Dive into Base64

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 an 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.