/securing-s3-buckets

S3 Encryption Reference

Choosing an Encryption Option

SSE-S3 (Recommended Default)

aws s3api put-bucket-encryption \
  --bucket <bucket-name> \
  --server-side-encryption-configuration \
  '{"Rules":[{"ApplyServerSideEncryptionByDefault":{"SSEAlgorithm":"AES256"},"BucketKeyEnabled":true,"BlockedEncryptionTypes":{"EncryptionType":["SSE-C"]}}]}'

SSE-KMS with Customer Managed Key

You MUST specify the KMS key by its full ARN (arn:aws:kms:<region>:<account>:key/<key-id>). Do NOT use a key alias.

You MUST apply a least-privilege key policy when creating a KMS key. Do NOT rely on the AWS default key policy — it grants kms:* to the account root, which allows any IAM principal with matching IAM permissions to perform all KMS operations on the key.

Important: The S3 API accepts both aws/s3 and key aliases (e.g. alias/my-key) without returning an error — it will store whatever value you provide. These restrictions are agent-enforced constraints, not API-enforced. Always verify the stored value with get-bucket-encryption after applying.

Least-Privilege KMS Key Policy Template

Save the following as key-policy.json before creating the key. Replace <account-id>, <region>, and <bucket-name> with actual values.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "AllowKeyAdministration",
      "Effect": "Allow",
      "Principal": {"AWS": "arn:aws:iam::<account-id>:role/<key-admin-role>"},
      "Action": [
        "kms:Create*",
        "kms:Describe*",
        "kms:Enable*",
        "kms:List*",
        "kms:Put*",
        "kms:Update*",
        "kms:Revoke*",
        "kms:Disable*",
        "kms:Get*",
        "kms:Delete*",
        "kms:TagResource",
        "kms:UntagResource",
        "kms:ScheduleKeyDeletion",
        "kms:CancelKeyDeletion"
      ],
      "Resource": "*"
    },
    {
      "Sid": "AllowS3EncryptionUsage",
      "Effect": "Allow",
      "Principal": {"AWS": "arn:aws:iam::<account-id>:root"},
      "Action": [
        "kms:Encrypt",
        "kms:Decrypt",
        "kms:ReEncrypt*",
        "kms:GenerateDataKey*",
        "kms:DescribeKey"
      ],
      "Resource": "*",
      "Condition": {
        "StringEquals": {
          "kms:ViaService": "s3.<region>.amazonaws.com",
          "kms:CallerAccount": "<account-id>"
        }
      }
    }
  ]
}

Key policy notes:

• AllowKeyAdministration — grants key management to a specific admin role. Replace <key-admin-role> with your actual admin role name.
• AllowS3EncryptionUsage — restricts encrypt/decrypt to S3 in the specified region and account via kms:ViaService and kms:CallerAccount conditions. Scope the Principal down to specific roles if cross-account access is not needed.
• Do NOT include a blanket kms:* statement. If you need to grant additional principals access, add narrowly scoped statements.

# Step 1: Create customer managed key with least-privilege policy
aws kms create-key \
  --description "S3 bucket encryption key" \
  --policy file://key-policy.json

# Step 2: Apply to bucket (use full key ARN, not alias)
aws s3api put-bucket-encryption \
  --bucket <bucket-name> \
  --server-side-encryption-configuration \
  '{"Rules":[{"ApplyServerSideEncryptionByDefault":{"SSEAlgorithm":"aws:kms","KMSMasterKeyID":"arn:aws:kms:<region>:<account>:key/<key-id>"},"BucketKeyEnabled":true,"BlockedEncryptionTypes":{"EncryptionType":["SSE-C"]}}]}'

Why S3 Bucket Keys (always enable)

• Reduces KMS API calls by up to 99%
• Lowers KMS costs significantly for high-throughput workloads
• No impact on security posture

Why Block SSE-C (always block)

• SSE-C keys are managed outside AWS — no CloudTrail audit trail
• Key loss = permanent data loss
• Cannot enforce rotation policies
• Incompatible with centralized compliance requirements

Enforce HTTPS in Transit

⚠️ If the bucket already has a policy, merge this DenyInsecureTransport statement into the existing Statement array rather than replacing the whole policy. See put-bucket-policy safety rules.

{
  "Version": "2012-10-17",
  "Statement": [{
    "Sid": "DenyInsecureTransport",
    "Effect": "Deny",
    "Principal": "*",
    "Action": "s3:*",
    "Resource": ["arn:aws:s3:::<bucket>/*", "arn:aws:s3:::<bucket>"],
    "Condition": {"Bool": {"aws:SecureTransport": "false"}}
  }]
}

Principal: "*" and Action: "s3:*" are required wildcards — a Deny policy must match all principals and actions to be effective. Do NOT narrow these.

Do NOT pin TLS certificates — AWS rotates them automatically.