5. Base 64 Encoding with URL and Filename Safe Alphabet

Base64url uses the same bit grouping as Base64 but replaces the two alphabet characters that are awkward in URLs and filenames.

Value 62: -
Value 63: _
pad: =

The = pad character can be omitted when the data length is known by some external rule, but a referring specification must say clearly whether padding is present or omitted.

Base64url and ordinary Base64 are different alphabets. Implementations should not silently treat them as interchangeable unless the protocol explicitly permits that behavior.