diff --git a/otp/cipher.go b/otp/cipher.go
index 993ab22..f9eb79d 100644
--- a/otp/cipher.go
+++ b/otp/cipher.go
@@ -6,10 +6,53 @@ import (
 	"io"
 )
 
+type cipherReader struct {
+	r    io.Reader
+	prng io.Reader
+}
+
+type cipherWriter struct {
+	w    io.Writer
+	prng io.Reader
+}
+
+func (c *cipherReader) Read(p []byte) (n int, err error) {
+	n, err = c.r.Read(p)
+	if err != nil {
+		if err == io.EOF {
+			p = p[:n]
+		} else {
+			return
+		}
+	}
+	rand := make([]byte, n)
+	_, rngErr := c.prng.Read(rand)
+	if rngErr != nil {
+		return
+	}
+	for i := 0; i < n; i++ {
+		p[i] ^= rand[i]
+	}
+	return
+}
+
+func (c *cipherWriter) Write(p []byte) (n int, err error) {
+	data := make([]byte, len(p))
+	_, err = c.prng.Read(data)
+	if err != nil {
+		return
+	}
+	for i := 0; i < len(p); i++ {
+		data[i] ^= p[i]
+	}
+	n, err = c.w.Write(data)
+	return
+}
+
 func NewReader(r io.Reader, prng io.Reader) io.Reader {
-	panic("implement me")
+	return &cipherReader{r, prng}
 }
 
 func NewWriter(w io.Writer, prng io.Reader) io.Writer {
-	panic("implement me")
+	return &cipherWriter{w, prng}
 }