@@ -39,12 +39,12 @@
 func testInit(c *qt.C, n int) (*Tree, *Tree) {
 	database1, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree1, err := NewTree(database1, 100, HashFunctionPoseidon)
+	tree1, err := NewTree(database1, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 
 	database2, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree2, err := NewTree(database2, 100, HashFunctionPoseidon)
+	tree2, err := NewTree(database2, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 
 	bLen := HashFunctionPoseidon.Len()
@@ -70,11 +70,11 @@ nLeafs := 1024
 
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, 100, HashFunctionPoseidon)
+	tree, err := NewTree(database, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree.db.Close() //nolint:errcheck
 
-	bLen := tree.HashFunction().Len()
+	bLen := 32
 	var keys, values [][]byte
 	for i := 0; i < nLeafs; i++ {
 		k := BigIntToBytes(bLen, big.NewInt(int64(i)))
@@ -93,7 +93,7 @@ time1 := time.Since(start)
 
 	database2, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree2, err := NewTree(database2, 100, HashFunctionPoseidon)
+	tree2, err := NewTree(database2, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree2.db.Close() //nolint:errcheck
 	tree2.dbgInit()
@@ -120,11 +120,11 @@ nLeafs := 1027
 
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, 100, HashFunctionPoseidon)
+	tree, err := NewTree(database, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree.db.Close() //nolint:errcheck
 
-	bLen := tree.HashFunction().Len()
+	bLen := 32
 	for i := 0; i < nLeafs; i++ {
 		k := BigIntToBytes(bLen, big.NewInt(int64(i)))
 		v := BigIntToBytes(bLen, big.NewInt(int64(i*2)))
@@ -135,7 +135,7 @@ }
 
 	database2, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree2, err := NewTree(database2, 100, HashFunctionPoseidon)
+	tree2, err := NewTree(database2, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree2.db.Close() //nolint:errcheck
 
@@ -167,13 +167,13 @@ func TestAddBatchTestVector1(t *testing.T) {
 	c := qt.New(t)
 	database1, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree1, err := NewTree(database1, 100, HashFunctionBlake2b)
+	tree1, err := NewTree(database1, 256, HashFunctionBlake2b)
 	c.Assert(err, qt.IsNil)
 	defer tree1.db.Close() //nolint:errcheck
 
 	database2, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree2, err := NewTree(database2, 100, HashFunctionBlake2b)
+	tree2, err := NewTree(database2, 256, HashFunctionBlake2b)
 	c.Assert(err, qt.IsNil)
 	defer tree2.db.Close() //nolint:errcheck
 
@@ -207,13 +207,13 @@
 	// 2nd test vectors
 	database1, err = badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree1, err = NewTree(database1, 100, HashFunctionBlake2b)
+	tree1, err = NewTree(database1, 256, HashFunctionBlake2b)
 	c.Assert(err, qt.IsNil)
 	defer tree1.db.Close() //nolint:errcheck
 
 	database2, err = badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree2, err = NewTree(database2, 100, HashFunctionBlake2b)
+	tree2, err = NewTree(database2, 256, HashFunctionBlake2b)
 	c.Assert(err, qt.IsNil)
 	defer tree2.db.Close() //nolint:errcheck
 
@@ -255,13 +255,13 @@ c := qt.New(t)
 
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree1, err := NewTree(database, 100, HashFunctionPoseidon)
+	tree1, err := NewTree(database, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree1.db.Close() //nolint:errcheck
 
 	database2, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree2, err := NewTree(database2, 100, HashFunctionPoseidon)
+	tree2, err := NewTree(database2, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree2.db.Close() //nolint:errcheck
 
@@ -300,13 +300,13 @@ c := qt.New(t)
 
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree1, err := NewTree(database, 100, HashFunctionPoseidon)
+	tree1, err := NewTree(database, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree1.db.Close() //nolint:errcheck
 
 	database2, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree2, err := NewTree(database2, 100, HashFunctionPoseidon)
+	tree2, err := NewTree(database2, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree2.db.Close() //nolint:errcheck
 
@@ -349,13 +349,13 @@ nLeafs := 8
 
 	database1, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree1, err := NewTree(database1, 100, HashFunctionBlake2b)
+	tree1, err := NewTree(database1, 256, HashFunctionBlake2b)
 	c.Assert(err, qt.IsNil)
 	defer tree1.db.Close() //nolint:errcheck
 
 	database2, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree2, err := NewTree(database2, 100, HashFunctionBlake2b)
+	tree2, err := NewTree(database2, 256, HashFunctionBlake2b)
 	c.Assert(err, qt.IsNil)
 	defer tree2.db.Close() //nolint:errcheck
 
@@ -699,7 +699,7 @@ time1 := time.Since(start)
 
 	database2, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree2, err := NewTree(database2, 100, HashFunctionPoseidon)
+	tree2, err := NewTree(database2, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree2.db.Close() //nolint:errcheck
 	tree2.dbgInit()
@@ -776,7 +776,7 @@ c := qt.New(t)
 
