fix invalid poseidon hash usage

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

hash.go

@@ -1,6 +1,8 @@
 package ssz
 
-import "hash"
+import (
+	"hash"
+)
 
 type HashFn func(dst []byte, input []byte) error
 
@@ -9,7 +11,14 @@ func NativeHashWrapper(hashFn hash.Hash) HashFn {
 		hash := func(dst []byte, src []byte) {
 			hashFn.Write(src[:32])
 			hashFn.Write(src[32:64])
-			hashFn.Sum(dst)
+			result := hashFn.Sum(nil)
+			if len(result) != 32 {
+				out := make([]byte, 32-len(result), 32)
+				out = append(out, result...)
+				_ = append(dst, out...)
+			} else {
+				_ = append(dst, result...)
+			}
 			hashFn.Reset()
 		}
 

hasher.go

@@ -43,10 +43,12 @@ func init() {
 
 func poseidonSum(input []byte) []byte {
 	res := poseidon.Sum(input)
-	if rest := len(res) % 32; rest != 0 {
-		res = append(res, zeroBytes[:32-rest]...)
+	if len(res) == 32 {
+		return res
 	}
-	return res
+	output := make([]byte, 32)
+	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) {
@@ -111,7 +108,7 @@ func TestVerifyCodeTrieProof(t *testing.T) {
 			valid: true,
 		},
 		{
-			root: "1b8170b45b3580f30d19aec698fbfe11c557f7398fa59a69dcbd6c6e53307732",
+			root: "03608ede03131a9f8d1bb968f071aa4704d789cc7b0a307e95a59c2875e2cd0c",
 			proof: []string{
 				"0000000000000000000000000000000000000000000000000000000000000000",
 				"0000000000000000000000000000000000000000000000000000000000000000",
@@ -165,7 +162,7 @@ func TestVerifyCodeTrieMultiProof(t *testing.T) {
 		valid   bool
 	}{
 		{
-			root: "12fe4f8049d94a8b967b29f4d4949969ef31615930656c46cad86df2de9d34bd",
+			root: "2b2bcc5615d1af1035ffd7ee1c3a5ae4accd10e1abf4b08ff991e37499806589",
 			proof: []string{
 				"0000000000000000000000000000000000000000000000000000000000000000",
 				"0000000000000000000000000000000000000000000000000000000000000000",