- 1. Internet and Intranet Protocols and Applications
- Network (Internet) Security
2. What is network security?
- Secrecy:only sender, intended receiver should understand msg contents
- Authentication:sender, receiver want to confirm identity of each other
- Message Integrity:sender, receiver want to ensure message not altered (in transit, or afterwards) without detection
- Non-repudiation:sender cannot claim other than what was sent
3. Internet security threats
- promiscuous NIC reads all packets passing by
- can read all unencrypted data (e.g. passwords)
- e.g.: C sniffs Bs packets
A B C src:B dest:Apayload 4. Internet security threats
- can generate raw IP packets directly from application, putting any value into IP source address field
- receiver cant tell if source is spoofed
A B C src:Bdest:Apayload 5. Internet security threats
- flood of maliciously generated packets swamp receiver
- Distributed DOS (DDOS): multiple coordinated sources swamp receiver
- e.g., C and remote host SYN-attack A
A B C SYN SYN SYN SYN SYN SYN SYN 6. Cryptography
- Encryptionis a process applied to a bit of information that changes the informations appearance, but not its (decrypted) meaning.
- Decryptionis the reverse process.
- If C is a bit ofcipher text(encrypted data) and M is a message ( plain text )then,
- C = E k (M)andM = D k (C)
- Where E kandD kare encryption and decryption processes respectively.
- E kandD kare both based on some key k.
7. Cryptography Algorithms
- symmetric keycrypto: sender, receiver keys identical
- public-keycrypto: encrypt keypublic , decrypt keysecret
Figure 7.3 goes here plaintext plaintext ciphertext K A K B 8. Friends and enemies: Alice, Bob, Trudy
- Well-known model in network security world
- Bob, Alice want to communicate securely
- Trudy, the intruder may intercept, delete, add messages
- Sometimes Trudys friend Mallory (malicious) may appear
Figure 7.1 goes here 9. Cryptography Basics
- Symmetric KeyCryptography:
- E k= D k (and must be kept SECRET!!!)
- E kis a public key (everyone can know it)
- D kis a private key and belongs toONEentity.
- Symmetric Key Algorithms are fast
- Public Key Algorithms are SLOW!!!
10. Symmetric Key Ciphers
- (c1 = c12, c2 = c5, c3 = c1, )
- Composition (both substitution and transposition, such as DES)
11. Symmetric key cryptography
- substitution cipher:substituting one thing for another
- monoalphabetic cipher: substitute one letter for another
plaintext:abcdefghijklmnopqrstuvwxyz ciphertext:mnbvcxzasdfghjklpoiuytrewq Plaintext: bob. i love you. alice ciphertext: nkn. s gktc wky. mgsbc E.g.: 12. DES: Data Encryption Standard
- US encryption standard [NIST 1993]
- 56-bit symmetric key, 64 bit plain-text input
- DES Challenge: 56-bit-key-encrypted phrase( Strong cryptography makes the world a safer place ) decrypted (brute force) in 4 months
- no known backdoor decryption approach
13. Symmetric keycrypto: DES
- 16 identical rounds of function application, each using different 48 bits of key
DES operation 14. Public key cryptography
15. How do public key algorithms work?
- They depend on the existence of some very hard mathematical problems to solve:
- Factoring VERY large numbers (example, a number containing 1024 bits!)
- Calculating discrete logarithms
- Find x where a x b (mod n)
- By hard we mean that it will take a super computer a very long time (months or years)
16. RSA encryption algorithm
- RSAdepends on factoring large numbers.Here is the algorithm :
Need d B ( ) and e B ( ) such that Need public and private keys for d B ( ) and e B ( ) Two inter-related requirements: d(e(m))=m B B 1 2 17. RSA: Choosing keys 1.Choose two large prime numbersp, q. (e.g., 1024 bits each) 2.Computen= pq,z = (p-1)(q-1 ) 3.Choosee( witheGoal: Bob wants Alice to prove her identity to him Protocol ap1.0: Alice says I am Alice Failure scenario?? 21. Authentication: another try Protocol ap2.0: Alice says I am Alice and sends her IP address along to prove it. Failure scenario? 22. Authentication: another try Protocol ap3.0: Alice says I am Alice and sends her secret password to prove it. Failure scenario? 23. Authentication: yet another try Protocol ap3.1: Alice says I am Alice and sends her encryptedsecret password to prove it. Failure scenario? I am Alice encrypt(password) 24. Authentication: yet another try Goal: avoid playback attack Failures, drawbacks? Figure 7.11 goes here Nonce: number (R) used only once in a lifetime ap4.0: to prove Alice live, Bob sends Alicenonce , R.Alice must return R, encrypted with shared secret key 25. Authentication: ap5.0
- ap4.0 requires shared symmetric key
- problem: how do Bob, Alice agree on key
- can we authenticate using public key techniques?
- ap5.0:use nonce, public key cryptography
Figure 7.12 goes here 26. ap5.0: security hole
- Man (woman) in the middle attack:Trudy poses as Alice (to Bob) and as Bob (to Alice)
Figure 7.14 goes here 27. Digital Signatures
- Cryptographic technique analogous to hand-written signatures.
- Sender (Bob) digitally signs document,establishing he is document owner/creator.
- Verifiable, nonforgeable:recipient (Alice) can verify that Bob, and no one else, signed document.
- Simple digital signature for message m:
- Bob encrypts m with his private key d B , creating signed message, d B (m).
- Bob sends m and d B (m) to Alice.
28. Digital Signatures (more)
- Suppose Alice receives msgm , and digital signatured B (m)
- Alice verifiesmsigned by Bob by applying Bobs public keye Btod B (m) thencheckse B (d B (m) ) = m.
- Ife B (d B (m) ) = m , whoever signedmmust have used Bobs private key.
- Alice thus verifies that:
- Alice can takem , and signatured B (m)to court and prove that Bob signedm .
29. Message Digests
- Computationally expensive to public-key-encrypt long messages
- Goal:fixed-length,easy to compute digital signature, fingerprint
- apply hash function H tom , get fixed size message digest,H(m).
- Hash function properties:
- Produces fixed-size msg digest (fingerprint)
- Given message digest x, computationally infeasible to find m such that x = H(m)
- computationally infeasible to find any two messages m and m such that H(m) = H(m).
30. Digital signature = Signed message digest
- Bob sends digitally signed message:
- Alice verifies signature and integrity of digitally signed message:
31. Hash Function Algorithms
- Internet checksum would make a poor message digest.
- Too easy to find two messages with same checksum.
- MD5 hash function widely used.
- Computes 128-bit message digest in 4-step process.
- arbitrary 128-bit string x, appears difficult to construct msg m whose MD5 hash is equal to x.
32. Trusted Intermediaries