On the Subject of Lempel-Ziv Ciphers
Precursor to modern zip files. Though nothing really in common anymore.
This module contains 3 screens, a keyboard, 2 arrows, and a submit button that displays the current page you’re on.
Pressing the left or right arrow takes you to the previous or next page. There are 2 pages.
To disarm the module, decrypt a word using the following three steps. Once you have the decrypted word, type it in using the keyboard. When you start typing, the screens go blank and the bottom screen will show what you are typing.
To clear your input, click one of the arrows.
Once you are satisfied with your input, press the button labeled “SUB” to submit your answer.
Step 1: Encrypted Binary Retrieval
For this step, use the letters from the top, middle and bottom screens on page 1 and the top screen on page 2. Concatenate them in this order and convert this encoded string to binary by replacing each letter with a binary code from the table on the right.
Step 2: Lempel-Ziv Decryption
Split the binary message into chunks as follows: 1 chunk of length 1, 2 chunks of length 2, 4 chunks of length 3, 8 chunks of length 4, etc. The number of chunks of each length is 2 to the power of one less than the length. The number of chunks may vary (e.g. you may have fewer than 8 chunks of length 4) but the message does not end in the middle of a chunk.
Convert each chunk from binary into a number, obtaining the “coded sequence”.
Next, you will replace each of these numbers with a bit sequence obtained from a numbered list, the “dictionary”. However, you will construct the dictionary as you go.