From Wikipedia: The term "Base64" refers to a specific MIME content transfer encoding. It is also used as a generic term for any similar encoding scheme that encodes binary data by treating it numerically and translating it into a base 64 representation. The specific choice of base 64 is due to the history of character set encoding: one can choose 64 characters that are both part of the subset common to most encodings, and also printable. This combination makes it unlikely for the data to be modified in transit through systems, such as email, which were traditionally not 8-bit clean.