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Questions and Answers
What is a primary characteristic of a cryptographically secure, pseudo-random bit generator used in stream ciphers?
What is a primary characteristic of a cryptographically secure, pseudo-random bit generator used in stream ciphers?
Why should two messages not be encrypted with the same key and Initialization Vector (IV)?
Why should two messages not be encrypted with the same key and Initialization Vector (IV)?
In symmetric stream ciphers, what does the term 'keystream' refer to?
In symmetric stream ciphers, what does the term 'keystream' refer to?
What occurs if an attacker gains knowledge of even a portion of the keystream?
What occurs if an attacker gains knowledge of even a portion of the keystream?
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What is the role of the key in the key stream generation process of symmetric stream ciphers?
What is the role of the key in the key stream generation process of symmetric stream ciphers?
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What is the primary purpose of symmetric cryptography?
What is the primary purpose of symmetric cryptography?
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In symmetric encryption, how is the plaintext transformed into ciphertext?
In symmetric encryption, how is the plaintext transformed into ciphertext?
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What is a common characteristic of symmetric ciphers?
What is a common characteristic of symmetric ciphers?
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What does cryptanalysis aim to reveal in encrypted data?
What does cryptanalysis aim to reveal in encrypted data?
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Which of the following methods is commonly used in cryptanalysis?
Which of the following methods is commonly used in cryptanalysis?
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Which aspect of symmetric encryption requires both parties to share the same information?
Which aspect of symmetric encryption requires both parties to share the same information?
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What term describes the art of concealing data within another medium?
What term describes the art of concealing data within another medium?
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What is a significant limitation of symmetric key cryptography compared to asymmetric cryptography?
What is a significant limitation of symmetric key cryptography compared to asymmetric cryptography?
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Which method is primarily used to ensure confidentiality in encryption?
Which method is primarily used to ensure confidentiality in encryption?
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What is a characteristic of block ciphers in encryption?
What is a characteristic of block ciphers in encryption?
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Which of the following describes a polyalphabetic cipher?
Which of the following describes a polyalphabetic cipher?
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What is the primary role of confusion in cipher techniques?
What is the primary role of confusion in cipher techniques?
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Which cipher type primarily uses XOR operations for encryption and decryption?
Which cipher type primarily uses XOR operations for encryption and decryption?
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What is the purpose of the keystream in stream ciphers?
What is the purpose of the keystream in stream ciphers?
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Which of the following is a feature of symmetric encryption?
Which of the following is a feature of symmetric encryption?
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What best defines a one-time pad in cryptography?
What best defines a one-time pad in cryptography?
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What is the difference between symmetric and asymmetric encryption?
What is the difference between symmetric and asymmetric encryption?
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Which cipher is known for being difficult to implement due to its impracticality?
Which cipher is known for being difficult to implement due to its impracticality?
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Which operation reduces the search space for potential keys in cryptography?
Which operation reduces the search space for potential keys in cryptography?
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What defines the substitution alphabet in monoalphabetic ciphers?
What defines the substitution alphabet in monoalphabetic ciphers?
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What is the impact of diffusion in cryptography?
What is the impact of diffusion in cryptography?
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Which of the following statements is true regarding polyphonic ciphers?
Which of the following statements is true regarding polyphonic ciphers?
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Which symmetric stream cipher was initially secret and reverse engineered, leading to vulnerabilities?
Which symmetric stream cipher was initially secret and reverse engineered, leading to vulnerabilities?
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Study Notes
Modern Symmetric Cryptography
- Cryptography is the art or science of hidden writing (confidential writing)
- Steganography is the art of concealing data
- Cryptanalysis is the art or science of breaking cryptographic systems
- Cryptology combines cryptography and cryptanalysis
Cryptography: How it Works
- A cipher (or cipher algorithm) is a specific cryptographic technique
- Encryption transforms original information into a cryptogram
- Decryption transforms a cryptogram back into the original information
- A key is an algorithm parameter that influences algorithm execution
- Plaintext is the original information
- Ciphertext is the encrypted form of the information
Use Cases for Symmetric Ciphers
- Self-protection: Alice encrypts plaintext with a secret key K, and only Alice can decrypt it.
- Secure communication: Alice and Bob communicate confidentially using a shared secret key K.
Goals of Cryptanalysis
- Reveal the plaintext hidden in a ciphertext, usually by discovering the key
- Sometimes, finding the algorithm used for encryption
- Methods like reverse engineering can be used in certain cases.
Cryptanalysis Attacks
- Brute-force search: Exhaustive search of all possible keys
- Clever attacks: Reducing the key space to smaller subsets (e.g. wordlists, numbers), identifying patterns in operations
Computer Ciphers: Strategies
- Substitution: Replacing symbols with other symbols
- Monoalphabetic: one substitution alphabet
- Polyalphabetic: multiple substitution alphabets
- Polyphonic: using more than one symbol for substitution.
- Stream ciphers: Mixing data streams (e.g. each bit is XORed with a keystream bit)
- Block ciphers: Working with fixed-length blocks of data using substitution and permutation.
- Monoalphabetic block ciphers
- Polyphonic block ciphers
Computer Ciphers: Symmetirc/Asymmetric
- Symmetric: Use the same key for encryption and decryption.
- Asymmetric: Use different keys for encryption and decryption.
