# 1) An affine cipher is a generalization of the Caesar cipher and ha...

## Question

1) An affine cipher is a generalization of the Caesar cipher and has the following form: E(x)=(ax+b) mod 26, where x is the plaintext letter and E(x) is the ciphertext letter. Now, assume that a ciphertext has been generated using an affine cipher where the first and second most frequent letter in the ciphertext are ‘B’ and ‘U’, respectively. Use the given information and what we’ve covered in class to break this code! Hint: You are being asked to solve for a and b.

2) Suppose that the Notifier of an audit system has been configured to send an e-mail to the administrator(s) of the system when the Notifier believes it has been compromised. Describe two potential problems with this configuration.

## Solution Preview

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1) E(x)=ax+b (mod 26)
Since affine is a linear mapping, it means that most frequent letters from the ciphertext correspond to most frequent letters from plaintext.
In English alphabet the most frequent letters are E and T; so E from plaintext is mapped by affine transformation into B in ciphertext and T from plaintext English is mapped by affine transformation into U in ciphertext....

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