1. Classical Techniques: Substitution
Substitute a character, digit or symbol for each character in the plaintext
Discussed:
The Caesar cipher
Monoalphabetic cipher
Playfair cipher
Polyalphabetic cipher

2. Caesar Cipher ci=E(3,pi)=(pi+3) mod 26; pi=D(3,ci)=(ci-3) mod 26
2000 years ago, by Julius Caesar
A simple substitution cipher, known as Caesar cipher
Replace each letter with the letter standing 3 places further down the alphabet
Plain: meet me after the toga party
Cipher: PHHW PH DIWHU WKH WRJD SDUWB
No key, just one mapping (translation)
Plain: abcdefghijklmnopqrstuvwxyz
Cipher: DEFGHIJKLMNOPQRSTUVWXYZABC
ci=E(3,pi)=(pi+3) mod 26;
pi=D(3,ci)=(ci-3) mod 26

3. Generalized Caesar Cipher
Can use any shift from 1 to 25, i.e., replace each letter by a letter a fixed distance away
ci=E(k,pi)=(pi+k) mod 26;
pi=D(k,ci)=(ci-k) mod 26
Shift cipher
Key = k
Key letter: the letter a plaintext A maps to
e.g. a key letter of F means A maps to F, B to G, …, Y to D, Z to E
Hence have 26 (25 useful) ciphers
Key space = 26

4. Brute-Force Cryptanalysis of Caesar Cipher
Ciphertext only attack
Charateristics for success
The encryption and decryption algorithms are known
There are only 25 keys to try
The language of the plaintext is known and easily recongnizable

5. Monoalphabetic Cipher
Increased key space compared to Caesar cipher
Alphabet sequence is not required
What is the key space?
26! > 4 x 1026
Monoalphabetic ciphers are still easy to break because they reflect frequency of alphabet
E = 12.75, T = 9.25, R = 8.50, N = 7.75, etc.

6. Relative Frequency of Letters in English Text

7. Frequency Statistics of Language
Substitution Techniques
Frequency Statistics of Language
In addition to the frequency info of single letters, the frequency info of two-letter (digram) or three-letter (trigram) combinations can be used for the cryptanalysis
Most frequent digrams
TH, HE, IN, ER, AN, RE, ED, ON, ES, ST, EN, AT, TO, NT, HA, ND, OU, EA, NG, AS, OR, TI, IS, ET, IT, AR, TE, SE, HI, OF
Most frequent trigrams
THE, ING, AND, HER, ERE, ENT, THA, NTH, WAS, ETH, FOR, DTH

8. Playfair Cipher
Best-known multiple-letter substitution cipher
Digram cipher (digram to digram, i.e., E(pipi+1) = cici+1 through keyword-based 5x5 transformation table)
Great advance over simple monoalphabetic cipher (26 letters  26x26=676 digrams)
Can be generalized to polygram cipher
Keyword = monarchy
Plaintext: H S E A A R M U
Ciphertext: B P I M R M C M M O N A R C H Y B D E F G
I/J K L P Q S T U V W X Z

9. Playfair Cipher - Rules
Repeating plaintext letters are separated with a filler letter, such as X.
Plaintext letters that fall in the same row of the matrix are each replaced by the letter to the right, with the first element of the row circularly following the last.
Plaintext letters that fall in the same column are each replaced by the letter beneath, with the top element of the row circularly following the last.
Otherwise, each plaintext letter is replaced by the letter that lies in its own row and the column occupied by the other plaintext letter.

10. Polyalphabetic Cipher
Typically a set of monoalphabetic substitution rules is used
Key determines which rule to use

11. Vigenère cipher ci = E(pi) = pi + ki mod(key length) mod 26 Example:
Best-known polyalphabetic ciphers
Each key letter determines one of 26 Caesar (shift) ciphers
ci = E(pi) = pi + ki mod(key length) mod 26
Example:
Keyword is repeated to make a key as long as the plaintext
Key: deceptivedeceptivedeceptive
Plaintext: wearediscoveredsaveyourself
Cipheretxt: ZICVTWQNGRZGVTWAVZHCQYGLMGJ

12. Vigenère cipher - 2

13. Example:
Encryption
Decryption

14. Vigenère cipher - 3
Vigenère autokey system: after key is exhausted, use plaintext for running key (to eliminate the periodic nature)
Key: deceptivewearediscoveredsav
Plaintext: wearediscoveredsaveyourself
Cipheretxt: ZICVTWQNGKZEIIGASXSTSLVVWLA

15. Transposition (Permutation) Techniques
Hide the message by rearranging the letter order without altering the actual letters used
Rail Fence Cipher
Write message on alternate rows, and read off cipher row by row
Example:
Block (Columnar) Transposition Ciphers
Message is written in rectangle, row by row, but read off column by column; The order of columns read off is the key
M e m a t r h t g p r y
e t e f e t e o a a t
MEMATRHTGPRYETEFETEOAAT
Key:
Plaintext: 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
Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ

16. Steganography “The art of covered writing” “Security by obscurity”
Hide messages in other messages
Conceal the existence of message
Conceal what you are communicating (Sending encrypted messages would make you a spy)
Character marking. Overwrite with a pencil
Invisible ink, - Pin punctures, - First letter of each word
Letter position on page, - Drawings, - Codes
Typewriter correction ribbon
Microdots
Digital steganography

17. Steganography - Example
News Eight Weather: Tonight increasing snow. Unexpected precipitation
Smothers Eastern towns. Be extremely cautious and use snowtires especially
heading east. The highways are knowingly slippery. Highway evacuation is
suspected. Police report emergency situations in downtown ending near
Tuesday
First letter of each word yields: Newt is upset because he thinks he is President
This example was created by Neil F. Johnson, and was published in
Steganography,Technical Report TR_95_11_nfj, URL: 
From WWII German spy (Kahn):
Apparently neutral’s protest is thoroughly discounted and ignored. Isman
hard hit. Blockade issue affects pretext for embargo on by products, ejecting
suets and vegetable Oils.
Second letter of each word yields: Pershing sails from NY June 1.