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, 140, HashFunctionBlake2b)
+	tree, err := NewTree(database, 256, HashFunctionBlake2b)
 	c.Assert(err, qt.IsNil)
 	defer tree.db.Close() //nolint:errcheck
 
@@ -796,7 +796,7 @@ c := qt.New(t)
 
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, 140, HashFunctionBlake2b)
+	tree, err := NewTree(database, 256, HashFunctionBlake2b)
 	c.Assert(err, qt.IsNil)
 	defer tree.db.Close() //nolint:errcheck
 
@@ -829,7 +829,7 @@
 	// 1
 	database1, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree1, err := NewTree(database1, 100, HashFunctionBlake2b)
+	tree1, err := NewTree(database1, 256, HashFunctionBlake2b)
 	c.Assert(err, qt.IsNil)
 	defer tree1.db.Close() //nolint:errcheck
 
@@ -843,7 +843,7 @@
 	// 2
 	database2, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree2, err := NewTree(database2, 100, HashFunctionBlake2b)
+	tree2, err := NewTree(database2, 256, HashFunctionBlake2b)
 	c.Assert(err, qt.IsNil)
 	defer tree2.db.Close() //nolint:errcheck
 
@@ -856,7 +856,7 @@
 	// 3
 	database3, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree3, err := NewTree(database3, 100, HashFunctionBlake2b)
+	tree3, err := NewTree(database3, 256, HashFunctionBlake2b)
 	c.Assert(err, qt.IsNil)
 	defer tree3.db.Close() //nolint:errcheck
 
@@ -891,7 +891,7 @@ nLeafs := 1024
 
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, 100, HashFunctionPoseidon)
+	tree, err := NewTree(database, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree.db.Close() //nolint:errcheck
 
@@ -927,11 +927,11 @@ nLeafs := 1024
 
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, 100, HashFunctionPoseidon)
+	tree, err := NewTree(database, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree.db.Close() //nolint:errcheck
 
-	bLen := tree.HashFunction().Len()
+	bLen := 32
 	var keys, values [][]byte
 	for i := 0; i < nLeafs; i++ {
 		k := BigIntToBytes(bLen, big.NewInt(int64(i)))
@@ -948,7 +948,7 @@ }
 
 	database2, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree2, err := NewTree(database2, 100, HashFunctionPoseidon)
+	tree2, err := NewTree(database2, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree2.db.Close() //nolint:errcheck
 	tree2.dbgInit()
@@ -962,7 +962,7 @@
 	// use tree3 to add nil value array
 	database3, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree3, err := NewTree(database3, 100, HashFunctionPoseidon)
+	tree3, err := NewTree(database3, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree3.db.Close() //nolint:errcheck
 

@@ -17,14 +17,13 @@ tree, err := NewTree(database, 4, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree.db.Close() //nolint:errcheck
 
-	bLen := tree.HashFunction().Len()
-
 	testVector := [][]int64{
 		{1, 11},
 		{2, 22},
 		{3, 33},
 		{4, 44},
 	}
+	bLen := 1
 	for i := 0; i < len(testVector); i++ {
 		k := BigIntToBytes(bLen, big.NewInt(testVector[i][0]))
 		v := BigIntToBytes(bLen, big.NewInt(testVector[i][1]))

@@ -18,14 +18,13 @@ c.Assert(err, qt.IsNil)
 	tree, err := arbo.NewTree(database, 4, arbo.HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 
-	bLen := tree.HashFunction().Len()
-
 	testVector := [][]int64{
 		{1, 11},
 		{2, 22},
 		{3, 33},
 		{4, 44},
 	}
+	bLen := 1
 	for i := 0; i < len(testVector); i++ {
 		k := arbo.BigIntToBytes(bLen, big.NewInt(testVector[i][0]))
 		v := arbo.BigIntToBytes(bLen, big.NewInt(testVector[i][1]))

@@ -229,8 +229,10 @@ }
 	}
 
 	// store root (from the vt) to db
-	if err := wTx.Set(dbKeyRoot, vt.root.h); err != nil {
-		return nil, err
+	if vt.root != nil {
+		if err := wTx.Set(dbKeyRoot, vt.root.h); err != nil {
+			return nil, err
+		}
 	}
 
 	// update nLeafs
@@ -310,17 +312,34 @@ }
 	return nil
 }
 
-func (t *Tree) add(wTx db.WriteTx, root []byte, fromLvl int, k, v []byte) ([]byte, error) {
-	keyPath := make([]byte, t.hashFunction.Len())
-	// if len(k) > t.hashFunction.Len() { // WIP
-	//         return nil, fmt.Errorf("len(k) > hashFunction.Len()")
-	// }
+// keyPathFromKey returns the keyPath and checks that the key is not bigger
+// than maximum key length for the tree maxLevels size.
+// This is because if the key bits length is bigger than the maxLevels of the
+// tree, two different keys that their difference is at the end, will collision
+// in the same leaf of the tree (at the max depth).
+func keyPathFromKey(maxLevels int, k []byte) ([]byte, error) {
+	maxKeyLen := int(math.Ceil(float64(maxLevels) / float64(8))) //nolint:gomnd
+	if len(k) > maxKeyLen {
+		return nil, fmt.Errorf("len(k) can not be bigger than ceil(maxLevels/8), where"+
+			" len(k): %d, maxLevels: %d, max key len=ceil(maxLevels/8): %d. Might need"+
+			" a bigger tree depth (maxLevels>=%d) in order to input keys of length %d",
+			len(k), maxLevels, maxKeyLen, len(k)*8, len(k)) //nolint:gomnd
+	}
+	keyPath := make([]byte, maxKeyLen) //nolint:gomnd
 	copy(keyPath[:], k)
+	return keyPath, nil
+}
 
