Add check on total weight and certificate threshold in `verifyValidat…

…orsUpdate` in LIP 0053 (#488)

* add check on total weight and certificate threshold

* Perform editorial edits

---------

Co-authored-by: janhack <29209167+janhack@users.noreply.github.com>

proposals/lip-0053.md

@@ -7,7 +7,7 @@ Discussions-To: https://research.lisk.com/t/introduce-cross-chain-update-mechani
 Status: Draft
 Type: Standards Track
 Created: 2021-05-22
-Updated: 2023-08-28
+Updated: 2023-09-26
 Requires: 0045, 0049, 0058, 0061
 ```
 
@@ -224,7 +224,25 @@ def verifyValidatorsUpdate(ccu: CCU) -> None:
     if len(newActiveValidators) < 1 or len(newActiveValidators) > MAX_NUM_VALIDATORS:
         raise Exception(f"Invalid validators array. It must have at least 1 element and at most {MAX_NUM_VALIDATORS} elements.")
 
+    totalWeight = 0
+    for validator in newActiveValidators:
+        # The bftWeight property of each element is a positive integer.
+        if validator.bftWeight == 0:
+            raise Exception("Invalid bftWeight property.")
+        totalWeight += validator.bftWeight
+        # Total BFT weight has to be less than or equal to MAX_UINT64.
+        if totalWeight > MAX_UINT64:
+            raise Exception("Total BFT weight exceeds maximum value.")
+
     certificateThreshold = ccu.params.certificateThreshold
+    # The range of valid values of the certificate threshold is given by the total sum of the validators weights:
+    # Minimum value: floor(1/3 * total BFT weight) + 1.
+    # Maximum value = total BFT weight.
+    if certificateThreshold < totalWeight//3 + 1:
+        raise Exception("Certificate threshold is too small.")
+    if certificateThreshold > totalWeight:
+        raise Exception("Certificate threshold is too large.")
+
     # computeValidatorsHash is defined in LIP 0058.
     certificate = decode(certificateSchema, ccu.params.certificate)
     if certificate.validatorsHash != computeValidatorsHash(newActiveValidators, certificateThreshold):