Depending on your requirements, one method of encryption may be more appropriate than another. To help you decide, here is a quick overview of each.

Server-side encryption with S3 managed keys, SSE-S3. This option requires minimal configuration and all management of encryption keys used are managed by AWS. All you need to do is to upload your data and S3 will handle all other aspects.

Server-side encryption with KMS managed keys, SSE-KMS. This method allows S3 to use the key management service to generate your data encryption keys. KMS gives you a far greater flexibility of how your keys are managed. For example, you are able to disable, rotate, and apply access controls to the CMK, and audit it against their usage using AWS CloudTrail.

Server-side encryption with customer provided keys, SSE-C. This option gives you the opportunity to provide your own master key that you may already be using outside of AWS. Your customer-provided key would then be sent with your data to S3, where S3 would then perform the encryption for you.

Client-side encryption with KMS, CSE-KMS. Similarly to SSE-KMS, this also uses the key management service to generate your data encryption keys. However, this time KMS is called upon via the client not S3. The encryption then takes place client-side and the encrypted data is then sent to S3 to be stored.

Client-side encryption with customer provided keys, CSE-C. Using this mechanism, you are able to utilize your own provided keys and use an AWS-SDK client to encrypt your data before sending it to S3 for storage.

Okay, that has given us a very high-level overview of the five different methods. Via a series of diagrams, I will now explain how the encryption and decryption process works for each.