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Difficulty: Very hardCIA Triad and Non-Repudiation

A healthcare analytics platform receives automated diagnostic telemetry from remote clinics. To comply with regulatory standards, the platform must verify that incoming data is unaltered during transit and ensure that sending clinics cannot deny originating a record. An engineer proposes applying a keyed-Hash Message Authentication Code (HMAC) using a symmetric key shared exclusively between each clinic and the platform. Which of the following security goals is compromised under this implementation when presenting log evidence to an independent third-party auditor?

  1. Non-repudiation, because symmetric key shared possession allows either the sender or the recipient to generate valid authentication tags.Answer
  2. B
    Data integrity, because cryptographic hashing mechanisms only protect against accidental transmission errors rather than intentional tampering.
  3. C
    Confidentiality, because computing a message digest exposes plaintext payload data during network transit.
  4. D
    Mutual authentication, because pre-shared keys are incapable of verifying endpoint identity during connection establishment.

Answer

Non-repudiation is compromised because symmetric key shared possession allows either the sender or the recipient to generate valid authentication tags.
Non-repudiation mandates that a sender cannot deny creating or transmitting a message. Because an HMAC relies on a shared symmetric secret key possessed by both parties, either the sender or the receiver can calculate a valid hash digest. Consequently, an independent third-party auditor cannot determine which party generated the payload, making symmetric HMAC insufficient for non-repudiation. Digital signatures using asymmetric private keys are required for non-repudiation.

Step-by-Step Solution

1
Identify the required security objectives in the scenario.
The system requires both data integrity (ensuring records are unaltered) and non-repudiation (proof of origin that cannot be disowned before a third-party auditor).
Regulatory compliance mandates verifying message integrity as well as unambiguous attribution of data sources.
2
Analyze the properties of the proposed cryptographic control (HMAC).
HMAC utilizes symmetric shared secret keys known to both the transmitting clinic and the central platform.
Symmetric cryptography allows all keyholders to encrypt, decrypt, or sign data using the identical key.
3
Determine which security pillar fails under third-party audit conditions.
Because the receiving platform also holds the symmetric key, it could synthesize valid HMAC tags itself. An auditor cannot verify whether the clinic or the platform generated the record, thereby violating non-repudiation.
Non-repudiation demands asymmetric digital signatures where only the originator holds the private key.

Key Concept

Distinction between symmetric HMAC (integrity and symmetric authentication) and asymmetric digital signatures (non-repudiation)
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CIA Triad and Non-Repudiation

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