- A
Configure the KMS key policy to allow the S3 service to use the key
The key policy must grant the S3 service principal permission to encrypt/decrypt.
- B
Enable default encryption on the S3 bucket with SSE-KMS
Default encryption ensures all objects are encrypted with KMS.
- C
Add a bucket policy that denies PutObject without encryption
Why wrong: This enforces encryption but does not set up the encryption itself.
- D
Enable encryption in transit using HTTPS for all S3 API calls
Why wrong: Encryption in transit is separate from encryption at rest.
- E
Use client-side encryption on all data before uploading
Why wrong: Client-side encryption is an alternative, not required if using server-side encryption.
Question 1,616 of 1,755
Enabling S3 Server-Side Encryption with AWS KMS
This MLS-C01 practice question tests your understanding of data engineering. 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 company is building a data lake on Amazon S3 and wants to ensure that data is encrypted at rest using AWS KMS. Which TWO actions are required to achieve this? (Choose TWO.)
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
✓
Configure the KMS key policy to allow the S3 service to use the key
Option A is correct because AWS KMS uses key policies to control access to the KMS key. For S3 to use a KMS key for server-side encryption (SSE-KMS), the key policy must grant the S3 service principal (or the bucket owner's account) the necessary permissions, such as kms:Encrypt and kms:Decrypt. Without this policy, S3 cannot access the key to encrypt or decrypt objects at rest. Option B is correct because enabling default encryption on the S3 bucket with SSE-KMS ensures that all objects uploaded to the bucket are automatically encrypted using the specified KMS key, meeting the requirement for encryption at rest.
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.
- ✓
Configure the KMS key policy to allow the S3 service to use the key
Why this is correct
The key policy must grant the S3 service principal permission to encrypt/decrypt.
Related concept
Read the scenario before looking for a memorised answer.
- ✓
Enable default encryption on the S3 bucket with SSE-KMS
Why this is correct
Default encryption ensures all objects are encrypted with KMS.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Add a bucket policy that denies PutObject without encryption
Why it's wrong here
This enforces encryption but does not set up the encryption itself.
- ✗
Enable encryption in transit using HTTPS for all S3 API calls
Why it's wrong here
Encryption in transit is separate from encryption at rest.
- ✗
Use client-side encryption on all data before uploading
Why it's wrong here
Client-side encryption is an alternative, not required if using server-side encryption.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse 'encryption at rest' with 'encryption in transit' or 'enforcing encryption via bucket policies,' and may select options like C or D, which address different security controls, instead of focusing on the specific mechanism (SSE-KMS) and the necessary KMS key policy configuration.
Detailed technical explanation
How to think about this question
SSE-KMS allows you to use a customer managed CMK or AWS managed KMS key, providing separate permissions via KMS key policies and enabling audit trails via AWS CloudTrail for every encryption/decryption operation. Under the hood, when S3 encrypts an object with SSE-KMS, it calls the KMS GenerateDataKey API to get a plaintext data key and an encrypted copy, then encrypts the object with the plaintext key and discards it, storing only the encrypted key alongside the object. A real-world scenario is compliance requirements that mandate separation of duties, where the KMS key policy restricts decryption to specific IAM roles, preventing even S3 bucket administrators from reading encrypted data.
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
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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: Configure the KMS key policy to allow the S3 service to use the key — Option A is correct because AWS KMS uses key policies to control access to the KMS key. For S3 to use a KMS key for server-side encryption (SSE-KMS), the key policy must grant the S3 service principal (or the bucket owner's account) the necessary permissions, such as kms:Encrypt and kms:Decrypt. Without this policy, S3 cannot access the key to encrypt or decrypt objects at rest. Option B is correct because enabling default encryption on the S3 bucket with SSE-KMS ensures that all objects uploaded to the bucket are automatically encrypted using the specified KMS key, meeting the requirement for encryption at rest.
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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