CS144 - Section 10Harsh NayyarAnnouncements● Final Exam scheduled for Monday December 12, 2011 (8:30AM-11:30AM)● Lab 5 late deadline has been extended to 11:59pm today, Good luck!Agenda● Review Cryptography ● Review TCP, Congestion Control ● Review Practice Final (Time Permitting)● Questions?● Final RemarksSymmetric Encryption● The crucial point is that there is a shared private key○ Must be known to both parties, and only known by two parties● Why?● Because it's symmetricK is important● Different types of security guarantees○ information theoretic○ computational● A completely random key provides us with information theoretic security (p(M|C) is uniform)● A shorter pseudo random key leads to computational security (p(M|C) may be 0 for all M except one)○ But we rely on it being a lot of work to guess and check● Why not completely random?○ Because it requires |K| = |M|, impractical to shareStream Ciphers● Generate pseudo-random stream of bits from short key● Encrypt/decrypt by XORing with stream as if one-time pad● But NOT one-time PAD! (People who claim so are frauds!)● In practice, many stream ciphers uses have run into problemsBlock Ciphers● Used in PGP (secure email), SSL (securing TCP), as well as IPsec● Message to be encrypted processed in blocks of k bits○ One-to-one mapping from clear-text to cipher block○ Generalizes to 2k! possible mappings for given k, resulting in cryptographic securityInput Output000 110001 111010 101011 100100 011101 010110 000111 001Look-up tablePlaintext:010110001111Input Blocks: 010,110,001,111Encrypted text:101000111001Output Blocks: 101,000,111,001Block Ciphers (contd)● How to we resolve the fact that if block is repeated in plaintext, ciphertext will be repeated? (secrecy compromised) ● Cipher-Block chaining!Integrity● Message Authentication Codes (abbreviated as MAC, distinct from media access control, i.e, link layer)● Alice and Bob have a shared secret key/authentication keyIntegrity (contd.)● We require a cryptographic hash function, H○ One for which it is computationally infeasible to find any two different messages x and y such that H(x)=H(y)● How do we distribute the shared authentication key?○ Public key cryptography (e.g., RSA)TCP Congestion ControlPractice Final● Questions?● Disclaimer: Please note that the practice final is meant as a preparation tool to help you think about different material presented during the quarter. It is not necessarily an indication of the exact coverage of materials or length of actual final examQuestions?Final Remarks● It's three hours (3>1.25)● You already have 5/100 if you attended Jon Peterson's guest lecture!● You already know what it looks like (very similar in format to the midterm)○ A mix of short answers, some multiple choice, some true/false● Questions are not necessarily in order of difficulty● Congratulations on making it to the end! You have learned a very valuable set of networking principles and implementation experience that will serve you
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