Cryptography for Cybersecurity Quiz

Study Notes

Message Authentication Codes (MAC): Alice and Bob use MACs to verify message integrity and authenticity. A shared secret key creates the MAC. Interception without the key prevents modification.

Key Generation: RSA uses a public and private key pair.
- Large Prime Numbers: Two large prime numbers (e.g. 2048-bit or 4096-bit) are randomly selected (p and q).
- Calculate n (Modulus): n is the product p x q. Factoring n back into p and q is computationally difficult.
- Calculate Euler's Function (φ(n)): φ(n) = (p-1) x (q-1)
- Public Exponent (e): A public exponent (e) is chosen such that 1 < e < φ(n) and gcd(e, φ(n)) = 1 (they are coprime). A common choice is 65537.
- Calculate Private Key (d): d is the modular multiplicative inverse of e modulo φ(n), such that e × d ≡ 1 (mod φ(n)).
Public Key: (n, e)
Private Key: (n, d)
Encryption Process:
- Convert message (M) to integer (m) where 0 ≤ m < n.
- Calculate ciphertext (c): c = m^e (mod n)
Decryption Process:
- Calculate original message (m): m = c^d (mod n)
- Convert m back to message (M)

Parameter Generation:
- Large Prime Number (p): A large prime number, critical to security (typically at least 2048 bits).
- Primitive Root (g): g is a number that generates all integers from 1 to p-1 under modulo p. These values are publicly shared.
Private Key Selection:
- Alice chooses a random integer 'a' where 1 < a < p-1.
- Bob chooses a random integer 'b' where 1 < b < p-1.
Public Key Computation:
- Alice computes her public key A: A = g^a (mod p)
- Bob computes his public key B: B = g^b (mod p)
Public Key Exchange: Alice sends A to Bob, Bob sends B to Alice.
Shared Secret Computation:
- Alice computes the shared secret S: S = B^a (mod p)
- Bob computes the shared secret S: S = A^b (mod p)
- S will always be the same for both parties.

Discrete Logarithm Problem (DLP): Difficult to compute private keys from public keys if p and g are large enough values.
Man-in-the-Middle Attack (MitM): Attackers can intercept and replace public keys. Combining the protocol with authentication methods (like digital signing and PKI) mitigates this.

Definition: Block ciphers operate on fixed-size bit blocks (e.g. 64 bits).
Encryption: Transforms plaintext blocks into ciphertext blocks using a secret key.
Decryption: Reverses the transformation using the same key, turning ciphertext back to plaintext.
Modes of Operation: Different modes combine blocks, to ensure data security during transfer. (e.g., ECB, CBC, CFB, OFB)
NIST (National Institute of Standards and Technology): Specifies cryptographic algorithms, including block ciphers.
Typical Block Cipher Scheme:
- Input: Block of plaintext (e.g. 64 bits)
- Key: Secret key (k bits)
- Output: Block of ciphertext (e.g. 64 bits)

Definition: A symmetric block cipher structure (also called Feistel Network).
Operation: Encryption and decryption utilize repeated round function calls on the input.

Concept: Security relies on the secrecy of the key, not the algorithm.
Application: A strong cryptographic system remains secure even if the algorithm is public.

Confusion: No simple relationship between plaintext and ciphertext.
Diffusion: Modifying a single bit in plaintext requires altering multiple bits within the ciphertext.

Definition: A widely used symmetric block cipher that encrypts data in 64-bit blocks using a 56-bit key.
Vulnerabilities: The short key length now makes DES vulnerable to brute-force attacks. -DES-16 Round Feistel Network - -Input: 64 bits plaintext, Key -Output: 64 bits ciphertext -Key Schedule: -Initial 56 bits are selected using Permuted Choice 1
- Divided into two 28 bit halves
- Rotated left in each round and
- Selected again into 48 bit subkey using Permuted Choice 2
Decryption: Rounds are applied in reverse order with subkeys in reverse order.

Definition: Applies DES three times to each block of data for enhanced security.
Key Size: Uses 128–192 bits keys.
Block Size: 64 bits

Definition: A one-way function that produces a fixed-size hash value from an input of arbitrary length (input: variable, output is fixed)
Characteristics:
- Fast to compute
- One-way property: Difficult or impossible to reverse.
- Small changes in the input result in large changes in the output.
Applications:
- Data integrity (verifying data hasn't changed)
- Password storage (storing hashed passwords)
- Digital signatures (creating unique message representation, to be signed and verified)