the network do not agree on the state of the ledger. 
+
+Security level
+--------------
+The factors that influence the success probability of a finality fork attack are the number of endorsments per block `E`, the portion of the total stake controlled by the attacker `beta` and a finality parameter `F`.
+The finality parameter `F` respresents a number of descendant a block must have, before being finalized. 
+The portion of the stake `beta` is positively correlated with the success probability of an attack, while `F` and `E` are inversly correlated. 
+We do not have control over `beta`, thus we define our security level as a portion `beta` and a maximum success probability of an attack.
+We chose `beta = 1/3` because it is the maximum proportion of Byzantine participants BFT based consensus protocols assume. We chose `p_success <= 1e-11` because it corresponds to a period of time of around 10 000 years (given one block every 0.5s).
+We must chose the minimal parameters `F` and `E` that match this security level.
+
+Simulation results
+------------------
+Our simulations results show that with `F = 64`, `E = 16` and given 32 thread and a portion `beta = 1/3` of the stake controlled by the attacker, the probability of success of an attack is in the order of `1e-11`.
+You can find more details in the [technical paper](https://arxiv.org/abs/1803.09029).
+
+.. image:: simulation_result.png
 
 
-See Massa's technical paper for an explanation on how `E` is chosen given performance and security considerations.
-
-Current testnet value: `E = 16`
-
 
 TODOS
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