In this paper, an ASIC suitable for cryptography applications based on modular arithmetic techniques, is presented. These applications, such as for example digital signature (DSA) and public key encryption and decryption (RSA), use, as basic operation, the modular exponentiation. This ASIC works as a coprocessor with a special set of instructions specialized on dealing with high accuracy integers, as well as on the rapid evaluation of modular multiplications and exponentiations. The algorithm, the hardware architecture, the design methodology and the results are described in detail.