US Patent 7936874 Information transfer system, encryption device, and decryption device

A content delivery system, enabling a ciphertext to be reduced in size when using the ElGamal cipher, includes a content delivery device performing elliptic curve encryption on a content key, generating an encrypted content key that includes an x coordinate of an elliptic curve point obtained by the elliptic curve encryption, and outputting the encrypted content key. Further, the content delivery system includes content reception device receiving the encrypted content key, calculating a y coordinate of the elliptic curve point using the x coordinate included in the encrypted content key, and performing elliptic curve decryption using the elliptic curve point and other information included in the encrypted content key, to generate a decrypted content key.