RSA algorithm private key extraction 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)?
Solving for $D$ in this case is known as the discrete logarithm problem. This is a known hard problem, as there are no known algorithms to calculate it in polynomial time. In other words, as the size (i.e. the number of digits) of the numbers increase, the time it takes to calculate gets higher exponentially. For this reason, it's used in cryptography as a one-way function for many things including RSA and Diffie-Hellman.