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Discrete Math · College

Discrete Math: Cryptography Basics

Foundational cryptography concepts built on discrete math, such as modular exponentiation and public key basics.

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What is modular arithmetic?
Arithmetic where computations are performed with respect to a modulus, working only with remainders after division by a fixed number.
What does the notation a ≡ b (mod n) mean?
a and b are congruent modulo n, meaning they have the same remainder when divided by n.
Define modular exponentiation.
Computing the result of a^b mod n efficiently without first calculating a^b.
Why is modular exponentiation important for cryptography?
It allows efficient computation of large powers while keeping intermediate results bounded, enabling practical implementation of cryptographic algorithms.
State Fermat's Little Theorem.
If p is prime and gcd(a, p) = 1, then a^(p-1) ≡ 1 (mod p).
State Euler's Theorem.
If gcd(a, n) = 1, then a^φ(n) ≡ 1 (mod n), where φ(n) is Euler's totient function.
What is Euler's totient function φ(n)?
The count of positive integers less than or equal to n that are coprime with n.
What is symmetric key cryptography?
An encryption method using a single shared secret key for both encryption and decryption.
What is public key cryptography?
An encryption method using a pair of mathematically related keys: a public key for encryption and a private key for decryption.
What two numbers form an RSA public key?
The modulus n and the public exponent e.
What two numbers form an RSA private key?
The modulus n and the private exponent d.
How is the RSA modulus n constructed?
By multiplying two distinct large prime numbers: n = p * q.
What is the relationship between the RSA exponents e and d?
e and d are multiplicative inverses modulo φ(n), meaning e * d ≡ 1 (mod φ(n)).
Define the discrete logarithm problem.
Given g, h, and prime p, find the exponent x such that g^x ≡ h (mod p), believed to be computationally hard.
Why is the discrete logarithm problem important to cryptography?
Many public key cryptosystems derive their security from the computational difficulty of solving the discrete logarithm problem.

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