S3 Bucket Policy Project 6: Enforce SSE-KMS Encryption on Upload

By Ravindra Bagale, trainer · LinkedIn · Last updated: 28 October 2026

1. Everyday project story

Pune health-tech CTO Shruti Kulkarni and Amsterdam insurer Pieter de Vries both need CloudTrail-friendly CMK encryption on every object at rest.

2. Why enforce KMS on Put?

Why do we need this?

Default bucket encryption helps, but a Deny on wrong encryption headers stops misconfigured SDKs from writing SSE-S3 or unencrypted objects.

• Require x-amz-server-side-encryption = aws:kms
• Optionally require specific key ARN
• Pair with default encryption = KMS
• Grant callers kms:GenerateDataKey + Decrypt

3. What is the policy?

What is it?

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "DenyIncorrectEncryption",
      "Effect": "Deny",
      "Principal": "*",
      "Action": "s3:PutObject",
      "Resource": "arn:aws:s3:::rb-bank-docs-ap-south-1/*",
      "Condition": {
        "StringNotEquals": {
          "s3:x-amz-server-side-encryption": "aws:kms"
        }
      }
    },
    {
      "Sid": "DenyWrongKmsKey",
      "Effect": "Deny",
      "Principal": "*",
      "Action": "s3:PutObject",
      "Resource": "arn:aws:s3:::rb-bank-docs-ap-south-1/*",
      "Condition": {
        "StringNotEquals": {
          "s3:x-amz-server-side-encryption-aws-kms-key-id": "arn:aws:kms:ap-south-1:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab"
        }
      }
    }
  ]
}

Line-by-line

1. First Deny — PutObject without aws:kms encryption header.
2. Condition StringNotEquals on s3:x-amz-server-side-encryption.
3. Second Deny — wrong CMK id.
4. Resource objects only (PutObject).
5. Callers still need IAM s3:PutObject + KMS perms.
6. Default bucket encryption should also be KMS.

4. CLI with output

How to do it (step by step)

KEY=arn:aws:kms:ap-south-1:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab
aws s3api put-bucket-policy --bucket rb-bank-docs-ap-south-1 --policy file://kms-enforce.json

# ALLOW
aws s3 cp report.pdf s3://rb-bank-docs-ap-south-1/report.pdf --sse aws:kms --sse-kms-key-id "$KEY"
# upload success

# DENY — no SSE
aws s3 cp report.pdf s3://rb-bank-docs-ap-south-1/bad.pdf
# An error occurred (AccessDenied) when calling the PutObject operation: Access Denied

5. Test allow and deny

Allow: Put with --sse aws:kms and correct key. Deny: Put with no SSE or SSE AES256.

# DENY SSE-S3
aws s3 cp report.pdf s3://rb-bank-docs-ap-south-1/aes.pdf --sse AES256
# AccessDenied

6. Key policy tip

7. Multi-industry

Banking (Bengaluru — Deepak Nair), health-tech (Pune — Shruti Kulkarni), insurance (Amsterdam — Pieter de Vries), govt archives (Delhi — Meenakshi Iyer).

8. AccessDenied debugging and Policy Simulator

When S3 returns AccessDenied, check in order: (1) Block Public Access, (2) matching bucket policy Deny, (3) missing IAM Allow, (4) KMS key policy, (5) VPC endpoint policy, (6) legacy ACL. IAM Policy Simulator can miss BPA/VPC/KMS — always re-test with real CLI from the real network path.

# Decode
aws s3api get-bucket-policy --bucket BUCKET --query Policy --output text | python3 -m json.tool
aws s3api get-public-access-block --bucket BUCKET
aws iam simulate-principal-policy --policy-source-arn ARN \
  --action-names s3:GetObject --resource-arns arn:aws:s3:::BUCKET/key.txt
aws cloudtrail lookup-events --lookup-attributes AttributeKey=EventName,AttributeValue=GetObject --max-items 5

9. Lab: enforce SSE-KMS

Do this in ap-south-1 (Mumbai) unless your org forbids it. Set a budget alert first.

How to do it (step by step)

1. Create CMK + lab bucket; set default encryption KMS.
2. Apply both Deny statements; replace key ARN.
3. Put with --sse aws:kms → success; without → AccessDenied.
4. Put with --sse AES256 → AccessDenied.

Cleanup (stop billing)

1. Schedule CMK deletion carefully; empty and delete bucket.
2. Remove lab IAM grants on the key.

10. Common errors and fixes

Each row is a real mistake students and juniors hit. Read the fix before you panic.

11. Troubleshooting checklist

1. Confirm region and endpoint hostname for the bucket.
2. Print get-bucket-policy and get-public-access-block.
3. Reproduce with a least-privilege lab IAM user.
4. Binary-search Denies by commenting one Sid at a time in lab.
5. Read CloudTrail errorCode and additionalEventData.
6. If KMS involved, test key permissions separately.
7. From VPC path: route to gateway prefix list + endpoint policy.
8. Cleanup versioning/Object Lock labs carefully.

