SHA-3: Fixes crash when cloning Shake state

hash/shake/sha3.go

@@ -65,15 +65,15 @@ func (d *state) Reset() {
 	d.buf = d.storage[:0]
 }
 
-func (d *state) clone() *state {
-	ret := *d
+func (d *state) clone(ret *state) {
+	// shallow copy
+	*ret = *d
+	// deep copy for a buf
 	if ret.state == spongeAbsorbing {
-		ret.buf = ret.storage[:len(ret.buf)]
+		ret.buf = ret.storage[:len(d.buf)]
 	} else {
-		ret.buf = ret.storage[d.rate-cap(d.buf) : d.rate]
+		ret.buf = ret.storage[d.rate-len(d.buf) : d.rate]
 	}
-
-	return &ret
 }
 
 // permute applies the KeccakF-1600 permutation. It handles
@@ -187,7 +187,8 @@ func (d *state) Read(out []byte) (n int, err error) {
 func (d *state) Sum(in []byte) []byte {
 	// Make a copy of the original hash so that caller can keep writing
 	// and summing.
-	dup := d.clone()
+	var dup state
+	d.clone(&dup)
 	hash := make([]byte, dup.outputLen)
 	dup.Read(hash)
 	return append(in, hash...)

hash/shake/sha3_test.go

@@ -284,6 +284,28 @@ func TestClone(t *testing.T) {
 	}
 }
 
+// Checks wether reset works correctly after clone
+func TestCloneAndReset(t *testing.T) {
+	// Shake 256, uses SHA-3 with rate = 136
+	d1 := NewShake256()
+	buf1 := make([]byte, 28, 28)
+	buf2 := make([]byte, 28, 28)
+	d1.Write([]byte{0xcc})
+	// Reading x bytes where x<168-136. This makes capability
+	// of the state buffer shorter.
+	d1.Read(buf1)
+	// This will crash if sha-3 code uses cap() instead
+	// of len() when calculating length of state buffer
+	d2 := d1.Clone()
+	d2.Reset()
+	d2.Write([]byte{0xcc})
+	d2.Read(buf2)
+
+	if !bytes.Equal(buf1, buf2) {
+		t.Error("Different value when reading after reset")
+	}
+}
+
 // BenchmarkPermutationFunction measures the speed of the permutation function
 // with no input data.
 func BenchmarkPermutationFunction(b *testing.B) {

hash/shake/shake.go

@@ -86,9 +86,11 @@ func (c *CShake) Reset() {
 
 // Clone returns copy of a cSHAKE context within its current state.
 func (c *CShake) Clone() CShake {
-	b := make([]byte, len(c.initBlock))
-	copy(b, c.initBlock)
-	return CShake{state: *c.clone(), initBlock: b}
+	var ret CShake
+	c.clone(&ret.state)
+	ret.initBlock = make([]byte, len(c.initBlock))
+	copy(ret.initBlock, c.initBlock)
+	return ret
 }
 
 // NewShake128 creates a new SHAKE128 variable-output-length CShake.