Mysterious Song Challenge

You have been given a task to decode the secret message that was found on Eve's machine.

Eve's machine is quite powerful and has more cores (aka processors) than a normal machine.
Apparently, it has as many cores (aka processors) as half of the ASCII number representing space.
Here is the message:

NTc3OTg1MDg0NzI3OTg1NzE3Mjg0MDg5Nzk4NTA2NTgyNjkwNjg3OTc4NjkzMTA1MjcyNjkwNzA4NTc4MDcyNjU4MzA3NDg1ODM4NDA2NjY5NzE4NTc4MTQwMzg3OTc2NzY3OTg3MDg0NzI2OTA3NjczNzg3NTA4NzczODQ3MjA2NTA4MzczODAwNzk3MDA4OTc5ODU4MjA2ODgyNzM3ODc1MjYwNzI4NDg0ODA4MzI2MTUxNTg2NzA3OTgyNjgxNDc3NjkxNTE4MTcyNDIwMTY2NzE3NjUxOTY5MTk2ODY2MjIyNTY5MTYxNzIwMjAyMTY3MjQyMzI0MTg2NTI1MjU3MDI1NjY

We also found the code that Eve used to encode the message but we failed to find a function that can decode it.
Here is the code for encoding:

import java.util.Base64;

public class SuperSecureSecretSocks {

    public static String messStuffUp(String input) {

        int numberOfCores = Runtime.getRuntime().availableProcessors();
        int[] ascii = new int[input.length()];

        for (int i = 0; i < input.length(); i++) {
            ascii[i] = (int) input.charAt(i);
        }

        for (int i = 0; i < input.length(); i++) {
            ascii[i] = ascii[i] - numberOfCores * 2;
        }

        String asciiString = "";
        for (int i = 0; i < input.length(); i++) {
            asciiString += ascii[i];
        }

        String base64encoded = Base64.getEncoder().withoutPadding().encodeToString(asciiString.getBytes());

        return base64encoded;
    }
}