The output padding is only relevant for decoding. For encoding, since the alphabet of Base64 is 6 bits wide, the padding is 0 when the input is not a multiple o

The output padding is only relevant for decoding. For encoding, since the alphabet of Base64 is 6 bits wide, the padding is 0 when the input is not a multiple of 6 (e.g. encoding two bytes (16 bits) needs two more bits to become a multiple of 6 (18)) Refer to the "examples" section of the wikipedia page