diff --git a/tree.go b/tree.go
index 7df1f91..bf59e49 100644
--- a/tree.go
+++ b/tree.go
@@ -14,6 +14,7 @@ package arbo
 import (
 	"bytes"
 	"fmt"
+	"math"
 	"sync/atomic"
 	"time"
 
@@ -114,12 +115,12 @@ func (t *Tree) Add(k, v []byte) error {
 	path := getPath(t.maxLevels, keyPath)
 	// go down to the leaf
 	var siblings [][]byte
-	_, _, siblings, err := t.down(k, t.root, siblings, path, 0)
+	_, _, siblings, err := t.down(k, t.root, siblings, path, 0, false)
 	if err != nil {
 		return err
 	}
 
-	leafKey, leafValue, err := t.newLeafValue(k, v)
+	leafKey, leafValue, err := newLeafValue(t.hashFunction, k, v)
 	if err != nil {
 		return err
 	}
@@ -148,7 +149,7 @@ func (t *Tree) Add(k, v []byte) error {
 }
 
 // down goes down to the leaf recursively
-func (t *Tree) down(newKey, currKey []byte, siblings [][]byte, path []bool, l int) (
+func (t *Tree) down(newKey, currKey []byte, siblings [][]byte, path []bool, l int, getLeaf bool) (
 	[]byte, []byte, [][]byte, error) {
 	if l > t.maxLevels-1 {
 		return nil, nil, nil, fmt.Errorf("max level")
@@ -177,6 +178,9 @@ func (t *Tree) down(newKey, currKey []byte, siblings [][]byte, path []bool, l in
 		}
 
 		if !bytes.Equal(currValue, emptyValue) {
+			if getLeaf {
+				return currKey, currValue, siblings, nil
+			}
 			oldLeafKey, _ := readLeafValue(currValue)
 			oldLeafKeyFull := make([]byte, t.hashFunction.Len())
 			copy(oldLeafKeyFull[:], oldLeafKey)
@@ -200,12 +204,12 @@ func (t *Tree) down(newKey, currKey []byte, siblings [][]byte, path []bool, l in
 			// right
 			lChild, rChild := readIntermediateChilds(currValue)
 			siblings = append(siblings, lChild)
-			return t.down(newKey, rChild, siblings, path, l+1)
+			return t.down(newKey, rChild, siblings, path, l+1, getLeaf)
 		}
 		// left
 		lChild, rChild := readIntermediateChilds(currValue)
 		siblings = append(siblings, rChild)
-		return t.down(newKey, lChild, siblings, path, l+1)
+		return t.down(newKey, lChild, siblings, path, l+1, getLeaf)
 	default:
 		return nil, nil, nil, fmt.Errorf("invalid value")
 	}
@@ -241,12 +245,12 @@ func (t *Tree) up(tx db.Tx, key []byte, siblings [][]byte, path []bool, l int) (
 	var k, v []byte
 	var err error
 	if path[l] {
-		k, v, err = t.newIntermediate(siblings[l], key)
+		k, v, err = newIntermediate(t.hashFunction, siblings[l], key)
 		if err != nil {
 			return nil, err
 		}
 	} else {
-		k, v, err = t.newIntermediate(key, siblings[l])
+		k, v, err = newIntermediate(t.hashFunction, key, siblings[l])
 		if err != nil {
 			return nil, err
 		}
@@ -265,8 +269,8 @@ func (t *Tree) up(tx db.Tx, key []byte, siblings [][]byte, path []bool, l int) (
 	return t.up(tx, k, siblings, path, l-1)
 }
 
-func (t *Tree) newLeafValue(k, v []byte) ([]byte, []byte, error) {
-	leafKey, err := t.hashFunction.Hash(k, v, []byte{1})
+func newLeafValue(hashFunc HashFunction, k, v []byte) ([]byte, []byte, error) {
+	leafKey, err := hashFunc.Hash(k, v, []byte{1})
 	if err != nil {
 		return nil, nil, err
 	}
@@ -292,14 +296,14 @@ func readLeafValue(b []byte) ([]byte, []byte) {
 	return k, v
 }
 
-func (t *Tree) newIntermediate(l, r []byte) ([]byte, []byte, error) {
-	b := make([]byte, PrefixValueLen+t.hashFunction.Len()*2)
+func newIntermediate(hashFunc HashFunction, l, r []byte) ([]byte, []byte, error) {
+	b := make([]byte, PrefixValueLen+hashFunc.Len()*2)
 	b[0] = 2
 	b[1] = byte(len(l))
-	copy(b[PrefixValueLen:PrefixValueLen+t.hashFunction.Len()], l)
-	copy(b[PrefixValueLen+t.hashFunction.Len():], r)
+	copy(b[PrefixValueLen:PrefixValueLen+hashFunc.Len()], l)
+	copy(b[PrefixValueLen+hashFunc.Len():], r)
 
