Web-Safe Base64-Encoding

Base 64 Encoding with URL and Filename Safe Alphabet ( web-safe Base64 )

RFC 4648 secion 5. 

5. Base 64 Encoding with URL and Filename Safe Alphabet

The Base 64 encoding with an URL and filename safe alphabet has been used in [12].

An alternative 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. The pad character "=" is typically percent-encoded when used in an URI [9], but if the data length is known implicitly, this can be avoided by skipping the padding; see section 3.2. 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, as indicated in Table 2.