Organic synthesis: explain question

A student plans to prepare 2-hydroxybutanenitrile, $\ce{CH3CH2CH(OH)CN}$, starting from propanal, $\ce{CH3CH2CHO}$. The proposed route is:

1. propanal is treated with an excess of hydrogen cyanide, $\ce{HCN}$, and a small amount of potassium cyanide, $\ce{KCN}$, is added.
2. the product of step 1 is warmed with dilute aqueous sulfuric acid, $\ce{H2SO4}$.

Explain, with reference to the functional groups and reactions involved, why step 1 produces the desired compound, why step 2 is necessary, and what by-product could form in step 1. In your answer, name the type of reaction in each step and state the role of the $\ce{KCN}$.