El Gamal variant of Diffie-Hellman key exchange

The El Gamal variant of Diffie-Hellman has the same setup with a public prime $q$ and generator $\alpha$.

Let $E$ be any symmetric encryption function. The schema is as follows:

1. $A$ chooses $X_A$ and computes $Y_A={\alpha }^{X_A}\text{ mod }q$. $A\to B:Y_A$
2. $B$ chooses integer $X_B$, computes $Y_B={\alpha }^{X_B}\text{ mod }q$ , and computes key $K=Y_A^{X_B}\text{ mod }q$. $B\to A:(E(M,K),Y_B)$
3. $A$ computes $K=Y_B^{X_A}\text{ mod }q$ $\text{and uses }K\text{ to decrypt }E(M,K)$.