12. FAQ

13. Interview Q&A

14. Tips in English, Marathi and Hindi

15. Policy design checklist before you save

1. Write the business rule in one sentence (who can do what from where).
2. Choose Allow vs Deny-unless consciously; remember explicit Deny wins.
3. List every Resource ARN shape you need (bucket vs bucket/*).
4. Name every Condition key exactly (case-sensitive).
5. Decide BPA settings before put-bucket-policy.
6. Plan the allow test and the deny test with expected HTTP/CLI codes.
7. Plan cleanup (versioning, Object Lock, KMS schedules).
8. Paste JSON through python3 -m json.tool before apply.

16. Policy Simulator walkthrough

IAM Policy Simulator is great for identity policies and a first pass on resource policies, but it can miss Block Public Access, VPC endpoint policies, and some KMS denials. Use it to catch typos, then always re-test with AWS CLI from the real network path (office IP, VPC, or CloudFront).

1. Open IAM → Policy simulator → pick the user/role.
2. Add action s3:GetObject / PutObject and the object ARN.
3. If you use resource policies, include the bucket policy text where the tool allows.
4. Note Allow/Deny; then run the matching CLI from the real path.
5. If Simulator Allow but CLI Deny, check BPA, VPCe, KMS, and explicit Denies next.

17. Copy-paste runbook (print this)

# 0) Region + names
export AWS_REGION=ap-south-1
export BUCKET=rb-lab-$(date +%s)-ap-south-1

# 1) Create + show BPA
aws s3api create-bucket --bucket $BUCKET --region $AWS_REGION \
  --create-bucket-configuration LocationConstraint=$AWS_REGION
aws s3api get-public-access-block --bucket $BUCKET || true

# 2) Validate + apply policy
python3 -m json.tool policy.json >/dev/null
aws s3api put-bucket-policy --bucket $BUCKET --policy file://policy.json
aws s3api get-bucket-policy --bucket $BUCKET --query Policy --output text | python3 -m json.tool | head

# 3) Allow test + Deny test (fill per project)
# ... project-specific curls / aws s3 cp ...

# 4) Cleanup
aws s3 rm s3://$BUCKET --recursive || true
aws s3api delete-bucket-policy --bucket $BUCKET || true
aws s3api delete-bucket --bucket $BUCKET

18. Industry story bank (reuse the pattern)

Each project above uses one industry story. Use the same policy pattern in these other industries without rewriting the JSON from scratch — only change the bucket name, principal, and Condition values.

• Healthcare (Priya Nair, Apollo-style clinic): patient PDFs — prefer SSE-KMS + OrgID + MFA delete.
• Fintech (Arjun Mehta, UPI wallet): statements — office IP + SecureTransport + CloudFront OAC for the customer portal.
• EdTech (Sofia Alvarez, online course): course videos — CloudFront OAC only; never public ACLs.
• Logistics (Kenji Watanabe, fleet app): GPS dumps — VPC endpoint only from the private subnet runners.
• Media (Fatima Khan, newsroom): raw footage — Object Lock COMPLIANCE + deny delete.
• Retail (Lucas Meyer, D2C brand): product images — public read for /images/* only; deny everything else.
• SaaS (Ananya Iyer, B2B CRM): tenant exports — ${aws:username} or ${aws:PrincipalTag/tenant} prefixes.
• Government (Rahul Deshmukh, state portal): RTI archives — OrgID + MFA + Object Lock.

19. AccessDenied debugging flowchart

1. Read the error code and the request ID. Save both.
2. Confirm you are calling the right bucket and region.
3. Check Block Public Access if the principal is * or a service that looks public.
4. Decode the policy: is there an explicit Deny that matches? Deny wins.
5. Check Condition keys: SecureTransport, SourceVpce, SourceIp, PrincipalOrgID, encryption headers.
6. For KMS: does the key policy allow the caller? Bucket policy alone is not enough.
7. For CloudFront: is OAC present and the distribution id matching?
8. For VPC: are you really going through the endpoint (no public IP leak)?
9. Simulate in IAM Policy Simulator, then re-test with CLI from the real path.
10. Only then open a support case with the request id and a redacted policy.

20. What good looks like in a PR review

• Policy JSON validates and is indented.
• Every Statement has Sid, Effect, Action, Resource.
• No Principal:* with write actions unless BPA and a Deny fence exist.
• Allow test and Deny test are documented with expected output.
• Cleanup steps will not strand Object Lock or KMS keys.

Revision, interview and comparison cards

Revise this lesson in a few minutes. Save the cards on your phone or download them as PNG files. The QR code on each card opens this lesson.

More cards: Ravi Sir's Cards.