The Base64 is a **binary-to-text encoding** that represents binary data in an ASCII string format by translating it into a radix-64 representation. Each Base64 digit represents exactly 6 bits of data. Therefore, three 8-bit bytes (total of 24 bits) can be represented by four 6-bit Base64 digits.

The particular set of 64 characters chosen to represent the 64 place-values for the base varies between implementations. For example, MIME's Base64 implementation uses A–Z, a–z, and 0–9 for the first 62 values.

Base64 encoding** converts three octets into four encoded characters**. Padding characters (represented by "=" sign) might be added to make the last encoded block contain four Base64 characters.