-	key, err := t.hashFunction.Hash(l, r)
+	key, err := hashFunc.Hash(l, r)
 	if err != nil {
 		return nil, nil, err
 	}
@@ -332,9 +336,67 @@ func getPath(numLevels int, k []byte) []bool {
 // GenProof generates a MerkleTree proof for the given key. If the key exists in
 // the Tree, the proof will be of existence, if the key does not exist in the
 // tree, the proof will be of non-existence.
-func (t *Tree) GenProof(k, v []byte) ([]byte, error) {
-	// unimplemented
-	return nil, fmt.Errorf("unimplemented")
+func (t *Tree) GenProof(k []byte) ([]byte, error) {
+	keyPath := make([]byte, t.hashFunction.Len())
+	copy(keyPath[:], k)
+
+	path := getPath(t.maxLevels, keyPath)
+	// go down to the leaf
+	var siblings [][]byte
+	_, value, siblings, err := t.down(k, t.root, siblings, path, 0, true)
+	if err != nil {
+		return nil, err
+	}
+
+	leafK, leafV := readLeafValue(value)
+	if !bytes.Equal(k, leafK) {
+		fmt.Println("key not in Tree")
+		fmt.Println(leafK)
+		fmt.Println(leafV)
+		// TODO proof of non-existence
+		panic(fmt.Errorf("unimplemented"))
+	}
+
+	s := PackSiblings(t.hashFunction, siblings)
+	return s, nil
+}
+
+// PackSiblings packs the siblings into a byte array.
+// [      1 byte        | L bytes |      32 * N bytes     ]
+// [  bitmap length (L) |  bitmap |  N non-zero siblings  ]
+// Where the bitmap indicates if the sibling is 0 or a value from the siblings array.
+func PackSiblings(hashFunc HashFunction, siblings [][]byte) []byte {
+	var b []byte
+	var bitmap []bool
+	emptySibling := make([]byte, hashFunc.Len())
+	for i := 0; i < len(siblings); i++ {
+		if bytes.Equal(siblings[i], emptySibling) {
+			bitmap = append(bitmap, false)
+		} else {
+			bitmap = append(bitmap, true)
+			b = append(b, siblings[i]...)
+		}
+	}
+
+	bitmapBytes := bitmapToBytes(bitmap)
+	l := len(bitmapBytes)
+
+	res := make([]byte, l+1+len(b))
+	res[0] = byte(l) // set the bitmapBytes length
+	copy(res[1:1+l], bitmapBytes)
+	copy(res[1+l:], b)
+	return res
+}
+
+func bitmapToBytes(bitmap []bool) []byte {
+	bitmapBytesLen := int(math.Ceil(float64(len(bitmap)) / 8)) //nolint:gomnd
+	b := make([]byte, bitmapBytesLen)
+	for i := 0; i < len(bitmap); i++ {
+		if bitmap[i] {
+			b[i/8] |= 1 << (i % 8)
+		}
+	}
+	return b
 }
 
 // Get returns the value for a given key
@@ -348,5 +410,3 @@ func CheckProof(k, v, root, mproof []byte) (bool, error) {
 	// unimplemented
 	return false, fmt.Errorf("unimplemented")
 }
-
-// TODO method to export & import the full Tree without values
diff --git a/tree_test.go b/tree_test.go
index 672a674..db0ac56 100644
--- a/tree_test.go
+++ b/tree_test.go
@@ -145,6 +145,24 @@ func TestAux(t *testing.T) {
 	assert.Nil(t, err)
 }
 
+func TestGenProof(t *testing.T) {
+	tree, err := NewTree(memory.NewMemoryStorage(), 100, HashFunctionPoseidon)
+	require.Nil(t, err)
+
+	defer tree.db.Close()
+	for i := 0; i < 10; i++ {
+		k := BigIntToBytes(big.NewInt(int64(i)))
+		v := BigIntToBytes(big.NewInt(int64(i * 2)))
+		if err := tree.Add(k, v); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	k := BigIntToBytes(big.NewInt(int64(7)))
+	_, err = tree.GenProof(k)
+	assert.Nil(t, err)
+}
+
 func BenchmarkAdd(b *testing.B) {
 	// prepare inputs
 	var ks, vs [][]byte