panic if rc4 xor'ed buffer doesn't fit into output buffer

rc4.go

@@ -52,6 +52,9 @@ func (c *RC4Cipher) XORKeyStream(dst, src []byte) {
 	if inexactOverlap(dst[:len(src)], src) {
 		panic("crypto/rc4: invalid buffer overlap")
 	}
+	// panic if len(dst) < len(src) with a runtime out of bound error,
+	// which is what crypto/rc4 does.
+	_ = dst[len(src)-1]
 	var outLen C.int
 	if C.go_openssl_EVP_EncryptUpdate(c.ctx, base(dst), &outLen, base(src), C.int(len(src))) != 1 {
 		panic("crypto/cipher: EncryptUpdate failed")

rc4_test.go

@@ -138,6 +138,29 @@ func TestRC4Block(t *testing.T) {
 	}
 }
 
+func TestRC4OutOfBoundsWrite(t *testing.T) {
+	// This cipherText is encrypted "0123456789"
+	cipherText := []byte{238, 41, 187, 114, 151, 2, 107, 13, 178, 63}
+	cipher, err := openssl.NewRC4Cipher([]byte{0})
+	if err != nil {
+		panic(err)
+	}
+	want := "abcdefghij"
+	plainText := []byte(want)
+	shorterLen := len(cipherText) / 2
+	defer func() {
+		err := recover()
+		if err == nil {
+			t.Error("XORKeyStream expected to panic on len(dst) < len(src), but didn't")
+		}
+		const plain = "0123456789"
+		if plainText[shorterLen] == plain[shorterLen] {
+			t.Errorf("XORKeyStream did out of bounds write, want %v, got %v", want, string(plainText))
+		}
+	}()
+	cipher.XORKeyStream(plainText[:shorterLen], cipherText)
+}
+
 func benchmarkRC4(b *testing.B, size int64) {
 	if !openssl.SupportsRC4() {
 		b.Skip("RC4 is not supported")