Programming

To exploit code that someone else has written, one must obviously know how to code. You need not be an advanced programmer, but you must be strong in your fundamentals and computational thinking.

Python is the standard for most CTF tasks - there are several essential libraries like pwntools and Z3Py, and it lets you get things done in a quick and dirty manner that’s perfect for a time-constrained scenario like a CTF.

Task

Learn the basics of Python and become accustomed to thinking in code.

Q1

Given a string(taken from the user), split it into a list of characters. Then reverse the list and convert it back to string. Eventually check whether the string is a palindrome string or not.

Note

Solve this question in python.

Topics covered: strings, if-else, for-loop, lists, nested for-loop

Bonus

Check out how palindrome check is done in best time complexity

Q2

Given a array of integers, sort them in any order, and find out the second highest element.

Note

Solve this question in c/cpp. The highest element and second highest element shouldn’t be same.

Topics covered: arrays, sorting

Q3

Given an integer, decide whether the number is sum of square of two other integers(return true or false).

Note

The integer is positive. Solve it in O(log n) time complexity. Solve using python/c/cpp

Topics covered: binary search, two pointer approach

Q4

Given a list of integers, find the first missing positive integer among them.

Input => 3,4,-1, 1
Output => 2

Hint: U can try sorting the list and then doing the program. Or u can explore little bit of math :).

Topics covered: math, time efficiency

Q5

Given a string, find the ascii of the characters of the string, and store them into a list. Now from the list reconstruct the string again.

Note

Use python to solve.

Topics covered: ascii, character and int array

Q6

Given a string, find the ascii of the characters of the string, then xor it with 0x69 and store it in a list. Now reconstruct the string again.

Note

Use python to solve.

Hint: If you try enough, every operation is reversible :)

Topics covered: ascii, xor operation.

Q7

Rahul is smart guy, who wants to keep his text files encrypted. The way he wants it to be done is by using a technique similar to the one used in Q6. Given a string, he wants to store the ascii of every characters of the string into a list and then do xor operation with the keys 0x01, 0x23, 0x45, 0x67 and 0x89 in this respective order and stores everything into a new list.

Note

Python makes life simple :)

Hint: Google is your best friend, he would never criticize you.

Topics covered: Repeated key xor.

Q8

Rahul can be very demanding sometimes, given a key as in Q7, he wants to check whether its a valid key or not. The following are the conditions for a valid key. -> Find the sum of product of the digit and its position -> If the sum is divisible by 11 then the key is valid else not

Input: 0112112420
Output: Valid

Explaination:
10*0 + 9*1 + 8*1 + 7*2 + 6*1 + 5*1 + 4*2 + 3*4 + 2*2 + 1*0 = 66
Since 66 is divisible by 11, it is valid

Input: 0112112425
Output: Invalid

Explaination:
10*0 + 9*1 + 8*1 + 7*2 + 6*1 + 5*1 + 4*2 + 3*4 + 2*2 + 1*0 = 71
Since 71 is not divisible by 11, it is not valid