+func (t *Tree) add(wTx db.WriteTx, root []byte, fromLvl int, k, v []byte) ([]byte, error) {
+	keyPath, err := keyPathFromKey(t.maxLevels, k)
+	if err != nil {
+		return nil, err
+	}
 	path := getPath(t.maxLevels, keyPath)
+
 	// go down to the leaf
 	var siblings [][]byte
-	_, _, siblings, err := t.down(wTx, k, root, siblings, path, fromLvl, false)
+	_, _, siblings, err = t.down(wTx, k, root, siblings, path, fromLvl, false)
 	if err != nil {
 		return nil, err
 	}
@@ -386,12 +405,10 @@ if bytes.Equal(newKey, oldLeafKey) {
 				return nil, nil, nil, ErrKeyAlreadyExists
 			}
 
-			oldLeafKeyFull := make([]byte, t.hashFunction.Len())
-			// if len(oldLeafKey) > t.hashFunction.Len() { // WIP
-			//         return nil, nil, nil,
-			//                 fmt.Errorf("len(oldLeafKey) > hashFunction.Len()")
-			// }
-			copy(oldLeafKeyFull[:], oldLeafKey)
+			oldLeafKeyFull, err := keyPathFromKey(t.maxLevels, oldLeafKey)
+			if err != nil {
+				return nil, nil, nil, err
+			}
 
 			// if currKey is already used, go down until paths diverge
 			oldPath := getPath(t.maxLevels, oldLeafKeyFull)
@@ -593,13 +610,10 @@ if !t.editable() {
 		return ErrSnapshotNotEditable
 	}
 
-	var err error
-
-	keyPath := make([]byte, t.hashFunction.Len())
-	// if len(k) > t.hashFunction.Len() { // WIP
-	//         return fmt.Errorf("len(k) > hashFunction.Len()")
-	// }
-	copy(keyPath[:], k)
+	keyPath, err := keyPathFromKey(t.maxLevels, k)
+	if err != nil {
+		return err
+	}
 	path := getPath(t.maxLevels, keyPath)
 
 	root, err := t.RootWithTx(wTx)
@@ -655,18 +669,17 @@
 // GenProofWithTx does the same than the GenProof method, but allowing to pass
 // the db.ReadTx that is used.
 func (t *Tree) GenProofWithTx(rTx db.ReadTx, k []byte) ([]byte, []byte, []byte, bool, error) {
-	keyPath := make([]byte, t.hashFunction.Len())
-	// if len(k) > t.hashFunction.Len() { // WIP
-	//         return nil, nil, nil, false, fmt.Errorf("len(k) > hashFunction.Len()")
-	// }
-	copy(keyPath[:], k)
+	keyPath, err := keyPathFromKey(t.maxLevels, k)
+	if err != nil {
+		return nil, nil, nil, false, err
+	}
+	path := getPath(t.maxLevels, keyPath)
 
 	root, err := t.RootWithTx(rTx)
 	if err != nil {
 		return nil, nil, nil, false, err
 	}
 
-	path := getPath(t.maxLevels, keyPath)
 	// go down to the leaf
 	var siblings [][]byte
 	_, value, siblings, err := t.down(rTx, k, root, siblings, path, 0, true)
@@ -793,18 +806,17 @@ // db.ReadTx that is used. If the key is not found, will return the error
 // ErrKeyNotFound, and in the leafK & leafV parameters will be placed the data
 // found in the tree in the leaf that was on the path going to the input key.
 func (t *Tree) GetWithTx(rTx db.ReadTx, k []byte) ([]byte, []byte, error) {
-	keyPath := make([]byte, t.hashFunction.Len())
-	// if len(k) > t.hashFunction.Len() { // WIP
-	//         return nil, nil, fmt.Errorf("len(k) > hashFunction.Len()")
-	// }
-	copy(keyPath[:], k)
+	keyPath, err := keyPathFromKey(t.maxLevels, k)
+	if err != nil {
+		return nil, nil, err
+	}
+	path := getPath(t.maxLevels, keyPath)
 
 	root, err := t.RootWithTx(rTx)
 	if err != nil {
 		return nil, nil, err
 	}
 
-	path := getPath(t.maxLevels, keyPath)
 	// go down to the leaf
 	var siblings [][]byte
 	_, value, _, err := t.down(rTx, k, root, siblings, path, 0, true)
@@ -827,7 +839,7 @@ if err != nil {
 		return false, err
 	}
 
