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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 technolog y 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 , orderin g 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 -
tut e 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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