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This PPT is for beginners and for intermediate developers who want to learn about Cryptography.

I have also explained some famous ciphers like AES, DES and RSA.

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- 1. CRYPTOGRAPHY
- 2. CONTENTS • Definition • Cryptography Issues • Cryptography Components • Cryptography Technique • Cryptography Categories • Symmetric key cryptography • Asymmetric key cryptography • Comparison of Symmetric and Asymmetric key cryptography • Famous Algorithms/Ciphers
- 3. DEFINITION
- 4. CRYPTOGRAPHY • A word with Greek origins, means “secret writing”. • The term to refer to the science and art of transforming messages to make them secure and immune to attacks. • Applications of cryptography includes ATM cards, computer passwords, and electronic commerce. Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan
- 5. CRYPTOGRAPHY ISSUES
- 6. CRYPTOGRAPHY ISSUES • Confidentiality: Only sender, intended receiver should “understand” message contents. • End-Point Authentication: Sender and receiver want to confirm identity of each other. • Message Integrity: Sender and receiver want to ensure message not altered (in transit, or afterwards) without detection. • Message Nonrepudiation: Message nonrepudiation means that a sender must not be able to deny sending a message that he or she, in fact, did send. • Entity Authentication: In entity authentication (or user identification) the entity or user is verified prior to access to the system resources
- 7. CRYPTOGRAPHY COMPONENTS
- 8. PLAINTEXT AND CIPHERTEXT • The original message, before being transformed, is called plaintext. • After the message is transformed, it is called ciphertext. • An encryption algorithm transforms the plaintext into ciphertext; a decryption algorithm transforms the ciphertext back into plaintext. • Example: • Plaintext: HELLO • Ciphertext: KHOOR Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan
- 9. CIPHER • Encryption and Decryption algorithms are referred as ciphers. • Also used to refer to different categories of algorithms in cryptography. • Example (Traditional Substitution Ciphers): • Monoalphabetic cipher • Polyalphabetic cipher • Plaintext: HELLO • Ciphertext: ABNZF Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan
- 10. KEY • A key is a number (or a set of numbers) that the cipher operates on, as an algorithm. • To encrypt a message, we need an encryption algorithm, an encryption key, and the plaintext. • To decrypt a message, we need a decryption algorithm, a decryption key, and the ciphertext. These reveal the original plaintext. • Types: • Shared key, Public key and Private key. Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan
- 11. USE OF KEY • Example: • Use Monoalphabetic Cipher to encrypt “HELLO” with key = 3. • Plaintext: HELLO • Ciphertext: KHOOR
- 12. ALICE, BOB, AND EVE • Alice is the person who needs to send secure data. • Bob is the recipient of the data. • Eve is the person who somehow disturbs the communication between Alice and Bob. Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan
- 13. CRYPTOGRAPHY TECHNIQUE
- 14. CRYPTOGRAPHY TECHNIQUE Plaintext Ciphertext Plaintext Alice Bob Eve Alice’s Encryption Key K1 Bob’s Decryption Key K2 If m = Plaintext, then • Ciphertext = K1(m) and • m = K2(K1(m))
- 15. CRYPTOGRAPHY CATEGORIES
- 16. CRYPTOGRAPHY CATEGORIES • We can divide all the cryptography algorithms (ciphers) into two groups: • Symmetric key (also called secret-key) cryptography algorithms and • Asymmetric key (also called public-key) cryptography algorithms. Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan
- 17. SYMMETRIC KEY CRYPTOGRAPHY • In symmetric-key cryptography, the same key is used by both parties. • The sender uses this key and an encryption algorithm to encrypt data; the receiver uses the same key and the corresponding decryption algorithm to decrypt the data. Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan
- 18. SYMMETRIC KEY CRYPTOGRAPHY TECHNIQUE Alice’s Encryption Key K Bob’s Decryption Key K Shared Secret Key K Plaintext Ciphertext Plaintext Alice Bob If m = Plaintext and K is the shared secret key, then • Ciphertext = K(m) and • m = K(K(m))
- 19. SYMMETRIC KEY CIPHERS Traditional Ciphers Modern Ciphers Substitution Ciphers Transpositio n Ciphers Mono-alphabetic Ciphers Poly-alphabetic Ciphers DES, AES, IDEA, CAST, RC4, RC5, etc. Simple Ciphers Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan Round Ciphers XOR, Rotation, S-box, P-box etc.
- 20. ASYMMETRIC KEY CRYPTOGRAPHY • In asymmetric or public-key cryptography, there are two keys: a private key and a public key. • The private key is kept by the receiver. • The public key is announced to the public. • If Alice wants to send a message to Bob, Alice uses the public key to encrypt the message. When the message is received by Bob, the private key is used to decrypt the message. • Some of the asymmetric algorithms are: • Diffie-Hellman, RSA, El Gamal, Elliptic Curve Cryptography (ECC), Man-in-the- Middle Attack etc. Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan
- 21. ASYMMETRIC KEY CRYPTOGRAPHY TECHNIQUE Bob’s Public Key K+ Bob’s Private Key K-If Plaintext Ciphertext Plaintext Alice Bob m = Plaintext, then • Ciphertext = K+(m) and • m = K-(K+(m)) To Public Used for encryption Used for decryption
- 22. COMPARISON OF SYMMETRIC AND ASYMMETRIC KEY CRYPTOGRAPHY
