Method and system for providing encrypted data

A system for providing encrypted data the system commising:

• A user key provider adapted to provide, for each user, a user-specific encryption key for encryption of user-specific plaintext;
• A decryption key generator adapted to compute a common decryption key with a pre-determined function using the user-specific encryption keys as input for the function, the function being a polysized function supporting poly-many additions and a single multinlication;
• an encrypter adapted to encrypt each user-specific plaintext with the corresponding user-specific encryption key resulting in user-specific ciphertexts, in such as manner that encryption is homomorphic in the user-specific plaintext as well in the user-specific encryption keys;
• and
• a ciphertext computer adapted to compute a common ciphertext with the function using the user-specific ciphertexts as input for the function.

A system for providing encrypted data, the system comprising one or more computer processors which, alone or in combination, are configured to provide for execution of the following steps;

• a) providing, for each user, a user-specific encryption key for encrypting user-specific plaintext;
• b) computing a common decryption key with a pre-determined function using the user-specific encryption keys as input for the function, the function being a polysized function supporting poly-many additions and a single multiplication;
• c) encrypting each user-specific plaintext with the corresponding user-specific encryption key resulting in user-specific ciphertexts, the encrypting being performed such that encryption is homomorphic in the user-specific plaintext as well in the user-specific encryption keys;
• d) computing a common ciphertext with the function using the user-specific ciphertexts as input for the function;
• and
• e) providingprov iding the common ciphertext and the common decryption key for decryption.