1. Examine the source code of the "C" program. We understand that it is DES.
2. Search the internet for an implementation of DES in "C". We find a strong correlation with this code.
3. We take the Decrypt function and paste it into our program. Change the names and paths of the files in our program.
4. The 64 bits given to us are the key. Decrypt the ciphertext. We get some sequence of bits.
5. We are given python code that inverts the bits. We use it to invert the resulting sequence. We convert bin to text.

Flag: vishwaCTF{3v3ryth1ng_15_m3s5y_4r0und_h3r3....}.