The original binary format is broken into six-bit chunks (e.g., 100101), which in decimal format correspond to the integers from 0 to 63. These are just mapped to letters in order:
000000 (0) = A,
000001 (1) = B,
000010 (2) = C,
etc.—it goes through the capital letters first, then lower-case letters, then digits, then “+” and “/”.
It’s so simple you could do it by hand from the above description, if you were looking at the data in binary format.
The original binary format is broken into six-bit chunks (e.g., 100101), which in decimal format correspond to the integers from 0 to 63. These are just mapped to letters in order:
000000 (0) = A,
000001 (1) = B,
000010 (2) = C,
etc.—it goes through the capital letters first, then lower-case letters, then digits, then “+” and “/”.
It’s so simple you could do it by hand from the above description, if you were looking at the data in binary format.