-	keyPath := make([]byte, hashFunc.Len())
+	keyPath := make([]byte, int(math.Ceil(float64(len(siblings))/float64(8)))) //nolint:gomnd
 	copy(keyPath[:], k)
 
 	key, _, err := newLeafValue(hashFunc, k, v)

@@ -2,7 +2,9 @@ package arbo
 
 import (
 	"encoding/hex"
+	"math"
 	"math/big"
+	"runtime"
 	"testing"
 	"time"
 
@@ -60,7 +62,7 @@
 func testAdd(c *qt.C, hashFunc HashFunction, testVectors []string) {
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, 10, hashFunc)
+	tree, err := NewTree(database, 256, hashFunc)
 	c.Assert(err, qt.IsNil)
 	defer tree.db.Close() //nolint:errcheck
 
@@ -68,7 +70,7 @@ root, err := tree.Root()
 	c.Assert(err, qt.IsNil)
 	c.Check(hex.EncodeToString(root), qt.Equals, testVectors[0])
 
-	bLen := hashFunc.Len()
+	bLen := 32
 	err = tree.Add(
 		BigIntToBytes(bLen, big.NewInt(1)),
 		BigIntToBytes(bLen, big.NewInt(2)))
@@ -92,11 +94,11 @@ func TestAddBatch(t *testing.T) {
 	c := qt.New(t)
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, 100, HashFunctionPoseidon)
+	tree, err := NewTree(database, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree.db.Close() //nolint:errcheck
 
-	bLen := tree.HashFunction().Len()
+	bLen := 32
 	for i := 0; i < 1000; i++ {
 		k := BigIntToBytes(bLen, big.NewInt(int64(i)))
 		v := BigIntToBytes(bLen, big.NewInt(0))
@@ -110,7 +112,7 @@ "296519252211642170490407814696803112091039265640052570497930797516015811235")
 
 	database, err = badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree2, err := NewTree(database, 100, HashFunctionPoseidon)
+	tree2, err := NewTree(database, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree2.db.Close() //nolint:errcheck
 
@@ -133,11 +135,11 @@ func TestAddDifferentOrder(t *testing.T) {
 	c := qt.New(t)
 	database1, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree1, err := NewTree(database1, 100, HashFunctionPoseidon)
+	tree1, err := NewTree(database1, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree1.db.Close() //nolint:errcheck
 
-	bLen := tree1.HashFunction().Len()
+	bLen := 32
 	for i := 0; i < 16; i++ {
 		k := BigIntToBytes(bLen, big.NewInt(int64(i)))
 		v := BigIntToBytes(bLen, big.NewInt(0))
@@ -148,7 +150,7 @@ }
 
 	database2, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree2, err := NewTree(database2, 100, HashFunctionPoseidon)
+	tree2, err := NewTree(database2, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree2.db.Close() //nolint:errcheck
 
@@ -173,11 +175,11 @@ func TestAddRepeatedIndex(t *testing.T) {
 	c := qt.New(t)
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, 100, HashFunctionPoseidon)
+	tree, err := NewTree(database, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree.db.Close() //nolint:errcheck
 
-	bLen := tree.HashFunction().Len()
+	bLen := 32
 	k := BigIntToBytes(bLen, big.NewInt(int64(3)))
 	v := BigIntToBytes(bLen, big.NewInt(int64(12)))
 
@@ -191,11 +193,11 @@ func TestUpdate(t *testing.T) {
 	c := qt.New(t)
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, 100, HashFunctionPoseidon)
+	tree, err := NewTree(database, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree.db.Close() //nolint:errcheck
 
-	bLen := tree.HashFunction().Len()
+	bLen := 32
 	k := BigIntToBytes(bLen, big.NewInt(int64(20)))
 	v := BigIntToBytes(bLen, big.NewInt(int64(12)))
 	if err := tree.Add(k, v); err != nil {
@@ -244,11 +246,11 @@ func TestAux(t *testing.T) { // TODO split in proper tests
 	c := qt.New(t)
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, 100, HashFunctionPoseidon)
+	tree, err := NewTree(database, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree.db.Close() //nolint:errcheck
 
-	bLen := tree.HashFunction().Len()
+	bLen := 32
 	k := BigIntToBytes(bLen, big.NewInt(int64(1)))
 	v := BigIntToBytes(bLen, big.NewInt(int64(0)))
 	err = tree.Add(k, v)
@@ -283,11 +285,11 @@ func TestGet(t *testing.T) {
 	c := qt.New(t)
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, 100, HashFunctionPoseidon)
+	tree, err := NewTree(database, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree.db.Close() //nolint:errcheck
 
-	bLen := tree.HashFunction().Len()
+	bLen := 32
 	for i := 0; i < 10; i++ {
 		k := BigIntToBytes(bLen, big.NewInt(int64(i)))
 		v := BigIntToBytes(bLen, big.NewInt(int64(i*2)))
@@ -307,11 +309,11 @@ func TestGenProofAndVerify(t *testing.T) {
 	c := qt.New(t)
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, 100, HashFunctionPoseidon)
+	tree, err := NewTree(database, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree.db.Close() //nolint:errcheck
 
