Week 4 - Symmetric Cryptography Flashcards
What are some terminologies in cryptography?
Cryptology
The study of communication over non-secure channels
Cryptography
The process of designing systems that achieve secure communications
Cryptanalysis
Attempt to discover the plaintext (or key)
Plaintext
Message before encryption
Ciphertext
Encrypted message
Key
Parameter to compute encryption/decryption algorithms
Encryption algorithm
Given plaintext and key, produce ciphertext
Decryption algorithm
Given ciphertext and key, produce plaintext
When is an encryption scheme computationally secure?
An encryption scheme is computationally secure if the ciphertext meets one or both of the following:
- Cost of breaking the ciphertext > the value of the plaintext
- Time of breaking the ciphertext > useful lifetime of the plaintext
What is a brute-force attack?
A brute-force attack is a method where every possible key is attempted until the ciphertext is translated into readable plaintext.
Process: Involves trying at least half of all possible keys, making it time-consuming.
Challenge: Recognizing successful decryption output requires knowledge of how the plaintext appears.
What are the three independent dimensions to classify in cryptography?
The three independent dimensions to classify in cryptography are:
- Type of Operations:
- Substitution: Maps plaintext elements to form ciphertext.
- Transposition: Rearranges plaintext elements. - Number of Keys:
- Single key: Symmetric encryption.
- Double key: Asymmetric encryption. - Processing Method:
- Block cipher: Processes plaintext and produces ciphertext blocks.
- Stream cipher: Processes plaintext and produces ciphertext elements.
What are the attacks in cryptanalysis?
- Ciphertext only
- Known plaintext
- Chosen plaintext
- Chosen ciphertext
- Chosen text
What is symmetric encryption?
Symmetric encryption is a cryptographic technique where a single key, shared between sender and receiver, is used for both encryption and decryption
Components: Plaintext, encryption and decryption algorithms, secret key, ciphertext.
Requirements: Strong encryption ensuring decryption without key is impractical.
Protection: Relies on secrecy of key, as algorithms are publicly available.
What are symmetric Block Ciphers?
Definition: Symmetric encryption algorithms that process input data in fixed-sized blocks, generating ciphertext of equal size for each block.
Operation: Utilizes a series of rounds, each applying substitutions and permutations based on a secret key.
Parameters:
Block Size & Key Size: Larger block size enhances security but slows encryption and decryption; common trade-off is a 128-bit block size.
Number of Rounds: More rounds increase security; typically 10 to 16 rounds.
Subkey Generation & Round
Function: Greater complexity improves security, balancing with ease of software implementation and analysis.
What are the 3 most important symmetric block ciphers?
- Data Encryption Standard (DES)
Key Length: 56 bits
Rounds: 16
Block size: 64 bits - Triple Data Encryption Standard (3DES)
Key Length: 168 bits
Rounds: 48
Block size: 64 bits - Advanced Encryption Standard (AES)
Key Length: 128, 192, or 256 bits
Rounds: 10, 12, or 14
Block size: 128 bits
What are the different methods of generating random numbers?
The methods of generating random numbers are:
- TRNG - True Random Number Generator
Uses source of true randomness (e.g. keystroke timing pattern) - PRNG - Pseudorandom Number Generator
Uses seed - PRF - Pseudorandom Function
What are symmetric stream ciphers?
Symmetric Stream Ciphers:
Definition: Within symmetric ciphers, these algorithms produce one output element for each input element at a time.
Characteristics: Less common than symmetric block ciphers, they utilize randomly generated encryption keys, typically one byte at a time.
Design Considerations: Focus on generating a large period for encryption sequence, ensuring pseudorandom keystream resembles true randomness, and employing sufficiently long keys to resist brute-force attacks.
What are the advantages and disadvantages of symmetric stream ciphers?
Advantages of Symmetric Stream Ciphers:
Fast: Operate on individual elements rather than entire blocks, resulting in faster processing.
Disadvantages of Symmetric Stream Ciphers:
Cannot Re-use Keys: Keys serve as inputs to the pseudorandom generator, preventing key reuse.
What are the 5 cipher block modes of operations?
Modes of Operation for Block Ciphers:
- Electronic Codebook Mode (ECB):
- Description: Processes plaintext in fixed-size blocks, generating a unique ciphertext block for each with a given key.
- Strength: Simple to implement.
- Weakness: Lacks security due to identical plaintext blocks producing identical ciphertext blocks.
- Cipher Block Chaining Mode (CBC):
- Description: Utilizes XOR of current plaintext block and preceding ciphertext block as input to encryption algorithm, with an Initialization Vector (IV) for the first block.
- Strength: No fixed relationship between input plaintext and output ciphertext blocks.
- Cipher Feedback Mode (CFB):
- Description: Transforms block ciphers into stream ciphers by dividing plaintext into smaller units and maintaining a register for encryption.
- Output Feedback Mode (OFB):
- Description: Generates a stream of ciphertext bits by repeatedly encrypting an IV and XORing it with plaintext.
- Counter Mode (CTR):
- Description: Employs a counter for each encryption, ensuring each plaintext block is encrypted with a unique counter value. Not a chaining mode and lacks a decryption algorithm for decryption tasks.
Advantages of CTR Mode:
- Hardware & Software Efficiency: Supports parallel processing without a chain.
- Preprocessing: Encryption does not rely on the plaintext block.
- Random Access: Supports random-access processing of plaintext and ciphertext blocks.
- Security: Offers comparable security to other modes.
- Simplicity: Does not require decryption algorithm implementation for decryption tasks.
