3.1 Symmetric-key cryptography and block ciphers

Symmetric-key cryptography is the backbone of secure communication. It uses a single secret key for both encryption and decryption, making it fast and efficient for protecting large amounts of data. However, key management is crucial for maintaining security.

Block ciphers are a fundamental component of symmetric-key cryptography. They operate on fixed-size blocks of data, using principles like confusion and diffusion to ensure security. Various modes of operation allow block ciphers to encrypt messages of any length securely.

Symmetric-key cryptography principles

Fundamentals of symmetric-key cryptography

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• Symmetric-key cryptography uses a single secret key for both encryption and decryption, which is shared between the communicating parties
• The security of symmetric-key cryptography relies on the secrecy of the shared key, requiring a secure key exchange mechanism (Diffie-Hellman key exchange)
• Symmetric-key algorithms are generally faster and more efficient than asymmetric-key algorithms, making them suitable for encrypting large amounts of data
• Common applications of symmetric-key cryptography include secure communication, data storage, and message authentication (HTTPS, full disk encryption)

Key management in symmetric-key cryptography

• Key management is a critical aspect of symmetric-key cryptography, involving key generation, distribution, storage, and rotation
• Symmetric-key cryptography is vulnerable to key compromise, requiring frequent key updates and secure key storage
• Secure key exchange mechanisms, such as key exchange protocols or out-of-band methods, are essential for establishing shared keys between parties
• Key derivation functions can be used to derive multiple keys from a single master key, simplifying key management
• Proper key storage techniques, such as hardware security modules or secure enclaves, help protect keys from unauthorized access

Block cipher security properties

Confusion and diffusion principles

• The security of block ciphers relies on the concept of confusion and diffusion, which obscure the relationship between the plaintext, key, and ciphertext
• Confusion involves complex substitution operations that make the relationship between the key and ciphertext as complex as possible
• Diffusion spreads the influence of each plaintext or key bit over as much of the ciphertext as possible, hiding statistical properties
• The combination of confusion and diffusion ensures that small changes in the plaintext or key result in significant changes in the ciphertext (avalanche effect)

Cryptanalytic attacks and security evaluation

• Block ciphers are susceptible to various cryptanalytic attacks, such as differential cryptanalysis, linear cryptanalysis, and related-key attacks
• Differential cryptanalysis exploits the differences in plaintext pairs and their corresponding ciphertext pairs to deduce information about the key
• Linear cryptanalysis approximates the block cipher as a linear function and exploits biases in the plaintext and ciphertext to recover the key
• Related-key attacks exploit the relationships between keys to break the cipher when the attacker can obtain encryptions under different related keys
• The security of block ciphers is often evaluated using the number of rounds, key size, and the strength of the underlying mathematical properties
• Increasing the number of rounds and using larger key sizes enhance the security of block ciphers against cryptanalytic attacks

Limitations and padding techniques

• Block ciphers have limitations in handling plaintext that is not a multiple of the block size, requiring padding or ciphertext stealing techniques
• Padding schemes, such as PKCS#7, add extra bytes to the plaintext to make it a multiple of the block size before encryption
• Ciphertext stealing techniques, such as CBC ciphertext stealing, avoid the need for padding by using the last incomplete block of plaintext to encrypt the second-to-last ciphertext block
• Improper padding or padding oracle attacks can lead to vulnerabilities if not implemented correctly

Block cipher modes of operation

Common modes and their characteristics

• Block cipher modes of operation define how a block cipher is repeatedly applied to securely encrypt and decrypt messages longer than the block size
• Electronic Codebook (ECB) mode encrypts each block independently, which is fast but vulnerable to pattern recognition and replay attacks
• Cipher Block Chaining (CBC) mode XORs each plaintext block with the previous ciphertext block before encryption, providing better security but requiring an initialization vector (IV)
• Counter (CTR) mode combines a nonce and a counter to generate a keystream, which is XORed with the plaintext, allowing parallel processing and random access
• Galois/Counter Mode (GCM) provides both confidentiality and authentication, using a combination of CTR mode and Galois field multiplication for generating an authentication tag

Choosing the appropriate mode

• The choice of block cipher mode depends on the specific security requirements, performance considerations, and the presence of padding or message authentication
• ECB mode is suitable for encrypting small, independent messages or for creating deterministic ciphertexts for searchable encryption
• CBC mode is commonly used for encrypting long messages or when the plaintext needs to be processed sequentially
• CTR mode is advantageous for parallel processing, random access, or when the plaintext length is not known in advance
• GCM mode is widely used in secure communication protocols (TLS) due to its authenticated encryption properties

Implementation considerations and vulnerabilities

• Proper implementation of block cipher modes is crucial to avoid vulnerabilities such as padding oracle attacks or IV reuse
• Initialization vectors (IVs) must be unique and unpredictable for each message to prevent vulnerabilities like the "two-time pad" attack
• Padding oracle attacks exploit the behavior of the decryption process when invalid padding is encountered, allowing an attacker to decrypt data without knowing the key
• Nonce reuse in CTR or GCM modes can lead to the exposure of the XOR of plaintext blocks, compromising confidentiality
• Careful management of nonces, IVs, and authentication tags is essential to maintain the security of block cipher modes

Symmetric-key algorithms

Commonly used algorithms

• Data Encryption Standard (DES) is a legacy block cipher with a 56-bit key, which is considered insecure due to its small key size
• Triple DES (3DES) applies DES three times with different keys, providing increased security but slower performance compared to modern algorithms
• Advanced Encryption Standard (AES) is a widely adopted block cipher with key sizes of 128, 192, or 256 bits, offering strong security and efficient hardware and software implementations
• Blowfish is a fast block cipher with a variable key size up to 448 bits, known for its simplicity and effectiveness in software implementations
• Twofish is a successor to Blowfish, with a block size of 128 bits and key sizes up to 256 bits, designed to be efficient on a wide range of platforms

Stream ciphers vs block ciphers

• Stream ciphers, such as RC4 or Salsa20, encrypt plaintext bit by bit or byte by byte, generating a pseudorandom keystream for XORing with the plaintext
• Stream ciphers have a lower latency and are suitable for real-time applications or when the plaintext length is not known in advance (video streaming)
• Block ciphers operate on fixed-size blocks and are more versatile, supporting various modes of operation for different security and functionality requirements
• Block ciphers are generally more secure and widely used compared to stream ciphers, which have been shown to have vulnerabilities (RC4 biases)

Algorithm selection considerations

• The choice of symmetric-key encryption algorithm depends on factors such as security requirements, performance, key size, and compatibility with existing systems
• Security considerations include the algorithm's resistance to known attacks, the strength of its underlying mathematical properties, and the key size
• Performance factors include the speed of encryption and decryption, memory usage, and suitability for hardware or software implementations
• Key size should be chosen based on the desired level of security and the expected lifetime of the protected data
• Compatibility with existing systems, standards, and protocols is important for interoperability and ease of integration