Henrik, thankyou too for explanation.
A little different approach...........

1, take 3 bytes at a time (from low array) and place into a LONG, lower 3 bytes, (zero high byte)
2, left-shift 6 bits, (puts next 6 bits alone in highbyte) place highbyte into array 1/2 way up, clear highbyte, shift again.........
for a total of 4 times (unpacks 3 base64 packed bytes to 4 bytes as you layed out)
3, load next 3 bytes to long etc........................up to end

not sure about padding at the part to have the correct unpacked string.

Don