[script] Comment PrecomputedTransactionData purpose

Antoine Riard · Antoine Riard · commit 56247242c1db · 2020-04-15T04:06:12.000-04:00
For the verification of each signature, amount of data hashing
is proportional to the size of the transaction. To avoid hashing
grows in O(n^2) as the number of sigops in a transaction increases,
cache transaction digest invariant across signature verification.
diff --git a/src/script/interpreter.h b/src/script/interpreter.h
@@ -118,6 +118,12 @@ enum
 
 bool CheckSignatureEncoding(const std::vector<unsigned char> &vchSig, unsigned int flags, ScriptError* serror);
 
+/** "Midstate" hashes cache.
+ *
+ * As some limbs of signature message are invariant to sigops
+ * evaluation, cache them to avoid performance hit and DoS vector
+ * See BIP143, CVE-2013-2292 for more information
+ */
 struct PrecomputedTransactionData
 {
     uint256 hashPrevouts, hashSequence, hashOutputs;
