- A
The public key used to verify the signature does not match the private key used to sign
Signature verification requires the matching public key. If the wrong key is configured, verification will fail.
- B
The image is not signed at all
Why wrong: The error says 'signature verification failed', which implies a signature was found but didn't validate.
- C
The webhook is not reachable
Why wrong: If the webhook is not reachable, the error would be about communication failure, not signature verification.
- D
The image tag is incorrect
Why wrong: Incorrect tag would cause a different error (e.g., image pull failure), not signature verification failure.
Question 524 of 997
Cosign Signature Verification Failure — Public Key Mismatch
This CKS practice question tests your understanding of supply chain security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A cluster administrator notices that a pod using an image from a public registry is failing to start. The image was signed with Cosign, and the cluster has an ImagePolicyWebhook configured to require signatures. The error message from the webhook indicates 'signature verification failed'. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The public key used to verify the signature does not match the private key used to sign
The error 'signature verification failed' indicates that the webhook attempted to verify the image's signature but the cryptographic check did not pass. Since the image was signed with Cosign, the most likely cause is that the public key configured in the ImagePolicyWebhook does not correspond to the private key used to sign the image. Cosign uses public-key cryptography (typically ECDSA-P256 or RSA) where the signature can only be verified with the matching public key.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The public key used to verify the signature does not match the private key used to sign
Why this is correct
Signature verification requires the matching public key. If the wrong key is configured, verification will fail.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The image is not signed at all
Why it's wrong here
The error says 'signature verification failed', which implies a signature was found but didn't validate.
- ✗
The webhook is not reachable
Why it's wrong here
If the webhook is not reachable, the error would be about communication failure, not signature verification.
- ✗
The image tag is incorrect
Why it's wrong here
Incorrect tag would cause a different error (e.g., image pull failure), not signature verification failure.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The CKS exam often tests the distinction between 'signature missing' and 'signature verification failed' — candidates mistakenly think any signature error means the image is unsigned, but the error message itself points to a key mismatch, not absence of a signature.
Detailed technical explanation
How to think about this question
Cosign signatures are stored as base64-encoded payloads in an OCI artifact (e.g., `.sig` tag) alongside the image. The ImagePolicyWebhook uses a configured public key (often stored in a Kubernetes Secret) to verify the signature against the image digest. A common subtlety is that the public key must be in PEM format and match the exact key algorithm used during signing; mismatched key types (e.g., RSA vs ECDSA) will also cause verification failure.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the CKS exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
- →
Supply Chain Security — study guide chapter
Learn the concepts, then practise the questions
- →
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Targeted practice on this topic area only
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Certified Kubernetes Security Specialist CKS study guide
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FAQ
Questions learners often ask
What does this CKS question test?
Supply Chain Security — This question tests Supply Chain Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The public key used to verify the signature does not match the private key used to sign — The error 'signature verification failed' indicates that the webhook attempted to verify the image's signature but the cryptographic check did not pass. Since the image was signed with Cosign, the most likely cause is that the public key configured in the ImagePolicyWebhook does not correspond to the private key used to sign the image. Cosign uses public-key cryptography (typically ECDSA-P256 or RSA) where the signature can only be verified with the matching public key.
What should I do if I get this CKS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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Last reviewed: Jul 4, 2026
This CKS practice question is part of Courseiva's free CNCF certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CKS exam.
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