How to calculate plaintext from modpower cipher Let $a$ be the plaintext message, $e$ be a small prime exponent (such as 11, 13, 17 etc.), $b$ be cipher text and $n$ be a large modulo (non-prime).
Suppose that:
$b \equiv a^e \space mod \space n$
If we know $b$, $e$ and $n$, can we possibly calculate $a$?
It is conjectured to be hard to recover $a$ given only $b$, $e$, and $n$. This is called the RSA assumption. If an efficient factorization algorithm exists, this assumption is false. 

However, it is possible that the RSA assumption is false and there exists no efficient factorization algorithm. In other words, the hardness of factoring might not imply the RSA assumption is true. 

In your question you merely call $n$ a large non-prime modulus. You have to be a little careful about the primes that make up $n$; there are lots of ways to screw up and accidentally make it easy to factor $n$.