construct expected wtxid commitment and compare with extracted wtxid …

…commitment

src/validation/coinbase.cairo

@@ -70,42 +70,45 @@ pub fn validate_coinbase(
 
         // validate BIP-141 segwit output
         if *tx.is_segwit {
-            let outputs = *tx.outputs;
+            let mut outputs = *tx.outputs;
             let mut is_wtxid_commitment_present: bool = false;
             let mut extracted_wtxid_commitment: ByteArray = "";
-            let mut i = 0;
-            let mut x = 6;
-
-            while i < outputs.len() {
-                let pk_script = *outputs[i].pk_script;
-
-                // check for OP_RETURN and the fixed prefix "0xaa21a9ed"
-                if pk_script.len() >= MINIMUM_WITNESS_COMMITMENT
-                    && pk_script[0] == 0x6a
-                    && pk_script[1] == 0x24
-                    && pk_script[2] == 0xaa
-                    && pk_script[3] == 0x21
-                    && pk_script[4] == 0xa9
-                    && pk_script[5] == 0xed {
+            let mut x = 0;
+
+            // construct expected wtxid commitment
+            let mut fixed_prefix_arr = array![0x6a, 0x24, 0xaa, 0x21, 0xa9, 0xed];
+            let mut fixed_prefix_byte: ByteArray = "";
+
+            for i in fixed_prefix_arr {
+                fixed_prefix_byte.append_byte(i);
+            };
+
+            let mut expected_wtxid_commitment = ByteArrayTrait::concat(
+                @fixed_prefix_byte, @wtxid_commitment.into()
+            );
+
+            while let Option::Some(output) = outputs.pop_back() {
+                let pk_script = *output.pk_script;
+
+                // check for pk_script with at least 38 bytes commitment length
+                if pk_script.len() >= MINIMUM_WITNESS_COMMITMENT {
                     // get wtxid commitment
-                    while x < pk_script.len() {
+                    while x < MINIMUM_WITNESS_COMMITMENT {
                         extracted_wtxid_commitment.append_word(pk_script[x].into(), 1);
                         x += 1;
                     };
 
-                    is_wtxid_commitment_present = true;
-                    break;
+                    // compare expected and extracted wtxid commitment
+                    if expected_wtxid_commitment == extracted_wtxid_commitment {
+                        is_wtxid_commitment_present = true;
+                        break;
+                    }
                 }
-                i += 1;
             };
 
             if !is_wtxid_commitment_present {
                 return Result::Err("No wtxid commitment found");
             }
-
-            if !(extracted_wtxid_commitment == wtxid_commitment.into()) {
-                return Result::Err("Wrong wtxid commitment");
-            }
         }
     }