Network Programming in .NET

https://github.com/infiniteloopltd/Google-Calendar-Redactor-Proxy/

A lightweight Google Cloud Run service that creates privacy-protected calendar feeds from Google Calendar. Share your availability with colleagues without exposing personal appointment details.

The Problem

You want to share your calendar availability with work colleagues, but:

• You have multiple calendars (work, personal, family) that you need to consolidate
• Google Calendar’s subscribed calendars (ICS feeds) don’t count toward your Outlook free/busy status
• You don’t want to expose personal appointment details to work contacts
• Outlook’s native calendar sharing only works with Exchange/Microsoft 365 calendars, not external ICS subscriptions

This service solves that problem by creating a privacy-filtered calendar feed that Outlook can subscribe to, showing you as “Busy” during your appointments without revealing what those appointments are.

How It Works

Google Calendar → This Service → Privacy-Protected ICS Feed → Outlook
   (full details)    (redaction)     (busy blocks only)      (subscription)

The service:

1. Fetches your Google Calendar ICS feed using the private URL
2. Strips out all identifying information (titles, descriptions, locations, attendees)
3. Replaces event summaries with “Busy”
4. Preserves all timing information (when you’re busy/free)
5. Returns a sanitized ICS feed that Outlook can subscribe to

Use Cases

• Multiple calendar consolidation: Combine work, personal, and family calendars into one availability view
• Privacy-protected sharing: Share when you’re busy without sharing what you’re doing
• Cross-platform calendaring: Bridge Google Calendar into Outlook environments
• Professional boundaries: Keep personal life private while showing accurate availability

Quick Start

1. Get Your Google Calendar Private URL

1. Open Google Calendar
2. Click the ⚙️ Settings icon → Settings
3. Select your calendar from the left sidebar
4. Scroll to “Integrate calendar”
5. Copy the “Secret address in iCal format” URL

Your URL will look like:

https://calendar.google.com/calendar/ical/info%40infiniteloop.ie/private-xxxxxxx/basic.ics

2. Deploy the Service

# Edit deploy.bat and set your PROJECT_ID
deploy.bat

# Or deploy manually
gcloud run deploy calendar-proxy --source . --platform managed --region europe-west1 --allow-unauthenticated

You’ll get a service URL like: https://calendar-proxy-xxxxxxxxxx-ew.a.run.app

3. Construct Your Privacy-Protected Feed URL

From your Google Calendar URL:

https://calendar.google.com/calendar/ical/info%xxxxx.xxx/private-xxxxxxx/basic.ics

Extract:

• calendarId: info@infiniteloop.ie (URL decoded)
• privateKey: xxxxxxxxxx (just the key, without “private-” prefix)

Build your proxy URL:

https://calendar-proxy-xxxxxxxxxx-ew.a.run.app/calendar?calendarId=info@infiniteloop.ie&privateKey=xxxxxxx

4. Subscribe in Outlook

Outlook Desktop / Web

1. Open Outlook
2. Go to Calendar
3. Click Add Calendar → Subscribe from web
4. Paste your proxy URL
5. Give it a name (e.g., “My Availability”)
6. Click Import

Outlook will now show:

• ✅ Blocked time during your appointments
• ✅ “Busy” status for those times
• ❌ No details about what the appointments are

What Gets Redacted

The service removes all identifying information:

What Gets Preserved

All timing and scheduling information remains intact:

• ✅ Event start times (DTSTART)
• ✅ Event end times (DTEND)
• ✅ Event duration
• ✅ Recurring events (RRULE)
• ✅ Exception dates (EXDATE)
• ✅ Event status (confirmed, tentative, cancelled)
• ✅ Time zones
• ✅ All-day events
• ✅ Unique identifiers (UID)

Technical Details

Stack: .NET 8 / ASP.NET Core Minimal API
Hosting: Google Cloud Run (serverless)
Cost: Virtually free for personal use (Cloud Run free tier: 2M requests/month)
Latency: ~200-500ms per request (fetches from Google, processes, returns)

