update typo and structures
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# Lecture 16
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## Continue on PRG
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## Chapter 3: Indistinguishability and Pseudorandomness
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PRG exists $\implies$ Pseudorandom function family exists.
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@@ -49,13 +49,13 @@ Pseudo random function familiy exists $\implies$
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Mult-message secure encryption exists.
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## Public key cryptography
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### Public key cryptography
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1970s.
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The goal was to agree/share a key without meeting in advance
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### Diffie-Helmann Key exchange
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#### Diffie-Helmann Key exchange
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A and B create a secret key together without meeting.
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@@ -75,7 +75,7 @@ And Alice do $(g^b)^a$ where Bob do $(g^a)^b$.
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With $g^a,g^b$ no one can compute $g^{ab}$.
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### Public key encryption scheme
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#### Public key encryption scheme
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Ideas: The recipient Bob distributes opened Bob-locks
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@@ -90,12 +90,12 @@ Public-key encryption scheme:
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Let $A, E$ knows $pk$ not $sk$ and $B$ knows $pk,sk$.
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Adversary can now encypt any message $m$ with the public key.
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Adversary can now encrypt any message $m$ with the public key.
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- Perfect secrecy impossible
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- Randomness necessary
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- Randomness necessary
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Security of public key
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#### Security of public key
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$\forall n.u.p.p.t D,\exists \epsilon(n)$ such that $\forall n,m_0,m_1\in \{0,1\}^n$
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@@ -113,7 +113,9 @@ We will achieve security in sending a single bit $0,1$
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Time for trapdoor permutation. (EX. RSA)
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Encryption Scheme: Given family of trapdoor permutation $\{f_i\}$ with hardcore bit $h(i)$
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#### Encryption Scheme via Trapdoor Permutation
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Given family of trapdoor permutation $\{f_i\}$ with hardcore bit $h(i)$
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$Gen(1^n):(f_i,f_i^{-1})$, where $f_i^{-1}$ uses trapdoor permutation of $t$
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