Base64 encoding / Base64 decoder
Base64 encoding schemes are commonly used when there is a need to encode binary data that needs be stored and transferred over media that are designed to deal with textual data. This is to ensure that the data remains intact without modification during transport. Base64 is used commonly in a number of applications including email via MIME, and storing complex data in XML or JSON.[1]
In computer science, Base64 is a group of binary-to-text encoding schemes that represent binary data (more specifically a sequence of 8-bit bytes) in an ASCII string format by translating it into a radix-64 representation. The term Base64 originates from a specific MIME content transfer encoding. Each non-final Base64 digit represents exactly 6 bits of data. Three 8-bit bytes (i.e., a total of 24 bits) can therefore be represented by four 6-bit Base64 digits.[2]
Common to all binary-to-text encoding schemes, Base64 is designed to carry data stored in binary formats across channels that only reliably support text content. Base64 is particularly prevalent on the World Wide Web where its uses include the ability to embed image files or other binary assets inside textual assets such as HTML and CSS files.[2]
Base64 is also widely used for sending e-mail attachments. This is required because SMTP in its original form was designed to transport 7 bit ASCII characters only. This encoding causes an overhead of 33–36% (33% by the encoding itself, up to 3% more by the inserted line breaks).[2]
URL safe
Using standard Base64 in URL requires encoding of '+', '/' and '=' characters into special percent-encoded hexadecimal sequences ('+' becomes '%2B', '/' becomes '%2F' and '=' becomes '%3D'), which makes the string unnecessarily longer.
For this reason, modified Base64 for URL variants exist (such as base64url in RFC 4648), where the '+' and '/' characters of standard Base64 are respectively replaced by '-' and '_', so that using URL encoders/decoders is no longer necessary and has no impact on the length of the encoded value.
Sources:
[1] www.base64encode.org
[2] en.wikipedia.org/wiki/Base64