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Published on December 29th, 2017

Cryptography

The Cryptopals Crypto Challenges: Set 1 - Detect AES in ECB Mode

Find the string that has been encrypted with AES-128 cipher with an ECB mode in a file of ciphertexts.

2 minutes read

This is the eighth challenge of Set 1 in The Cryptopals Crypto Challenges website. Previously, I spoke about these challenges and provided walkthroughs for the previous challenges, if you haven't read them, here are the links:

For this challenge, you are given a file, which contains a bunch of ciphertexts that has been encrypted using AES-128 Cipher but only one of them has an ECB (Electronic Codebook) mode. Find the string the string that has the ECB mode.

In the previous post, I had explained that ECB is a cipher mode that is used to repeat the key until it covers the entire plaintext i.e. the same 16 byte plaintext will have the same 16 byte ciphertext.

Well, the solution is pretty much trivial, so here's the solution:

Implementation of the method(s):

    
    //Detect ECB Mode in AES Cipher
    bool CryptoLib::detect_ecb_mode(string str, int keyLength)
    {
        //Divide into equal amount of blocks
        int blocks = str.size() / keyLength;

        /*
            Theory: the problem with ECB as I had mentioned
            in the previous post, it uses the exact number of bytes of the ciphertext
            to encrypt the plaintext repeatedly.

            In that case, just do the reverse.

            Divide it into equal amount of blocks, in this case, we
            know the key is "YELLOW SUBMARINE", which is 16 bytes.

            Then, all you have to do is take two substrings of a string and compare,
            if they have the same string, we found it!
        */
        for(int i=0; i<blocks; i++)
        {
            //Take a substring of x number of bytes
            string strA = str.substr(i*keyLength, keyLength);

            for(int j=i+1; j<blocks; j++)
            {
                //Take another substring of x number of bytes
                string strB = str.substr(j*keyLength, keyLength);
                if(strA == strB)
                {
                    //Found
                    return true;
                }
            }
        }
        return false;
    }
    

Final code:

    
    //The Cryptopals Crypto Challenges - Set 1 Challenge 8
    #include "crypto.h"

    int main()
    {
        CryptoLib crypt;

        string str;

        ifstream infile;
        infile.open("8.txt");

        int count = 0;
        while(!infile.eof())
        {
            getline(infile, str);

            //Check if this string has ECB mode
            if(crypt.detect_ecb_mode(str, 16) == true)
            {
                cout << "FOUND AT LINE " << count << " => " << str << endl;
                break;
            }
            count++;
        }
    }
    

Note: This solution and the library named crypto.h was built using the C++ programming language. The source code for this solution can be found on Github.

Well, this challenge marks the end of Set 1 from The Cryptopals Crypto Challenges. However, I do intend to continue solving all these crypto challenges, let's see how it goes!

Until next time, then!