API Endpoint

GET /calendar?calendarId={id}&privateKey={key}

Parameters:

• calendarId (required): Your Google Calendar ID (usually your email)
• privateKey (required): The private key from your Google Calendar ICS URL

Response:

• Content-Type: text/calendar; charset=utf-8
• Body: Privacy-redacted ICS feed

Local Development

# Run locally
dotnet run

# Test
curl "http://localhost:8080/calendar?calendarId=test@example.com&privateKey=abc123"

Deployment

Prerequisites

• Google Cloud SDK installed
• .NET 8 SDK installed
• GCP project with Cloud Run API enabled
• Billing enabled on your GCP project

Deploy

# Option 1: Use the batch file
deploy.bat

# Option 2: Manual deployment
gcloud run deploy calendar-proxy ^
  --source . ^
  --platform managed ^
  --region europe-west1 ^
  --allow-unauthenticated ^
  --memory 512Mi

The --allow-unauthenticated flag is required so that Outlook can fetch your calendar without authentication. Your calendar data is still protected by the private key in the URL.

Security & Privacy

Is This Secure?

Yes, with caveats:

✅ Your calendar data is already protected by Google’s private key mechanism
✅ No data is stored – the service is stateless and doesn’t log calendar contents
✅ HTTPS encrypted – All traffic is encrypted in transit
✅ Minimal attack surface – Simple pass-through service with redaction

⚠️ Considerations:

• Your private key is in the URL you share (same as Google’s original ICS URL)
• Anyone with your proxy URL can see your busy/free times (but not details)
• The service runs as --allow-unauthenticated so Outlook can fetch it
• If you need stricter access control, consider adding authentication

Privacy Features

• Strips all personally identifying information
• Marks all events as CLASS:PRIVATE
• No logging of calendar contents
• No data persistence
• Stateless operation

Recommendations

• Don’t share your proxy URL publicly
• Treat it like a password – it grants access to your availability
• Regenerate your Google Calendar private key if compromised
• Monitor your Cloud Run logs for unexpected access patterns

Cost Estimation

Google Cloud Run pricing (as of 2025):

• Free tier: 2M requests/month, 360,000 GB-seconds/month
• Typical calendar: Refreshes every 30-60 minutes
• Monthly cost: $0 for personal use (well within free tier)

Even with 10 people subscribing to your calendar refreshing every 30 minutes:

• ~14,400 requests/month
• ~$0.00 cost

Troubleshooting

“404 Not Found” when subscribing in Outlook

• Verify your service is deployed: gcloud run services list
• Check your URL is correctly formatted
• Ensure --allow-unauthenticated is set

“Invalid calendar” error

• Verify your Google Calendar private key is correct
• Test the URL directly in a browser first
• Check that your calendarId doesn’t have URL encoding issues

Events not showing up

• Google Calendar ICS feeds can take 12-24 hours to reflect changes
• Try re-subscribing to the calendar in Outlook
• Verify the original Google Calendar ICS URL works

Deployment fails

# Ensure you're authenticated
gcloud auth login

# Set your project
gcloud config set project YOUR_PROJECT_ID

# Enable required APIs
gcloud services enable run.googleapis.com
gcloud services enable cloudbuild.googleapis.com

Limitations

• Refresh rate: Calendar clients typically refresh ICS feeds every 30-60 minutes (not real-time)
• Google’s ICS feed: Updates can take up to 24 hours to reflect in the ICS feed
• Authentication: No built-in authentication (relies on URL secrecy)
• Multi-calendar: Requires one proxy URL per Google Calendar

Alternatives Considered

Contributing

Contributions welcome! Areas for enhancement:

•  Add basic authentication support
•  Support multiple calendars in one feed
•  Caching layer to reduce Google Calendar API calls
•  Health check endpoint
•  Metrics/monitoring
•  Custom “Busy” text per calendar

License

MIT License – free to use, modify, and distribute.

Author

Created by Infinite Loop Development Ltd to solve a real business need for calendar privacy across platforms. https://github.com/infiniteloopltd/Google-Calendar-Redactor-Proxy/