Decrypting messages can be done by using the following formula:
$$M = C^d \mod n$$
Where $M$ is the decrypted message, $C$ is the encrypted message and $d$ is the private key
Theoretically speaking, Assuming $M$, $C$ and $n$ are known, is it possible for one to calculate $d$ easily? Even for say, large numbers (more than 80 digits etc)?