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Network security

Network security. Basic encryption techniques. Luk Stoops VUB - programming laboratory. Security Problems. Student To have fun snooping on people’s email Hacker To test out someone’s security system; steal data Sales rep To claim to represent all of Europe, not just Andorra Businessman

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Network security

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  1. Network security Basic encryption techniques Luk Stoops VUB - programming laboratory

  2. Security Problems • Student • To have fun snooping on people’s email • Hacker • To test out someone’s security system; steal data • Sales rep • To claim to represent all of Europe, not just Andorra • Businessman • To discover a competitor’s strategic marketing plan

  3. Security Problems • Ex-employee • To get revenge for being fired • Accountant • To Embezzle money from a company • Stockbroker • To deny a promise made to a customer by email

  4. Security Problems • Con man • To steal creditcard numbers for sale • Spy • To learn an enemy’s military strength • Terrorist • To steal germ warfare secrets • … • ...

  5. Network Security • Secrecy • user authorisation • Authentication • determining who you are talking to • Nonrepudiation • signatures • Integrity control • received message, is it not modified ?

  6. Traditional Cryptography Decryption Key Dk Encryption key Ek Ciphertext C=Ek(P) Encryption Decryption Plaintext P Plaintext P Intruder

  7. Some Terminology • Cryptanalysis • The art of breaking ciphers • Cryptography • The art of devising ciphers • Cryptology • Cryptanalysis & Cryptography

  8. Method of Encryption • Cryptanalyst knows method of encryption • Amount of effort to invent, test and install a new method every time the old method is compromised or thought to be compromised has always made it impractical to keep this secret • This is where the key enters

  9. The Key • Combination Lock • 2 digits = 100 possibilities • 3digits = 1000 possibilities • Workfactor for the cryptanalyst by exhaustive search of the key space is exponential in the key length • range from 64-bit to 256-bits keys

  10. Cryptoanalyst Variations • Ciphertext only • only ciphertext • Known plaintext (“please login”) • matched ciphertext and plaintext • Chosen plaintext • matched ciphertext and chosen plaintext

  11. Substitution Cipher ABCDEFGHIJKLMNOPQRSTUVWXYZ BCDEFGHIJKLMNOPQRSTUVWXYZA Caesar cipher Monoalphabetic substitution ABCDEFGHIJKLMNOPQRSTUVWXYZ QWERTYUIOPASDFGHJKLZXCVBNM

  12. Monoalphabetic Substitution 26 • 26! = 4 . 10 possible keys • 1 msec per solution = 10 years • Statistical properties • most common letters: e, t, o, a, n, i, … • most common digrams: th, in, er, re, an • most common trigrams: the, ing, and, ion • Counting relative frequencies and assigning most common letters, then digrams .. • Guessing likely words (financial) 13

  13. Transposition Cipher • Reorder the letters but do not disguise them • afllskaselawabtoosscdlnmoman esilyntrnntsoepaedobueriricc • Breaking transposition ciphers • Frequency analysis: normal pattern • guess number of columns • digrams depend on keylength • ordering found by english plaintext digrams MEGABUCK 74512836 pleasetr ansferon emillion dollarst omyswiss bankacco untabcde

  14. One-Time Pads • Unbreakable cipher • A B C 010000010100001001000011 data 101100110101010011010100 random bit key 111100100001011010010111 ciphertext • key on CD ?

  15. Cryptographic principles • All encrypted messages must contain some redundancy information • active intruders cannot send random junk • easier to break the message • Prevent active intruders from playing back old messages (timestamps)

  16. Secret-Key Algorithms • Complex and involuted encryption algorithm Permutation Substitution 212 = 4096 8 x 4 = 32

  17. Data Encryption Standard (DES)

  18. DES (cont.) • IBM 1977 • No longer secure in its original form • Parameterised by 56-bit key • 19 stages • Decryption with same key in reverse order • Complexity lies in iteration function (S- & P boxes)

  19. DES Subversion • DES is basically a monoalphabetic substitution • Encrypting a longer message is done by breaking it up in consecutive 8-byte (64-bit) blocks

  20. DES Chaining • Chaining makes block i a function of all previous blocks • Same plaintext no longer maps onto the same cyphertext Random initialisation vector IV

  21. DES Cipher Feedback Mode • Byte-by-byte encryption • initialisation vector needed

  22. Breaking DES • Original 128-bit key reduced to 56 under NSA pressure • 1977 20 million $ DES breaking machine in one day • 140.000 people checking 7x1016 keys in a month • Chinese Lottery • 1.2 billion people with chip that searches 1 million encryption's / sec • within 60 sec key is found • Congratulations ! You have won the Chinese Lottery. • To collect, please call 0800-11111111

  23. Des Cracker • July 17, 1998: EFF Builds DES Cracker • $220,000 device • Average of 4.5 days to crack a key • June 8, 1998: government : • “FBI is unable to crack DES” • 56-bit key is too short http://www.eff.org/descracker/

