Create README.md

Author: akrasuski1

2017-07-15-ctfzone/decrypt_the_undecryptable/README.md

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+# Decrypt the undecryptable (reverse, ppc, 724p)
+
+> Oh noes! It looks like we've lost decryption keys to our super-secret archive that contains all the blackmail material we've accumulated over the years!
+> Luckly, the encryption system was built with the government money by the lowest bidder, so maybe it contains some flaws.
+
+> decrypt_the_undecryptable.tar
+
+In this task we were given a randomish BMP picture and a binary for en-/de-crypting.
+
+I didn't even really look at the binary - one quick glance at the image was enough to see distinct repetition patterns.
+This very likely meant it was encrypted using some block cipher in ECB mode or, alternatively, xored using short repeating
+key. I quickly found an offset containing the repeating part, produced a quick and dirty Python script to xor
+the whole image with that 128-byte long block and save the result. The final image was not perfect, but good enough to be
+able to read the message.