Alphanumeric shellcode
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In computer science (or more likely, in computer underground terms) Alphanumeric code is mashine code that is written entierly in readable ASCII-letters such as "a"-"z", "A"-"Z", "1"-"9", "#", "!", "@", et.c., et.c., et.c..
This is possible to do with a (very) good understanding of the assembly language for the specific computer that the code is intended for.