Glossary

5.3 Message authentication codes (MACs)

5 min read•august 15, 2024

Message Authentication Codes (MACs) are crucial tools in cryptography, ensuring data and . They generate fixed-size tags from messages and secret keys, protecting against tampering and verifying sender identity. MACs are essential in secure communication protocols and financial transactions.

MACs differ from hash functions and digital signatures in key ways. While hash functions only provide integrity, MACs offer both integrity and authentication. Digital signatures add non-repudiation. MACs are faster and use symmetric keys, making them ideal for high-speed communication when parties can share secrets securely.

Message Authentication Codes: Purpose and Concept

Ensuring Data Integrity and Authentication

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  • Message Authentication Codes (MACs) verify integrity and authenticity of messages transmitted over insecure channels
  • Generate fixed-size tag derived from message and secret key, appended to message before transmission
  • Detect unauthorized modifications to message during transmission, ensuring data integrity
  • Provide authentication by verifying message sent by claimed sender possessing shared secret key
  • MAC generation process typically one-way function, computationally infeasible to recover original message or key from MAC tag
  • Protect against active attacks (message tampering, insertion, deletion, replay attacks)
  • Do not provide confidentiality, often used with encryption for both integrity and confidentiality

MAC Generation and Verification Process

  • MAC algorithm takes message and secret key as inputs
  • Produces fixed-length output called MAC tag or
  • Sender appends MAC tag to original message before transmission
  • Receiver uses same secret key and MAC algorithm to recompute tag from received message
  • Compares recomputed tag with received tag to verify integrity and authenticity
  • If tags match, message considered authentic and unaltered
  • Mismatch indicates potential tampering or transmission errors

Applications and Use Cases

  • Secure communication protocols (TLS, IPsec)
  • Protect integrity of stored data (file systems, databases)
  • Authenticate financial transactions (banking systems, cryptocurrencies)
  • Ensure integrity of software updates and patches
  • Verify authenticity of IoT device communications
  • Secure message passing in distributed systems
  • Protect integrity of digital signatures in some schemes

MACs vs Hash Functions vs Digital Signatures

Comparison of Security Properties

  • MACs and hash functions produce fixed-size outputs, MACs incorporate secret key while hash functions do not
  • Hash functions provide integrity but not authentication, MACs offer both integrity and authentication
  • Digital signatures provide integrity, authentication, and non-repudiation, MACs only offer first two properties
  • MACs typically faster to compute and verify compared to digital signatures, suitable for high-speed communication
  • Hash functions used in various applications beyond security (data indexing, fingerprinting), MACs and digital signatures primarily for security purposes
  • Digital signatures use asymmetric cryptography allowing public verification, MACs use symmetric cryptography requiring shared secret keys
  • MACs preferred when parties can securely share secret key, digital signatures used when public verification necessary or key distribution challenging

Key Differences in Implementation

  • Hash functions:
    • No secret key required
    • Same input always produces same output
    • Examples: , SHA-3, Blake2
  • MACs:
    • Require shared secret key
    • Same input with different keys produces different outputs
    • Examples: -SHA256, -
  • Digital Signatures:
    • Use public-private key pair
    • Signature generation requires private key, verification uses public key
    • Examples: RSA signatures, ECDSA, EdDSA

Use Case Scenarios

  • Hash functions:
    • Password storage (salted and hashed)
    • Data deduplication in storage systems
    • Blockchain proof-of-work (Bitcoin mining)
  • MACs:
    • API authentication (HMAC for request signing)
    • Secure cookies in web applications
    • Authenticating packets in network protocols (IPsec)
  • Digital Signatures:
    • Signing software releases and updates
    • Authenticating SSL/TLS certificates
    • Non-repudiable electronic documents (legal contracts)

Applying MAC Algorithms: HMAC and CMAC

HMAC (Hash-based Message Authentication Code)

  • Widely used MAC algorithm combining cryptographic hash function with secret key
  • HMAC construction involves two rounds of hashing, using inner and outer padded keys
  • Provides additional security against length extension attacks
  • General form: HMAC(K,m)=H((K′⊕opad)∥H((K′⊕ipad)∥m))HMAC(K, m) = H((K' \oplus opad) \| H((K' \oplus ipad) \| m))
    • H: underlying hash function
    • K: secret key
    • K': derived key
    • opad, ipad: outer and inner padding constants
  • Common implementations:
    • HMAC-SHA256
    • HMAC-SHA3
    • HMAC-Blake2
  • Used in various protocols and standards (TLS, IPsec, OAuth)