- 23. SYMMETRIC KEY CRYPTOGRAPHY • Only 1 shared key is involved. • The same key encrypts and decrypts the plaintext. • The shared key is kept secret between Alice and Bob. • Examples of Symmetric algorithms: • DES, 3DES, AES, IDEA, BLOWFISH, TWOFISH, RC4, RC5, SAFER etc. ASYMMETRIC KEY CRYPTOGRAPHY • Here 2 keys : a private and a public key are involved. • The Public key encrypts the plaintext while the private key decrypts it. • The private key is just kept secret by the Bob while the public key is made public. • Examples of Asymmetric algorithms: • Diffie-Hellman, RSA, El Gamal, Elliptic Curve Cryptography
- 24. FAMOUS ALGORITHMS/CIPHERS
- 25. MODERN ROUND CIPHERS • The ciphers of today are called round ciphers because they involve multiple rounds, where each round is a complex cipher made up of the simple ciphers. • The key used in each round is a subset or variation of the general key called the round key. • If the cipher has N rounds, a key generator produces N keys, K1, K2,...., KN, where K1 is used in round 1, K2 in round 2, and so on. • Modem symmetric-key ciphers: DES and AES are referred to as block ciphers because they divide the plaintext into blocks and use the same key to encrypt and decrypt the blocks. Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan
- 26. DES - DATA ENCRYPTION STANDARD • The algorithm encrypts a 64-bit plaintext block using a 64-bit key. • DES has two transposition blocks (P-boxes) and 16 complex round ciphers (they are repeated). • Although the 16 iteration round ciphers are conceptually the same, each uses a different key derived from the original key. • The initial and final permutations are keyless straight permutations that are the inverse of each other. The permutation takes a 64-bit input and permutes them according Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan to predefined values.
- 27. DES TECHNIQUE Initial Permutation Round 1 Round 2 Round 16 Final Permutation Round Key Generator K1 K2 K16 64-Bit Plaintext 64-Bit Ciphertext 64-Bit Key DES
- 28. AES - ADVANCED ENCRYPTION STANDARD • The Advanced Encryption Standard (AES) was designed because DES's key was too small. • AES is designed with three key sizes: 128, 192, or 256 bits. • Table shows the relationship between the data block, number of rounds, and key size. Size of Data Block Number of Rounds Key Size 10 128 128-Bits 12 192 14 256 • The structure and operation of the other configurations are similar. The difference lies in the key generation. Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan
- 29. AES TECHNIQUE Pre Round Transformation Round 1 Round 2 Round N (slightly different) Round Key Generator K1 K2 KN 128-Bit Plaintext 128-Bit Ciphertext Cipher Key (128-bits for N =10, 192-bits for N =12 or 256-bits for N =14) AES K0 • Each round of AES, except for the last, is a cipher with four operations that are invertible. • The last round has only three operations.
- 30. RSA ALGORITHM • It uses two numbers, e and d, having a special relationship to each other, as the public and private keys. • Selecting Keys Bob use the following steps to select the private and public keys: 1. Bob chooses two very large prime numbers p and q. 2. Bob multiplies the above two primes to find n, the modulus for encryption and decryption. In other words, n = p X q. 3. Bob calculates another number φ = (p -1) X (q - 1). 4. Bob chooses a random integer e. He then calculates d so that d x e = 1 mod φ. 5. Bob announces e and n to the public; he keeps φ and d secret. Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan
- 31. RSA ALGORITHM • Encryption • Anyone who needs to send a message to Bob can use n and e. • For example, if Alice needs to send a message to Bob, she can change the message, usually a short one, to an integer. This is the plaintext. • She then calculates the ciphertext, using e and n, as c = pe (mod n). • Alice sends C, the ciphertext, to Bob. • Decryption • When Bob receives the ciphertext, he uses his private key d to decrypt the message: p = cd (mod n) Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan
- 32. CONCLUSION • By using of encryption techniques a fair unit of confidentiality, authentication, integrity, access control and availability of data is maintained. • Using cryptography Electronic Mail Security, Mail Security, IP Security, Web security can be achieved.
- 33. REFERENCE CHAPTER 30, CRYPTOGRAPHY DATA COMMUNICATIONS AND NETWORKING BY BEHROUZ A. FOROUZAN
- 34. THANK YOU
- 35. EXAMPLES
- 36. MONOALPHABETIC CIPHER • Use Monoalphabetic Cipher to encrypt “HELLO” with key = 3. Key=3 means shift each character of plaintext by character at 3 places ahead of it. Hence replacing H by K, E by H, L by O and O by R in the plaintext. • Plaintext: HELLO • Ciphertext: KHOOR
- 37. POLYALPHABETIC CIPHER • Use Polyalphabetic Cipher to encrypt “HELLO” with key = 3,2. Divide HELLO in group of 3 characters viz. HEL-LO and use key=3 to encrypt HEL and key=2 to encrypt LO. • Plaintext: HELLO • Ciphertext: KHONB
- 38. TRANSPOSITION CIPHER • Encrypt the message "HELLO MY DEAR" using the key: • Plaintext: 1234 • Ciphertext: 2413 In encryption, we move the character at position 2 to position 1, the character at position 4 to position 2, and so on. We first remove the spaces in the message. We then divide the text into blocks of four characters. We add a bogus character Z at the end of the third block. The result is HELL OMYD EARZ. We create a three-block ciphertext ELHLMDOYAZER. • Plaintext: HELLO MY DEAR • Ciphertext: ELHLMDOYAZER

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