Cryptography exam prep study guide

The study and practice of securing communication through encoding.

Cryptography

What type of key is used to compute a MAC?

symmetric

? secure encryption algorithm property that ensures that changing a single bit of an encryption key impacts most of the ciphertext bits. The ? property hides the relationship between a ciphertext and the encryption key

confusion

? is a secure encryption algorithm property that ensures that changing a single plaintext bit changes about half of the ciphertext bits and changing a single ciphertext bit changes about half of the plaintext bits. The ? property hides the relationship between a plaintext and a ciphertext.

diffusion

the original readable message

plaintext

the encrypted (unreadable) message

ciphertext

converting plaintext > ciphertext using an algorithm and a key

encryption

converting ciphertext > plaintext using the key

decryption

the mathematical function used for encryption/decryption

algorithm (cipher)

the secret value that controls encryption/decryption

key

keeps data secret , ex : AES encryption

confidentiality

prevents undetected changes , ex: hashing, HMAC

integrity

keeps data accessible, ex : redundancy, backups

availability

verifies identity, ex : digital certificates, kerberos

authentication

prevents denial of actions, Ex : digital signatures

nonrepudiation

What are three Asymmetric ( public key ) algorithms ?

RSA, EIGamal, ECC

This asymmetric algorithm is computationally efficient because it uses smaller key sizes.

• ideal for use in battery powered IoT devices

ECC

Which asymmetric encryption algorithm should not be used in a device with limited memory?

EIGamal

Which asymmetric encryption algorithm can provide a high level of security with a small key size?

ECC

CIA stands for ?

confidentiality, integrity, and availability

what are the three types of cryptography

symmetric, assymetric, hashing

this type of cryptography, same key for encryption and decryption, shared secret

ex : AES , DEs, 3DES, blowfish

symmetric

this type of cryptography , key pair : public and private , mathematically related, ex: RSA, ECC, Diffie-Hellman

asymmetric

RSA, Diffie-hellman, ECC are examples of ?-key cryptography

public

this type of cryptography, one way mathematical function, no key, ex : SHA-256, SHA-3, MD5

Hashing

try every key until one works

brute force

looks for patterns in ciphertext ( used on classical ciphers)

frequency analysis

attacker knows or chooses plaintext and compares ciphertext

known plaintext / chosen plaintext

exploit physical characteristics ( timing, power use )

side-channel attacks

targets hash collisions

birthday attack

algorithm for encryption/decryption

cipher

all possible keys. larger=more secure

keyspace

measure of randomness

entropy

adds ransomness to encryption

initialization vector (IV)

random data added to a password before hashing

salt

this substitution cipher …

• simple shift , A>D , key=shift number

caesar

this substitution cipher….

reverse alphabet , a<>z , fixed substitution

atbash

substitution cipher , polyalphabetic, keyword , resists frequency analysis

vigenere

substitution cipher, digraph substitution, 5×5 grid , replaces letter pairs

playfair

substitution cipher, matrix-based, linear algebra, requires invertible key matrix

hill cipher

substitution cipher, mechanical polyalphabetic , WWII german cipher, used rotors and plugboard

Enigma

this type of cipher rearranges letters instead of replacing them

transposition ciphers

transposition cipher, zigzag pattern across rows

rail fence cipher

transposition cipher , write in columns, read by key order

columnar transposition

transposition cipher, ancient greek wrapping stick

scytale cipher

replace =

substitution

rearrange =

transposition

data formatting for transmission/storage , reversible, example : Base64, ASCII, UTF8

Encoding

data protection from unauthorized access , reversible with key, example : AES , RSA

Encryption

data integrity verification , not reversible, example : SHA-256, MD5

Hashing

? is often used for representing keys, hashes and ciphertext

hexadecimal

this number system is human readable

decimal

this number system is compact byte representation, 0×2A, base 16

hexadecimal

this number system is used in older computing formats, example : 052, base 8

octal

Value that “undoes” multiplication mod n , key to RSA’s private key math

modular inverse

Remainder after division

17 mod 5 = 2

Modulo (mod)

Divisible only by 1 and itself

Used in RSA, DH

Prime Numbers

Numbers with no shared factors

Needed for key generation

Relatively Prime

Largest shared divisor

gcd(9, 12) = 3

Greatest Common Divisor (GCD)

Converts binary data to ASCII text.

Base64

Represents text using numeric codes

ASCII

This encoding scheme , Supports all characters

Unicode/UTF-8

Commonly used in digital certificates and hashes.

Hex

? is often used for representing keys , hashes, and ciphertext

hexadecimal

Which classic cipher is this ?

