i-4 security

1. i-4 security

2. Security taxonomy • Physical security • Resource exhaustion • Key-based security • cryptography

3. Security dichotomy • Computer (system) Security • automated tools and mechanisms to protect data in a computer, even if the computers are connected to a network • against hackers (intrusion) • against viruses • against Denial of Service attacks • Access control, authorization, … • Internet (network) Security • measures to prevent, detect, and correct security violations that involve the transmission of information in a network or interconnected network • Everything on the network can be a target • Every transmitted bit can be tapped

4. Friends and enemies: Alice, Bob, Trudy • well-known in network security world • Bob, Alice want to communicate “securely” • Trudy (intruder) may tap, delete, add, modify messages Alice Bob data, control messages channel secure sender secure receiver data data Trudy Source: Kurose at UMass

5. There are bad guys (and girls) out there! Q: What can a “bad guy” do? A: A lot! • eavesdrop: intercept messages • Insert/modify/delete messages into connection • impersonation: can fake (spoof) source address in packet (or any field in packet) • hijacking: “take over” ongoing connection by removing sender or receiver, inserting himself in place • denial of service: prevent service from being used by others (e.g., by overloading resources) Source: Kurose at UMass

6. Thwart the attacks! • Basic Security services • authentication • Access control • confidentiality • Data (ormessage) integrity • Non-repudiation

7. More Security services • Anonymity • Availability • Accountability • Privacy • forensics

8. Security mechanisms • Encipherment • Encryption and decryption • Keys • Message digest • Hash function characteristics • it is easy to compute the hashed value for any given message, • it is infeasible to find a message that has a given hash, • it is infeasible to find two different messages with the same hash • Can have a key (Cryptographic) • Digital Signatures • demonstrating the authenticity of a digital message or document

9. Meaning of Cryptography • from Greek • Cryptos: secret, hidden • graphos: writing • cryptography: study of secret writing

10. Basics Encryption key Decryption key Encryption (Encipherment) Decryption (Decipherment) Message (plaintext, cleartext) Ciphertext (cryptogram) plaintext cipher - algorithm for transforming plaintext to ciphertext key - info used in cipher known only to sender/receiver encipher (encrypt) - converting plaintext to ciphertext decipher (decrypt) - recovering ciphertext from plaintext cryptography - study of encryption principles/methods cryptanalysis (codebreaking) - the study of principles/methods of deciphering ciphertext without knowing key

11. Classification of Cryptosystems • The way in which keys are used • Symmetric cryptography • Single key • Public key cryptography • Two keys • the way in which plaintext is processed • Block cipher • Stream cipher

12. Symmetric cryptography

13. Symmetric Encryption • also known as • Classical, conventional • private-key • single-key • Secret key • sender and recipient share a common key • was only type prior to invention of public-key cryptography • until second half of 1970’s

14. Symmetric Cipher Model there must be a secure mechanism for the distribution of this key a priori

15. Requirements • two requirements for secure use of symmetric encryption: • a strong encryption algorithm • a secret key known only to sender / receiver Y = EK(X) X = DK(Y) • assume encryption algorithm is known • imply a secure channel to distribute the key

16. X-or() in cryptography • Sender wants to send M to receiver • M (Original plaintext): 1010 • K (Key): 0011 • M  K = 1001 (Encrypted ciphertext) 1001 transmitted • Receiver already knows K • (M  K)  K= 1001  0011 = 1010 = M -> original message is restored!

17. Some primitives • Substitution • Permutation

18. Two types of symmetric ciphers • Stream cipher • Encrypts one bit at a time • RC4 • Block cipher • Encrypts a block of bits at a time • DES, AES

19. Asymmetric cryptography Or Public key cryptography (PKC)

20. PKC – General Characteristics • public-key/two-key/asymmetric cryptography • uses 2 keys • public-key • may be known by anybody, and can be used to encrypt messages, and verify signatures • private-key • known only to the recipient, used to decrypt messages, and sign (create) signatures • keys are related to each other but it is not feasible to find out private key from the public one • Modular arithmetic

21. PKC – General Characteristics • It is computationally easy to en/decrypt messages when the relevant keys are known • RSA • Trap-door one-way function • ku: public-key, kr: private key Y=fku(X) easy, if ku and X are known X=fkr-1(Y)easy, if kr and Y are known, but infeasible if Y is known but kr is not known

22. Public-Key Cryptography: Encryption Bob Alice

23. Bob Alice 1. Construct m 2. Compute c= F(m,kp) c 3. Send c to Bob 4. Receive c from Alice 5. Compute d=F-1(c,ks) 6. m = d Another notation • Alice has a public key, kp, and a secret key, ks • Alice’s public key is known to Bob • Asymmetric Cipher: F-1(F(m,kp),ks) = m

24. Public-Key Cryptography - Authentication Commutative! Alice Bob

25. Why PKC? • Initially developed to address two challenging issues: • key distribution • symmetric crypto requires how to securely share the key • in PKI you do not need to distribute/know secret keys, but you need trusted third parties • digital signatures (non-repudiation) • not possible with symmetric crypto

26. Diffie-Hellman (D-H) Algorithm • D-H model’s primary contribution: • Take a prime p and a primitive element g • Cyclic group in finite field • Publicize both g and p • Alice chooses some x  Zp* and sends (gx mod p) to Bob • Bob chooses some y  Zp* and sends (gy mod p) to Alice • Eve can see both (gx mod p) and (gy mod p) but she cannot calculate x or y • Discrete logarithm problem

27. D-H Algorithm gx mod p • Alice calculates the key; k = (gy)x mod p • Bob calculates the same key; k = (gx)y mod p • Since Eve does not know x or y, she cannot calculate the key k • Diffie and Hellman developed this method to share a key using some publicly available information gy mod p Alice Bob

28. PKC Applications • 3 categories • encryption/decryption • to provide secrecy • digital signatures • to provide authentication and non-repudiation • key exchange • to agree on a session key (symmetric cipher) to encrypt data packets • Why not use public/private keys?

29. MESSAGE INTEGRITY

30. Function H( ) that takes as input an arbitrary length message and outputs a fixed-length string: “message signature” Note that H( ) is a many-to-1 function H( ) is often called a “hash function” MD5, SHA-1 Desirable properties: Easy to calculate Irreversibility: Can’t determine m from H(m) Collision resistance: Computationally difficult to produce m and m’ such that H(m) = H(m’) Seemingly random output large message m H: Hash Function H(m) Message Digest Source: Kurose at UMass

31. s = shared secret s s message message message H( ) H( ) compare Message Authentication Code (MAC) • Authenticates sender • Verifies message integrity • No encryption ! • Also called “keyed hash” • Notation: MDm = H(s||m) ; send m||MDm • HMAC (Hash-based Message Authentication Code) Source: Kurose at UMass

32. Digital Signatures • data integrity, non-repudiation, authentication • Basic idea • use private key on the message to generate a piece of information that can be generated only by yourself • because you are the only person who knows your private key • public key can be used to verify the signature • so everybody can verify • Generally signatures are created and verified over the hash of the message • Notover the original message. Why?

33. Digital Signature – RSA approach Sender a Receiver M: message to be signed H: Hash function E: RSA Private Key Operation KRa: Sender’s Private Key D: RSA Public Key Operation KUa: Sender’s Public Key EKRa[H(M)] Signature of A over hash of M