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Learning Opportunities
This puzzle can be solved using the following concepts. Practice using these concepts and improve your skills.
Statement
Goal
Have you ever wondered how the OpenSSH Random Art is produced?They used 'The Drunken Bishop algorithm', designed by Alexander von Gernler.
"Bishop Peter finds himself in the middle of an ambient atrium. There are walls on all four sides and apparently there is no exit. The floor is paved with square tiles, strictly alternating between black and white. His head heavily aching—probably from too much wine he had before—he starts wandering around randomly. Well, to be exact, he only makes diagonal steps—just like a bishop on a chess board. When he hits a wall, he moves to the side, which takes him from the black tiles to the white tiles (or vice versa). And after each move, he places a coin on the floor, to remember that he has been there before. After 64 steps, just when no coins are left, Peter suddenly wakes up. What a strange dream!"
The algorithm is well described in the document "The drunken bishop: An analysis of the OpenSSH fingerprint visualization algorithm" (link in reference).
To summarize, OpenSSH uses MD5 to generate a 128 bits fingerprint for the server's key, for example in hexadecimal
fc:94:b0:c1:e5:b0:98:7c:58:43:99:76:97:ee:9f:b7
You should create a chess board 17x9 (origin 0,0 at top left corner) and place a Bishop (the letter
1111111
01234567890123456
+---[CODINGAME]---+ x
0| |
1| |
2| |
3| |
4| S |
5| |
6| |
7| |
8| |
+-----------------+
y
Then, break the 128 bit fingerprint in input into pairs of bits that define 4 possible moves:
00: up-left (↖ North West )
01: up-right (↗ North East)
10: down-left(↙ South West)
11: down-right (↘ South East)
During the 64 steps of the algorithm, the bit pairs are processed byte-wise, from left to right and least significant bits first.
Example: The fingerprint
F C 9 4 ...
11 11 11 00 : 10 01 01 00 ...
| | | | | | | |
Step 4 3 2 1 8 7 6 5
For each pair of bits in the input we move the bishop one space on the board and increment a counter recording how many times we visit each square. Instead of moving off the board at the edges, the bishop slides along the sides as if they were walls. For example, if the bishop is on the right edge of the board and the next move is 01, we simply slide the bishop one space vertically up, incrementing the counter on the new square as normal. If the Bishop ends up at a corner and he cannot slide, he simply does not move (and we should increment counter for that corner cell). Here the four cases:
Position | bit pair | action
-------------+----------+--------
top left | 00 | no move
top right | 01 | no move
bottom left | 10 | no move
bottom right | 11 | no move
At the end of the 64 steps, the board is drawn assigning a symbol to each position on the board according to how many times it was visited. OpenSSH uses these symbols:
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
. o + = * B O X @ % & # / ^
We apply the wrap-around logic if the number of times visited is greater than 14, e.g. we use
The special values
References:
http://www.dirk-loss.de/sshvis/drunken_bishop.pdf
https://www.jfurness.uk/the-drunken-bishop-algorithm/
https://dev.to/krofdrakula/improving-security-by-drawing-identicons-for-ssh-keys-24mc
Input
Line 1: Server fingerprint string format as group of 2 hexadecimal digit separated by : .
Output
11 lines: Random ASCII Art
Constraints
The input string is always 16 bytes (separated by : ), so there are exactly 64 steps that can be produced by the input string.
Example
Input
fc:94:b0:c1:e5:b0:98:7c:58:43:99:76:97:ee:9f:b7
Output
+---[CODINGAME]---+ | .=o. . | | . *+*. o | | =.*..o | | o + .. | | S o. | | o . | | . . . | | o .| | E.| +-----------------+
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