Patent ID: 8781113

Claim:
A method of decrypting a ciphertext, the method comprising: pre-storing a plurality of polynomial functions by a display apparatus, the plurality of polynomial functions into which a secret key for decrypting a ciphertext to a plaintext according to a public-key cryptography algorithm is broken down; receiving, by the display apparatus, the ciphertext generated based on the secret key which is broken down into the plurality of polynomial functions, from a ciphertext generating device; and decrypting, by the display apparatus, the received ciphertext into the plaintext based on the pre-stored polynomial functions, wherein the decrypting the received ciphertext into the plaintext comprises processing the polynomial functions, and decrypting the ciphertext into the plaintext based on the processed polynomial functions, wherein an element selected at random is added to each of the polynomial functions broken down in the ciphertext generating device, and the added elements to each of the polynomial functions are determined such that the elements are offset by multiplication of the broken down polynomial functions in a decryption apparatus to calculate an identity element.