What type of encryption is applied via Server-Side Encryption (SSE) in Amazon S3?

Server-Side Encryption (SSE) in Amazon S3 utilizes AWS-managed encryption services to handle data encryption at rest automatically. This means that when you store objects in Amazon S3, you can opt for SSE, allowing AWS to manage the encryption keys and processes without requiring you to intervene in the encryption logic or management of keys directly.

AWS provides different types of SSE, including SSE-S3, where Amazon manages the encryption keys; SSE-KMS, which integrates with AWS Key Management Service for more controlled access to encryption keys; and SSE-C, which allows you to manage your encryption keys yourself, though this option doesn't fall under AWS-managed services specifically. The main point is that SSE simplifies data protection and compliance by integrating securely within the AWS ecosystem while allowing you to scale effectively.

In contrast, the other options involve different scopes of encryption that do not reflect the nature of SSE as thoroughly. Client-side encryption would require the user to handle encryption before sending data to S3, while encryption before data upload suggests a process that doesn't involve S3's on-the-fly encryption functionality. User-defined encryption indicates a manual management approach, which is not indicative of the AWS-managed processes that define SSE.