4. The apparatus according to claim 1 , wherein the one or more hardware processors are further configured to generate identification information for identifying the plaintext, to generate the concealing key based on the identification information, and to output the concealed text, the identification information, and the tag as a set.
4. The apparatus according to claim 1 , wherein the one or more hardware processors are further configured to generate identification information for identifying the plaintext, to generate the concealing key based on the identification information, and to output the concealed text, the identification information, and the tag as a set. 5. The apparatus according to claim 4 , wherein the one or more hardware processors generate the concealing key based on a master key and the identification information.