An apparatus (14-1, 50) for encrypting image data is disclosed, which includes: a processor (42) configured to divide encoded still image data or video data into a first element (a) and a second element (b); generate a first key (k1); generate a second key (k2) for at least every one image of the still image data or video data; encrypt a first portion (b1) of the second element (b) of the image using the first key (k1) and the second key (k2) corresponding to the at least one image; replace a second portion (b2) of the second element (b) of the at least one image other than the first portion (b1) with the second key (k2) corresponding to the image; and compose the first element (a) of the image, the encrypted first portion (b1′) of the second element (b), and the second key (k2), for at least every one image to generate encrypted data. An apparatus (16-2, 17-2, 70) for decrypting which decrypts the encrypted data generated by the encryption apparatus is also disclosed.