Summary of Lecture 1•Security Concepts: –Confidentiality; Integrity; Availability–Authenticity; Assurance; Anonymity•Overview on the crypto tools–Symmetric/public crypto., cryptographic hash, digital signature, digital certificate. •Secure Password–Common means for authentication–Usually stored via hash values –long psw. + odd char. are better and saferCS4293 Topics on Cybersecurity1
Lecture 2 – Basic Crypto ToolsDr. Cong WangCS Department City University of Hong KongCS4293 Topics on Cybersecurity2Slides partially adapted from lecture notes by M. Goodrich&R. Tamassia, W. Stallings&L. Brown, and Dan Boneh.
Symmetric CryptographyCS4293 Topics on CybersecurityAssumes parties already share a secret key3
Symmetric Cryptosystem•Scenario–Alice wants to send a message (plaintext P) to Bob. –The communication channel is insecure and can be eavesdropped–If Alice and Bob have previously agreed on a symmetric encryption scheme and a secret key K, the message can be sent encrypted (ciphertext C)•Issues–What is a good symmetric encryption scheme?–What is the complexity of encrypting/decrypting?–What is the size of the ciphertext, relative to the plaintext?CS4293 Topics on CybersecurityCmmencryptKdecryptK4
Basics•Notation–Secret key K–Encryption function EK(M), or Ek(P).–Decryption function DK(C) –Plaintext length typically the same as ciphertext length–Encryption and decryption are PRP, i.e., pseudorandom permutation functions (bijections),on the set of all n-bit arrays•Efficiency–functions EKand DKshould have efficient algorithms•Consistency–Decrypting the ciphertext yields the plaintext–DK(EK(M)) = M or DK(EK(P)) = P.CS4293 Topics on Cybersecurity5
Basics (Cont’d)E, D: cipher k: secret key (e.g. 128 bits)m, c: plaintext, ciphertextn: nonce (aka IV)Encryption algorithm is publicly known•Never use a proprietary cipherAliceEm, nE(k,m,n)=cBobDc, nD(k,c,n)=mkknonceCS4293 Topics on Cybersecurity6
Use CasesSingle use key:(one time key)•Key is only used to encrypt one message•encrypted email: new key generated for every email•No need for nonce (set to 0)Multi use key: (many time key)•Key used to encrypt multiple messages•SSL: same key used to encrypt many packets•Need either uniquenonce or randomnonceCS4293 Topics on Cybersecurity7
Attacks•Attacker may havea)collection of ciphertexts (ciphertext only attack)b)collection of plaintext/ciphertext pairs (known plaintext attack)c)collection of plaintext/ciphertext pairs for plaintexts selected by the attacker (chosen plaintext attack, CPA)d)collection of plaintext/ciphertext pairs for ciphertexts selected by the attacker (chosen ciphertextattack, CCA)CS4293 Topics on CybersecurityHi, Bob.Don’t invite Eve to the party! Love, AliceEncryptionAlgorithmPlaintextCiphertextkeyEveHi, Bob.