it is encoded in Base64 but it doesn't always start with the same few characters and it doesn't nessaasrily end with an "=".
the equal sign (=) is for padding.
from Wikipedia:"The final '==' sequence indicates that the last group contained only one byte, and '=' indicates that it contained two bytes."
you can see the examples there for more information.
this can also be helpful:
https://stackoverflow.com/questions/4080988/why-does-base64-encoding-require-padding-if-the-input-length-is-not-divisible-by
