CHAPTER 1
Introduction
According t o legend , Juliu s Caesa r sen t encrypte d communication s t o
Cicero becaus e h e di d no t trus t th e messenger s deliverin g hi s messages . Hi s
scheme, whic h i s extremel y simple , i s base d o n elementar y mathematics .
More tha n 2,05 0 year s later , compute r scientist s a t th e Massachusett s In -
stitute fo r Technolog y (MIT ) develope d a n encryptio n metho d i n which th e
enciphering detail s ar e mad e publi c an d ye t th e messag e remain s secure !
This schem e use s th e sam e mathematic s a s Caesar' s scheme , albei t i n a
more sophisticate d way .
A ciphe r i s a schem e fo r obscurin g a messag e i n orde r t o exchang e in -
formation securely . Th e purpos e o f using a ciphe r i s to kee p a message con -
cealed fro m al l except th e intende d receiver . Privac y i s something w e value,
even whe n th e conten t o f a messag e i s no t particularl y significant . Tha t
is wh y w e us e envelopes , instea d o f postcards , fo r al l bu t th e mos t trivia l
of ou r writte n communications . I n thi s curren t "informatio n age, " w e sen d
many differen t type s o f messages , includin g e-mail , ban k transactions , an d
on-line shopping. Banks , stores , an d librarie s al l hold a tremendous amoun t
of informatio n abou t us . Th e nee d fo r privac y i s greater tha n ever .
The us e o f cipher s i s probably a s ol d a s writin g itself . On e o f th e earli -
est recorde d instance s occur s i n th e boo k o f Jeremia h i n th e Hebre w Bible .
Throughout history , a variety o f ingenious technique s hav e been use d t o ob -
scure o r hid e messages . Thes e includ e writin g wit h invisibl e ink , employin g
coding machines , an d hidin g th e rea l messag e insid e a large r one . A s a n
example o f th e latte r technique , conside r th e message :
(1)
A TRICKY TIGE R ALWAY S COME S KNOWINGLY.
^ ' A THREATENED DEE R ALWAY S WALK S NEARBY.
Can yo u fin d th e rea l message ? I f yo u writ e dow n th e firs t lette r o f eac h
word i n (1), you wil l fin d th e hidde n message :
ATTACK AT DAWN.
This schem e is , o f course , no t particularl y secure .
Dedicated codin g machine s provid e mor e securit y tha n th e schem e illus-
trated i n (1). The y have the additiona l advantag e o f enabling the sender an d
receiver t o cod e an d decod e message s quickly . Thoma s Jefferso n invente d
one o f th e oldes t know n codin g machines ; i t i s o n displa y a t Monticell o i n
Virginia. I n Worl d Wa r II , th e German s use d th e ENIGM A machine . A
turning poin t i n th e wa r occurre d whe n th e allie s buil t thei r ow n cop y o f
l
http://dx.doi.org/10.1090/mawrld/025/01
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