The answer is that the Lambda execution role lacks the required kms:Encrypt and kms:Decrypt permissions on the customer-managed KMS key. This is because when DynamoDB is configured with a customer-managed KMS key for encryption at rest, every write operation like PutItem internally triggers KMS Encrypt and Decrypt calls to manage the item-level encryption, even though the developer already granted dynamodb:PutItem. On the AWS Certified Developer Associate DVA-C02 exam, this scenario tests your understanding that DynamoDB’s integration with customer-managed KMS keys extends IAM permission requirements beyond the DynamoDB actions themselves—a common trap is assuming the DynamoDB action alone is sufficient. The fix requires adding a policy statement to the Lambda execution role granting kms:Encrypt and kms:Decrypt on the specific KMS key ARN. Memory tip: “DDB writes need KMS keys” — if your table uses a customer-managed key, your role must have both the DynamoDB action and the KMS crypto actions.
DVA-C02 Security Practice Question
This DVA-C02 practice question tests your understanding of 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 developer is building a serverless application that processes personally identifiable information (PII). The application uses API Gateway, Lambda, and DynamoDB. The developer needs to ensure that the PII is encrypted at rest in DynamoDB. The company already uses AWS KMS with a customer-managed key for other services. The developer wants to reuse the same KMS key for DynamoDB. After enabling encryption with the KMS key, the Lambda function fails to write to the table with an AccessDenied error. The Lambda execution role has dynamodb:PutItem permission. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Lambda execution role lacks kms:Encrypt and kms:Decrypt permissions on the customer-managed KMS key.
When a DynamoDB table is encrypted with a customer-managed KMS key, any operation that reads or writes data to the table requires the caller to have permissions to use that KMS key. Even though the Lambda execution role has dynamodb:PutItem permission, the PutItem operation internally triggers KMS Encrypt and Decrypt calls to manage the encryption of the item. Without kms:Encrypt and kms:Decrypt permissions on the specific KMS key, the request fails with an AccessDenied error.
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 Lambda execution role lacks kms:Encrypt and kms:Decrypt permissions on the customer-managed KMS key.
Why this is correct
Required for encrypted DynamoDB table.
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 Lambda execution role does not have DynamoDB write permissions.
Why it's wrong here
It has dynamodb:PutItem.
✗
The DynamoDB table has a resource-based policy that denies access.
Why it's wrong here
Unlikely and not mentioned.
✗
The Lambda function is not in a VPC, so it cannot access the KMS key.
Why it's wrong here
KMS is accessible over the internet.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume DynamoDB's built-in encryption with a KMS key is transparent and does not require additional IAM permissions beyond the DynamoDB actions, but in reality, the caller must have explicit KMS permissions on the key for any read or write operation.
Detailed technical explanation
How to think about this question
DynamoDB uses envelope encryption with KMS: when you write an item, DynamoDB calls KMS to generate a data key (using GenerateDataKey) and then encrypts the item with that data key. The Lambda function's execution role must have kms:Encrypt and kms:Decrypt permissions on the customer-managed key to allow DynamoDB to perform these operations on its behalf. This is a common pitfall because developers often focus on the data-plane permissions (DynamoDB actions) and forget the key-level permissions required for server-side encryption with a customer-managed KMS key.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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.
Security — This question tests Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Lambda execution role lacks kms:Encrypt and kms:Decrypt permissions on the customer-managed KMS key. — When a DynamoDB table is encrypted with a customer-managed KMS key, any operation that reads or writes data to the table requires the caller to have permissions to use that KMS key. Even though the Lambda execution role has dynamodb:PutItem permission, the PutItem operation internally triggers KMS Encrypt and Decrypt calls to manage the encryption of the item. Without kms:Encrypt and kms:Decrypt permissions on the specific KMS key, the request fails with an AccessDenied error.
What should I do if I get this DVA-C02 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.
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Question Discussion
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