3D-Reality

My friend has set me this 'test' of my skills and provided a test string and sample strings:


Test String - a6cb53c20131f38bfecc8df72b2b09b29da18335acb7

a - b4
aa - b4fd
aaa - b4fd2f
aaaa - b4fd2fdc

b - b7
bb - b751
bbb - b75195
bbbb - b7519569

c - b6
cc - b641
ccc - b64164
cccc - b641644c

d - b1
dd - b176
ddd - b176a3
dddd - b176a32d

abcd - b4fe49f9
dcba - b171cc4e
baab - b752b940

Please help me work out how this encryption works so I can decode the sample string!

Thanks
John

P.S. I have so far worked out that the first block of hex is produced by this:
(Ascii Value of Letter XOR 213)


If you give any code, please use Visual Basic syntax or plain English - thanks!