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SAA-C03 Practice Question 83: KMS Multi Region Keys

AWS Certified Solutions Architect – Associate (SAA-C03) · Test 9 · question 3 of 10

Data Protection & Secretshard

A trading application runs active-active in two Regions. It encrypts message payloads client-side with the AWS Encryption SDK before writing them to a DynamoDB global table, and either Region may read and decrypt a record written by the other. Today each Region has its own single-Region KMS key, so a reader must make a cross-Region KMS call to decrypt a payload written elsewhere, which adds latency and fails during a Regional KMS disruption.

What should the architect do?

Select one answer
Show answer and explanation

Answer:

  • D. Create a multi-Region primary key, replicate it to the second Region, and re-encrypt the existing data under the new related keys

Related multi-Region keys share the same key ID and the same key material in different Regions, so data encrypted in one Region can be decrypted in another without re-encrypting and without a cross-Region call to KMS. That removes both the added latency and the dependency on the other Region's KMS endpoint, and AWS names active-active multi-Region applications and client-side encryption libraries such as the AWS Encryption SDK as the scenarios these keys exist for. The re-encryption step is unavoidable: existing ciphertext is bound to the old single-Region key.

Option A makes the cross-Region call legal rather than unnecessary. The reader still has to reach the other Region's KMS endpoint on the decrypt path, so the latency stays and the Regional dependency, the thing that breaks during a disruption, is untouched.

Option B removes the cross-Region call at read time by paying for it at write time. Every record now needs two KMS operations and double the stored ciphertext, and a write during a Regional KMS disruption fails outright, which is worse than the situation today.

Option C is the closest wrong answer and is simply not possible. AWS KMS does not allow converting an existing single-Region key into a multi-Region key, or the reverse, deliberately, so that data already protected keeps its data residency properties. That constraint is what forces the re-encryption in option D.

Reference: Multi-Region keys in AWS KMS

Practice Test 9 ›

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