Encrypt each file directly by calling AWS KMS Encrypt with a customer managed KMS key is the correct choice because AWS KMS accepts plaintext up to 4 KB per Encrypt request which covers the roughly 2 KB JSON files and this approach keeps key material inside KMS rather than exposing data keys outside the service.

Calling Encrypt each file directly by calling AWS KMS Encrypt with a customer managed KMS key means the application sends the small plaintext over TLS to KMS and KMS performs the encryption and returns ciphertext so you do not need to store or manage plaintext data keys in your application. Using a customer managed key also gives you control over access policies and key rotation which improves security for sensitive patient data.

Generate a data key with a customer managed KMS key and use that data key to envelope encrypt each file is a valid pattern for larger objects but it requires handling plaintext data keys in application memory or storage which increases exposure and operational complexity when each file is small.

Encrypt the files directly using an AWS managed KMS key is not appropriate because AWS managed keys are owned and managed by AWS for service integrations and they are generally not available for arbitrary direct Encrypt API calls from customer applications.

Use the AWS Encryption SDK with KMS to generate data keys and encrypt the files relies on envelope encryption and therefore exposes plaintext data keys in the client process and adds complexity that is unnecessary for payloads that fit within KMS Encrypt size limits.

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