It is non-standard base64: it uses '_' and '-' instead of '+' and '/'. You can read in Wikipedia at https://en.wikipedia.org/wiki/Base64 https://en.wikipedia.or

It is non-standard base64: it uses '_' and '-' instead of '+' and '/'. You can read in Wikipedia at https://en.wikipedia.org/wiki/Base64 https://en.wikipedia.org/wiki/Base64