Is the key of Hill cipher for example for $m=2$, the determinant always an odd number?
Because when I try to make a key and the determinant is even number, I can't find the inverse in modulo 26.
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1Hint: If $a$ is invertible mod $b$, $a$ and $b$ must be coprime. – Robert Israel Mar 31 '20 at 13:55
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The determinant must be odd and not 13. This is because of the fact that a matrix with entries mod $n$ is invertible if and only if its determinant is invertible mod $n$. Since the only invertible elements mod 26 are the odds except 13, the same must be required of your determinant.
Randall
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