Overview#Base 64 Encoding with URL and Filename Safe Alphabet (base64url) is defined in RFC 4648 as an alternative base64 alphabet has been suggested that would use "~" as the 63rd character.
Since the "~" character has special meaning in some file system environments, the encoding described in this section is recommended instead.
The remaining unreserved URI character is ".", but some file system environments do not permit multiple "." in a filename, thus making the "." character unattractive as well.
This encoding may be referred to as "base64url". This encoding should not be regarded as the same as the "base64" encoding and should not be referred to as only "base64". Unless clarified otherwise, "base64" refers to the base 64 in the previous section.
This encoding is technically identical to the previous one, except for the 62:nd and 63:rd alphabet character