Amazon S3 Default Encryption vs. Bucket Policies | Overview.

 

Here's twtech comparison of Amazon S3 Default Encryption vs. Bucket Policies- Overview.

Scope:

• Amazon S3 – Default Encryption,
• Amazon S3 – Bucket Policies,
• Best Practice: Use Both Together,
• Sample Bucket Policy to Enforce SSE-KMS,

 Amazon S3 – Default Encryption

Feature	Description
Purpose	Automatically encrypts objects when they are uploaded to the bucket, ensuring compliance with encryption-at-rest requirements.
Applies To	Every new object uploaded to the bucket after default encryption is configured.
Control Level	Bucket-level setting – applies encryption automatically to all objects in the bucket.
Supported Encryption Methods	SSE-S3 (AES-256), SSE-KMS (AWS Key Management Service)
Use Case	Ensures that all data stored in S3 is encrypted without needing each upload request to explicitly specify encryption headers.
Limitations	Doesn't prevent unencrypted uploads if you’re not using bucket policies; encryption is automatic but not enforced by default if clients specify their own encryption.
Auditability	Encryption status is visible in object metadata. Combined with CloudTrail for audit logs.

 Amazon S3 – Bucket Policies

Feature	Description
Purpose	Define fine-grained access control rules for who can perform what actions on the bucket and its contents.
Applies To	IAM principals (users, roles, accounts), request conditions, actions (e.g., GetObject, PutObject)
Control Level	IAM policy-style JSON document attached to the bucket – highly customizable.
Use Case	Restrict access to specific IPs, enforce MFA, require HTTPS, enforce encryption policies (e.g., deny unencrypted uploads)
Example Security Enforcement	twtech can write a bucket policy to deny all PutObject requests that do not include encryption using SSE-KMS or SSE-S3.
Auditability	Changes can be tracked using AWS CloudTrail. Can be combined with AWS Config for compliance tracking.

Best Practice: Use Both Together

Combine Them For	Benefit
Default Encryption + Bucket Policy	Ensures automatic encryption and enforces encryption compliance via policy.
Default Encryption	Handles cases where the user doesn’t specify encryption – ensures data is still encrypted.
Bucket Policy	Can reject uploads if the correct encryption method is not used (e.g., mandate SSE-KMS).

 Sample Bucket Policy to Enforce SSE-KMS

# json 

{

  "Version": "2012-10-17",

  "Statement": [

    {

      "Sid": "twtechDenyUnEncryptedObjectUploads",

      "Effect": "Deny",

      "Principal": "*",

      "Action": "s3:PutObject",

      "Resource": "arn:aws:s3:::twtech-s3bucke/*",

      "Condition": {

        "StringNotEquals": {

          "s3:x-amz-server-side-encryption": "aws:kms"

        }

      }

    }

  ]

}