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Cryptology - Unofficial St. Mary's College of California Web Site

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Chapter 12<br />

RSA<br />

Take two large prime numbers, q and p.<br />

Find the product n, and the totient φ.<br />

If e and φ have GCD one<br />

and d is d’s inverse, then you’re done!<br />

For sending m raised to the e reduced mod n gives<br />

secre-c.<br />

Daniel G. Treat<br />

We have now reached our final chapter. So it is natural that we have reached<br />

the more difficult and useful cipher in the book. Fortunately, our cipher also<br />

brings us back around to the beginning <strong>of</strong> the book.<br />

We started out, in Chapter 1, with the Caesar Cipher: translate letters into<br />

numbers and add three. This was too easy to decrypt, so Chapters 3 and 4 we<br />

turned to Decimation Ciphers: translate letters into numbers and multiply by<br />

three. This also turned out to be easy to decrypt. After several other stops along<br />

the way, we now complete the triple: having used addition and multiplication<br />

how about exponentiation<br />

Example: Encipher power by raising to the 3-rd power modulo 26.<br />

plaintext p o w e r<br />

plainnumbers 16 15 23 5 18<br />

cubed 4096 2275 12167 125 5832<br />

ciphernumbers %26 14 13 25 21 8<br />

ciphertext N M Y U H<br />

The ciphertext is NMYUH<br />

⋄<br />

231

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