An alphanumeric string is a group of letters and numbers, which are put together to form a special sort of identification. A character stored in alphanumeric form is considerably smaller than storing an 8-bit ASCII character, as each character is only 6 bits in length.
There is no standard for storing 6-bit alphanumeric data. A 6-bit field has 64 possible values, so if only 36 are used in single case, there is room for another 28 characters (usually slashes and other punctuation), making alphanumeric data useful for storing text and website addresses.
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 underground terminology, alphanumeric code is machine code that is written so that it assembles into entirely alphanumeric ASCII characters such as 0-9, A-Z and a-z. This type of encoding was created by hackers to be able to pass machine code through a filter that removes any non-alphanumeric character and still be able to have it perform its intended tasks successfully. (Because normal machine code frequently uses non-alphanumeric characters, these would get removed by such a filter and the code would not be able to pass the filter without being modified to the point where it no longer works). A slightly less restrictive form of this type of encoding is printable code, which uses all printable characters such as 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.
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