PURPOSE: To decrease memory capacity, by encrypting a digital signal using a public key encryption with a vector comprising elements satisfying a prescribed relating equation.

CONSTITUTION: An input binary data is converted into an M-notation (M=natural number) data xj (j=1∼K) which is ≥2 at a converting circuit 10 and inputted to a multiplication circuit 12. The circuit 12 multiplies a signal bj from a memory 11 storing a K-element vector (b) by xj to obtain a product MOdL between the xj and bj, and the total sum of xj.bj is obtained at an operating circuit 13. K-set of natural numbers satisfying Equation 1, a natural number L sufficiently larger than a number obtained from Equation 2, and a natural number W being mutually prime with the L, are determined, bj=Waj (modL) is obtained to each aj and only the K-element vector b=(b1,∼bK) is opened. A transmitting personnel resolves Equation 3 to K-set of M-adic data x1∼xK to be encrypted, and transmits the result as an encrypted data.