By introducing inequality to the information dispersal/sharing storage method, a ciphertext management method or the like is provided to support novel ciphertext data management. After the ciphertext and key data are each divided, pairs of the divided ciphertext and key data are generated. Specifically, they are one-to-one paired as with conventional techniques. Furthermore, additional one-to-many pairs are generated. The generated one-to-one pairs provide equality as with conventional techniques. When the number of the one-to-one pairs of the divided ciphertext and key data that can be used is equal to or greater than a threshold number, both the ciphertext data and the key data can be reconstructed, and accordingly, the secret data can be decoded. In contrast, even when the one-to-many pairs that can be used is equal to or greater than a threshold number, the ciphertext data and/or the key data cannot be reconstructed. This provides inequality.
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