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Difficulty: HardCryptographic Concepts and Algorithms

A cloud compliance team is configuring an automated log aggregation vault to collect audit trails from independent third-party SaaS vendors. The organization mandates two primary security requirements for all ingested logs: first, the vault must be able to prove that a log file was not modified after creation; second, the log file must provide non-repudiation, ensuring that an untrusted vault administrator who possesses read access cannot forge a valid log entry on behalf of any vendor. Which of the following cryptographic techniques should each vendor apply to the log files prior to transmission to meet all audit requirements?

  1. A
    Compute a SHA-256 cryptographic hash digest of the log file and append the resulting hash to the header of the log prior to transmission.
  2. Sign the log file using the vendor's private key to generate an asymmetric digital signature appended to the payload.Answer
  3. C
    Generate a Hash-based Message Authentication Code (HMAC-SHA256) tag using a pre-shared secret key shared between the vendor and the vault administrator.
  4. D
    Encrypt the log file using the central vault's public RSA key prior to sending the payload over the network.

Answer

The correct technique is signing the log file using the vendor's private key to generate an asymmetric digital signature.
Generating an asymmetric digital signature by hashing the log file and encrypting the hash digest with the vendor's private key guarantees both integrity and non-repudiation. Anyone can verify the signature using the vendor's public key, but no one—including the untrusted vault administrator—can forge a valid signature without access to the vendor's private key.

Step-by-Step Solution

1
Analyze the security requirements specified in the scenario
Identified the need for both data integrity (detecting post-creation modifications) and non-repudiation (preventing an untrusted vault administrator with key access from forging log entries).
Non-repudiation requires asymmetric cryptography where only the producing entity possesses the signing key.
2
Evaluate symmetric vs. asymmetric cryptographic primitives for proof of origin
Symmetric primitives (like HMAC with shared keys) allow any key holder to generate valid tags, failing non-repudiation. Asymmetric digital signatures (private key signing, public key verification) ensure only the private key owner can produce valid signatures.
The vault administrator cannot forge signatures without the vendor's private key.
3
Select the option fulfilling both integrity and non-repudiation
Digitally signing the log payload using the vendor's private key fulfills all stated audit and security criteria.
Verification using the vendor's public key confirms both that the file was unchanged and that it originated from that specific vendor.

Key Concept

Digital Signatures and Non-Repudiation
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Topics

Cryptographic Concepts and Algorithms

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