| 摘要 |
PROBLEM TO BE SOLVED: To prevent a receiver from reading document contents before the date or anytime other than the period designated by an original document producer. SOLUTION: The original document is fetched and enciphered. A cryptographic key related to a date or a period designated by the producer of the original document is acquired from an encipher date distribution site, and the cryptographic key enciphering the original document is enciphered by using it. The enciphered cryptographic key 20 is added to the enciphered original document 19 together with a designated date 18 and a decoding program 17 to prepare a date-designated document 16. The decoding program fetches the present date for a clock, etc., incorporated in a computer and compares the present date with the designated date or period added to the date-designated document, and when they are matched with each other, a decoding key is acquired from the encipher date distribution site. The cryptographic key enciphering the original document is decoded by using the decoding key. The enciphered original document is decoded by using the decoded cryptographic key. |
