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content/CSE4303/CSE4303_L13.md

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+# CSE4303 Introduction to Computer Security (Lecture 13)
+
+## Asymmetric Encryption
+
+### Public-key building block: Trapdoor function (TDF)
+
+#### Definition of trapdoor function
+
+A trapdoor function $X\to Y$ is a triple of efficient algorithms $(G,F,F^{-1})$ such that:
+
+- $G(\circ)$ is randomized algorithm outputs a key pair $(pk,sk)$.
+- $F(pk,\circ)$ is a deterministic algorithm that takes as input a public key $pk$ and a message $m$ and outputs a ciphertext $c$.
+- $F^{-1}(sk,\circ)$ is a deterministic algorithm that takes as input a secret key $sk$ and a ciphertext $c$ and outputs a message $m$.
+
+more precisely: $\forall(pk,sk)$ outputs by $G$, $\forall x\in X: F^{-1}(sk,F(pk,x))=x$.
+
+### RSA cryptosystem
+
+[RSA cryptosystem](https://notenextra.trance-0.com/CSE442T/CSE442T_L10/#theorem-rsa-is-a-trapdoor)
+
+Setup
+
+- $n = pq$, with $p$ and $q$ primes
+- $e$ relatively prime to $\varphi(n) = (p-1)(q-1)$
+- $d$ inverse of $e$ in $\mathbb{Z}_{\varphi(n)}$
+
+Keys
+
+- **Public key:*- $K_E = (n, e)$
+- **Private key:*- $K_D = d$
+
+Encryption
+
+- Plaintext $M \in \mathbb{Z}_n$
+- $C = M^e \bmod n$
+
+Decryption
+
+- $M = C^d \bmod n$
+
+<details>
+<summary>Example</summary>
+
+Setup
+
+- $p = 7,\ q = 17$
+- $n = 7\cdot 17 = 119$
+- $\varphi(n) = 6\cdot 16 = 96$
+- $e = 5$
+- $d = 77$
+
+Keys
+
+- **public key:*- $(119, 5)$
+- **private key:*- $77$
+
+Encryption
+
+- $M = 19$
+- $C = 19^5 \bmod 119 = 66$
+
+Decryption
+
+- $M = 66^{77} \bmod 119 = 19$
+
+</details>
+
+#### RSA cryptosystem: challenge
+
+- The implementation of the RSA cryptosystem requires various algorithms.
+
+- Overall
+  - Representation of integers of arbitrarily large size and arithmetic operations on them
+
+- Encryption
+  - **Modular power**
+
+- Decryption
+  - **Modular power**
+
+- Setup
+  - Generation of **random numbers** with a given number of bits (to generate candidates $p$ and $q$)
+  - **Primality testing** (to check that candidates $p$ and $q$ are prime)
+  - Computation of the **GCD** (to verify that $e$ and $\varphi(n)$ are relatively prime)
+  - Computation of the **multiplicative inverse** (to compute $d$ from $e$)
+
+#### RSA: basis of security
+
+For all efficient algorithms $A$:
+$$
+\Pr\!\left[ A(N,e,y) = y^{1/e} \right] < \text{negligible},
+$$
+where $p,q \leftarrow$ $n$-bit primes, $N \leftarrow pq$, and $y \leftarrow \mathbb{Z}_N$.
+
+### Diffie-Hellman key exchange
+
+Based on hardness of “discrete log problem”:
+
+Given $p$, $g$, $y=g^x \pmod p$, what is $x$?
+
+- Eavesdropper sees: $p$, $g$, $A=g^a \pmod p$, and $B=g^b \pmod p$.
+- How hard is it to compute $g^{ab} \pmod p$?
+- More generally: define $DH_g(g^a, g^b) = g^{ab} \pmod p$.
+
+### Elliptic Curve Cryptography (ECC) 
+
+- Parameters: curve, modulus, initial point
+  - Curve: $y^2 = x^3 + ax^2 + bx + c$
+  - Modulus: large prime number
+  - Initial point: large $(x, y)$
+- Operations: addition, point doubling, dot (see tutorial)
+  - Repeated addition $\sim$ multiplication
+  - Point doubling $\sim$ multiplying by $2$
+  - Repeated point doubling $\sim$ multiplying by powers of $2$
+
+Hard problem: analogue of discrete-log problem using elliptic curves in particular geometric space
+
+- See ArsTechnica tutorial, or many videos online
+- Reversing the dot and point-doubling operators in the finite field defined by the curve and modulus
+- Example: Let the finite field be defined by $y^2 = x^3 + 7 \pmod{31}$ with initial point $(x, y)$.
+  - Question: Suppose we see a new point $(x_2, y_2)$ and we know $(x_2, y_2) = n \cdot (x, y)$. What is $n$?
+  - I.e., how many times must we add $(x, y)$ to itself to get $(x_2, y_2)$?
+  - Public key: $(x_2, y_2)$ and parameters of the ECC system
+  - Private key: $n$
+  - Encryption: embed message as points on the EC, run EC ops on them
+
+### Public-key encryption from TDFs 
+
+Security Theorem:
+
+- If $(G, F, F^{-1})$ is a secure trapdoor function (TDF),
+- $(E_s, D_s)$ provides authenticated encryption,
+- and $H : X \to K$ is modeled as a random oracle (RO),
+
+then $(G, E, D)$ is CCA$_{\text{RO}}$ secure.
+
+- That is, it is CCA-secure in the random oracle model.
+- An additional extension is required to obtain full CCA security in the standard model, and such constructions are known.
+
+## Summary
+
+Wrapup: symmetric vs. asymmetric systems
+
+1. Symmetric: faster, but key distribution hard
+2. Asymmetric: slower, but key distribution/management
+easier
+3. Application: secure web sessions (e.g. online shopping visit)
+   1. Use symmetric-key-encrypted sessions
+   2. Exchange symmetric keys with asymmetric scheme
+   3. Authenticate public keys (using PKI or web of trust)

content/CSE4303/_meta.js

@@ -16,4 +16,5 @@ export default {
     CSE4303_L10: "Introduction to Computer Security (Lecture 10)",
     CSE4303_L11: "Introduction to Computer Security (Lecture 11)",
     CSE4303_L12: "Introduction to Computer Security (Lecture 12)",
+    CSE4303_L13: "Introduction to Computer Security (Lecture 13)",
 }