-	bLen := tree.HashFunction().Len() - 1
+	bLen := 32
 	for i := 0; i < 10; i++ {
 		k := BigIntToBytes(bLen, big.NewInt(int64(i)))
 		v := BigIntToBytes(bLen, big.NewInt(int64(i*2)))
@@ -339,11 +341,11 @@ func TestDumpAndImportDump(t *testing.T) {
 	c := qt.New(t)
 	database1, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree1, err := NewTree(database1, 100, HashFunctionPoseidon)
+	tree1, err := NewTree(database1, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree1.db.Close() //nolint:errcheck
 
-	bLen := tree1.HashFunction().Len()
+	bLen := 32
 	for i := 0; i < 16; i++ {
 		k := BigIntToBytes(bLen, big.NewInt(int64(i)))
 		v := BigIntToBytes(bLen, big.NewInt(int64(i*2)))
@@ -357,7 +359,7 @@ c.Assert(err, qt.IsNil)
 
 	database2, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree2, err := NewTree(database2, 100, HashFunctionPoseidon)
+	tree2, err := NewTree(database2, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree2.db.Close() //nolint:errcheck
 	err = tree2.ImportDump(e)
@@ -376,11 +378,11 @@ func TestRWMutex(t *testing.T) {
 	c := qt.New(t)
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, 100, HashFunctionPoseidon)
+	tree, err := NewTree(database, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree.db.Close() //nolint:errcheck
 
-	bLen := tree.HashFunction().Len()
+	bLen := 32
 	var keys, values [][]byte
 	for i := 0; i < 1000; i++ {
 		k := BigIntToBytes(bLen, big.NewInt(int64(i)))
@@ -469,7 +471,7 @@ c.Assert(err, qt.IsNil)
 
 	var keys, values [][]byte
 	for i := 0; i < 16; i++ {
-		k := BigIntToBytes(32, big.NewInt(int64(i)))
+		k := BigIntToBytes(1, big.NewInt(int64(i)))
 		v := k
 
 		keys = append(keys, k)
@@ -492,10 +494,10 @@ c.Assert(root1, qt.DeepEquals, root2)
 
 	// get all key-values and check that are equal between both trees
 	for i := 0; i < 16; i++ {
-		auxK1, auxV1, err := tree1.Get(BigIntToBytes(32, big.NewInt(int64(i))))
+		auxK1, auxV1, err := tree1.Get(BigIntToBytes(1, big.NewInt(int64(i))))
 		c.Assert(err, qt.IsNil)
 
-		auxK2, auxV2, err := tree2.Get(BigIntToBytes(32, big.NewInt(int64(i))))
+		auxK2, auxV2, err := tree2.Get(BigIntToBytes(1, big.NewInt(int64(i))))
 		c.Assert(err, qt.IsNil)
 
 		c.Assert(auxK1, qt.DeepEquals, auxK2)
@@ -504,7 +506,7 @@ }
 
 	// try adding one more key to both trees (through Add & AddBatch) and
 	// expect not being added due the tree is already full
-	k := BigIntToBytes(32, big.NewInt(int64(16)))
+	k := BigIntToBytes(1, big.NewInt(int64(16)))
 	v := k
 	err = tree1.Add(k, v)
 	c.Assert(err, qt.Equals, ErrMaxVirtualLevel)
@@ -518,13 +520,13 @@ func TestSetRoot(t *testing.T) {
 	c := qt.New(t)
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, 100, HashFunctionPoseidon)
+	tree, err := NewTree(database, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 
 	expectedRoot := "13742386369878513332697380582061714160370929283209286127733983161245560237407"
 
 	// fill the tree
-	bLen := tree.HashFunction().Len()
+	bLen := 32
 	var keys, values [][]byte
 	for i := 0; i < 1000; i++ {
 		k := BigIntToBytes(bLen, big.NewInt(int64(i)))
@@ -574,11 +576,11 @@ func TestSnapshot(t *testing.T) {
 	c := qt.New(t)
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, 100, HashFunctionPoseidon)
+	tree, err := NewTree(database, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 
 	// fill the tree
-	bLen := tree.HashFunction().Len()
+	bLen := 32
 	var keys, values [][]byte
 	for i := 0; i < 1000; i++ {
 		k := BigIntToBytes(bLen, big.NewInt(int64(i)))
@@ -624,11 +626,11 @@ c := qt.New(t)
 
 	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, 100, HashFunctionPoseidon)
+	tree, err := NewTree(database, 256, HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)
 	defer tree.db.Close() //nolint:errcheck
 
-	bLen := tree.HashFunction().Len()
+	bLen := 32
 	k := BigIntToBytes(bLen, big.NewInt(int64(3)))
 
 	root, err := tree.Root()
@@ -638,6 +640,133 @@ c.Assert(err, qt.IsNil)
 
