Disclosed is an Academy, Research Institute, and Agency (ARIA) encryption/decryption apparatus for encrypting and decrypting input data by repeating a plurality of rounds. The ARIA encryption/decryption apparatus includes a first register storing input data or an intermediate calculation value according to a first control signal; a second register storing a input round key for every round; an exclusive OR operation unit performing an exclusive OR operation on values stored in the first and second registers; a substitution unit substituting a result of the exclusive OR operation on a basis of an ARIA substitution algorithm; a diffusion unit diffusing a result of the substitution in the substitution unit on a basis of an ARIA diffusion algorithm if a current round is not a final round; and a control unit outputting the first control signal so that an output of the diffusion unit is used as the intermediate calculation value if the current round is the final round or an output of the substitution unit is used as the intermediate calculation value if the current round is the final round, and outputting an output of the exclusive OR operation unit as a result of the ARIA encryption/decryption.