what is cryptography? symmetric encryption ,shift cipher, rot 13, ceasar cipher

1. LECTURE 2
Symmetric encryption

2. Symmetric Encryption
An encryption system in which the sender and receiver of a message share a single, common
key that is used to encrypt and decrypt the message.
In other terms, Data is encrypted and decrypted using the same key.
Symmetric-key cryptography is sometimes called secret key cryptography.
Examples of symmetric algorithms are DES, 3DES and AES.
Symmetric-key systems are simpler and faster, but their main drawback is that the two parties
must somehow exchange the key in a secure way. Public-key encryption avoids this problem
because the public key can be distributed in a non-secure way, and the private key is never
transmitted, Public-key encryption avoids this problem because the public key can be
distributed in a non-secure way, and the private key is never transmitted.

3. Some basic terminologies
Plain text: original text.
Cipher text: coded message.
cipher - algorithm for transforming plaintext to cipher text.
key - info used in cipher known only to sender/receiver.
encipher (encrypt) - converting plaintext to cipher text.
decipher (decrypt) - recovering cipher text from plaintext.
cryptography - study of encryption principles/methods.
cryptanalysis (codebreaking) - study of principles/ methods of deciphering cipher text ( with
out knowing key).
cryptology - field of both cryptography and cryptanalysis.

4. Symmetric
encryption
Same key used for
encryption and decryption.

5. Requirements
Two things we require for secure encryption.
Strong algorithm
Secret key ( only known by sender and receiver.
Mathematically
Y=EK(X)
X=EK(Y)

6. Shift Cipher
 Each letter we identify with a number
 A = 0
 B = 1
 C = 2
 ...
 Z = 25
 The key k is a number in the range 0 − 25
 Encryption is add k onto each letter modulo 26.
 Use the key k = 3.
 HELLO becomes
KHOOR

8. ROT-13 CIPHER

9. ROT-13 CIPHER

10. CAESAR CIPHER

11. Caesar Cipher
 Earliest known substitution cipher and first attested use in military
affairs
 The Roman emperor Julius Caesar used to substitute each letter in
his diplomatic communications with the letter that was three letters
further along in the alphabet.
 Replaces each letter by 3rd letter on
 p : ABCDEFGHIJKLMNOPQRSTUVWXYZ
 F(p) : DEFGHIJKLMNOPQRSTUVWXYZABC

12. Caesar Cipher
 Example:
 Plaintext : ET TU BRUTUS
 Ciphertext : HW WX EUXWXV

13. Caesar Cipher
 Can define transformation as:
 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
 D E F G H I J K L M N O P Q R S T U V W X Y Z A B C
 Mathematically give each letter a number
 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
 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
 Then have Caesar cipher as:
 c = E(p) = (p + k) mod (26)
 p = D(c) = (c – k) mod (26)

14. Caesar Cipher

15. LECTURE 3
Cryptography

16. What is Cryptography?
◦ Cryptography is the science of using mathematics to encrypt and decrypt data.
◦ Cryptography is a technique used to convert plain text into cipher text and then
again convert into plain text.
◦ Cryptography enables you to store sensitive information or transmit it across
insecure networks so that it cannot be read by anyone except the intended recipient.
◦ Single/private key
◦ 2-keys public/private
◦ Used to achieve confidentiality

17. Classification of cryptography

18. CRYPTANALYSIS

19. Cryptanalysis
Hacker used this technique to discover not just plain text but the key
also
 objective to recover key not just message
general approaches:
cryptanalytic attack
brute-force attack
if either succeed all key use compromised
To find the weakness

20. Brute force attack
• try every possible key on a piece of cipher text until an intelligible translation
into plaintext is obtained.
• On average, half of all possible keys must be tried to achieve success.
◦ If either type of attack succeeds in deducing the key, the effect is catastrophic:
All future and past messages encrypted with that key are compromised.
◦ Time consuming
◦ If the key size 128 bit or greater then possible combinations are increase
◦ Example: DES(56 bit) , AES(128 bit)