16-page4 - “Encoding E with key S” means converting...

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Key Ciphers • A better method is to use multiple translation systems, and rotate among them every letter. A convenient way to do this is to adopt a short word as a key . Let’s look at encoding the message “ET TU BRUTE” with the key “SPQR”. • The frst, fFth, and ninth letters get encoded with key S, the second and sixth with key P, the third and seventh with key Q, and the Fourth and eighth with key R.
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Unformatted text preview: “Encoding E with key S” means converting both E and S to numbers (using A=0, B=1,. .., Z=25, so that E=4 and S=18), adding the numbers mod 26 (getting 4+18=22), and converting the result back to a letter (W). • Deciphering means fnding the key letter For each ciphertext letter and subtracting it to get the plaintext letter. Plaintext: ET TU BRUTE Repeated Key: SP QR SPQRS Ciphertext: WH IK TFJJW...
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This note was uploaded on 11/22/2011 for the course COMPSCI 187 taught by Professor Barrington during the Fall '11 term at UMass (Amherst).

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