**The Basics of Cryptography**

When Julius Caesar sent messages to his generals, he didn’t trust hismessengers. So he replaced every A in his messages with a D, every B with anE, and so on through the alphabet. Only someone who knew the “shift by 3”rule could decipher his messages.

**Encryption and decryption**

Data that can be read and understood without any special measures is calledplaintext or cleartext. The method of disguising plaintext in such a way as tohide its substance is called encryption. Encrypting plaintext results inunreadable gibberish called ciphertext. You use encryption to ensure thatinformation is hidden from anyone for whom it is not intended, even thosewho can see the encrypted data. The process of reverting ciphertext to itsoriginal plaintext is called decryption.

**Q: What is cryptography?**

A:

Cryptography is the science of using mathematics to encrypt and decrypt data. Cryptography enables you to store sensitive information or transmit it acrossinsecure networks (like the Internet) so that it cannot be read by anyone exceptthe intended recipient.

While cryptography is the science of securing data, cryptanalysis is the scienceof analyzing and breaking secure communication. Classical cryptanalysisinvolves an interesting combination of analytical reasoning, application ofmathematical tools, pattern finding, patience, determination, and luck.Cryptanalysts are also called attackers.Cryptology embraces both cryptography and cryptanalysis.

**How does cryptography work?**

A cryptographic algorithm, or cipher, is a mathematical function used in theencryption and decryption process. A cryptographic algorithm works incombination with a key—a word, number, or phrase—to encrypt the plaintext. The same plaintext encrypts to different ciphertext with different keys. Thesecurity of encrypted data is entirely dependent on two things: the strength ofthe cryptographic algorithm and the secrecy of the key. A cryptographic algorithm, plus all possible keys and all the protocols thatmake it work comprise a cryptosystem. PGP is a cryptosystem.

Conventional cryptographyIn conventional cryptography, also called secret-key or symmetric-keyencryption, one key is used both for encryption and decryption. The

**Data Encryption Standard (DES)**is an example of a conventional cryptosystem thatis widely employed by the Federal Government.

**Caesar’s Cipher**

An extremely simple example of conventional cryptography is a substitutioncipher. A substitution cipher substitutes one piece of information for another.This is most frequently done by offsetting letters of the alphabet. Two examplesare Captain Midnight’s Secret Decoder Ring, which you may have owned whenyou were a kid, and Julius Caesar’s cipher. In both cases, the algorithm is tooffset the alphabet and the key is the number of characters to offset it.For example, if we encode the word

**“SECRET”**using Caesar’s key value of 3, we offset the alphabet so that the 3rd letter down (D) begins the alphabet.So starting with

**ABCDEFGHIJKLMNOPQRSTUVWXYZ**

and sliding everything up by 3, you get

**DEFGHIJKLMNOPQRSTUVWXYZABC**

where D=A, E=B, F=C, and so on.

Using this scheme, the plaintext, “SECRET” encrypts as “VHFUHW.” Toallow someone else to read the ciphertext, you tell them that the key is 3.Obviously, this is exceedingly weak cryptography by today’s standards, buthey, it worked for Caesar, and it also illustrates how conventionalcryptography works.

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