Cryptography Ch-1 prepared by: Diwan. Essential Terms Cryptography Encryption Plain text Cipher text Decryption Cipher text Plain text Cryptanalysis Cryptology.

Cryptography

Ch-1

prepared by:Diwan

Essential Terms• Cryptography• Encryption

Plain text Cipher text

• DecryptionCipher text Plain text

• Cryptanalysis• Cryptology Source: http://www.unmuseum.org/enigma.jpg

3

Basic Terminologies• Cryptography deals with creating documents that can be

shared secretly over public communication channels• Cryptographic documents are decrypted with the key

associated with encryption, with the knowledge of the encryptor

• The word cryptography comes from the Greek words: Krypto (secret) and graphein (write)

• Cryptanalysis deals with finding the encryption key without the knowledge of the encryptor

• Cryptology deals with cryptography and cryptanalysis• Cryptosystems are computer systems used to encrypt

data for secure transmission and storage

4

Basic Terminologies• Keys are rules used in algorithms to convert

a document into a secret document• Keys are of two types:– Symmetric– Asymmetric

• A key is symmetric if the same key is used both for encryption and decryption

• A key is asymmetric if different keys are used for encryption and decryption

5

Basic Terminologies

• Examples:– Symmetric key methods• DES 56-bit• Triple DES 128-bit • AES 128-bit and higher• Blowfish 128-bit and higher

– Asymmetric key methods• RSA • PGP

6

Basic Terminologies• Plaintext is text that is in readable form• Ciphertext results from plaintext by applying the

encryption key• Notations:– M message, C ciphertext, E encryption,

D decryption, k key– E(M) = C– E(M, k) = C

• Fact: D(C) = M, D(C, k) = M

Cryptographic Methods

• Symmetric– Same key for encryption and decryption– Key distribution problem

• Asymmetric– Mathematically related key pairs for encryption and

decryption– Public and private keys

7

Cryptographic Methods

• Hybrid– Combines strengths of both methods– Asymmetric distributes symmetric key• Also known as a session key

– Symmetric provides bulk encryption– Example:• SSL negotiates a hybrid method

8

Types of Secret Writing

Secret writing

Steganography Cryptography

10

Basic Terminologies

• Steganography is the method of hiding secret messages in an ordinary document

• Steganography does not use encryption• Steganography does not increase file size for

hidden messages• Example: select the bit patterns in pixel colors

to hide the message

Steganography

• Hiding a message within another medium, such as an image

• No key is required• Example– Modify color map of JPEG image

11

Steganography

• Steganography – covered writing – is an art of hiding information

• Popular contemporary steganographic technologies hide information in images

New York Times, August 3rd, 2001http://www.nytimes.com/images/2001/10/30/science/sci_STEGO_011030_00.jpg

Hiding information in pictures

Image in which to hide another image

Image to hide within the other image

http://www.cl.cam.ac.uk/~fapp2/steganography/image_downgrading/

Types of Cryptography

• Stream-based Ciphers– One at a time, please– Mixes plaintext with key stream– Good for real-time services

• Block Ciphers– Amusement Park Ride– Substitution and transposition

14

Types of Secret WritingSecret writing

Steganography Cryptography

Substitution Transposition

Mon alphabetic

Polyalphabetic

Encryption Systems• Substitution Cipher– Convert one letter to another– Cryptoquip

• Transposition Cipher– Change position of letter in text– Word Jumble

• Monoalphabetic Cipher Caesar

• Polyalphabetic Cipher– Vigenère

16

Public Key Cryptography• Private (symmetric, secret) key – the same

key used for encryption/decryption• Problem of key distribution• Public (asymmetric) key cryptography – a

public key used for encryption and private key for decryption

• Key distribution problem solved

Currently Available Crypto Algorithms (private key)

• DES (Data Encryption Standard) and derivatives: double DES and triple DES

• IDEA (International Data Encryption Standard)

• Blowfish• RC5 (Rivest Cipher #5)• AES (Advance Encryption Standard)

• RSA (Rivest, Shamir, Adleman)• DH (Diffie-Hellman Key Agreement

Algorithm)

Currently Available Crypto Algorithms (public key)

Hashing Algorithms

• MD5– Computes 128-bit hash value– Widely used for file integrity checking

• SHA-1– Computes 160-bit hash value

20