A keygen (an abbreviated form of "key generator") is a small program that will generate valid CD keys or serial/registration numbers for a piece of software. These are made available by software cracking groups for free download on various websites dedicated to software piracy. In some countries, the use of keygens to activate software without purchasing a genuine code is unlawful.

The author of a keygen typically uses a disassembler to look at the raw assembly code of the targeted program, checking either the software itself or the installer. Once access has been obtained to the program's code, the location of the subroutine(s) responsible for verifying that the key entered is valid are found. Using this knowledge, the algorithm may be reverse engineered to generate valid keys, which is then incorporated into the keygen.

With weaker serial protection schemes a complex reverse is not required as the key-checking code itself in the original application can be effectively copied and incorporated into a keygen. Weaker schemes sometimes internally generate a correct key inside the original application for comparison purposes (to determine whether the entered key is correct). Some keygens use a brute force approach or brute force hybrid approach to creating valid keys.

In these instances, rather than produce an exact reverse of the key check algorithm, the attacker uses a search technique, testing many possible combinations per second against the key validation check until a given combination produces a valid key.

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