Base64 index table
Upper case (26) and lower case (26) letters, with 10 digitals,contribute 62 characters.Index 63 and 64 are + and / in standard Base64,- and _ in base64url.
Padding
Assuming a byte has 8 bits, we have 8 **2 = 64.Thus odd numbers of bytes would have needed padding.In fact, base64 works directly on bits, not assuming a 8-bit clean environment.For bits, we have 2** 6 = 64.Thus bytes are grouped by 3 (lcm(6, 8) / 8 = 18 / 8 = 3).When encoding a 1-byte or 2-byte group, it is padded with 0.In encoded output, a final ==/= sequence indicates the last group contains only 1/2 bytes.
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