Digital signature server and user terminal

A user terminal operative to receive, together with content data, signature data indicating that the content data is a certified one, comprising a processor configured to:

• receive from outside, together with the content data, signature data generated by encrypting a content digest by a plurality of signature keys included in a signature key column set which is an aggregate of signature key columns each including a plurality of signature keys, the content digest being generated based on the content data;
• receive from outside, a signature key set which is an aggregate of the signature keys selected one by one from the signature key columns of the signature key column set respectively;
• and
• verify the signature data by decrypting the content digest included in the signature data by using the signature keys included in the signature key set, and then comparing this content digest with a content digest obtained from the content data received from outside.

A method of verifying signature data indicating that the content data is a certified one at a user terminal, the method comprising:

• receiving, from outside, at the user terminal, together with content data, signature data generated by encrypting a content digest by a plurality of signature keys included in a signature key column set which is an aggregate of signature key columns each including a plurality of signature keys, the content digest being generated based on the content data;
• receiving, from outside, at the user terminal, a signature key set which is an aggregate of the signature keys selected one by one from the signature key columns of the signature key column set respectively;
• and
• verifying the signature data by decrypting the content digest included in the signature data by using the signature keys included in the signature key set, and then comparing this content digest with a content digest obtained from the content data received from outside.

A non-transitory computer-readable medium configured to perform a method of verifying signature data indicating that the content data is a certified one at a user terminal, the method comprising:

• receiving, from outside, at the user terminal, together with content data, signature data generated by encrypting a content digest by a plurality of signature keys included in a signature key column set which is an aggregate of signature key columns each including a plurality of signature keys, the content digest being generated based on the content data;
• receiving, from outside, at the user terminal, a signature key set which is an aggregate of the signature keys selected one by one from the signature key columns of the signature key column set respectively;
• and
• verifying the signature data by decrypting the content digest included in the signature data by using the signature keys included in the signature key set, and then comparing this content digest with a content digest obtained from the content data received from outside.

A method of generating signature data indicating that content data is a certified one, at a server, the method comprising:

• storing, at the server, a signature key column set which is an aggregate of signature key columns each of which is an aggregate of a plurality of signature keys;
• storing, at the server, correspondence relationship information indicating a correspondence relationship between a signature key set and a user terminal, the signature key set being an aggregate of any signature keys selected from the signature key columns respectively;
• and
• generating signature data by encrypting a content digest by using the plurality of signature keys included in the signature key column set, the content digest being generated based on the content data.

A non-transitory computer-readable medium configured to perform a method of generating signature data indicating that content data is a certified one, at a server, the method comprising:

• storing, at the server, a signature key column set which is an aggregate of signature key columns each of which is an aggregate of a plurality of signature keys;
• storing, at the server, correspondence relationship information indicating a correspondence relationship between a signature key set and a user terminal, the signature key set being an aggregate of any signature keys selected from the signature key columns respectively;
• and
• generating signature data by encrypting a content digest by using the plurality of signature keys included in the signature key column set, the content digest being generated based on the content data.

A signature data server operative to generate signature data indicating that content data is a certified one, the signature data server comprising:

• a signature key column set database configured to store a signature key column set which is an aggregate of signature key columns each of which is an aggregate of a plurality of signature keys;
• a correspondence relationship information database configured to store correspondence relationship information indicating a correspondence relationship between a signature key set and a user terminal, the signature key set being an aggregate of any signature keys selected from the signature key columns respectively;
• and
• a processor configured to generate signature data by encrypting a content digest by using the plurality of signature keys included in the signature key column set, the content digest being generated based on the content data.