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Classical Cryptography
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Transposition Ciphers
Also called permutation ciphers.
Shuffle the plaintext, without altering the actual letters
used.
Example: Row Transposition Ciphers
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Row Transposition Ciphers
Plaintext is written row by row in a rectangle.
Ciphertext: write out the columns in an order
specified by a key.
Key: 3 4 2 1 5 6 7
Plaintext:
Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ
a t t a c k p
o s t p o n e
d u n t i l t
w o a m x y z
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Transposition Decrypt Example
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Cipher Text A L E S T Y E C R O O W A W O A S N R I D N I X F L T P I
Z
Key Column = 5
Key Line = 35124
A R A F E
W I L L CO D E T R
A N S P O
S I T I O
N X Y Z W
A R A F E W I L L C O D E T R A N S P O S I T I O N X Y Z W
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Affine Cypher
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where a-1
is the modular multiplicative inverse of amodulo m. I.e., it satisfies the equation
1 = a a-1 mod m.
The multiplicative inverse of a only exists if a and m are
coprime.
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Affine Cipher
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Assume K=(5,8)
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Affine Cipher -Encryption
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Affine Cipher - Decryption
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Affine Cipher - Example
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Decipher HPCCXAQ if the encipherment function is
E(x) = (5x+ 8)MOD26
Solution: We begin by finding the decipherment function,
Since 5x≡1 (mod26) is solved with x≡ 21 (mod26) we see 5−1 (mod26) = 21
Therefore, E−1(y) = 21(y−8)MOD26
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