 	_, _, err = tree.Get(k)
 	c.Assert(err, qt.Equals, ErrKeyNotFound) // and not equal to db.ErrKeyNotFound
+}
+
+func TestKeyLen(t *testing.T) {
+	c := qt.New(t)
+	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
+	c.Assert(err, qt.IsNil)
+	// maxLevels is 100, keyPath length = ceil(maxLevels/8) = 13
+	maxLevels := 100
+	tree, err := NewTree(database, maxLevels, HashFunctionBlake2b)
+	c.Assert(err, qt.IsNil)
+
+	// expect no errors when adding a key of only 4 bytes (when the
+	// required length of keyPath for 100 levels would be 13 bytes)
+	bLen := 4
+	k := BigIntToBytes(bLen, big.NewInt(1))
+	v := BigIntToBytes(bLen, big.NewInt(1))
+
+	err = tree.Add(k, v)
+	c.Assert(err, qt.IsNil)
+
+	err = tree.Update(k, v)
+	c.Assert(err, qt.IsNil)
+
+	_, _, _, _, err = tree.GenProof(k)
+	c.Assert(err, qt.IsNil)
+
+	_, _, err = tree.Get(k)
+	c.Assert(err, qt.IsNil)
+
+	k = BigIntToBytes(bLen, big.NewInt(2))
+	v = BigIntToBytes(bLen, big.NewInt(2))
+	invalids, err := tree.AddBatch([][]byte{k}, [][]byte{v})
+	c.Assert(err, qt.IsNil)
+	c.Assert(len(invalids), qt.Equals, 0)
+
+	// expect errors when adding a key bigger than maximum capacity of the
+	// tree: ceil(maxLevels/8)
+	maxLevels = 32
+	database, err = badgerdb.New(badgerdb.Options{Path: c.TempDir()})
+	c.Assert(err, qt.IsNil)
+	tree, err = NewTree(database, maxLevels, HashFunctionBlake2b)
+	c.Assert(err, qt.IsNil)
+
+	maxKeyLen := int(math.Ceil(float64(maxLevels) / float64(8))) //nolint:gomnd
+	k = BigIntToBytes(maxKeyLen+1, big.NewInt(1))
+	v = BigIntToBytes(maxKeyLen+1, big.NewInt(1))
+
+	expectedErrMsg := "len(k) can not be bigger than ceil(maxLevels/8)," +
+		" where len(k): 5, maxLevels: 32, max key len=ceil(maxLevels/8): 4." +
+		" Might need a bigger tree depth (maxLevels>=40) in order to input" +
+		" keys of length 5"
+
+	err = tree.Add(k, v)
+	c.Assert(err.Error(), qt.Equals, expectedErrMsg)
+
+	err = tree.Update(k, v)
+	c.Assert(err.Error(), qt.Equals, expectedErrMsg)
+
+	_, _, _, _, err = tree.GenProof(k)
+	c.Assert(err.Error(), qt.Equals, expectedErrMsg)
+
+	_, _, err = tree.Get(k)
+	c.Assert(err.Error(), qt.Equals, expectedErrMsg)
+
+	// check AddBatch with few key-values
+	invalids, err = tree.AddBatch([][]byte{k}, [][]byte{v})
+	c.Assert(err, qt.IsNil)
+	c.Assert(len(invalids), qt.Equals, 1)
+
+	// check AddBatch with many key-values
+	nCPU := flp2(runtime.NumCPU())
+	nKVs := nCPU + 1
+	var ks, vs [][]byte
+	for i := 0; i < nKVs; i++ {
+		ks = append(ks, BigIntToBytes(maxKeyLen+1, big.NewInt(1)))
+		vs = append(vs, BigIntToBytes(maxKeyLen+1, big.NewInt(1)))
+	}
+	invalids, err = tree.AddBatch(ks, vs)
+	c.Assert(err, qt.IsNil)
+	c.Assert(len(invalids), qt.Equals, nKVs)
+
+	// check that with maxKeyLen it can be added
+	k = BigIntToBytes(maxKeyLen, big.NewInt(1))
+	err = tree.Add(k, v)
+	c.Assert(err, qt.IsNil)
+
+	// check CheckProof check with key longer
+	kAux, vAux, packedSiblings, existence, err := tree.GenProof(k)
+	c.Assert(err, qt.IsNil)
+	c.Assert(existence, qt.IsTrue)
+
+	root, err := tree.Root()
+	c.Assert(err, qt.IsNil)
+	verif, err := CheckProof(tree.HashFunction(), kAux, vAux, root, packedSiblings)
+	c.Assert(err, qt.IsNil)
+	c.Assert(verif, qt.IsTrue)
+
+	// use a similar key but with one zero, expect that CheckProof fails on
+	// the verification
+	kAux = append(kAux, 0)
+	verif, err = CheckProof(tree.HashFunction(), kAux, vAux, root, packedSiblings)
+	c.Assert(err, qt.IsNil)
+	c.Assert(verif, qt.IsFalse)
+}
+
+func TestKeyLenBiggerThan32(t *testing.T) {
+	c := qt.New(t)
+	maxLevels := 264
+	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
+	c.Assert(err, qt.IsNil)
+	tree, err := NewTree(database, maxLevels, HashFunctionBlake2b)
+	c.Assert(err, qt.IsNil)
+
+	bLen := 33
+	err = tree.Add(
+		randomBytes(bLen),
+		randomBytes(bLen))
+	c.Assert(err, qt.IsNil)
+
+	// 2nd key that we add, will find a node with len(key)==32 (due the
+	// hash output size, expect that next Add does not give any error, as
+	// internally it will use a keyPath of size corresponent to the
+	// maxLevels size of the tree
+	err = tree.Add(
+		randomBytes(bLen),
+		randomBytes(bLen))
+	c.Assert(err, qt.IsNil)
 }
 
 func BenchmarkAdd(b *testing.B) {

@@ -1,6 +1,8 @@
 package arbo
 
-import "math/big"
+import (
+	"math/big"
+)
 
