# Tried to draw a network protocol but always get error

I tried to draw the following diagram in LaTeX

I reference the method from here:

I can successfully compile the code from above link, but when I attempted to draw more lines between A and B by modifying the number in the:

\draw[shorten <=-1cm,shorten >=-1cm,-latex] (m-4-2.south west)--(m-4-2.south east);
\draw[shorten <=-1cm,shorten >=-1cm,-latex] (m-7-2.south east)--(m-7-2.south west);

I always got the error message said:

Package pgf Error: No shape named m-5-2 is known.

Any solution for this problem? Thanks.

$\begin{array}{@{} c c c @{}} A & & B \\ & \XLR{\text{KE}} \\ \text{obtain }k,T^A_{\text{KE}} & & \text{obtain }k,T^B_{\text{KE}} \\ K:= \text{PRF}(k,\text{"KE"}) & & K:= \text{PRF}(k,\text{"KE"}) \\ K_\text{mac}:= \text{PRF}(k,\text{"MAC"}) & & K_\text{mac}:= \text{PRF}(k,\text{"MAC"}) \\ & \XR{r_A} \\ & \XL{r_B} \\ \text{[...]} \end{array}$