CTF: Derbycon 2016 Hilary E-Mail Buster

derbycon2016crypto.py

                                                                                                    
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# iterate through a file by chunk_size-chunks
def read_in_chunks(infile, chunk_size):
    chunk = infile.read(chunk_size)
    while chunk:
        yield chunk
        chunk = infile.read(chunk_size)


# get the location of a chunk in another file
def get_chunk_location(infile, search_string):
    location = infile.tell()
    chunky = infile.read(len(search_string))
    while chunky:
        if (search_string == chunky):
            infile.seek(0, 0)
            return location
        location = infile.tell()
        chunky = infile.read(len(search_string))
    infile.seek(0, 0)
    return -1


# open ye olde files
femail = open("email.enc", 'rb')
fclasst = open("class.txt", 'rb')
fclasse = open("class.enc", 'rb')
fout = open("email.txt", 'wb')
foutunk = open("unk.enc", 'r+b')
finunk = open("unk.txt", 'rb')
paddedspace = b'\x20\x00\x00\x00\x20\x00\x00\x00'
paddednull = b'\x00\x00\x00\x00\x00\x00\x00\x00'

# magic stuff
for chunk in read_in_chunks(femail, 8):
    # Search for each chunk in encrypted file
    location = get_chunk_location(fclasse, chunk)
    # if location isn't -1 (not found), find the chunk in the plaintext
    if location != -1:
        fclasst.seek(location)
        plaintext = fclasst.read(8)
        fout.write(plaintext)
    else:
        # is it in unknown file?
        location = get_chunk_location(foutunk, chunk)
        if (location == -1):
            foutunk.seek(0, 2)
            foutunk.write(chunk)
            fout.write(paddedspace)
        else:
            finunk.seek(location)
            plaintext = finunk.read(8)
            if (plaintext == paddednull):
                # write spaces if we don't have a value yet
                # fout.write(chunk)
                fout.write(paddedspace)
            else:
                fout.write(plaintext)

femail.close()
fclasst.close()
fclasse.close()
fout.close()