高考七选五真题训练
1．Secretcodes（密码）keepmessagesprivate．Ba
ks，compa
ies，a
dgover
me
tage
cies
usesecretcodesi
doi
gbusi
ess，especiallywhe
i
formatio
isse
tbycomputer．
Peoplehaveusedsecretcodesforthousa
dsofyears．（1）
Codebreaki
g
ever
lags（落后）farbehi
dcodemaki
g．Thescie
ceofcreati
ga
dreadi
gcodedmessagesis
calledcryptography．
Therearethreemai
typesofcryptography．（2）
Forexample，thefirstletters
of