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Message Authentication Codes (MACs)↑

Message Authentication Codes (MACs) are essential tools in cryptography for ensuring the integrity and authenticity of data in secure communications. A MAC is generated by processing a message and a secret key through a cryptographic function, producing a fixed-length tag. This tag enables the recipient to verify the message’s authenticity by detecting unauthorized modifications and confirming that it originated from a trusted source with access to the shared secret key.

Using secure MAC constructions such as HMAC1 and MAC modes like CMAC2 helps prevent unauthorized data modification. Weak MAC implementations can expose vulnerabilities such as data manipulation, unauthorized access, and compliance violations.

By following best practices—such as adopting standardized MAC algorithms, managing cryptographic keys securely, and ensuring protocols are up-to-date—organizations can significantly reduce their exposure to security risks. Secure MAC implementation helps prevent unauthorized message modification and data corruption in communication protocols like TLS, IPsec, and SSH, and supports compliance with security standards and regulations.

Introduction to MACs↑

Message Authentication Codes (MACs) play a vital role in secure cryptographic systems. They work by applying a cryptographic function—typically a hash function (HMAC) or block cipher (CMAC)—to a combination of a message and a secret key, generating a tag that allows recipients to verify message authenticity and integrity.

MACs are used in secure communication protocols such as IPsec (for secure network communication), SSH (for secure remote access), and TLS 1.3 (for secure web browsing) to protect message authentication and integrity. In TLS 1.3, MACs are also used within key derivation and handshake authentication. Common MAC constructions include HMAC, CMAC, GMAC, and Poly1305.

Improper MAC implementation—such as using deprecated algorithms, mismanaging keys, or introducing side-channel vulnerabilities—can weaken data security. Side-channel vulnerabilities leak information through the physical implementation of a cryptographic system, such as power consumption or timing. This allows attackers to manipulate messages or compromise authentication mechanisms. Ensuring proper MAC implementation is crucial for secure data transmission, protecting sensitive information, and supporting compliance with cryptographic security standards.

Risk↑

The severity of insecure MAC implementations varies depending on the context. Vulnerabilities stem from incorrect usage, poorly designed constructions, and insecure implementation.

Several attacks exploit timing irregularities, allowing attackers to compromise data integrity or authenticity. Other risks include improper key management, using obsolete algorithms, or failure to enforce strict verification, such as constant-time comparison or proper nonce handling. These weaknesses can make MACs ineffective against forgery or tampering, enabling attackers to bypass integrity checks, alter data, or expose sensitive information.

CBC-MAC, when used with variable-length messages, is vulnerable to forgery attacks. While it provides strong security for fixed-length inputs, variable-length messages introduce weaknesses that attackers can exploit. NIST acknowledges these flaws in SP 800-38B3 and recommends CMAC as a secure alternative. Practical attacks exploiting these issues have been demonstrated in TLS and DTLS, with some known since 2002.

Some of these attacks exploit predictable patterns between message blocks, bypassing integrity checks and enabling tampering or forgery. Attacks such as these emphasize the importance of addressing timing irregularities and adhering to robust cryptographic standards. While such attacks often require specialized expertise and precise timing measurements, they can severely impact systems with flawed implementations. To mitigate these risks, organizations should adopt recommended MAC algorithms, such as HMAC-SHA256 or AES-CMAC, implement modern protocols like TLS 1.3, and rely on well-tested cryptographic libraries designed to prevent such vulnerabilities.

Security recommendations↑

To mitigate risks associated with insecure MAC implementations, follow these straightforward recommendations:

  1. Adopt Standardized and Secure MAC Algorithms: Use recommended algorithms like HMAC-SHA256 or AES-CMAC, which are supported by cryptographic standards and provide strong security guarantees.
  2. Avoid Deprecated and Weak Algorithms: Discontinue the use of insecure algorithms, such as MD5-based or SHA1-based HMAC, or raw CBC-MAC without additional safeguards. These algorithms are vulnerable to known attacks and should not be used in modern systems.

The use of HMAC and CMAC are recommended in NIST SP 800-224, amongst others. NIST-approved versions of HMAC are listed in Table 2 in NIST SP 800-224, which also approves versions of CMAC using AES. By following these recommendations, organizations can effectively address the risks associated with insecure MAC usage while maintaining a practical and manageable approach.

Sources↑

  1. HMAC is specified in FIPS 198-1, which is currently being converted into a NIST Special Publication 800-224. NIST SP 800-224 is in an initial public draft state, and FIPS 198-1 will be withdrawn once the final version of NIST SP 800-224 is published. ↩

  2. CMAC is a block cipher mode specified in NIST SP 800-38B, “Recommendation for Block Cipher Modes of Operation: the CMAC Mode for Authentication”. ↩

  3. Section 3, Introduction, page 1. ↩