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- 1. Network Security Lecture 2
- 2. Security goals
- 3. Attacks
- 4. Security Services
- 5. Model for Network Security
- 6. Cryptogrphy <ul><li>Symmetric Encryption and Message Confidentiality. </li></ul><ul><li>Public Key Cryptography and Message Authentication </li></ul>
- 7. Symmetric Enryption <ul><li>DES, 3-DES, AES </li></ul><ul><li>Principles of Encryption </li></ul><ul><ul><li>Five Ingredients </li></ul></ul><ul><ul><li>Plain text, Encryption Algorithm, Secret Key, Cipher text, Decryption Algorithm </li></ul></ul>
- 8. Model for Network Security <ul><li>using this model requires us to: </li></ul><ul><ul><li>design a suitable algorithm for the security transformation </li></ul></ul><ul><ul><li>generate the secret information (keys) used by the algorithm </li></ul></ul><ul><ul><li>develop methods to distribute and share the secret information </li></ul></ul><ul><ul><li>specify a protocol enabling the principals to use the transformation and secret information for a security service </li></ul></ul>
- 9. Symmetric Cipher Model
- 10. Requirements <ul><li>two requirements for secure use of symmetric encryption: </li></ul><ul><ul><li>a strong encryption algorithm </li></ul></ul><ul><ul><li>a secret key known only to sender / receiver </li></ul></ul><ul><li>mathematically have: </li></ul><ul><ul><li>Y = E K ( X ) </li></ul></ul><ul><ul><li>X = D K ( Y ) </li></ul></ul><ul><li>assume encryption algorithm is known </li></ul><ul><li>implies a secure channel to distribute key </li></ul>
- 11. Model of Conventional Crypto System
- 12. Cryptography <ul><li>characterize cryptographic system by: </li></ul><ul><ul><li>type of encryption operations used </li></ul></ul><ul><ul><ul><li>substitution / transposition / product </li></ul></ul></ul><ul><ul><li>number of keys used </li></ul></ul><ul><ul><ul><li>single-key or private / two-key or public </li></ul></ul></ul><ul><ul><li>way in which plaintext is processed </li></ul></ul><ul><ul><ul><li>block / stream </li></ul></ul></ul>
- 13. Cryptanalysis <ul><li>objective to recover key not just message </li></ul><ul><li>general approaches: </li></ul><ul><ul><li>cryptanalytic attack </li></ul></ul><ul><ul><li>brute-force attack </li></ul></ul>
- 14. Cryptanalytic Attacks <ul><li>ciphertext only </li></ul><ul><ul><li>only know algorithm & ciphertext, is statistical, can identify plaintext </li></ul></ul><ul><li>known plaintext </li></ul><ul><ul><li>also have plaintext for the ciphertext </li></ul></ul><ul><li>chosen plaintext </li></ul><ul><ul><li>Also can select plaintext and obtain ciphertext </li></ul></ul><ul><li>chosen ciphertext </li></ul><ul><ul><li>Also can select ciphertext and obtain plaintext </li></ul></ul>
- 15. Level of Security <ul><li>unconditional security </li></ul><ul><ul><li>no matter how much computer power or time is available, the cipher cannot be broken since the ciphertext provides insufficient information to uniquely determine the corresponding plaintext </li></ul></ul><ul><li>computational security </li></ul><ul><ul><li>Cost exceed the value of information </li></ul></ul><ul><ul><li>Time exceeds the useful life of information </li></ul></ul>
- 16. Brute Force Search <ul><li>always possible to simply try every key </li></ul><ul><li>most basic attack, proportional to key size </li></ul><ul><li>assume either know / recognise plaintext </li></ul>6.4 10 6 years 2 10 26 µs = 6.4 10 12 years 26! = 4 10 26 26 characters permutation 5.9 10 30 years 2 167 µs = 5.9 10 36 years 2 168 = 3.7 10 50 168 5.4 10 18 years 2 127 µs = 5.4 10 24 years 2 128 = 3.4 10 38 128 10.01 hours 2 55 µs = 1142 years 2 56 = 7.2 10 16 56 2.15 milliseconds 2 31 µs = 35.8 minutes 2 32 = 4.3 10 9 32 Time required at 10 6 decryptions/µs Time required at 1 decryption/µs Number of Alternative Keys Key Size (bits)
- 17. Feistel Cipher <ul><li>Many encryption algorithims are based on Feistel Cipher. </li></ul>
- 18. Confusion and Diffusion <ul><li>cipher needs to completely obscure statistical properties of original message </li></ul><ul><li>a one-time pad does this </li></ul><ul><li>diffusion – dissipates statistical structure of plaintext over bulk of ciphertext </li></ul><ul><li>confusion – makes relationship between ciphertext and key as complex as possible </li></ul>
- 19. Feistel Cipher Structure <ul><li>Horst Feistel devised the feistel cipher </li></ul><ul><ul><li>based on concept of invertible product cipher </li></ul></ul><ul><li>partitions input block into two halves </li></ul><ul><ul><li>process through multiple rounds which </li></ul></ul><ul><ul><li>perform a substitution on left data half </li></ul></ul><ul><ul><li>based on round function of right half & subkey </li></ul></ul><ul><ul><li>then have permutation swapping halves </li></ul></ul><ul><li>implements Shannon’s S-P net concept </li></ul>
- 20. Feistel Cipher Structure
- 21. Feistel Cipher Design Elements <ul><li>block size </li></ul><ul><li>key size </li></ul><ul><li>number of rounds </li></ul><ul><li>subkey generation algorithm </li></ul><ul><li>round function </li></ul><ul><li>fast software en/decryption </li></ul><ul><li>ease of analysis </li></ul>
- 22. Feistel Cipher Decryption
- 23. On the Encryption side

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