diff --git a/PROTOCOL.md b/PROTOCOL.md
index 71174db..b390bbc 100644
--- a/PROTOCOL.md
+++ b/PROTOCOL.md
@@ -188,7 +188,7 @@ When receiving an invalid token, the server may send back to the client a challe
 | Step | Calculation |
 |:--------:|:------|
 | `1` | $t\_s = \mid \frac{t\_{now}}{W}\mid, m\_s = t\_s \ \mathbb{Z}\_{(2)}$ |
-| `2` | $y\_1 = T \oplus h(t\_c)$ |
+| `2` | $y\_1 = T \oplus h(t\_s)$ |
 | `3` | $y\_2 = y\_1 \oplus m\_s$ |
 
 Send $y_1$ and $y_2$.
\ No newline at end of file