A system and method include an encryptor, a decryptor, and a communication link. The encryptor includes a first processor and a first memory, and the decryptor includes a second processor and a second memory. The encryptor and the decryptor communicate data via the communication link. The encryptor and the decryptor are configured to: exchange public messages comprising keying materials, and calculate a common key based on the keying materials, first private modular integers known only to the encryptor, and second private modular integers known only to the decryptor. The keying materials disclosed in the public messages form an under-determined system of equations in variables of the first and the second private modular integers.