You're right that there will never be a case where you will add three padding characters to your encoded text.
The encoding process is like this:
1. Take a string and break it up into sets of 3 bytes each.
2. For every set of three bytes, break it up into 4 numbers of 6 bits each.
3. The numbers are indexes into the base 64 lookup table, giving you your base 64 character.
The last set will either contain 3 bytes (no padding needed), or 2 bytes (we pad with one '='),
or 1 byte (we pad with 2 '=').
If it has 2 bytes, we add one 0 byte to the end. This lets us extract three numbers out of it: 6 bits + 6 bits + 6 bits (the final 4 bits + 2 bits from the 0 byte). We use the three numbers as indexes in the lookup table. Since bas64 takes three characters and outputs 4, we add a single '=' byte to our output.
If we have 1 byte in the final set, we add two 0 bytes to the end. We can extract two numbers: 6 bits + 6 bits (the final 2 bits + 4 bits from the neighbouring 0 byte). We add two '=' bytes to that our final set gives us 4 characters.
Let's try an example with the string "Hello".
H e l l o = 0x48 0x65 0x6C 0x6C 0x6F
01001000 01100101 01101100 01101100 01101111
We have 2 sets: one with 3 bytes, and one with 2 bytes. So we add one 0 byte to the end:
{01001000 01100101 01101100} {01101100 01101111 00000000}
Let's take the first set and produce our first 4 numbers:
01001000 01100101 01101100
010010000110010101101100
010010 000110 010101 101100
0x12 0x06 0x15 0x2C
18 6 21 44
Those last four numbers are the decimal values we got, we'll use them to index in to the lookup table.
But first, let's encode the second set:
01101100 01101111 00000000
011011000110111100000000
011011 000110 111100 000000
0x1B 0x06 0x3C
27 6 60
Notice how we skipped that last set of 6 bits? That's because the 0 byte isn't really part of the input text, we added it so that we can extract those three numbers. Since we need a way to indicate to whoever's decoding the text that we did this, we need to add a single padding byte.
Now let's use the base 64 lookup table to generate our encoded text:
{18 6 21 44} {27 6 60} =
{S G V s} {b G 8} =
Putting it all together:
SGVsbG8=
I hope that makes sense!
