Alice and Bob: read their histroy
Cryptography Application
sharing secret key via public communication
online voting system
digital signature
computation on encrypted data
zero-knowledge proof
\mathbb{Z}_N = \{0, 1, ..., N - 1\} is a good universe for addition / subtraction \mathbb{Z}_N^* = \{A \in \mathbb{Z}_N | \text{gcd}(A, N) = 1\} is a good universe for multiplication / division
\mathbb{Z}_N^* is closed under multiplication
Multiplication table permutation property: every row and column is a permutation of the elements \mathbb{Z}_N^*
\varphi(N) = |\mathbb{Z}_N^*|
when N is prime P, then \varphi(P) = P - 1
when P, Q distinct primes, then \varphi(QP) = (P - 1)(Q - 1)
there always exists a generator in \mathbb{Z}_P^* where P is a prime
Euler's Theorem: in a universe \mathbb{Z}_N^*, all number in \mathbb{Z}_N^* with power equal to \varphi(N) is 1 ((\forall A \in \mathbb{Z}_N^*)(A^{\varphi(N)} = 1)) and repeat a circle.
\text{isPrime}:
O(n^6) time, deterministic, result from 2002
O(n^2) time, Monte Carlo, result from 1975, Miller-Rabin
\text{randomPrime}:
Prime Number Theorem: about \frac{1}{n} fraction of n-bit numbers are prime.
Assuming \text{isPrime} \in O(n^2), then \text{randomPrime} \in O(n^3)
No known deterministic method to solve this problem
Sender:
M: PICK original message
K_A: PICK encryption key
Public:
C: encrypted message
Enc: encryption algorithm
Dec: decryption algorithm
Receiver:
K_B: OBTAIN decryption key
M: CALCULATE decrypted message
Variations:
number shifting: only 24 possible keys
number mapping: frequency analysis
Enigma: turing machine
One-time pad:
Encryption: C = M \oplus K (bitwise or with one-time padding for every bit)
Encryption: M = C \oplus K (C \oplus K = (M \oplus K) \oplus K = M \oplus (K \oplus K) = M)
perfectly secure: (\forall M)(K \text{ uniformly random} \implies C \text{ uniformly random}) (Shannon's Thorem: |K| < |M| \implies \text{ adversary can learn some information about } M \text{ given unlimited computational power})
padding length: has to be size of message
one-time: given \begin{cases} C_1 = M_1 \oplus K\\ C_2 = M_2 \oplus K\\ \end{cases}, then C_1 \oplus C_2 = M_1 \oplus M_2 reveals some information
Sender:
P: PICK modular universe (must be prime)
G: PICK for base (must be generator)
R_1: PICK exponent
G^{R_1R_2}: CALCULATE secret key
Public:
P: modular universe
G: generator for base
G^{R_1}: half key 1
G^{R_2}: half key 2
Receiver:
R_2: PICK exponent
G^{R_1R_2}: CALCULATE secret key
Process:
Sender send above information to Receiver
Sender pick a secret exponent R_1 \in \mathbb{Z}_{\varphi(P)}^*
Sender send G^{R_1}
Receiver pick a secret exponent R_2 \in \mathbb{Z}_{\varphi(P)}^*
Receiver send G^{R_2}
Both compute G^{R_1R_2} and let it be secret.
Breaking Encryption: solving distinct log problem (obtaining R_1 from G^{R_1} and R_2 from G^{R_2}) - proving is as hard as deciding P \text{ v.s. } NP
DH Assumption: Hard to compute G^{R_1R_2} form P, G, G^{R_1}, G^{R_2}
Decisional DF Assumption: You learn no info about G^{R_1R_2} - so universe need to be picked carefully
Sender:
Public:
N (public key): universe (must be N = PQ)
E (public key): exponent (must be E \in \mathbb{Z}_{\varphi(N)} to have inverse)
C: encrypted message
Receiver:
P, Q: PICK two distinct prime for universe (must be distinct primes, otherwise \varphi(N) can be discover and E^{-1} can be calculated. If not distinct, calculate square root P = \sqrt{N} can be efficiently done)
N = PQ: CALCULATE universe (must be N = PQ)
\varphi(N) = (P - 1)(Q - 1): CALCULATE exponent universe
E: PICK exponent (must be E \in \mathbb{Z}_{\varphi(N)} to have inverse)
E^{-1}(private key): CALCULATE exponent inverse with Extended Euclid Algorithm with \varphi(N) (remember to make the result positive)
Process:
Breaking Encryption: solving distinct root problem
to compute M from M^E \mod N
to compute \varphi(N) from N
to factor P, Q from N to obtain \varphi(N) = (P - 1)(Q - 1)
to pick M \notin \mathbb{Z}_N^* (there are only PQ - (P - 1)(Q - 1) = PQ - (PQ - P - Q + 1) = P + Q - 1 many such M in the pool of PQ possible space) and do Euclid's algorithm on M, N to reveal P, Q.
