rng: Fix subkeys not initialized

rng/reader.go

@@ -79,15 +79,14 @@ func (r *Reader) init() error {
 		r.bufferSeeded = !r.o.fullReset
 	}
 	// Always reset subkeys.
-	var tmp [32]byte
 	_, err := io.ReadFull(r.o.rng, r.tmp[:])
 	if err != nil {
 		return err
 	}
-	r.subxor[0] = binary.LittleEndian.Uint64(tmp[:8])
-	r.subxor[1] = binary.LittleEndian.Uint64(tmp[8:16])
-	r.subxor[2] = binary.LittleEndian.Uint64(tmp[16:24])
-	r.subxor[3] = binary.LittleEndian.Uint64(tmp[24:32])
+	r.subxor[0] = binary.LittleEndian.Uint64(r.tmp[:8])
+	r.subxor[1] = binary.LittleEndian.Uint64(r.tmp[8:16])
+	r.subxor[2] = binary.LittleEndian.Uint64(r.tmp[16:24])
+	r.subxor[3] = binary.LittleEndian.Uint64(r.tmp[24:32])
 	r.offset = 0
 	return nil
 }

rng/reader_test.go

@@ -25,6 +25,55 @@ import (
 	"testing"
 )
 
+func TestSubkeysInitialized(t *testing.T) {
+	src := rand.New(rand.NewSource(12345))
+	r, err := NewReader(WithRNG(src))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	allZero := true
+	for _, v := range r.subxor {
+		if v != 0 {
+			allZero = false
+			break
+		}
+	}
+	if allZero {
+		t.Fatal("subxor keys are all zero after initialization")
+	}
+
+	// With fullReset=false (default), the buffer is reused across Reset.
+	// If subkeys were always zero, output would be identical after reset.
+	buf1 := make([]byte, 1024)
+	io.ReadFull(r, buf1)
+
+	r.Reset()
+
+	buf2 := make([]byte, 1024)
+	io.ReadFull(r, buf2)
+
+	if bytes.Equal(buf1, buf2) {
+		t.Fatal("output identical after reset — subkeys not re-initialized")
+	}
+}
+
+func TestResetSizeProducesUniqueOutput(t *testing.T) {
+	const size = 64 << 20 // 64 MiB
+	r, _ := NewReader(WithSize(size))
+
+	out1 := make([]byte, size)
+	io.ReadFull(r, out1)
+
+	r.ResetSize(size)
+	out2 := make([]byte, size)
+	io.ReadFull(r, out2)
+
+	if bytes.Equal(out1, out2) {
+		t.Fatal("ResetSize produced identical output: subxor keys are not being randomized")
+	}
+}
+
 func BenchmarkReader(b *testing.B) {
 	for _, size := range []int{1000, 1024, 16384, 1 << 20} {
 		r, err := NewReader()