HACKDAY Qualif 2023
Astronaut's Poem [342 pts]
Challenge Description:
## Challenge Description
This is an Stegnography Challenge, in which we are given a pdf containing 1 page of poem only. We are asked to find the constellation in French and in lower case of the constellation containing the found NGC.
## Solution
We start with the metadata of the pdf file.
exiftool Astronauts_Poem.pdf
We will notice this line: Language : https://esolangs.org/wiki/Poetic
Poetic is an esoteric programming language created by User:JWinslow23 in 2018. It is a derivative of brainfuck that encodes instructions into the lengths of words. Programs generally look like abstract poetry, and upon execution, they are turned into a list of numbers denoting the lengths of the words, and then executed as instructions.
And there exists an Official online interpreter. After executing the poem we receive:
We return to our poem and search for the line 10. Remember that we need to find a number representing a NGC, so we should try execute the line 10 ourselves. According to the official site of Poetic language, we have:
| Value | Brainfuck Equivalent | Description |
|---|---|---|
| 1 | [ |
If the current byte is equal to 0, jump execution to after the matching 2. |
| 2 | ] |
If the current byte is not equal to 0, jump execution to the matching 1. |
| 3 | + |
Increment the value of the current byte by next_digit. If next_digit is 0, the value used instead is 10. |
| 4 | - |
Decrement the value of the current byte by next_digit. If next_digit is 0, the value used instead is 10. |
| 5 | > |
Increment the memory pointer by next_digit. If next_digit is 0, the value used instead is 10. |
| 6 | < |
Decrement the memory pointer by next_digit. If next_digit is 0, the value used instead is 10. |
| 7 | . |
Output the value of the current byte as an ASCII character. |
| 8 | , |
Read a character from the input stream, and write its ASCII value to the current byte. (Note: This fails to update the current byte upon end of input.) |
| 9 | N/A | Set the current byte to a random value from 0 to 255. |
| 0 | N/A | End program execution here. This does not end program execution if it is used as an argument for 3, 4, 5, or 6. |
And the number is derived from the word following these rules:
- If a word is less than 10 letters long, it is converted to that single digit.
- If a word is exactly 10 letters long, it is converted to the digit
0. - If a word is more than 10 letters long, it is converted to a series of digits, representing the number as written in decimal notation (for example, a 12-letter word becomes
12).
Applying these to line 10, we obtain this:
5 2 3 9 1 5 2 3 10 3 10 3 10 6 2 4 1 2 5 2 0
5239 : move the memory_pointer and + 9
152310310310 : start a loop in which we add 10 three times and get back to the position of the 9
the rest is to break out of loop and end. So basically we have 9 * (10 + 10 + 10) = 270.
After this we receive the number 270, and the NGC 270 is in the constellation Cetus, or la constellation de la Baleine in French. So the flag is HACKDAY{baleine}.