Basic Text Encryption Schemes
plaintext
cyphertext
Encryption
encrypted
(coded)
message
humanreadable
message
Caesar Cypher
Replace each letter with the letter k positions to the right of it using circular wrapping, where k
is an integer constant between 1 and 25. Here are examples of two different Caesar cipher
schemes:
A
E
Plaintext Letters
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
F G H I J K L M N O P Q R S T U V W X Y Z A B C D
Cyphertext Substitutions (k = 4)
Plaintext Letters
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
P Q R S T U V W X Y Z A B C D E F G H I J K L M N O
Cyphertext Substitutions (k = 15)
If the plaintext message SPRING IS COMING SOON is encrypted using the Caesar
cipher scheme with k = 4, the encrypted message is
WTVMRK MW GSQMRK WSSR
If the plaintext message SPRING IS COMING SOON is encrypted using the Caesar
cipher scheme with k = 15, the encrypted message is
HEGXCV XH RDBXCV HDDC
To decode a message coded using a Caesar cypher, we must know the key value used to
code the message and we use the corresponding Caesar coding table for character
substitutions in reverse (cyphertext to plaintext).
Basic Text Encryption Schemes
Vignere Cypher
Add offsets for letters in a special keyword to plaintext letter offsets modulo 26. The keyword
must contain only letters.
A B C D E F G H I J K
0 1 2 3 4 5 6 7 8 9 1
0
Plaintext Letters
L M N O P Q
1 1 1 1 1 1
1 2 3 4 5 6
Letter Offsets
R
1
7
S
1
8
T
1
9
U
2
0
V
2
1
W X
2 2
2 3
Y
2
4
Z
2
5
Suppose the plaintext message is SPRING IS COMING SOON. This message is
encrypted using the Vignere cipher scheme with keyword BEMIDJI as follows:
S(18)
P(15)
R(17)
I(8)
N(13)
G(6)
I(8)
S(18)
C(2)
O(14)
M(12)
I(8)
N(13)
G(6)
S(18)
O(14)
O(14)
N(13)
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
B(1)
E(4)
M(12)
I(8)
D(3)
J(9)
I(8)
B(1)
E(4)
M(12)
I(8)
D(3)
J(9)
I(8)
B(1)
E(4)
M(12)
I(8)
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
T(19)
T(19)
D(3)
Q(16)
Q(16)
P(15)
Q(16)
T(19)
G(6)
A(0)
U(20)
L(11)
W(22)
O(14)
T(19)
S(18)
A(0)
V(21)
Encrypted message: TTDQQP QT GAULWO TSAV
To decode a message coded using a Vignere cipher, we must have the keyword used to code
the message and then we reverse the coding process by subtracting (modulo 26) the letter
offset for each letter in the keyword from the offset for each matching letter in the coded
message.
Basic Text Encryption Schemes
Permutation Cypher
Divide the characters in the plaintext message into fixed length blocks and permute the letters
in each block using the same permutation order. Suppose we want to encrypt the message
SPRING IS COMING SOON using a permutation cipher with block size 4. The first step is
to put the plaintext characters in groups of 4, padding with spaces at the end, if necessary:
S P R I
N G I S
C O M I
N G S O
O N
Now we pick a permutation of the numbers 1 through 4, say 3 1 4 2, and use this pattern
to permute the letters in each group:
1
S
3
R
2
P
1
S
3
R
4
I
4
I
2
P
1 2 3
N G
3 1 4
N I
4
I
2
G
1 2 3 4
S
C O
3 1 4 2
C S O
1
M
3
N
2
I
1
M
3
N
4
G
4
G
2
I
1 2
S
3 1
O
3
O
4
O
4
O
2
S
1 2 3 4
N
3 1 4 2
N
Encrypted message: RSIP NIGCSO NMGIO OS N
If we use the permutation 2 4 3 1 instead, the message has a different coding:
1
S
2
P
2
P
4
I
3
R
3
R
4
I
1
S
1
N
2
G
2 3 4
G
I
4 3 1
I
N
1 2 3 4
S
C O
2 4 3 1
O C S
1
M
2
I
2
I
4
G
Encrypted message: PIRSGI N OCSIGNMSOO
3
N
3
N
4
G
1
M
1 2
S
2 4
S O
3 4
O O
3 1
O
1 2 3 4
N
2 4 3 1
N
N
To decode a message coded with a permutation cipher, we must know the permutation used
for the coding and then repeatedly apply it to the letters in the coded message to permute
them back to their plaintext positions.