 // SwapEndianness swaps the order of the bytes in the byte slice.
 func SwapEndianness(b []byte) []byte {

@@ -37,22 +37,32 @@ k       []byte
 	v       []byte
 }
 
-func (p *params) keysValuesToKvs(ks, vs [][]byte) ([]kv, error) {
+func (p *params) keysValuesToKvs(ks, vs [][]byte) ([]kv, []int, error) {
 	if len(ks) != len(vs) {
-		return nil, fmt.Errorf("len(keys)!=len(values) (%d!=%d)",
+		return nil, nil, fmt.Errorf("len(keys)!=len(values) (%d!=%d)",
 			len(ks), len(vs))
 	}
-	kvs := make([]kv, len(ks))
+	var invalids []int
+	var kvs []kv
 	for i := 0; i < len(ks); i++ {
-		keyPath := make([]byte, p.hashFunction.Len())
-		copy(keyPath[:], ks[i])
-		kvs[i].pos = i
-		kvs[i].keyPath = keyPath
-		kvs[i].k = ks[i]
-		kvs[i].v = vs[i]
+		keyPath, err := keyPathFromKey(p.maxLevels, ks[i])
+		if err != nil {
+			// TODO in a future iteration, invalids will contain
+			// the reason of the error of why each index is
+			// invalid.
+			invalids = append(invalids, i)
+			continue
+		}
+
+		var kvsI kv
+		kvsI.pos = i
+		kvsI.keyPath = keyPath
+		kvsI.k = ks[i]
+		kvsI.v = vs[i]
+		kvs = append(kvs, kvsI)
 	}
 
-	return kvs, nil
+	return kvs, invalids, nil
 }
 
 // vt stands for virtual tree. It's a tree that does not have any computed hash
@@ -94,9 +104,9 @@ }
 
 	l := int(math.Log2(float64(nCPU)))
 
-	kvs, err := t.params.keysValuesToKvs(ks, vs)
+	kvs, invalids, err := t.params.keysValuesToKvs(ks, vs)
 	if err != nil {
-		return nil, err
+		return invalids, err
 	}
 
 	buckets := splitInBuckets(kvs, nCPU)
@@ -186,7 +196,6 @@ }(i)
 	}
 	wg.Wait()
 
-	var invalids []int
 	for i := 0; i < len(invalidsInBucket); i++ {
 		invalids = append(invalids, invalidsInBucket[i]...)
 	}
@@ -284,7 +293,10 @@ }
 
 // add adds a key&value as a leaf in the VirtualTree
 func (t *vt) add(fromLvl int, k, v []byte) error {
-	leaf := newLeafNode(t.params, k, v)
+	leaf, err := newLeafNode(t.params, k, v)
+	if err != nil {
+		return err
+	}
 	if t.root == nil {
 		t.root = leaf
 		return nil
@@ -366,16 +378,18 @@
 	return pairs, nil
 }
 
-func newLeafNode(p *params, k, v []byte) *node {
-	keyPath := make([]byte, p.hashFunction.Len())
-	copy(keyPath[:], k)
+func newLeafNode(p *params, k, v []byte) (*node, error) {
+	keyPath, err := keyPathFromKey(p.maxLevels, k)
+	if err != nil {
+		return nil, err
+	}
 	path := getPath(p.maxLevels, keyPath)
 	n := &node{
 		k:    k,
 		v:    v,
 		path: path,
 	}
-	return n
+	return n, nil
 }
 
 type virtualNodeType int

@@ -2,6 +2,7 @@ package arbo
 
 import (
 	"encoding/hex"
+	"math"
 	"math/big"
 	"testing"
 
@@ -9,28 +10,62 @@ qt "github.com/frankban/quicktest"
 	"go.vocdoni.io/dvote/db/badgerdb"
 )
 
+// testVirtualTree adds the given key-values and tests the vt root against the
+// Tree
+func testVirtualTree(c *qt.C, maxLevels int, keys, values [][]byte) {
+	c.Assert(len(keys), qt.Equals, len(values))
+
+	// normal tree, to have an expected root value
+	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
+	c.Assert(err, qt.IsNil)
+	tree, err := NewTree(database, maxLevels, HashFunctionSha256)
+	c.Assert(err, qt.IsNil)
+	for i := 0; i < len(keys); i++ {
+		err := tree.Add(keys[i], values[i])
+		c.Assert(err, qt.IsNil)
+	}
+
+	// virtual tree
+	vTree := newVT(maxLevels, HashFunctionSha256)
+
+	c.Assert(vTree.root, qt.IsNil)
+
+	for i := 0; i < len(keys); i++ {
+		err := vTree.add(0, keys[i], values[i])
+		c.Assert(err, qt.IsNil)
+	}
+
+	// compute hashes, and check Root
+	_, err = vTree.computeHashes()
+	c.Assert(err, qt.IsNil)
+	root, err := tree.Root()
+	c.Assert(err, qt.IsNil)
+	c.Assert(vTree.root.h, qt.DeepEquals, root)
+}
+
 func TestVirtualTreeTestVectors(t *testing.T) {
 	c := qt.New(t)
 
