fix invalid poseidon hash usage

We need to pad high-order bytes with zeros in case if result is less than 32 bytes.

hasher.go

@@ -42,11 +42,10 @@ func init() {
 }
 
 func poseidonSum(input []byte) []byte {
+	output := make([]byte, 32)
 	res := poseidon.Sum(input)
-	if rest := len(res) % 32; rest != 0 {
-		res = append(res, zeroBytes[:32-rest]...)
-	}
-	return res
+	copy(output[32-len(res):], res)
+	return output
 }
 
 // HashWithDefaultHasher hashes a HashRoot object with a Hasher from

tests/codetrie_test.go

@@ -11,14 +11,11 @@ import (
 	"github.com/iden3/go-iden3-crypto/poseidon"
 )
 
-var zeroBytes = make([]byte, 32)
-
 func poseidonSum(input []byte) []byte {
+	output := make([]byte, 32)
 	res := poseidon.Sum(input)
-	if len(res) != 32 {
-		res = append(res, zeroBytes[:32-len(res)]...)
-	}
-	return res
+	copy(output[32-len(res):], res)
+	return output
 }
 
 func TestVerifyMetadataProof(t *testing.T) {