01 3 intro annotated tủ tài liệu bách khoa
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Online
Cryptography
Course
Dan
Boneh
Introduc/on
History
Dan
Boneh
History
David
Kahn,
“The
code
breakers”
(1996)
Dan
Boneh
Symmetric
Ciphers
Dan
Boneh
Few
Historic
Examples
(all
badly
broken)
1.
Subs/tu/on
cipher
k
:=
Dan
Boneh
Caesar
Cipher
(no
key)
Dan
Boneh
What
is
the
size
of
key
space
in
the
subs/tu/on
cipher
assuming
26
leSers?
|𝒦| = 26
|𝒦|=26! (26
factorial)
|𝒦| = 2↑26
|𝒦| = 26↑2
Dan
Boneh
How
to
break
a
subs/tu/on
cipher?
What
is
the
most
common
leSer
in
English
text?
“X”
“L”
“E”
“H”
Dan
Boneh
How
to
break
a
subs/tu/on
cipher?
(1)
Use
frequency
of
English
leSers
(2)
Use
frequency
of
pairs
of
leSers
(digrams)
Dan
Boneh
An
Example
UKBYBIPOUZBCUFEEBORUKBYBHOBBRFESPVKBWFOFERVNBCVBZPRUBOFERVNBCVBPCYYFVUFO
FEIKNWFRFIKJNUPWRFIPOUNVNIPUBRNCUKBEFWWFDNCHXCYBOHOPYXPUBNCUBOYNRVNIWN
CPOJIOFHOPZRVFZIXUBORJRUBZRBCHNCBBONCHRJZSFWNVRJRUBZRPCYZPUKBZPUNVPWPCYVF
ZIXUPUNFCPWRVNBCVBRPYYNUNFCPWWJUKBYBIPOUZBCUIPOUNVNIPUBRNCHOPYXPUBNCUB
OYNRVNIWNCPOJIOFHOPZRNCRVNBCUNENVVFZIXUNCHPCYVFZIXUPUNFCPWZPUKBZPUNVR
B
36
è
E
N
34
NC
11
è
IN
UKB
6
PU
10
è
AT
RVN
6
U
33
è
T
UB
10
P
32
è
A
UN
C
26
9
FZI
è
THE
4
trigrams
digrams
Dan
Boneh
2.
Vigener
cipher
(16’th
century,
Rome)
k
=
C R Y P T O C R Y P T O C R Y P T
m
=
W H A T A N I C E D A Y T O D A Y
(+
mod
26)
c
=
Z Z Z J U C L U D T U N W G C Q S !
suppose
most
common
=
“H”
first
leSer
of
key
=
“H”
–
“E”
=
“C”
Dan
Boneh
3.
Rotor
Machines
(1870-‐1943)
Early
example:
the
Hebern
machine
(single
rotor)
A
B
C
.
.
X
Y
Z
key
K
S
T
.
.
R
N
E
E
K
S
T
.
.
R
N
N
E
K
S
T
.
.
R
Dan
Boneh
Rotor
Machines
(cont.)
Most
famous:
the
Enigma
(3-‐5
rotors)
#
keys
=
264
=
218
(actually
236
due
to
plugboard)
Dan
Boneh
4.
Data
Encryp/on
Standard
(1974)
DES:
#
keys
=
256
,
block
size
=
64
bits
Today:
AES
(2001),
Salsa20
(2008)
(and
many
others)
Dan
Boneh
End
of
Segment
Dan
Boneh
