We are given a compiled Java class file in the challenge and asked to reverse engineer it. I downloaded the file on my computer and searched for online java class file decompilers.
JDec was the website I chose. I uploaded the class file and got the following code:
/* Decompiler 70ms, total 584ms, lines 83 */
import java.util.Scanner;
public class KeygenMe {
public static void main(String[] var0) {
Scanner var1 = new Scanner(System.in);
System.out.println("Enter key:");
String var2 = var1.nextLine();
if (var2.length() != 34) {
System.out.println("Invalid key");
} else if (var2.charAt(33) != '}') {
System.out.println("Invalid key");
} else if (var2.charAt(32) != 'd') {
System.out.println("Invalid key");
} else if (var2.charAt(31) != '0') {
System.out.println("Invalid key");
} else if (var2.charAt(30) != 'a') {
System.out.println("Invalid key");
} else if (var2.charAt(29) != '1') {
System.out.println("Invalid key");
} else if (var2.charAt(28) != 'e') {
System.out.println("Invalid key");
} else if (var2.charAt(27) != 'f') {
System.out.println("Invalid key");
} else if (var2.charAt(26) != 'b') {
System.out.println("Invalid key");
} else if (var2.charAt(25) != '2') {
System.out.println("Invalid key");
} else if (var2.charAt(24) != '_') {
System.out.println("Invalid key");
} else if (var2.charAt(23) != 'd') {
System.out.println("Invalid key");
} else if (var2.charAt(22) != '3') {
System.out.println("Invalid key");
} else if (var2.charAt(21) != 'r') {
System.out.println("Invalid key");
} else if (var2.charAt(20) != '1') {
System.out.println("Invalid key");
} else if (var2.charAt(19) != 'u') {
System.out.println("Invalid key");
} else if (var2.charAt(18) != 'q') {
System.out.println("Invalid key");
} else if (var2.charAt(17) != '3') {
System.out.println("Invalid key");
} else if (var2.charAt(16) != 'r') {
System.out.println("Invalid key");
} else if (var2.charAt(15) != '_') {
System.out.println("Invalid key");
} else if (var2.charAt(14) != 'g') {
System.out.println("Invalid key");
} else if (var2.charAt(13) != 'n') {
System.out.println("Invalid key");
} else if (var2.charAt(12) != '1') {
System.out.println("Invalid key");
} else if (var2.charAt(11) != 'l') {
System.out.println("Invalid key");
} else if (var2.charAt(10) != '0') {
System.out.println("Invalid key");
} else if (var2.charAt(9) != '0') {
System.out.println("Invalid key");
} else if (var2.charAt(8) != '7') {
System.out.println("Invalid key");
} else if (var2.charAt(7) != '{') {
System.out.println("Invalid key");
} else if (var2.charAt(6) != 'F') {
System.out.println("Invalid key");
} else if (var2.charAt(5) != 'T') {
System.out.println("Invalid key");
} else if (var2.charAt(4) != 'C') {
System.out.println("Invalid key");
} else if (var2.charAt(3) != 'o') {
System.out.println("Invalid key");
} else if (var2.charAt(2) != 'c') {
System.out.println("Invalid key");
} else if (var2.charAt(1) != 'i') {
System.out.println("Invalid key");
} else if (var2.charAt(0) != 'p') {
System.out.println("Invalid key");
} else {
System.out.println("Valid key");
}
}
}
We can see it compares each position of the user input nextLine from the end, nextLine at index 0 should be p, nextLine at index 1 should be i etc.
Final flag for me was: picoCTF{700l1ng_r3qu1r3d_2be1a0d}.
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