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F F j Hill Cipher Cryptanalysis
Assume that you know ciphertext and part of the plaintext.
Example:
Ciphertext: ZWSENIUSPLJVEULNVPVJPW
Beginning of the Plaintext: howareyout.
We know that the block size is 2, so the size of the key-matrix is 2X2
Find a key-matrix and decipher the message.
Solution:
The first two blocks yield the following equations:
EMBED Equation.3
The next step is to check determinant of the matrix EMBED Equation.3
EMBED Equation.3
-308 is not relatively prime to 26, this means that unfortunately we will not be able to find a matrix-key from the first two blocks.
Next step will be to try a third block:
EMBED Equation.3
EMBED Equation.3
Unfortunately, -210 is not relatively prime to 26 also.
As next step, we will try to work with first and fifth blocks:
EMBED Equation.3
Finally, 9 is relatively prime with 26, so we can proceed to the next step.
As next step, will be to find the inverse of matrix B = EMBED Equation.3
EMBED Equation.3 EMBED Equation.3
The key-matrix is found and it is: A= EMBED Equation.3
To decipher, use the web-resources.
The final plaintext is:
How are you today students
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