In general, in computing, an alphanumeric code is a series of letters and numbers (hence the name) which are written in a form that can be processed by a computer.
Specifically, in computer hacker terminology, alphanumeric code is machine code that is written so that it assembles into entirely alphanumeric ASCII or Unicode characters such as 0-9, A-Z and a-z. This type of encoding was created by hackers to hide working machine code inside what appears to be text. This can be useful to avoid detection of the code and to allow the code to pass through filters that modify or remove non-alphanumeric characters unchanged. A similar type of encoding is called printable code and uses all printable characters (0-9, A-Z, a-z, !@#%^&* etc...) It has been shown that it is possible to create shellcode that looks like normal text in English.
Writing alphanumeric or printable code requires good understanding of the instruction set architecture of the machine(s) on which the code is to be executed. It has been demonstrated that it is possible to write alphanumeric code that is executable on more than one machine.
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