Chapter 1 – Introduction Part 3

1. Chapter 1 – IntroductionPart 3

2. Cryptographic Concepts • Encryption: a means to allow two parties, customarily called Alice and Bob, to establish confidential communication over an insecure channel that is subject to eavesdropping. Alice Bob Eve

3. Encryption and Decryption Communicationchannel Sender Recipient • The message M is called the plaintext. • Alice will convert plaintext M to an encrypted form using an encryption algorithm E that outputs a ciphertext C for M. encrypt decrypt ciphertext plaintext plaintext shared secret key shared secret key Attacker (eavesdropping)

4. Encryption and Decryption • As equations: C = E(M) M = D(C) • The encryption and decryption algorithms are chosen so that it is infeasible for someone other than Alice and Bob to determine plaintext M from ciphertext C. Thus, ciphertext C can be transmitted over an insecure channel that can be eavesdropped by an adversary.

5. Cryptosystem • The set of possible plaintexts • The set of possible ciphertexts • The set of encryption keys • The set of decryption keys • The correspondence between encryption keys and decryption keys • The encryption algorithm to use • The decryption algorithm to use

6. Caesar Cipher • Replace each letter with the one “three over” in the alphabet. Public domain image from http://commons.wikimedia.org/wiki/File:Caesar3.svg

7. Example: Cæsar cipher ABCDEFGHIJKLMNOPQRSTUVWXYZ DEFGHIJKLMNOPQRSTUVWXYZABC • Plaintext is HELLO WORLD • Change each letter to the third letter following it (X goes to A, Y to B, Z to C) • Key is 3, usually written as letter ‘D’ • Ciphertext is KHOOR ZRUOG CIT 380: Securing Computer Systems

8. Example: Cæsar cipher key=3 ABCDEFGHIJKLMNOPQRSTUVWXYZ DEFGHIJKLMNOPQRSTUVWXYZABC Decrypt: FRPSXWHU CIT 380: Securing Computer Systems

9. Symmetric Cryptosystems Communicationchannel Sender Recipient • Alice and Bob share a secret key, which is used for both encryption and decryption. encrypt decrypt ciphertext plaintext plaintext shared secret key shared secret key Attacker (eavesdropping)

10. Symmetric Key Distribution • Requires each pair of communicating parties to share a (separate) secret key. shared secret shared secret shared secret shared secret shared secret n (n-1)/2 keys shared secret

11. Example of Symmetric Cryptography • DES (Data Encryption Standard) 1977-8 • 56 bit key • Triple DES • AES (Advanced Encryption Standard) 1997 • Rijndael • 128, 192, or 256 bit key size

12. Symmetric Cryptosystems Communicationchannel Sender Recipient • Alice and Bob share a secret key, which is used for both encryption and decryption. encrypt decrypt ciphertext plaintext plaintext shared secret key shared secret key Attacker (eavesdropping)

13. Public-Key Cryptography • Bob has two keys: a private key, SB, which Bob keeps secret, and a public key, PB, which Bob broadcasts widely. • In order for Alice to send an encrypted message to Bob, she need only obtain his public key, PB, use that to encrypt her message, M, and send the result, C = EPB (M), to Bob. Bob then uses his secret key to decrypt the message as M = DSB (C).

14. Public-Key Cryptography Communicationchannel Sender Recipient • Separate keys are used for encryption and decryption. encrypt decrypt plaintext plaintext ciphertext plaintext public key private key Attacker (eavesdropping)

15. Public Key Distribution private private public public • Only one key is needed for each recipient n key pairs public public private private

16. Disadvantage of Public Key Encryption • 1,000 times slower than symmetric key

17. Sharing Secret Key • Figure 1.12 on page 30

18. Digital Signatures • Public-key encryption provides a method for doing digital signatures • To sign a message, M, Alice just encrypts it with her private key, SA, creating C = ESA(M). • Anyone can decrypt this message using Alice’s public key, as M’ = DPA(C), and compare that to the message M.

19. Sign and Encrypt • Alice encrypts with Bob’s public key and signs with her private key • C = ESA(DPB(M)) • Bob first applies Alice’s public key. This proves the message came from Alice. • Then Bob decrypt with his private key. • M= DSB(EPA(C))

20. Man-in-the-Middle Attack • Page 32 Figure 1.13

21. Brute Force Attack • Page 33 – Figure 1.14 • Caesar Cipher attack

22. Cryptographic Hash Functions • A checksum on a message, M, that is: • One-way: it should be easy to compute Y=H(M), but hard to find M given only Y • Collision-resistant: it should be hard to find two messages, M and N, such that H(M)=H(N). • Examples: MD5,SHA-1, SHA-256.

23. Hashing • http://hash.online-convert.com/