1. A data processor comprising: An arithmetic unit that performs an RSA decryption process using the Chinese remainder theorem, wherein the arithmetic unit performs the modulo exponentiation operation expressed as Y=X d modN to calculate C0=X d′ modN, C1=X d modN, and T=X 2 ̂ n modN, where d′ denotes two's complement of d and n denotes the number of bits in d and 2 ̂ n denotes 2 n ; wherein the arithmetic unit determines whether or not a remainder resulting from the product of a value of C0 and a value of C1 modulo N matches a value of T; wherein the arithmetic unit assigns a value of C1 to Y if a match is found, and wherein the arithmetic unit reports an error if a match is not found.