Complexity in Continuous Space:
Addition: grade school
Subtraction: grade school
Multiplication: grade school
Division: grade school (output remainder)
Exponentiation: exponential time, cannot write it down
Logarithm:
Root:
Modular Complexity: finite set \mathbb{Z}_N = \{0, 1, 2, ..., N - 1\}
Addition (polynomial time): A +_N B = (A + B) \mod N
Subtraction (polynomial time): B +_N -B = 0 (additive inverse)
Multiplication (polynomial time): A \cdot_N B = (A \cdot B) \mod N
Division (polynomial time Euler's Algorithm): B \cdot_N B^{-1} = 1 (multiplicative inverse)
Exponentiation(polynomial time):
Root(no poly time known)
Log(no poly time known)
Modular Division Algorithm:
Compute B^{-1}: Extension of Euclid's Algorithm
miix: integral linear combination where constants are integers \text{gcd}(A, B) = kA + lB (\text{gcd}(A, B) is a mix of A and B)
if we find 1 = kB + lN, B^{-1} = k by lN \equiv 0 \mod N
Modular Exponentiation Algorithm: repeatedly square and mode Modular Log Algorithm: no known poly-time algorithm Modular Root Algorithm: no known poly-time algorithm
Euclidean Algorithm (O(k^2) for mods and remainders, O(k^3) worst when inputs are two consecutive Fibonacci numbers): gcd(x, y) = gcd(y, x \mod y) Extended Euclidean Algorithm: gcd(a, b) = x \cdot a + y \cdot b Diophantine Equation: a \cdot x + b \cdot y = c for solution of variable x, y. Such solution is not unique, when (x_0, y_0) is a solution (x_0 + tb/d, y_0 - ta/d) is also a solution Lattice: \langle{\mathbb{N}}, |\rangle - a partial order where any two elements have a join (supremum: least common multiple) and a meet (infimum: greatest common divisor)
Theorem: Given polynomial p(x) = ax^3 + bx^2 + cx + d, if p(0), p(1) odd, then (\forall x \in \mathbb{R}(p(x) \neq 0))
Arithmetic of remainders
(x + y) \mod m = ((x \mod m) + (y \mod m)) \mod m
(xy) \mod m = ((x \mod m) \cdot (y \mod m)) \mod m
We define arithmetic on equivalence classes of space \mathbb{Z}_m:
[x]_{\mathbb{Z}_m} +_{\mathbb{Z}_m} [y]_{\mathbb{Z}_m} = [x+y]_{\mathbb{Z}_m}
[x]_{\mathbb{Z}_m} \cdot_{\mathbb{Z}_m} [y]_{\mathbb{Z}_m} = [x \cdot y]_{\mathbb{Z}_m}
Notation of _{\mathbb{Z}_m} is a lot, therefore we think \mathbb{Z}_m = \{0, 1, ..., m-1\} \subseteq \mathbb{Z}
It is hard to solve even quadratic modular equations.
simplifies: ab \equiv ac (\mod m) \implies b \equiv c (\mod (m/gcd(a, m))) (we can drop a when a, m are coprime)
Inhomogeneous Equation 1: ax \equiv 1 (\mod m) has a solution iff a, m are coprime. The solution is unique in modulo m if exists.
Inhomogeneous Equation 2: ax \equiv c (\mod m) has a solution iff gcd(a, m) | c. Also, the number of solutions is gcd(a, m)
also when p is prime, \mathbb{Z}_p^* = \{1, 2, ..., p - 1\}, then we can solve all ax = b (\mod p) when a \not\equiv 0 (\mod p)
Multiplicative Inverse
multiplicative inverse: x in ax = 1 (\mod m) is multiplicative inverse of a. (x = a^{-1}) !Multiplicative Inverse Chart](imgs/2021-11-18-22-48-39.png)
\mathbb{Z}_m^* = \{a \in \mathbb{Z}_m | gcd(a, m) = 1\} is multiplicative subgroup of Z_m
Euler's Totient Function: cardinality of Z_m^* is \varphi(m)
Orbit (Power) of Modulo: \langle{a}\rangle = \{a^k | k \geq 0\}
multiplicative order: in prime modulo orbit, a^k \equiv 1 such that k < m - 1, then k | m - 1. The least value of k such that k > 0 is called "multiplicative order" of a.
multiplicative generator: for prime modulo, there exists a in orbit such that \langle{a}\rangle = \mathbb{Z}_m^*
Solving System of Equation a_1 x \equiv b_i (\mod m_i)
since we need gcd(a_i, m_i) | b_i, we can reduce equation to a_i'x \equiv b_i' (\mod m_i') where a_i', m_i' are coprime. This is equivalent to x \equiv c_i (\mod m_i') for some c_i
for coprime moduli of x \equiv a_i (\mod m_i) for n = 1, 2
Chinese Remainder Theorem: Let m_i, i = 1, ..., n be pairwise coprime, then x \equiv a_i (\mod m_i) have a unique solution in \mathbb{Z}_m: m = m_1m_2...m_n. (for n > 2, repeat)
Better Method for n > 2:
Decompose \mathbb{Z}_m into \mathbb{Z}_{p_1^{e_1}} \times \mathbb{Z}_{p_2^{e_2}} \times ... \times \mathbb{Z}_{p_k^{e_k}} where m = \prod p_i^{e_i} is prime decomposition.
Addition, Multiplication: wheel level Multiplication: wheel level Exponentiation: change base in sector does not affect output setctor
Prime wheel: any number raise to p-1 sector -> on 1 sector Non-Prime wheel: prime divisor of m sector, as well as other composit in first ring sector, will not contain prime number
p1 * p2 = filling up all non-prime number in prime sector
p1 * p1 = filling up non-prime number in 1 sector
Euler: got to be 1 when exponent phi exponent cycles, to the 1, 2, 3, 4..., repeat there are generator to the 1, 2, 3, 4 each with new number
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