Task 2: Guessing the Key On April 17, 2018, Alice finished her tax return, and she saved the return (a PDF file) on her disk. To protect the file, she encrypted the PDF file using a key generated from the program described in Task 1. She wrote down the key in a notebook, which is securely stored in a safe. A few month later, Bob broke into her computer and gets a copy of the encrypted tax return. Since Alice is CEO of a big company, this file is very valuable. Bob cannot get the encryption key, but by looking around Alices computer, he saw the key-generation program, and suspected that Alices encryption key may be generated by the program. He also noticed the timestamp of the encrypted file, which is "2018-04-17 23:08:49". He guessed that the key may be generated within a two-hour window before the file was created. Since the file is a PDF file, which has a header. The beginning part of the header is always the version number. Around the time when the file was created, PDF-1.5 was the most common version, i.e., the header starts with %PDF-1.5, which is 8 bytes of data. The next 8 bytes of the data are quite easy to predict as well. Therefore, Bob easily got the first 16 bytes of the plaintext. Based on the meta data of the encrypted file, he knows that the file is encrypted using aes-128-cbc. Since AES is a 128-bit cipher, the 16-byte plaintext consists of one block of plaintext, so Bob knows a block of plaintext and its matching ciphertext. Moreover, Bob also knows the Initial Vector (IV) from the encrypted file (IV is never encrypted). Here is what Bob knows:

Plaintext:	255044462d312e350a25d0d4c5d80a34
Ciphertext: d06bf9d0dab8e8ef880660d2af65aa82
IV:	09080706050403020100A2B2C2D2E2F2

Your job is to help Bob find out Alices encryption key, so you can decrypt the entire document. You should write a program to try all the possible keys. If the key was generated correctly, this task will not be possible. However, since Alice used time() to seed her random number generator, you should be able to find out her key easily. You can use the date command to print out the number of seconds between a specified time and the Epoch, 1970-01-01 00:00:00 +0000 (UTC). See the following example.

$ date -d "2018-04-15 15:00:00" +%s 1523818800