Calculate the size to a Base 64 decoded message

Well, base64 represents 3 bytes in 4 characters... so to start with you just need to divide by 4 and multiply by 3.

You then need to account for padding:

• If the text ends with "==" you need to subtract 2 bytes (as the last group of 4 characters only represents 1 byte)
• If the text ends with just "=" you need to subtract 1 byte (as the last group of 4 characters represents 2 bytes)
• If the text doesn't end with padding at all, you don't need to subtract anything (as the last group of 4 characters represents 3 bytes as normal)