[tangent128]

Your questions are related.

For context: since a base64 character represents 6 bits, every block of three data bytes corresponds to a block of four base64 encoded characters. (83 == 24 == 64)

That means it's often convenient to process base64 data 4 characters at a time. (in the same way that it's often convenient to process hexadecimal data 2 characters at a time)

1) You use = to pad the encoded string to a multiple of 4 characters, adding zero, one, or two as needed to hit the next multiple-of-4.

So, "543210" becomes "543210==", "6543210" becomes "6543210=", and "76543210" doesn't need padding.

(You'll never need three = for padding, since one byte of data already needs at least two base64 characters)

2) Leftover bits should just be set to zero; the decoder can see that there's not enough bits for a full byte and discard them.

3) In almost all modern cases, the padding isn't necessary, it's just convention.

The Wikipedia article is pretty exhaustive: https://en.wikipedia.org/wiki/Base64