add sbox explanatino

README.md

@@ -255,12 +255,14 @@ As shown next, the Clyde module is separated in two mechanisms: the Clyde comput
 generation/handling of the randomness. The computation takes as input the sharing of the key (i.e., the `sharing_key` bus), the 
 tweak (i.e., the `tweak` bus) and either the plaintext or the ciphertext (i.e., the `data_in` bus). The control signal `inverse` is used to 
 specify to the core which operation (i.e., encryption or decryption) is currently perfomed. Next, control signals will be represented in blue.  
+
 The randomness is generated by using two (similar) instances of a maximum length
 [128-bits LFSR](https://www.xilinx.com/support/documentation/application_notes/xapp210.pdf) (i.e., the `prng_unit` module). Seeds can be
 sent to these instances through the `feed_data` signal by means of a `SEED` segment, as explained above. 
 A specific controller (i.e., the `stalling_unit`) is used to properly handle the interaction between the PRNGs and the Clyde logic (using the 
 `control_sig*`, `control_status*`and `stall_control*` signals). Basically, the latter enables the LFSRs and stalls the computation 
-core when randomness is required and not ready. It is also used as a control wrapper interface for the Clyde computation logic.
+core when randomness is required and not ready. It is also used as a control wrapper interface for the Clyde computation logic. Note 
+that in the case of the CTF, the randomness is generated in 1 clock cycle and the computation core is thus not stalled. 
 
 <div align="center">
  
@@ -336,11 +338,12 @@ inversely (the `MSKcols2bundle` module). These only consist in wiring modificati
 
 </div>
 
-A `MSKspook_sbox_dual` instance is implemented using dedicated logic for the Sbox operation. The inverse Sbox 
-operation is performed by reusing the Sbox logic with two additional linear layers (`MSKpre_inv_sbox` before
-the sbox logic and `MSKpost_inv_sbox` after). In particular, the input of the Sbox logic comes either 
-from `MSKpre_inv_sbox` or from the instance input depending on the value of the `inverse` control signal. 
-Similarly, the output of the instance comes either from `MSKpost_inv_sbox` or from the output of the sbox.
+A`MSKspook_sbox_dual` instance is mainly composed by the logic dedicated to perform the Sbox operation
+(i.e., `MSKspook_spook`). The inverse Sbox operation is performed by reusing the Sbox logic with two 
+additional linear layers (`MSKpre_inv_sbox` before the sbox logic and `MSKpost_inv_sbox` after). In 
+particular, the input of the Sbox logic comes either from `MSKpre_inv_sbox` or from the instance 
+input depending on the value of the `inverse` control signal. Similarly, the output of the instance 
+comes either from `MSKpost_inv_sbox` or from the output of the sbox.
 
 <div align="center">
  
@@ -367,6 +370,19 @@ The different configurations are summed up in the following table:
 
 </div>
 
+The specific (and quite difficult) architure of a `MSKspook_sbox` instance is shown next. The labels used
+directly reflect the related HDL code. Each color used represent a level in the pipeline. 
+A specificity is to be noted for the AND gates: these are implemented with one pipeline level and two 
+different latencies for the inputs. More especially, the inputs are expected to enter the 
+core at two successive clock cycles. Denoting these cycles c<sub>0</sub> and c<sub>1</sub>, the gate is 
+drawn with the color of the input expected at the cycle c<sub>0</sub>.
+
+<div align="center">
+
+![MSKspook_sbox](/spook_msk/schematics/MSKspook_sbox.jpg)
+
+</div>
+
 ### Simulations Script (unix-like)
 
 As mentionned above, the [simu](spook_msk/simu) contains the simulation script