The "base64url" encoding specified by
RFC 4648 section 5,
Base 64 Encoding with URL and Filename Safe Alphabet.
Uses "The URL and Filename safe Base 64 Alphabet" as specified in Table 1 of RFC 4648 for encoding and decoding. Encode operation does not add any line separator character. Decode operation throws if it encounters a character outside the base64url alphabet.
'=' is used for padding.