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

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Presentation on theme: "Cryptography Ch-1 prepared by: Diwan. Essential Terms Cryptography Encryption Plain text Cipher text Decryption Cipher text Plain text Cryptanalysis Cryptology."— Presentation transcript:

1 Cryptography Ch-1 prepared by: Diwan

2 Essential Terms Cryptography Encryption Plain text Cipher text Decryption Cipher text Plain text Cryptanalysis Cryptology Source:

3 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 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 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 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

7 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

8 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

9 Types of Secret Writing Secret writing SteganographyCryptography

10 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

11 Steganography Hiding a message within another medium, such as an image No key is required Example – Modify color map of JPEG image 11

12 Steganography Steganography – covered writing – is an art of hiding information Popular contemporary steganographic technologies hide information in images New York Times, August 3rd, ence/sci_STEGO_011030_00.jpg

13 Hiding information in pictures Image in which to hide another image Image to hide within the other image /

14 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

15 Types of Secret Writing Secret writing SteganographyCryptography SubstitutionTransposition Mon alphabetic Polyalphabetic

16 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

17 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

18 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)

19 RSA (Rivest, Shamir, Adleman) DH (Diffie-Hellman Key Agreement Algorithm) Currently Available Crypto Algorithms (public key)

20 Hashing Algorithms MD5 – Computes 128-bit hash value – Widely used for file integrity checking SHA-1 – Computes 160-bit hash value 20


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