CMAC (Cipher-based Message Authentication Code)

  • Block cipher-based MAC algorithm using symmetric key cipher in CBC mode to generate MAC tag
  • Addresses weaknesses in older CBC-MAC schemes
  • Incorporates special key derivation process
  • Uses different keys for last block based on message length
  • General form: CMAC(K,m)=MSBt(Cn)CMAC(K, m) = MSBt(Cn)
    • K: secret key
    • m: message
    • Cn: final block cipher output
    • MSBt: most significant t bits
  • Common implementations:
    • CMAC-AES
    • CMAC-3DES
  • Suitable for constrained environments (embedded systems, smart cards)

MAC Generation and Verification Process

  • Generating MAC:
    1. Choose appropriate MAC algorithm (HMAC or CMAC)
    2. Input message and secret key into chosen algorithm
    3. Obtain resulting authentication tag
    4. Append tag to original message for transmission
  • Verifying MAC:
    1. Separate received message and MAC tag
    2. Recompute MAC using received message and shared secret key
    3. Compare recomputed MAC with received MAC tag
    4. Accept message as authentic if MACs match, reject otherwise

Security Analysis of MAC Schemes

Attack Vectors and Vulnerabilities

  • Key recovery attacks aim to discover secret key used in MAC generation
  • Forgery attacks attempt to create valid MAC tags for unauthorized messages without knowing secret key
  • Length extension attacks mitigated in properly designed MAC schemes (HMAC)
  • Collision attacks on underlying hash function or block cipher may weaken MAC security
  • Birthday attack exploits probability of collisions in hash functions, applicable to some MAC schemes
  • Timing attacks and side-channel attacks potentially reveal information about secret key

Cryptanalysis Techniques

  • Differential cryptanalysis: analyzes differences in MAC outputs for related inputs
  • Linear cryptanalysis: exploits linear approximations of the MAC algorithm
  • Algebraic attacks: attempt to express MAC algorithm as system of equations to solve for key
  • Meet-in-the-middle attacks: applicable to some multi-key MAC constructions
  • Slide attacks: exploit self-similarity in key schedule of underlying cipher (for CMAC)

Security Considerations and Best Practices

  • Use cryptographically strong random number generators for key generation
  • Implement secure key management practices (regular key rotation, secure storage)
  • Choose appropriate key lengths based on security requirements (128 bits minimum for symmetric keys)
  • Use well-vetted, standardized MAC algorithms (HMAC, CMAC) instead of custom designs
  • Ensure MAC tag length is sufficient to prevent brute-force attacks (at least 128 bits)
  • Implement constant-time comparison for MAC verification to prevent timing attacks
  • Use different keys for different purposes (MAC key separate from encryption key)
  • Consider combining MAC with encryption in authenticated encryption schemes (GCM, ChaCha20-Poly1305)

Key Terms to Review (16)

