To prove triangles \( \triangle ABC \cong \triangle CDA \) using the ASA (Angle-Side-Angle) Congruence Theorem, we must confirm that the included side between the two angles is congruent. Since we already have that \( \angle BCA \cong \angle DAC \) and \( \angle BAC \cong \angle DCA \), we just need to show the congruence of the side \( AC \).
The additional information needed and justified in this case is:
\( AC \cong AC \) by the Reflexive Property of Congruence.
This shows that the diagonal \( AC \) is the same length in both triangles, satisfying the requirements of the ASA theorem to conclude that \( \triangle ABC \cong \triangle CDA \).