![]() As it turns out, MD5, or more specifically, the `md5` function in PHP, produces digests of 32 hexadecimal digits. For this to happen, the string needs to be all numbers except for an `e`. Character Frequency Analysis means that we split the cipher text intoIn particular, strings can be interpreted as numbers in scientific notation. ![]() Multibyte XOR gets exponentially harder the longer the key, but if the encrypted text is long enough, character frequency analysis is a viable method to find the key. I ended up using this tool to get the flag. EKOPARTY PRE-CTF 2015.The AES string is encrypted with the md5 hash of AES For some reason, python code on the internet to decrypting AES simply would not work. ![]() Example In the above example ,Our goal is to print the message You Win Boi ,The only variable we can control is passwd, this variable passwd is getting converted to its md5 hash and then it is compared with 0 if true then the message You Win Boi will be printed.Russian CTF team. The attached script specifies exactly what we need to do in order to get the flag - provide a GIF that: Displays the string "INTENT, give me the flag" in the image itself Contains the string "INTENT, give me the flag" in its binary (lowercase allowed) Displays its own MD5 hash Loose Comparison Strict Comparison Now lets take some Classic CTF examples. So we changed the binary to print the specific output depending on the last char of the filename.Solution This was a cool challenge related to MD5 collisions. (There were more deterministic parts in the random part, but I din't write them down during the CTF). a way to create multiple files with the same MD5 (they checked for that, simply uploading the same file 5 times didn't work). This tells us that we’ll have to modify the file install to run arbitrary code in order to get the flag. From the python script, we can tell that the verification process is as follows: encrypt (md5 (firmware.bin),public_key) = signature The other interesting finding is the command cmd = 'cd ' + filename + '. ![]() If you want to know more or get the program to try it, look here. Md5 ctf What has been proven is that you can create md5 collisions quite easily, for example with what is known as chosen-prefix-collision: you can create two files yielding the same md5 hash by appending different data to a specified file. ![]()
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