I have a frequency distribution of letters for the plaintext and the ciphertext.

I'm trying to determine the conditional probabilities to identify the plaintext/ciphertext pairings.

If I encrypt with Caesar cipher with the key of $3$, then $e$ will become $h$.

As I understand it, the conditional probability of $\Pr(M=e\mathbin|C=h)$ should be $1$ (or very close to it).

How do I calculate the conditional probability to prove that $\Pr(M=e\mathbin|C=h) = 1$?

  • $\begingroup$ Oops sorry. It's deleted now. How do I actually calculate the conditional probability? I understand the expected result but not how to calculate it. $\endgroup$ – huffandpuff Jan 29 at 18:02
  • $\begingroup$ If P(M=e) = 0.012702, P(C=h) = 0.012702. To get P(M=e|C=h), the calculation will be (P(C=h|M=e)*P(M=e))/P(C=h). Where P(C=h|M=e) is1/26. Is calculation correct? $\endgroup$ – huffandpuff Jan 29 at 19:59

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