Method: Shift each letter by a fixed number (the key).

Example: Key = 3 → A→D, B→E, etc.

Keyspace: 26 possible keys if using the English alphabet.

Attack: Brute force or frequency analysis.

Weakness: Small keyspace, predictable letter patterns.

simple substitution , caesar cipher

Which classic cipher is this ?

•Method: Write plaintext into a grid → permute rows and columns.

•Key: Row and column order (permutations).

•Strength: Scrambles text structure, disperses statistical info.

•Weakness: Letters themselves unchanged (frequency preserved).

•Modern Connection: Concept of diffusion used in block ciphers.

Double Transposition Cipher

Which classic cipher is this ?

•Method: XOR plaintext bits with a random key of equal length.

•Security: Provably unbreakable if key is random, secret, and used once.

•Drawback: Impractical key distribution (pad must be as long as message).

•Modern Connection: Basis for stream cipher design.

One-Time Pad (Vernam Cipher)

Convert plaintext → ciphertext.

Encryption

Convert ciphertext → plaintext

Decryption

Algorithm used for encryption/decryption

Cipher

Secret value used by cipher

Key

All possible key combinations , larger = more secure

Keyspace

Strengthens weak keys by increasing complexity

Key Stretching

what are the 3 properties of a secure cipher

confusion , diffusion, kerckhoffs’s principle

Changes in key → large changes in ciphertext (hides key relationship).

confusion

Changes in plaintext → large changes in ciphertext (hides structure)

Diffusion

Security must depend on the secrecy of the key, not the algorithm

Kerckhoffs’s Principle

what are the two symmetric crytography ciphers ?

stream & block

this type of symmetric cipher …..

encrypts data bit-by-bit or byte-by-byte using a keystream

modeled after one-time pad ( but with shorter, stretched keys)

strengths : fast, efficient for streaming data

weaknesses : key reuse leads to severe vulnerabilities

stream cipher

what are 2 examples of symmetric stream ciphers

A5/1 (GSM) , RC4 (SSL/WEP)

this type of symmetric cipher ….

• encrypted fixed-size blocks ( 64 bit and 128 bit )

• requires padding if plaintext not a multiple of block size

  • used in hash functions, MACs, and encryption modes

block cipher

what are 5 example algorithms for block cipher?

DES, 3DES, AES, IDEA, TEA

what are the 5 modes of block cipher operation?

ECB, CBC, CFB, OFB, CTR

The process of adding bits to a plaintext's last block is called ?

padding

The ? is a block cipher which encrypts a plaintext 64-bit block at a time

IDEA

Block Cipher Modes of Operation…..

Encrypts each block separately

NO error propagation

Identical plaintext → identical ciphertext; least secure

Electronic code book ( ECB )

Block Cipher Modes of Operation…..

XORs each plaintext block with previous ciphertext block, computationally chained together

Error propagates : YES

Uses IV( randomly generated sequence of bits of equal size )

Each plaintext block is XORed with the previous ciphertext block before being encrypted.

cipher block chaining ( CBC )

Block Cipher Modes of Operation…..

Treats block cipher as stream cipher

XOR with previous ciphertext

error propagation : YES

encryption and decryption same process, IV used in the first plaintext block and ciphertext blocks are computationally chained together

Each plaintext block is encrypted and XORed with the previous ciphertext block.

CFB ( cipher feedback )

Block Cipher Modes of Operation…..

Uses keystream XOR; independent of ciphertext

error propagation : NO

Each block is created independently of plaintext and ciphertext blocks

OFB ( output feedback )

Block Cipher Modes of Operation…..

Uses counter + nonce; XOR with plaintext

Error propagation : NO

Parallelizable; common in modern use

A ciphertext block depends on the position of the current plaintext block

Counter ( CTR )

what are the 3 major block ciphers

DES , 3DES, and AES

this block cipher ….

•Block size: 64 bits

•Key size: 56 bits

•Weakness: Small key size → brute-force possible.

DES

this block cipher ….

•Process: DES applied 3 times.

Triple DES (3DES)

3DES : option ? 3 independent keys (168-bit) > most secure

option 1

3DES : option ? 2 keys (112-bit)

2

3DES : option ? 1 key (same as DES) → least secure

option 3

this symmetric block cipher ….

•Used in: VPNs, Wi-Fi, HTTPS, disk encryption.

•Properties: Fast, strong confusion/diffusion, resists modern attacks.

• most widely used block cipher

AES

Why is AES-192 more secure than AES-128?

bc 192 has a larger key space

What are the 3 versions of AES ?

128 . 192. 256

All versions of AES have ? bits as their block size

128 bits