A digital encrypting and decrypting unit (PMEU) that operates according to a Rivest-Shamir-Adleman (RSA) cryptosystem based on Residue Numeral System (RNS) and Chinese Reminder Theorem (CRT). The unit includes two modular exponentiation calculating units (MES-1, MES-2) to process a two residual signals (X mod p; X mod q) to calculate a result of a modular exponentiation by a binary method. The calculating units have inputs (I-k[i], I-SM, I-MM) and outputs (O-k[i], O-SM, O-MM) for signals representing partial results of the modular exponentiation. A modular exponentiation controlling unit (MECU) is connected to the inputs and outputs of the calculating units to control flow of the signals representing the partial results of the modular exponentiation.