文档介绍：Keyword Cipher
1. select a keyword - if any letters are repeated drop the second and all other occurrences from the keyword
,如果这个关键词中有重复的字母,则去除第一次出现之外的所有相同字母。
2. write the keyword below the alphabet, fill in the rest of the space with the remaining letters in the alphabet in their standard order
,并用字母表中除了关键词之外的其他字母按标准的顺序填写余下的空间。
Example
The keyword is
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
plaintext
ciphertext
C
N
E
T
B
F
D
J
H
L
K
P
O
R
Q
U
S
W
V
X
Z
Y
I
M
A
G
这样的话,明文字母A就由密文字母M来替换,B则由A来替换,. . .
明文:HELP I AM LOST
密文:TCHO B MJ HLRS
Exercise
The keyword is COUNT
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
plaintext
ciphertext
T
A
B
D
E
G
F
I
H
K
J
M
L
Q
P
S
R
W
V
X
Z
Y
N
C
O
U
Starting Position
The keyword does not have to start at the beginning of the plaintext alphabet(关键词不从第一字母开始)
it could start at any letter
for example, “count” could start at “k”
ciphertext
T
A
B
D
E
G
F
I
H
K
J
L
M
Q
P
S
R
W
V
X
Z
Y
N
C
O
U
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
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U
V
W
X
Y
Z
plaintext
Example
If the keyword is “visit”(note, the second “i” in visit is dropped below) starting at “a” and the plaintext is “next”, the application is:
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
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X
Y
Z
plaintext
ciphertext
L
M
N
O
P
R
Q
W
U
Y
X
Z
V
S
I
A
T
C
B
D
F
E
K
G
H
J
N E X T
K
A
X
Q
Keyword in CAP
运行CAP软件,输入一些明文,然后从Cipher菜单选取Keyword菜单项,将弹出一个Keyword Cipher窗口。
关键词加密法的分析法
关键词加密法当然比简单移位加密法功能更强,因为要尝试所有的关键词及其位置,显然比所有25种移位加密密钥要困难得多。
这种攻击法的原理就是计算机用字典中的每个词进行尝试,直至找到一个能破解该加密法的为止。
用穷举查找攻击法(即尝试所有可能的关键词)在今天也是可行的,但不是可取的。
Breaking a Keyword
The keyword cipher is not secure, in fact it is easy to break
One reason why it is useful to study such a cipher is that in order to break this cipher you must use some of the most fundamental