-	bLen := 32
+	maxLevels := 32
+	keyLen := int(math.Ceil(float64(maxLevels) / float64(8))) //nolint:gomnd
 	keys := [][]byte{
-		BigIntToBytes(bLen, big.NewInt(1)),
-		BigIntToBytes(bLen, big.NewInt(33)),
-		BigIntToBytes(bLen, big.NewInt(1234)),
-		BigIntToBytes(bLen, big.NewInt(123456789)),
+		BigIntToBytes(keyLen, big.NewInt(1)),
+		BigIntToBytes(keyLen, big.NewInt(33)),
+		BigIntToBytes(keyLen, big.NewInt(1234)),
+		BigIntToBytes(keyLen, big.NewInt(123456789)),
 	}
 	values := [][]byte{
-		BigIntToBytes(bLen, big.NewInt(2)),
-		BigIntToBytes(bLen, big.NewInt(44)),
-		BigIntToBytes(bLen, big.NewInt(9876)),
-		BigIntToBytes(bLen, big.NewInt(987654321)),
+		BigIntToBytes(keyLen, big.NewInt(2)),
+		BigIntToBytes(keyLen, big.NewInt(44)),
+		BigIntToBytes(keyLen, big.NewInt(9876)),
+		BigIntToBytes(keyLen, big.NewInt(987654321)),
 	}
 
 	// check the root for different batches of leafs
-	testVirtualTree(c, 10, keys[:1], values[:1])
-	testVirtualTree(c, 10, keys[:2], values[:2])
-	testVirtualTree(c, 10, keys[:3], values[:3])
-	testVirtualTree(c, 10, keys[:4], values[:4])
+	testVirtualTree(c, maxLevels, keys[:1], values[:1])
+	testVirtualTree(c, maxLevels, keys[:2], values[:2])
+	testVirtualTree(c, maxLevels, keys[:3], values[:3])
+	testVirtualTree(c, maxLevels, keys[:4], values[:4])
 
 	// test with hardcoded values
 	testvectorKeys := []string{
@@ -53,8 +88,8 @@ values = append(values, []byte{0})
 	}
 
 	// check the root for different batches of leafs
-	testVirtualTree(c, 10, keys[:1], values[:1])
-	testVirtualTree(c, 10, keys, values)
+	testVirtualTree(c, 256, keys[:1], values[:1])
+	testVirtualTree(c, 256, keys, values)
 }
 
 func TestVirtualTreeRandomKeys(t *testing.T) {
@@ -69,45 +104,14 @@ keys[i] = randomBytes(32)
 		values[i] = randomBytes(32)
 	}
 
-	testVirtualTree(c, 100, keys, values)
-}
-
-func testVirtualTree(c *qt.C, maxLevels int, keys, values [][]byte) {
-	c.Assert(len(keys), qt.Equals, len(values))
-
-	// normal tree, to have an expected root value
-	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
-	c.Assert(err, qt.IsNil)
-	tree, err := NewTree(database, maxLevels, HashFunctionSha256)
-	c.Assert(err, qt.IsNil)
-	for i := 0; i < len(keys); i++ {
-		err := tree.Add(keys[i], values[i])
-		c.Assert(err, qt.IsNil)
-	}
-
-	// virtual tree
-	vTree := newVT(maxLevels, HashFunctionSha256)
-
-	c.Assert(vTree.root, qt.IsNil)
-
-	for i := 0; i < len(keys); i++ {
-		err := vTree.add(0, keys[i], values[i])
-		c.Assert(err, qt.IsNil)
-	}
-
-	// compute hashes, and check Root
-	_, err = vTree.computeHashes()
-	c.Assert(err, qt.IsNil)
-	root, err := tree.Root()
-	c.Assert(err, qt.IsNil)
-	c.Assert(vTree.root.h, qt.DeepEquals, root)
+	testVirtualTree(c, 256, keys, values)
 }
 
 func TestVirtualTreeAddBatch(t *testing.T) {
 	c := qt.New(t)
 
 	nLeafs := 2000
-	maxLevels := 100
+	maxLevels := 256
 
 	keys := make([][]byte, nLeafs)
 	values := make([][]byte, nLeafs)
@@ -151,7 +155,7 @@ vTree2 := newVT(7, HashFunctionPoseidon) // used for addBatch
 
 	var keys, values [][]byte
 	for i := 0; i < 128; i++ {
-		k := BigIntToBytes(32, big.NewInt(int64(i)))
+		k := BigIntToBytes(1, big.NewInt(int64(i)))
 		v := k
 
 		keys = append(keys, k)