Changing the typo in section 8.4 hmac (#45)

crypto/macs.md

@@ -53,7 +53,7 @@ There are a number of schemes out there, but one good one is AES-CMAC, an algori
 In AES-EMAC, the key $$K$$ is 256 bits, viewed as a pair of 128-bit AES keys: $$K=\langle K_1,K_2 \rangle$$. The message $$M$$ is decomposed into a sequence of 128-bit blocks: $$M = P_1 \Vert P_2 \Vert ... \Vert P_n$$. We set $$S_0 = 0$$ and compute
 
 $$
-S_i = \textrm{AES}_{K_1}(S*{i-1} \oplus P_i),\qquad
+S_i = \textrm{AES}_{K_1}(S_{i-1} \oplus P_i),\qquad
     \text{for $i=1,2,\dots,n$.}
 $$