Abstract: An encryption key generating method and apparatus based on homomorphic encryption, and a ciphertext operation method and apparatus using the generated encrypt key are disclosed. The method of generating an encryption key for performing encryption based on homomorphic encryption includes receiving data, generating a first encryption key and a second encryption key used for encrypting the data based on a secret key, and transmitting the first and second encryption keys.

Abstract: Disclosed is a method and apparatus for modulus refresh, where the method for modulus refresh of a ciphertext in homomorphic encryption includes receiving a first ciphertext corresponding to a first modulus, generating a second ciphertext by performing a blind rotation on the first ciphertext, and generating a target ciphertext corresponding to a second modulus greater than the first modulus based on the first ciphertext and the second ciphertext.

Abstract: Disclosed are a communication system and an operating method thereof. A plurality of electronic devices for forming a group may be configured to share a homomorphic encryption (HE) key. A server may be configured to generate a Boolean circuit for performing an operation based on input values corresponding to the number of electronic devices. Each of the electronic devices may be configured to transmit an encrypted session ephemeral key to the server through HE. The server may be configured to calculate an HE-evaluated value by applying the encrypted session ephemeral keys as the input values of the Boolean circuit. The server may be configured to transmit the HE-evaluated value and a given random number to each electronic device. Each electronic device may be configured to generate a common session key for the group based on the HE key, the HE-evaluated value and the given random number through homomorphic decryption.

Abstract: Disclosed are a communication system and an operating method thereof. A plurality of electronic devices for forming a group may be configured to share a homomorphic encryption (HE) key. A server may be configured to generate a Boolean circuit for performing an operation based on input values corresponding to the number of electronic devices. Each of the electronic devices may be configured to transmit an encrypted session ephmeral key to the server through HE. The server may be configured to calculate an HE-evaluated value by applying the encrypted session ephmeral keys as the input values of the Boolean circuit. The server may be configured to transmit the HE-evaluated value and a given random number to each electronic device. Each electronic device may be configured to generate a common session key for the group based on the HE key, the HE-evaluated value and the given random number through homomorphic decryption.