  24. Breaking DES (cont.) • Insecure DES is still widely used for secure applications such as banking • Using DES 2 times ? • 2112 = 1033 • 1 billion DES chips, 1 billion operations / sec takes 100 million years • meet-in-the-middle attack: 257 operations

  25. Triple Encryption DES • EDE for backward compatibility with single-key DES systems (k1 = k2) • No breaking method known • Not enough silicon in the universe. • Not enough time before sun burns out. • 3-key EEE system is even better (168-bits)

  26. International Data Encryption Algorithm IDEA • Swiss • 128-bit • generates 52 16-bit keys • 6 / iteration • 4 / transform

  27. Advanced Encryption Standard • 1972: National Institute of Standards and Technology • public request for encryption algorithm • DES • January 1997: new public request for AES • 15 submissions in June 1998 (12 survived) • March 1999 (5 candidates) • January 2000 (winner) http://www.nist.gov/aes/

  28. Eb(p) Ea Da Eb Db B A Public-Key Algorithms • Key protection and distribution problem • 1976 Diffie and Hellman (Stanford univ.) • encryption and decryption key are different • decryption key can not be derived from the encryption key • encryption key is made public

  29. RSA Algorithm • Rivest, Shamir, Adleman • based on the difficulty of factoring large numbers • Factoring a 200-digit number requires 4 billion years of computer time • too slow for encrypting large volumes of data • Used to distribute a one-time session keys for use with DES or IDEA

  30. Knapsack algorithm • Merkle and Hellmann 1978 • Large number of objects with different weight • selecting a subset in knapsack • total weight and list of possible objects is public • broken by Shamir and Rivest

  31. Key Distribution Centre Authentication • Technique used by a process to verify that its communication partner is who it is supposed to be Alice Bob Trudy

  32. Authentication Shared Secret Key • Key agreed upon on the telephone or in person • R random 128-bit number

  33. Authentication Shared Secret Key (cont.) • Combining information • But ...

  34. Authentication The Reflection Attack • Designing a correct authentication protocol is harder than it looks

  35. Authentication Safety rules • Have the initiator prove who she is before the responder has to • Have the initiator and responder use different keys for proof (Kab Kab’) • Have the initiator and responder draw their challenges from different sets (even & odd numbers)

  36. Authentication Establishing a Shared Key • Diffie-Hellman key exchange • n, g, (n-1)/2 : large prime numbers 512-bit 512-bit • But ...

  37. Authentication The Bucket Brigade Attack • (wo)man-in-the-middle attack • Interlock protocol (a/2 - b/2 - a/2 - b/2)

  38. Authentication Key Distribution Center • Ks = session key • Authentication happens for free • But ...

  39. Authentication Replay Attack • Trudy gets a job from Alice • Alice ask Bob to transfer money to Trudy • Trudy, snooping on the network copies the message (2) and the money--transfer request that follows it • Trudy replays this messages KDBC

  40. Authentication Needman-Schroeder • Timestamps • Unique message number (nonce) • Multiway challenge-response protocol

  41. Authentication Kerberos • Used in many real systems • Authentication server • Ticket-Granting server

  42. Authentication Public-Key • Public keys must be known • Public keys in a public database ? • bucket brigade attack possible

  43. Digital Signatures • Signing (an order to buy a ton of gold) 1 The receiver can verify the claimed identity of the sender 2 The sender cannot later repudiate the contents of the message 3 The receiver cannot possibly have concocted the message himself

  44. Digital Signatures Secret-Key Signatures • Proof that the message came from Alice • BB only accept messages from A if encrypted with Ka • Bob produces exhibit KBB(A,t,P) • RA(random number) + t (timestamp) to prevent replay attacks Big Brother

  45. Digital Signatures Public-Key Signatures • Does not require a trusted authority • P = E(D(P)) = D(E(P)) • Proof: Bob produces exhibit P and DA(P) • But...

  46. Digital Signatures Public-Key Problems • Alice discloses her secret key • If the price of gold drops, she can repudiate here message to Bob by telling the police here key was stolen • Alice decides to change her key • Some authority should record all key changes and their dates

  47. Digital Signatures Message Digests • Authentication without secrecy • Given P, it is easy to compute MD(P) • Given MD(P), it is effectively impossible to find P • No one can generate two messages that have the same message digest • MD5 Rivest (RSA) • SHA Secure Hash Algorithm (NIST, 1993)

  48. Digital Signatures Digest in SET • Changing a single bit in the message will change roughly half the bits in the message digest.

  49. Digital Signatures Digital Signature Standard • 1991, National Institute of Standards and Technology • El Gamal public-key algorithm • difficulty of computing discrete logarithms Too secret (NSA) Too new (not yet thoroughly analysed) Too slow (10 to 40 times slower than RSA) Too insecure (512-bit key)

  50. Email Privacy • Pretty Good Privacy (Zimmermann) • Privacy • Authentication • Digital signatures • Compression • Free of charge via the Internet • Privacy Enhanced Mail

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