AES: AES, or Advanced Encryption Standard, is a symmetric encryption algorithm widely used for securing data through encryption and decryption processes. It plays a vital role in modern cryptography by providing robust security for sensitive information, ensuring confidentiality and integrity across various applications.
Authentication tag: An authentication tag is a short piece of information generated during the encryption process that ensures the integrity and authenticity of a message. It acts as a digital fingerprint, verifying that the message has not been altered in transit and confirming the identity of the sender. Authentication tags are crucial for secure communication, particularly in systems using message authentication codes (MACs) to provide validation and protect against tampering.
Authenticity: Authenticity in cryptography refers to the assurance that a message, data, or transaction is genuine and can be verified as coming from the stated source. This concept is crucial in ensuring that information has not been altered and that it originates from a trusted sender, preventing impersonation or forgery. Authenticity connects to various cryptographic mechanisms that ensure that communications and transactions maintain integrity and trustworthiness.
Block cipher modes: Block cipher modes are techniques used to encrypt and decrypt data in fixed-size blocks, enhancing the security and usability of block ciphers. These modes determine how the plaintext is divided into blocks and how each block is processed with respect to previous blocks, which can provide different security properties like confidentiality and integrity. Understanding these modes is crucial for implementing secure cryptographic systems, especially in the context of ensuring that messages remain authentic and unaltered.
CMAC: CMAC, or Cipher-based Message Authentication Code, is a type of message authentication code that uses a block cipher to provide authenticity and integrity for a message. By combining a secret key with the message and processing it through a block cipher, CMAC generates a unique tag that can verify that the message has not been altered and indeed comes from the claimed sender. This mechanism is crucial for ensuring secure communications in various applications.
Collision attack: A collision attack is a method used in cryptography to find two different inputs that produce the same output hash value, thereby compromising the integrity of the hashing function. This type of attack exploits the fact that hash functions have a finite output size, which means that there are more possible inputs than outputs, leading to potential collisions. If an attacker can create a collision, it undermines the reliability of digital signatures and message authentication codes, making them susceptible to forgery and manipulation.
Data integrity verification: Data integrity verification is the process of ensuring that data remains accurate, consistent, and reliable over its lifecycle. It involves techniques and methods used to confirm that data has not been altered or corrupted during storage, transmission, or processing, thus maintaining trust in the information being used.
Forgery attack: A forgery attack is an attempt by an unauthorized entity to create a false representation of a legitimate entity's signature or message in order to gain access, mislead, or commit fraud. In the realm of message authentication codes (MACs), forgery attacks challenge the integrity and authenticity of messages by manipulating or replicating valid MACs to deceive the recipient into believing that the forged message is legitimate.
Hash-based authentication: Hash-based authentication is a method of verifying the integrity and authenticity of a message or data using a hash function to generate a fixed-size output from variable-sized input data. This technique ensures that any change in the input will produce a different hash value, allowing for the detection of tampering or unauthorized modifications. It is often employed in conjunction with message authentication codes (MACs) to provide a secure way of confirming both the source and integrity of messages.
HMAC: HMAC, or Hash-based Message Authentication Code, is a specific type of message authentication code that combines a cryptographic hash function with a secret key. It ensures both the integrity and authenticity of a message by generating a unique code that can only be verified by someone who has access to the secret key. This makes HMAC an essential component in securing communications and verifying data integrity in various applications.
Integrity: Integrity in cryptography refers to the assurance that data has not been altered in an unauthorized manner. This concept is essential for maintaining trust in communications and data storage, ensuring that the information received is exactly what was sent. Integrity is achieved through various mechanisms like digital signatures, hash functions, and message authentication codes, which help verify that data remains unchanged during transmission or storage.
ISO/IEC 9797: ISO/IEC 9797 is a standard for message authentication codes (MACs) that specifies methods for data integrity and authenticity in cryptographic applications. It defines various MAC algorithms, particularly focusing on the construction and use of MACs to ensure that messages have not been altered and come from a verified source. This standard is crucial in enhancing the security of information systems by providing a way to validate the authenticity of messages.
Keyed hash function: A keyed hash function is a cryptographic mechanism that takes an input message and a secret key to produce a fixed-size output, known as a hash or message digest. This process ensures both the integrity and authenticity of the message, as the same input with the same key will always yield the same hash, allowing verification of data integrity while protecting against unauthorized tampering.
RFC 2104: RFC 2104 is a Request for Comments document that specifies the HMAC (Hash-based Message Authentication Code) algorithm, which combines cryptographic hash functions with secret keys to provide message integrity and authenticity. This standard helps ensure that data has not been tampered with during transmission, allowing two parties to verify the integrity of their messages securely.
Secure Communications: Secure communications refer to methods and protocols that protect information from unauthorized access during transmission. This is crucial in maintaining confidentiality, integrity, and authenticity of data exchanged between parties. Various cryptographic techniques, including encryption and digital signatures, are employed to ensure that sensitive information remains private and is not tampered with, making secure communications a fundamental aspect of modern digital interactions.
SHA-256: SHA-256 (Secure Hash Algorithm 256-bit) is a cryptographic hash function that produces a fixed-size 256-bit hash value from any input data, which is often used in various security applications and protocols. This function plays a critical role in ensuring data integrity, authentication, and security across various digital systems, particularly in blockchain technology, digital signatures, and secure communications.
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