- A
Use AWS KMS with automatic key rotation
Why wrong: This manages keys but does not enforce encryption.
- B
Enable S3 default encryption and set a bucket policy to deny PutObject without encryption headers
This ensures all objects are encrypted.
- C
Enable S3 default encryption only
Why wrong: Default encryption does not prevent clients from overriding with no encryption.
- D
Enable S3 Block Public Access
Why wrong: This prevents public access, not encryption.
Question 1,122 of 1,755
Enforcing S3 Encryption at Rest
This MLS-C01 practice question tests your understanding of data engineering. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 company is building a data lake on Amazon S3. They need to enforce encryption at rest for all objects. Which combination of actions will achieve this? (Assume the bucket is versioned.)
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
✓
Enable S3 default encryption and set a bucket policy to deny PutObject without encryption headers
Option B is correct because combining S3 default encryption with a bucket policy that denies PutObject requests lacking encryption headers ensures that every object stored in the bucket is encrypted at rest, even if the PutObject call does not include encryption parameters. Default encryption alone can be overridden by a client that explicitly sets encryption headers, but the bucket policy enforces encryption for all uploads, closing that loophole. This dual approach guarantees compliance with encryption-at-rest requirements for a versioned bucket.
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.
- ✗
Use AWS KMS with automatic key rotation
Why it's wrong here
This manages keys but does not enforce encryption.
- ✓
Enable S3 default encryption and set a bucket policy to deny PutObject without encryption headers
Why this is correct
This ensures all objects are encrypted.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Enable S3 default encryption only
Why it's wrong here
Default encryption does not prevent clients from overriding with no encryption.
- ✗
Enable S3 Block Public Access
Why it's wrong here
This prevents public access, not encryption.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume S3 default encryption alone is sufficient, but the exam tests the nuance that default encryption can be overridden by client-supplied headers, requiring a bucket policy to enforce encryption for all PutObject requests.
Detailed technical explanation
How to think about this question
Under the hood, S3 default encryption uses a bucket-level setting that automatically applies SSE-S3 or SSE-KMS to objects when no encryption header is present in the PutObject request. However, the S3 API allows clients to explicitly set the `x-amz-server-side-encryption` header to `AES256` or `aws:kms`, which overrides the default; a bucket policy with a `Deny` effect using the `s3:x-amz-server-side-encryption` condition key (e.g., `Null` or `StringNotEquals`) blocks such overrides. In real-world scenarios, this combination is critical for regulated industries like healthcare or finance, where auditors require proof that every object is encrypted at rest, regardless of how it was uploaded.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
Quick reference
AWS S3 Storage Class Comparison
| Storage Class | Min Duration | Retrieval | Use Case |
|---|---|---|---|
| S3 Standard | None | Immediate | Frequently accessed data |
| S3 Standard-IA | 30 days | Immediate | Infrequent access, rapid retrieval |
| S3 One Zone-IA | 30 days | Immediate | Non-critical infrequent data |
| S3 Intelligent-Tiering | None | Immediate–hours | Unknown or changing access patterns |
| S3 Glacier Instant | 90 days | Milliseconds | Archive with instant retrieval |
| S3 Glacier Flexible | 90 days | Minutes–hours | Archive, flexible retrieval |
| S3 Glacier Deep Archive | 180 days | Hours | Long-term compliance archive |
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this MLS-C01 question test?
Data Engineering — This question tests Data Engineering — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Enable S3 default encryption and set a bucket policy to deny PutObject without encryption headers — Option B is correct because combining S3 default encryption with a bucket policy that denies PutObject requests lacking encryption headers ensures that every object stored in the bucket is encrypted at rest, even if the PutObject call does not include encryption parameters. Default encryption alone can be overridden by a client that explicitly sets encryption headers, but the bucket policy enforces encryption for all uploads, closing that loophole. This dual approach guarantees compliance with encryption-at-rest requirements for a versioned bucket.
What should I do if I get this MLS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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