Base64

From AbiWiki

Base64 is an encoding scheme to allow binary files to be represented using ASCII text. It uses all printable characters to encode the data. There are 64 characters used which is where the name comes from. 64 characters need 6 bits of binary data to represent them and thus the 8 bit binary must be expanded in size. 4 characters are used to represent 3 bytes of data thus increasing the file size by 33%. The actual increase is more like 37% for email due to the need to limit line lengths to 72 characters and the inclusion of header data.

Base64 is used to transmit binary data inside of email documents, for RTF images, AbiWord (ABW) images, and FB2 images. It is a MIME (Multipurpose Internet Mail Extensions) encoding standard for data transfer. The mapping usually uses the uppercase letters, the lowercase letter, and 0-9. However, this only gets to 62 characters. The last two vary depending on the implementation. + and / are often used. Since there is no guarantee that there will be an exact match of the number of bytes to the number of character the = sign is used to pad the field when needed. When the decode encounters the = it knows the input data has ended. Data can be split over multiple files and concatenated together prior to the decode.