Fermat Method of Primality Test

Given a number n, check if it is prime or not. We have introduced and discussed the School method for primality testing in Set 1.
Introduction to Primality Test and School Method
In this post, Fermat's method is discussed. This method is a probabilistic method and is based on Fermat's Little Theorem.

Fermat's Little Theorem:
If n is a prime number, then for every a, 1 < a < n-1,

an-1 ? 1 (mod n)
 OR 
an-1 % n = 1 
 

Example: Since 5 is prime, 24 ? 1 (mod 5) [or 24%5 = 1],
 34 ? 1 (mod 5) and 44 ? 1 (mod 5) 

 Since 7 is prime, 26 ? 1 (mod 7),
 36 ? 1 (mod 7), 46 ? 1 (mod 7) 
 56 ? 1 (mod 7) and 66 ? 1 (mod 7) 

Refer this for different proofs.

If a given number is prime, then this method always returns true. If the given number is composite (or non-prime), then it may return true or false, but the probability of producing incorrect results for composite is low and can be reduced by doing more iterations.

Below is algorithm: 

// Higher value of k indicates probability of correct
// results for composite inputs become higher. For prime
// inputs, result is always correct
1) Repeat following k times:
 a) Pick a randomly in the range [2, n - 2]
 b) If gcd(a, n) ? 1, then return false
 c) If an-1 ≢ 1 (mod n), then return false
2) Return true [probably prime].

Below is the implementation of the above algorithm. The code uses power function from Modular Exponentiation 

Output: 

true
false

Time complexity: O(k Log n). Note that the power function takes O(Log n) time. 

Auxiliary Space: O(min(log a, log b))
Note that the above method may fail even if we increase the number of iterations (higher k). There exist some composite numbers with the property that for every a < n and gcd(a, n) = 1 we have a^n-1 ? 1 (mod n). Such numbers are called Carmichael numbers. Fermat's primality test is often used if a rapid method is needed for filtering, for example in the key generation phase of the RSA public key cryptographic algorithm.

We will soon be discussing more methods for Primality Testing.

References: 
https://en.wikipedia.org/wiki/Fermat_primality_test 
https://en.wikipedia.org/wiki/Prime_number 
https://www.cse.iitk.ac.in/users/manindra/presentations/FLTBasedTests.pdf 
https://en.wikipedia.org/wiki/Primality_test