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Difficulty: MediumCIA Triad and Non-Repudiation

A cloud operations team configures an automated logging mechanism to record high-privilege configuration changes across production servers. The system computes a Hash-based Message Authentication Code (HMAC) for each log entry using a single symmetric key shared among all system administrators. Following an unauthorized system modification, an administrator denies executing the change, claiming that any user with access to the shared key could have forged the log entry. Which of the following security goals failed to be established by this logging design?

  1. Non-repudiationAnswer
  2. B
    Integrity
  3. C
    Confidentiality
  4. D
    Authorization

Answer

Non-repudiation is the security goal that failed to be established because shared symmetric keys cannot uniquely trace an action to a specific individual.
Non-repudiation provides indisputable proof of the origin and integrity of data such that the sender/creator cannot deny having performed the action. When symmetric keys are shared among multiple users (such as in standard HMAC implementations), any holder of the key can generate valid message authentication codes. As a result, individual attribution is lost, preventing the establishment of non-repudiation. Digital signatures utilizing asymmetric cryptography (where each user holds a unique private key) are required to achieve non-repudiation.

Step-by-Step Solution

1
Analyze the technical control used in the scenario.
The logging system uses a Hash-based Message Authentication Code (HMAC) with a shared symmetric key.
Symmetric cryptography uses the same key for generation and verification across all authorized parties.
2
Evaluate the security guarantees of the control.
HMAC provides data integrity and proof that someone possessing the secret key created the log, but it cannot identify which specific user signed it.
Because all system administrators possess the identical symmetric key, any administrator could have produced the valid signature.
3
Map the limitation to the corresponding core security principle.
Inability to prove individual accountability means non-repudiation is lost.
To achieve non-repudiation, asymmetric cryptography (such as private key digital signatures) must be used so that only one unique individual could have authored the entry.

Key Concept

Non-Repudiation vs Integrity in Cryptographic Controls
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Topics

CIA Triad and Non-Repudiation

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