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Learning Opportunities

This puzzle can be solved using the following concepts. Practice using these concepts and improve your skills.



You are Catherine, you know that Alice and Bob are chatting secretly and you want to be the woman in the middle; ie you want to intercept Alice and Bob’s messages, to read them and to send them messages that you cipher in place of theirs. If Alice sent Bob a message, Bob will never receive Alice’s message but yours instead.
You are lucky: you caught a ciphered text and its clear version and you know that they are using the Hill cipher method.
You have to find the Hill matrix in order to decipher a text from Alice or Bob and to cipher the text you want to send instead.

The texts are using the 45 symbols of the alphanumeric encoding of the QR codes:
0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ $%*+-./: (yes, it’s a space between the Z and the $)

How to:
The Hill cipher uses matrix multiplication (here, with a modulus of 45).
For example, you have to cipher CODINGAME. with A, a 2×2 matrix.
CODINGAME. is 12 24 13 18 23 16 10 22 14 42 (because C’s index is 12).

If A is:
⎡6 5⎤
⎣7 6⎦

The first pair of ciphered symbols will be:
6×12+5×24 mod 45 = 192 mod 45 = 12 that is C and 7×12+6×24 mod 45 = 228 mod 45 = 3 that is 3.
Thus, CO is ciphered C3, the product A×T mod 45 where T is the column matrix whose elements are 12 and 24.
Finally, the ciphered text is C3XJ%WZMOZ.

If the matrix’s size is 3, you will cipher 3 characters at a time.

For this puzzle, you have to find the cipher matrix (and its size) and how to decipher.
Line 1: The ciphered text
Line 2: The clear text
Line 3: The ciphered text you have to decipher
Line 4: The clear text you have to cipher
Line 1: Line 3 deciphered
Line 2: Line 4 ciphered
You can assume that the size of the cipher matrix divides the length of all the messages (but it’s not 1).

Just to help you:
* The clear texts of a test and validator pair will be the same but the Hill matrix won’t be the same.
* Test 1/validator 1 and test 2/validator 2’s Hill matrices are the same.
* Test 2/validator 2 and test 3/validator 3’s clear texts are the same.
* There are no trailing spaces in the clear texts but there might be some in the ciphered texts (as in the example).

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