If you want to Base64 encode or decode a text without writing a program or using tools online/offline, well notepad++ text editor has Base64 encoding/decoding options under Plugins -> MIME Tools.
How to use it
Open your file in notepad++ ( or write/paste text in the editor tab).
Select the text that you want to encode or decode
Go-to Plugins -> MIME Tools -> Base64 Encode/Decoder.
- You can see here that the text is now base64 encoded.