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Difficulty: Very hardCIA Triad and Non-Repudiation

An enterprise API gateway receives automated configuration updates from external vendors. To satisfy auditing requirements, security policies dictate that the system must establish indisputable proof of origin for every payload, ensuring vendors cannot deny sending a specific update. The system architects propose utilizing SHA-256 hashing paired with pre-shared symmetric keys (HMAC) between the gateway and each vendor. Which security goal remains unfulfilled by this proposed architecture, and why?

  1. Non-repudiation, because symmetric shared keys allow either the sender or receiver to generate a valid message authentication code, preventing third-party proof of origin.Answer
  2. B
    Integrity, because hashing with shared keys verifies network transmission paths rather than payload consistency.
  3. C
    Authorization, because cryptographic message digests assign system privileges rather than validating payload authenticity.
  4. D
    Confidentiality, because asymmetric encryption algorithms are required to obfuscate data contents during transit.

Answer

Non-repudiation remains unfulfilled because symmetric pre-shared keys allow either party possessing the key to create a valid HMAC, preventing proof of payload origin to a third-party auditor.
Non-repudiation requires unforgeable proof of origin that can be verified by a third party. Because symmetric HMAC relies on a secret key known to both the sender and the receiver, either party can compute the valid hash. Therefore, the vendor can plausibly claim the receiver generated the hash, failing non-repudiation. Asymmetric digital signatures (private key signing) are required to achieve non-repudiation.

Step-by-Step Solution

1
Analyze the security requirement stated in the scenario.
The scenario requires indisputable proof of origin so vendors cannot deny sending a specific payload (non-repudiation).
Non-repudiation provides legal or technical proof that a specific sender created and transmitted a message.
2
Evaluate the proposed cryptographic mechanism (SHA-256 HMAC with pre-shared symmetric keys).
Symmetric HMAC keys are known to both the vendor (sender) and the enterprise API gateway (receiver).
Because both parties hold the identical secret key, either side could theoretically generate the matching HMAC digest.
3
Determine which security pillar is missing.
While HMACs satisfy integrity and data origin authentication between two mutually trusting endpoints, they fail to provide non-repudiation to an external auditor.
True non-repudiation requires asymmetric cryptography (digital signatures using private keys) where only the sender possesses the signing key.

Key Concept

Non-repudiation vs. Integrity in Cryptographic Mechanisms
Estimated Time:1m 30s
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Topics

CIA Triad and Non-Repudiation

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