Modular exponentiation

Modular exponentiation

Modular exponentiation is a type of exponentiation performed over a modulus. It is particularly useful in computer science, especially in the field of cryptography.
A "modular exponentiation" calculates the remainder when a positive integer b (the base) raised to the e-th power (the exponent), and the total quantity is divided by a positive integer m, called the modulus. In symbols, this is, given base b, exponent e, and modulus m, the modular exponentiation cis:
For example, given b = 5, e = 3, and m = 13, the solution c is the remainder of dividing by 13, which is the remainder of 125 / 13, or 8.
If b, e, and m are non-negative, and b < m, then a unique solution c exists with the property 0 ≤ c < m.
Modular exponentiation can be performed with a negative exponent e by finding the modular multiplicative inverse d of b modulo m using the extended Euclidean algorithm. That is:

where e < 0 and

Modular exponentiation problems similar to the one described above are considered easy to do, even when the numbers involved are enormous. On the other hand, computing the discrete logarithm - that is, the task of finding the exponent e if given b, c, and m - is believed to be difficult. This one way function behavior makes modular exponentiation a candidate for use in cryptographic algorithms.