Network storage for processing cryptographic files while keeping private key secret in key terminal

Abstract

In a storage system (), a key terminal () secretly records a primary private key included together with a primary public key in a primary key pair. An upload terminal () encrypts an object file into an encrypted file with a generated common key, encrypts the common key into a first cipher with the primary public key, and stores the encrypted file and the first cipher in a storage server (). A download terminal () generates a temporary key pair including a temporary public key and a temporary private key, secretly records the temporary private key, transmits the temporary public key to the key terminal (), and signs in to the storage server (). Then, the key terminal () and the storage server () in cooperation with each other generate a second cipher acquired by encrypting the common key with the temporary public key while the common key is kept secret from the storage server (), and transmit the second cipher to the download terminal (). The download terminal () decrypts the shared key from the second cipher with the temporary private key, and decrypts, with the common key, the object file from the encrypted file acquired from the storage server ().