Week2-Crypto-basics.pdf - Summary of Lecture 1 \u2022 Security Concepts \u2013 Confidentiality Integrity Availability \u2013 Authenticity Assurance Anonymity \u2022

# Week2-Crypto-basics.pdf - Summary of Lecture 1 u2022...

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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 safer CS4293 Topics on Cybersecurity 1
Lecture 2 – Basic Crypto Tools Dr. Cong Wang CS Department City University of Hong Kong CS4293 Topics on Cybersecurity 2 Slides partially adapted from lecture notes by M. Goodrich&R. Tamassia, W. Stallings&L. Brown, and Dan Boneh.
Symmetric Cryptography CS4293 Topics on Cybersecurity Assumes parties already share a secret key 3
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 Cybersecurity C m m encrypt K decrypt K 4
Basics Notation Secret key K Encryption function E K (M), or E k (P). Decryption function D K (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 E K and D K should have efficient algorithms Consistency Decrypting the ciphertext yields the plaintext D K (E K (M)) = M or D K (E K (P)) = P. CS4293 Topics on Cybersecurity 5
Basics (Cont’d) E, D: cipher k: secret key (e.g. 128 bits) m, c: plaintext, ciphertext n: nonce (aka IV) Encryption algorithm is publicly known Never use a proprietary cipher Alice E m, n E(k,m,n)=c Bob D c, n D(k,c,n)=m k k nonce CS4293 Topics on Cybersecurity 6
Use Cases Single 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 unique nonce or random nonce CS4293 Topics on Cybersecurity 7
Attacks Attacker may have a) 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 ciphertext attack, CCA ) CS4293 Topics on Cybersecurity Hi, Bob. Don’t invite Eve to the party! Love, Alice Encryption Algorithm Plaintext Ciphertext key Eve Hi, Bob.