March 17th, 2011, 02:29 PM
When to work in modp and mod phi(p)
while reading the el gamal signature scheme , i came across the following steps:
1. select a random k such that gcd ( k, phi(p) )=1 where p is a prime number
2. compute r= (alpha) ^ k (mod p) where ( 0<r<p)
3. compute s= k^-1 (m - ar) (mod (phi(p))
i do not understand why the steps keep on changing the modulus to work in? I have found the same thing happening in many of other signature functions and asymmetric encryption function.
please help thank you