- Key pair for asymmetric cryptography
- A public and a private component
- A private component can be used to produce a public component
- Key pair for asymmetric cryptography
Techniques Used by Ciphers
- Confusion: Complex relationship between the key, plaintext, and ciphertext.
- Diffusion: Dissipating plaintext statistics in the ciphertext. The changes will not be predictable.
(Symmetric) Stream Ciphers: Examples
- A5/1, A5/2 are used in cellular communications.
- RC4 is used in Wi-Fi communications.
- Salsa20, Chacha20, etc, are other common stream ciphers.
(Symmetric) Stream Ciphers: Approach
- Use a cryptographically secure, pseudo-random bit generator.
- This generator produces the keystream. A key and an initialization vector are used to control the stream generator.
Stream Ciphers: Exploitation Considerations
- No two messages should be encrypted with the same key and IV.
- Keystreams can be periodic.
- Ciphertexts are deterministically manipulated.
- Integrity control is crucial to detect manipulation of the ciphertext.
Symmetric Block Ciphers: Examples
- DES: Input/output 64-bit blocks; key: 56 bits. Deprecated.
- AES: Input/output 128-bit blocks; key: 128, 192, or 256 bits. Widely used in applications.
- Other include IDEA, CAST, Twofish, Blowfish, RC5, RC6, Kasumi.
Symmetric Block Ciphers: Approach
- Use a pipeline of transformation rounds
- Each round adds confusion and diffusion
- Each round is usually controlled by a subkey (aka key schedule)
- Rounds need to be reversable
- Standard structures are used like Feistel Networks and Substitution-Permutation Networks.
Substitution-Permutation Network (SPN)
- S-Boxes: Tables mapping input to output values (usually reversible), often key-dependent
- P-Boxes: Restructure the bit positions without changing the bit values.
AES Algorithm
- Input (128 bits), output (128 bits) with transformations.
- Round keys (128/192/256 bits) based on the key size, and several rounds of transformation.
- Encryption rounds that use AddRoundKey, SubBytes, ShiftRows, and MixColumns to scramble the data
- Rounds to perform decryption that do the inverse of encryption.
AES in CPU Instruction Sets
- AES-NI (Intel): Instructions for AES encryption/decryption, key generation, ...
Cipher Modes: Electronic Code Book (ECB)
- Encrypts each block independently.
- Block patterns in the plaintext are clearly exposed in the ciphertext.
Cipher Modes: Cipher Block Chaining (CBC)
- Combines with the previous block's ciphertext.
- More secure than ECB, but introduces a dependency, reducing parallelism opportunities.
- Uses an initialization vector for the first block.
ECB/CBC Cipher Modes: Contents Not Block-aligned
- Padding is required for incomplete blocks (e.g., PKCS#7).
- Padding introduces additional overhead.
Stream Cipher Modes
- Used with a Pseudo-random generator that creates a keystream.
- Output Feedback (OFB), Ciphertext Feedback (CFB), and Counter (CTR) modes for stream ciphers.
- Output Feedback (OFB), Ciphertext Feedback (CFB), and Counter (CTR) modes for stream ciphers.
Stream Ciphers Modes: Galois/Counter Mode (GCM)
- Use counter mode to encrypt messages.
- Incorporates an authentication tag for security.
Cipher Modes Comparison
- ECB: Simple, but vulnerable to patterns.
- CBC: More secure, sequential and non-parallel processing.
- OFB, CFB, CTR: Stream ciphers, more secure than ECB
Cipher Modes: Multiple Encryption
- Triple encryption (3DES) provides added security compared to single encryption, but at a cost.
- DESX offers better speed compared to 3DES by using extra keys.
Digests, Integrity Control, and Key Derivation
- Digest functions produce summaries, have fixed size, and depend on entire data set.
- Message integrity codes provide data integrity assurance and require a secret key.
- Key derivation functions transform input data into a key with specific properties.
Digest Functions: Approaches
- Merkle-Damgård Structure: Iterative compression function
- Sponge Functions: Absorbs data, then squeezes bits for digest
Message Integrity Code (MIC)
- MIC=H(T).
- Used to protect data from modification.
Message Authentication Code (MAC)
- Used with a key K (shared secret between initiator and receiver)
- MAC=H(T, K)
- Requires a secret key, providing better security than simple MIC.
Key derivation
- Password based Key Derivation Functions (PBKDF2 and scrypt)
- Techniques to make key derivation computationally harder.
Security Domains
- Boundaries separating systems and operations for security reasons.
- Example is like castle walls in a drawing.
Security Policies
- Guidelines within a security domain.
- Defining which activities are allowed and not allowed.
Security Mechanisms
- Procedures, technical tools, and policies that secure resources.
- Mechanisms are used to implement policies, and secure the resources within a domain.
Core Concepts: domains, policies, mechanisms, and controls
- Organizational domains for defining security scope.
- Security policies for defining what's allowed or not allowed.
- Security mechanisms to implement these policies, such as authentication or access control.
- Security controls which are the processes and techniques used for security management.
Security in Computing Systems
- Systems' complexity.
- Attacker's capabilities.
- User behavior.
Threat Research
- Techniques, tools for analyzing and defending against threats.
- Gathering information like open source intelligence, reverse engineering, etc.
- Using graphs for threat analysis and relations between threats.
MITRE Att&ck Matrix
- Knowledge base for adversary tactics and techniques.
SOC - Security Operations Center
- Continuous system monitoring detects security threats.
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