2 1. INTRODUCTIO N this machine fro m tha t poin t on , the y wer e abl e t o rea d Germa n commu - nications create d b y ENIGMA . Computers hav e significantl y change d th e wa y i n whic h code d message s are created . Wit h th e increas e o f compute r speed , i t i s no w possibl e t o implement encryptio n scheme s tha t wer e impractica l jus t a fe w year s ago . On th e othe r hand , faste r computer s als o mea n tha t intercepte d message s can b e mor e quickl y analyze d b y unintende d receivers . S o scheme s tha t might hav e bee n secur e befor e ca n no w b e broke n easily . The widesprea d us e o f technology ha s increase d th e necessit y fo r encryp - tion. Unti l recently , th e nee d fo r secre t code s wa s limite d t o governmen t an d perhaps som e businesses . Computer s an d th e Interne t hav e change d tha t forever. Today , man y peopl e hav e thei r paycheck s deposite d directl y int o their checkin g accounts . T o d o this , th e employer' s an d th e employees ' ban k account number s mus t b e transmitted electronically . Likewise , ordering mer - chandise on-lin e require s sendin g a credi t car d numbe r ou t int o cyberspace . And, w e al l expec t tha t ou r e-mai l message s ar e private . We wil l begi n ou r stud y o f encryptio n wit h th e syste m use d b y Juliu s Caesar. Buildin g o n tha t simpl e system , w e wil l the n conside r mor e com - plicated schemes . Al l o f thes e differen t cipher s wil l buil d towar d th e ciphe r developed b y th e MI T compute r scientists , whic h i s calle d th e RS A Cipher . It i s on e o f th e cipher s tha t i s use d t o provid e securit y fo r th e Internet . This introductor y chapte r cover s som e basi c terminology . Th e origina l message i s know n a s th e plaintext, whil e th e code d messag e i s calle d th e ciphertext. Th e proces s o f convertin g fro m plaintex t t o ciphertex t i s know n as enciphering o r encryption restorin g th e plaintex t fro m th e ciphertex t i s deciphering o r decryption. Th e man y scheme s use d fo r encipherin g consti - tute th e are a o f stud y know n a s cryptography. Suc h a schem e i s know n a s a cryptographic system o r a cipher. Technique s use d fo r decipherin g a messag e without an y knowledg e o f th e encipherin g detail s fal l int o th e are a o f crypt- analysis. Cryptanalysi s i s wha t th e layperso n call s "breakin g th e code. " The area s o f cryptograph y an d cryptanalysi s togethe r ar e calle d cryptology. When devisin g a n encipherin g scheme , i t i s importan t tha t ther e b e a corresponding decipherin g scheme . I t i s useles s t o enciphe r a messag e i f it i s impossibl e t o recove r th e plaintext . Thus , fo r eac h syste m tha t w e develop, w e wil l describ e bot h th e encipherin g an d decipherin g processes . We wil l als o us e eac h syste m a s a n opportunit y t o discus s differen t aspect s of cryptanalysis . Fo r th e firs t fe w cryptographi c schemes , w e wil l conside r the followin g question . I f w e di d no t kno w th e encipherin g details , ho w might w e g o abou t discoverin g th e plaintex t message ? Here i s a ciphertex t tha t w e wil l analyz e i n a late r chapter : ZK YR J FEC P SVV E Z E KY V CRJ K WV N UVTRUV J KYR K TIPGKFXIRGYVIJYRM V LJV U DRK Y K F UVJZX E TZGYVI J Can yo u deciphe r it ?
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