Encryption device, encryption system, encryption method, and encryption program

An encryption system comprising an encryption device and a decryption device,

• wherein the encryption device includes:
• circuitry configured to provide:
• a one-time pad cipher key storage part which stores a one-time pad cipher key formed of a plurality of bits and used in a one-time pad cipher;
• a one-time pad encrypting part which encrypts communication data by the one-time pad cipher by sequentially using part of the one-time pad cipher key stored in the one-time pad cipher key storage part, to generate encrypted data;
• a block-cipher key storage part which stores a block-cipher key used in a block cipher;
• a block-encrypting part which encrypts communication data by the block cipher by using the block-cipher key stored in the block-cipher key storage part, to generate encrypted data;
• and
• an encryption control part which controls whether the communication data is to be encrypted by the one-time pad encrypting part, or by the block-encrypting part, depending on a remaining bit count of the one-time pad cipher key stored in the one-time pad cipher key storage part, and notifies to the decryption device a message indicating which one of the one-time pad encrypting part and the block-encrypting part is to perform encryption, and
• wherein the decryption device includes:
• circuitry configured to provide:
• a one-time pad decipher key storage part which stores a one-time pad decipher key formed of a plurality of bits and used in the one-time pad cipher;
• a one-time pad decrypting part which decrypts encrypted data by the one-time pad cipher by sequentially using part of the one-time pad decipher key stored in the one-time pad decipher key storage part, to generate communication data;
• a block-decipher key storage part which stores a block-decipher key used in the block cipher;
• a block-decrypting part which decrypts encrypted data by the block cipher by using the block-decipher key stored in the block-decipher key storage part, to generate communication data;
• and
• a decryption control part which controls whether the encrypted data is to be decrypted by the one-time pad decrypting part, or by the block-decrypting part, in accordance with a message notified by the encryption control part.

An encryption method comprising:

• a one-time pad encrypting step, by a processing device, of encrypting communication data by a one-time pad cipher by sequentially using part of a one-time pad cipher key formed of a plurality of bits and stored in a storage device, to generate encrypted data;
• a block-encrypting step, by the processing device, of encrypting the communication data by a block cipher by using a block-cipher key stored in the storage device, to generate encrypted data;
• and
• an encryption control step, by the processing device, of controlling whether the communication data is to be encrypted in the one-time pad encrypting step by the one-time pad cipher, or in the block-encrypting step by the block cipher, depending on a remaining bit count of the one-time pad cipher key stored in the storage device, wherein when the communication data is sequentially encrypted in the one-time pad encrypting step in units of predetermined unitary data, if the bit count of the one-time pad cipher key stored in the storage device becomes smaller than a bit count of the unitary data to be encrypted next, the method includes performing switching so that the block-encrypting part encrypts the communication data.

A non-transitory computer readable medium storing an encryption program which causes a computer to execute a method comprising:

• a one-time pad encrypting process of encrypting communication data by a one-time pad cipher by sequentially using part of a one-time pad cipher key formed of a plurality of bits and stored in a storage device, to generate encrypted data;
• a block-encrypting process of encrypting the communication data by a block cipher by using a block-cipher key stored in the storage device, to generate encrypted data;
• and
• an encryption control process of controlling whether the communication data is to be encrypted in the one-time pad encrypting process by the one-time pad cipher, or in the block-encrypting process by the block cipher, depending on a remaining bit count of the one-time pad cipher key stored in the storage device, wherein when the communication data is sequentially encrypted in the one-time pad encrypting process in units of predetermined unitary data, if the bit count of the one-time pad cipher key stored in the storage device becomes smaller than a bit count of the unitary data to be encrypted next, the method includes performing switching so that the block-encrypting part encrypts the communication data.