solution of http://crackmes.de/users/ascii/kgm1/

keygen.c

#include<stdio.h>
#include<stdlib.h>

void print_key(char p[], int f) {
	printf("key: ");
	int cycle;
	for (cycle = 0 ; cycle < 8 ; cycle++) {
		printf("%c", p[cycle]);
	}

	printf("%c", f);
}

int main(int argc, char* argv[]) {
	unsigned char key[] = {0x45, 0x36, 0xab, 0xc8, 0xcc, 0x11, 0xe3, 0x7a};
	unsigned char password[9];
	unsigned int final = 0;

	int cycle;

	srand( time( NULL ) );

	do {
		final = 0;

		for (cycle = 0 ; cycle < 8 ; cycle++) {
			password[cycle] = rand() % (0x7a - 0x21) + 0x21;

			/*
			if (!isalnum(password[cycle])) {
				final = 0;

				break;
			}*/

			final += (password[cycle]) ^ key[cycle];

		}

		print_key(password, 0x41);
		printf("\r");
		final &= 0xff;

	} while( (final > 0x7a) || (final < 0x61) );

	print_key(password, final);
	printf("\n");

	return 0;
}