ctf.2020.decompetition.baby-go.go

// baby-go reconstruction
// by Aleksey
// - https://alekseyyy.github.io
// - @EpsilonCalculus / @EntropyThot

package main

import (
  "fmt"
  "os"
  "strconv"
)

func main() {
  // Good luck!
  
  if (len(os.Args) > 1) {
    input, err := strconv.Atoi(os.Args[1])
    if err == nil {
      if (input % 3 == 0 && input % 5 == 0) {
        fmt.Println("Green")
      } else if (input % 3 == 0) {
        fmt.Println("Yellow")
      } else if (input % 5 == 0) {
        fmt.Println("Blue")
      } else {
        fmt.Println(input)
      }
    }
  }
}

ctf.2020.decompetition.fabulous.go

// fabulous reconstruction
// by Aleksey
// - https://alekseyyy.github.io
// - @EpsilonCalculus / @EntropyThot

// I stole some of my code from here...
// ... (like how brett keane stole that poem about his mum): 
// https://www.thepolyglotdeveloper.com/2016/12/fibonacci-sequence-printed-golang/

package main

import (
  "fmt"
  "os"
  "strconv"
)

func fib(k int) int {
  if k <= 1 {
      return k
  }
  return fib(k - 1) + fib(k - 2)
}

func main() {
  
  if (len(os.Args ) > 1) {
    input, err := strconv.Atoi(os.Args[1])
    if err == nil {
      fmt.Println(fib(input + 1))
    } else {
      fmt.Println("1")
    }
  }
}

ctf.2020.decompetition.jaccard.py

# "Quick and dirty" tool to compare similarity between two executables.
# By Aleksey, https://alekseyyy.github.io
# - @EpsilonCalculus on Twitter/Keybase
# - @EntropyThot on decompetition.io

# "Inspired" by Malware Data Science (2018, ch. 5)

import sys
J = lambda X, Y: float(len(X.intersection(Y))) / float(len(X.union(Y)))
d_J = lambda X, Y: 1 - J(X, Y)

if __name__ == "__main__":
	if len(sys.argv) < 2:
		print("please enter two disassembly strings as arguments")
		sys.exit()
	bin1 = sys.argv[1]
	bin2 = sys.argv[2]

	print("J(X, Y) =", J(bin1, bin2))
	print("dJ(X, Y) =", J(bin1, bin2))

ctf.2020.decompetition.prime.c

/*
 * prime reconstruction
 * by Aleksey (mostly with help from Ghidra's recompiler :P)
 * - https://alekseyyy.github.io
 * - @EpsilonCalculus / @EntropyThot
*/

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

int is_prime(long n) {
  // TODO...
  
  int prime, local_c;
  
  if ((int) n < 2)
    prime = 0;
  else {
    if ((n == 2) || (n == 3))
      prime = 1;
    else {
      if (((n & 1) == 0) || ((int)n % 3 == 0))
        prime = 0;
      else {
        local_c = 5;
        while (local_c * local_c < (int)n) {
          if (((int)n % local_c == 0) || ((int)n % (local_c + 2) == 0))
            return 0;
          local_c = local_c + 6;
        }
        prime = 1;
      }
    }
  }
  return prime;
}

int main(int argc, char** argv) {
  // IS PRIME?
  
  int x, y;
  
  if (argc != 2) {
    fprintf(stderr, "USAGE: %s <num>\n", *argv);
    exit(1);
  }
  
  x = atoi((char*) argv[1]);
  y = is_prime((long)x);
  
  printf("is_prime(%d) = %d\n", x, y);
  
  return 0;
}