As hinkley said, it is basically impossible to get the seed from the output of the generator without breaking the algorithm itself. CSPRNG can also be used as

As hinkley said, it is basically impossible to get the seed from the output of the generator without breaking the algorithm itself. CSPRNG can also be used as the basis for a stream cipher. The output must be the same for the receiver of the message to be able to decrypt it. Being able to extract the seed would be problematic because an attacker could know some part of the message being sent, calculate the CSPRNG output from it and get the seed, thus being able to read all the messages. CSPRNG are specifically designed to be a one-way function. Calculating the output from the seed should be easy, but calculating the seed from any output must be impossible.