1. A Technical Seminar Presentation - 2004 CLASSICAL CRYPTOGRAPHY
Under the guidance of
Dr. Partha S. Mallick
Presented by
Jyotishman Baishya
Roll # EC

2. Introduction Basic Concepts Key Cryptography Plaintext
Cipher text
Cipher
Key
Encipher (encode)
Decipher (decode)
Cryptanalysis
Cryptology
Code

3. Types of Ciphers Substitution Transposition Mono-alphabetic Scytale
Poly-alphabetic
Poly-graphic
Transposition
Scytale
Reverse
Rail Fence
Geometric
Row/Column

4. Mono-alphabetic Substitution
Caesar Cipher
2000 years ago Julius Caesar used a simple substitution cipher, now known as the Caesar cipher
first attested use in military affairs (Gallic Wars)
replace each letter by 3rd letter on, eg.
L FDPH L VDZ L FRQTXHUHG is
I CAME I SAW I CONQUERED
can describe this mapping (or translation alphabet) as:
Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cipher: EFGHIJKLMNOPQRSTUVWXYZABC

5. General Mono-alphabetic Substitution
rather than just shifting the alphabet could shuffle (jumble) the letters arbitrarily
each plain text letter maps to a different random cipher text letter
hence key is 26 letters long
have a total of 26! ~ 4 x 1026 keys!
but … can be cracked through frequency analysis

6. Poly-alphabetic Substitution Cipher
Vigenère Cipher
Blaise de Vigenère is generally credited as the inventor of the “polyalphabetic substitution cipher”
To improve security use many monoalphabetic substitution alphabets
Hence each letter can be replaced by many others
Use a key to select which alphabet is used for each letter of the message
ith letter of key specifies ith alphabet to use
Use each alphabet in turn
Repeat from start after end of key is reached

7. Vigenère Example
Write the plaintext out and under it write the keyword repeated
Then using each key letter in turn as a Caesar cipher key
Encrypt the corresponding plaintext letter Plaintext THISPROCESSCANALSOBEEXPRESSED
Keyword CIPHERCIPHERCIPHERCIPHERCIPHE
Ciphertext VPXZTIQKTZWTCVPSWFDMTETIGAHLH
In this example the keyword is "CIPHER". Hence the following translation alphabets are mapped to the plaintext letters:
C: - CDEFGHIJKLMNOPQRSTUVWXYZAB
I: - IJKLMNOPQRSTUVWXYZABCDEFGH… …
for ABCDEFGHIJKLMNOPQRSTUVWXYZ

8. Hill Cipher
A multiple-letter encryption method – encrypts m letters of plaintext at each step
The encryption key K is a m x m matrix of coefficients
To encrypt – multiply the matrix K by a vector of m plaintext letters to receive a vector of m ciphertext letters. (Arithmetic is modulo the size of the alphabet.)
The decryption key K-1 is the m x m matrix of coefficients that is the inverse of matrix K: K * K-1 = I
To decrypt – multiply the matrix K-1 by the vector of m ciphertext letters to receive the vector of m plaintext letters. (Arithmetic is modulo the size of the alphabet.)

9. Transposition Ciphers
Scytale cipher
a strip of paper was wound round a staff
message written along staff in rows, then paper removed leaving a strip of seemingly random letters
not very secure as key was width of paper & staff
Reverse cipher
write the message backwards
Rail Fence cipher
write message with letters on alternate rows
Geometric Cipher
Write message following one pattern and read out with another.

10. Transposition Ciphers (contd…)
Row Transposition ciphers
in general write message in a number of columns and then use some rule to read off from these columns.
key could be a series of number being the order to: read off the cipher; or write in the plaintext.
Block (Columnar) Transposition ciphers
the message is written in rows, but read off by columns in order given by key.
for ease of recovery may insist matrix is filled.

11. Poly-alphabetic Cipher Machines
Jefferson Wheel
Rotor Based Machines

12. Conclusion
Cryptography has undergone tremendous development since the days it had come into existence some 4000 years ago.
The world of research and development continues to experiment with new concepts such as quantum cryptography, cellular automata, chaotic cryptosystems, elliptic curves based on cryptography and visual cryptography.
Developments are also taking place in the area of most sophisticated methods of digital steganography.
Simultaneously some challenging area of cryptanalysis, being the other side of development exercise will gain importance in view of the plethora applications demanding assured security.

13. Thank You.