Cryptography
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This document provides an overview of cryptography. It defines cryptography as the science of secret writing and discusses its use in applications like ATM cards and passwords. It describes the basic components of cryptography including plaintext, ciphertext, ciphers, keys, and algorithms. It differentiates between symmetric and asymmetric key cryptography. It provides examples of traditional and modern ciphers, including DES, AES, and RSA algorithms. In conclusion, it states that cryptography techniques help maintain data security, privacy, and integrity.
Cryptography
- 2. 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.
- 4. 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
- 5. 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
- 6. 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.
- 7. USE OF KEY • Example: • Use Monoalphabetic Cipher to encrypt “HELLO” with key = 3. • Plaintext: HELLO • Ciphertext: KHOOR
- 8. 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.
- 10. CRYPTOGRAPHY TECHNIQUE Alice Bob Eve Plaintext PlaintextCiphertext Alice’s Encryption Key K1 Bob’s Decryption Key K2 If m = Plaintext, then • Ciphertext = K1(m) and • m = K2(K1(m))
- 12. 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.
- 13. 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.
- 14. SYMMETRIC KEY CRYPTOGRAPHY TECHNIQUE Alice Bob Alice’s Encryption Key K Bob’s Decryption Key K Plaintext PlaintextCiphertext If m = Plaintext and K is the shared secret key, then • Ciphertext = K(m) and • m = K(K(m)) Shared Secret Key K
- 15. SYMMETRIC KEY CIPHERS Traditional Ciphers Substitution Cipher Monoalphabetic Ciphers Polyalphabetic Cipher Transposition Cipher Modern Cipher Simple Ciphers XOR, Rotation etc. Round Ciphers DES, AES, IDEA etc.
- 16. 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
- 17. 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
- 18. 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
- 19. 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.
- 20. ASYMMETRIC KEY CRYPTOGRAPHY TECHNIQUE Alice Bob CiphertextPlaintext Plaintext Bob’s Public Key K+ Bob’s Private Key K- If m = Plaintext, then • Ciphertext = K+(m) and • m = K-(K+(m)) To Public Used for encryption Used for decryption
- 21. COMPARISON OF SYMMETRIC AND ASYMMETRIC KEY CRYPTOGRAPHY
- 22. 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,
- 24. 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.
- 25. DES - DATA ENCRYPTION STANDARD • The algorithm encrypts a 64-bit plaintext block using a 64-bit key. • DES has two transposition blocks 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. The permutation takes a 64-bit input and permutes them according to predefined values.
- 26. DES TECHNIQUE Initial Permutation Final Permutation Round 1 Round 2 Round 16 Round Key Generator K1 K2 K16 64-Bit Plaintext 64-Bit Ciphertext 64-Bit Key DES
- 27. 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. • The structure and operation of the other configurations are similar. The difference lies in the key generation. Size of Data Block Number of Rounds Key Size 10 128 128-Bits 12 192 14 256
- 28. AES TECHNIQUE Pre Round Transformation Round N (slightly different) Round 1 Round 2 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
- 29. RSA ALGORITHM • RSA named for its inventors Rivest, Shamir & Adleman. • 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.
- 30. 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)
- 31. 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.
- 32. REFERENCE • DATA COMMUNICATIONS AND NETWORKING BY BEHROUZ A. FOROUZAN • WWW.GOOGLE.COM