Integer multiplication is one of the fundamental functions in cryptographic systems. Although much research has already been done on the testing of multiplication functions, most does not meet the need of cryptographic systems, where very large numbers are involved. Others provide only probabilistic algorithms. In this paper, we propose an efficient deterministic algorithm for verifying large number multiplications in cryptographic systems. A deterministic oracle for large integer multiplication functions will result. In addition, our method can also be used to verify selected segments of digits in the product of two numbers.