Cyphertext management method, cyphertext management apparatus, and program

A ciphertext management method for managing ciphertext data, comprising:

• encrypting, by means of an encryption unit included in a ciphertext management apparatus, secret data using key data so as to generate the ciphertext data;
• dividing, by means of a dividing unit included in the ciphertext management apparatus, the ciphertext data and the key data so as to generate m (m represents an integer) pieces of divided ciphertext data and n (n represents an integer) pieces of divided key data;
• and
• linking, by means of a distribution unit included in the ciphertext management apparatus, the divided ciphertext data and the divided key data,
• storing, by means of a pair storage unit, pairs of the divided ciphertext data and the divided key data,
• transmitting, by means of a transmission unit, the pairs of the divided ciphertext data and the divided key data to a reconstruction unit,
• reconstructing, by means of the reconstruction unit, pairs of the ciphertext data and the key data from the pairs of the divided ciphertext data and the divided key data,
• wherein, in the linking, equal pairs are generated such that each piece of divided ciphertext data and each piece of divided key data are used only once to perform pairing,
• wherein unequal pairs are generated such that one piece from among at least one from among the divided ciphertext data and the divided key data is used multiple times to perform pairing,
• wherein, in the dividing, the dividing unit divides the ciphertext data and the key data such that the ciphertext data can be reconstructed based on s (s represents an integer that is equal to or greater than 2 and that is smaller than m) pieces of divided ciphertext data, and such that the key data can be reconstructed based on t (t represents an integer that is equal to or greater than 2 and that is smaller than n) pieces of divided key data,
• wherein, in the linking, all the unequal pairs are generated such that the number of the pieces of divided ciphertext data to be used is smaller than s and/or the number of the pieces of divided key data to be used is smaller than t, and
• wherein, in the transmitting, in a case that the reconstruction unit has a permission to one or more unequal pairs and no permission to equal pairs, the transmission unit transmits one or more equal pairs after the reconstruction unit acquires a permission to one or more equal pairs.