How Sudoku Challenge handles your data — built on blockchain with privacy by design.
Effective Date: 14 February 2026 | Last Updated: October 2026
Sudoku Challenge (sudokuchallenge.com) is built on the Internet Computer Protocol (ICP), a decentralized blockchain network. Unlike traditional web applications, your game data is stored in on-chain smart contracts — not in a private corporate database. This architecture gives you stronger privacy guarantees than conventional websites.
Key Privacy Features
No Personal Data Required
No email, name, phone, or account registration needed to play
No Personal Info Required
No email, phone number, real name, or address is ever collected
Decentralized Storage
Game data lives on blockchain smart contracts, not a private server
Cryptographic Sign-In
Sign in via Internet Identity — no passwords stored anywhere
What We Store
The following data is stored on the Internet Computer blockchain in our smart contract. This data is necessary to provide the game's functionality:
Player Profile
Your username, selected avatar, and country flag — all chosen by you. No real name or personal identifiers.
Competitive Data
Tournament results, duel outcomes, daily challenge completions, and your competitive rating history.
Game Records
Move-by-move records of completed games for the replay system, including the puzzle grid, moves, and solving time.
Replay Detail
For games saved to your Dashboard only: your pencil marks, cell colours, multi-cell inputs, and when you left or returned to the game. Used to show your working in replays; competitive games (Daily Challenge, Daily Triple, tournaments, duels) are also checked automatically for fair play. Others can see it only once that day's puzzle is over. Puzzles you keep with the Save button stay in your browser until you finish them.
Feedback & Reviews
Star ratings and written reviews you submit on the Feedback page, along with your username and the submission timestamp.
Sign-In ID
Your Internet Identity sign-in ID — a unique identifier used to sign you in. It is not linked to any personal information.
What We Don't Collect
We are committed to collecting the absolute minimum data needed to run the platform. We do not collect:
No email addresses
No real names
No phone numbers
No physical addresses
No ip addresses
No location / gps data
No browser fingerprints
No browsing history
No payment information
Local Browser Storage
Some data is stored locally in your browser to improve your experience. This data never leaves your device and is not sent to any server:
Game settingsTheme, difficulty preferences, and board display options
Saved progressCurrent puzzle progress, so you can resume where you left off
Guest usernameRandomly generated guest name, used when you play without signing in
You can clear this data at any time through your browser settings. Clearing local data will not affect your on-chain profile, rating, or game history.
Blockchain & Decentralization
Sudoku Challenge runs on the Internet Computer Protocol (ICP) — a public blockchain that hosts both our website and the game logic and data behind it. Key privacy advantages include:
No Central Server
There is no private corporate server storing your data. All data lives in a smart contract on a decentralized network of independent nodes.
Cryptographic Authentication
Internet Identity provides passwordless, anonymous authentication. Your identity is a cryptographic key pair — not an email/password combination.
Auditable Code
Smart contract code on the Internet Computer can be verified by anyone. You don't have to trust us — you can verify.
No Middleman
Your browser communicates directly with the blockchain. There are no intermediary services collecting data between you and the application.
Note: As platform administrators, we retain the ability to enforce our community guidelines. This includes suspending or banning accounts that violate our policies, removing inappropriate content such as abusive reviews, and adjusting ratings in cases of confirmed manipulation. These administrative actions are necessary to maintain a fair and safe environment for all players.
Third-Party Services & Advertising
Sudoku Challenge uses the following third-party services that may collect data through cookies and similar technologies:
•Google Analytics is used to understand how visitors interact with our website (pages visited, session duration, general geographic region). This helps us improve the platform. Google Analytics uses cookies to collect anonymized usage data, and it only loads if you accept analytics cookies in our cookie banner — until then, no Google Analytics script runs and no analytics cookie is set.
•We do not show ads on Sudoku Challenge today. If we add advertising in the future, ad partners may use cookies and similar technologies to serve and measure ads; we will ask for your consent first and update this policy.
•You may opt out of Google Analytics tracking by installing the Google Analytics Opt-out Browser Add-on (tools.google.com/dlpage/gaoptout).
•You may opt out of personalized advertising through your browser settings or by using ad-blocking extensions.
•Any future advertising partners will be selected to maintain a safe and quality user experience.
•We do not sell any data to third parties. These services are used solely for platform improvement and to support the free availability of the game.
Apart from Google Analytics (and any advertising service added in future, as described above), we do not integrate with any other third-party social media tracking or data collection services.
Ad-Free Experience
We understand that not everyone wants to see advertisements. Sudoku Challenge offers a Premium subscription that removes all ads from the platform. When ads are removed, third-party advertising scripts do not load — meaning no ad-related cookies or tracking will be active on your session. Google Analytics is separate: it only runs if you accept analytics cookies, and you can turn it off any time under Cookie settings.
User-Generated Content
When you submit feedback, reviews, or choose a username, you are creating public-facing content that other users can see. By submitting this content, you agree to the following:
•All content is screened both in your browser and again on-chain to filter profanity, hate speech, slurs, and other inappropriate language.
•We reserve the right to remove, modify, or reject any user-generated content that violates our community guidelines — at our sole discretion and without prior notice.
•Accounts that repeatedly violate content guidelines may be banned by administrators.
•Reviews and feedback are displayed publicly alongside your username and avatar. Do not include personal information in your reviews.
Your Rights
As a user of Sudoku Challenge, you have the right to:
Clear your local browser data at any time without affecting your on-chain account.
Stop using the service at any time without penalties.
Leave tournaments before they start (your participation data is removed).
Request information about what data is stored on-chain associated with your sign-in ID.
Opt out of personalized advertising through your browser or ad network settings.
Contact us at any time with questions or concerns about your data.
Children's Privacy
Sudoku Challenge does not knowingly collect personal information from children under 13. Since we don't collect personal information from any user (no email, no name, no location), the platform is safe for users of all ages. However, we encourage parents and guardians to supervise their children's online activities. If you believe a child has somehow provided personal information through the platform, please contact us and we will take appropriate steps.
Changes to This Policy
We may update this privacy policy from time to time to reflect changes in our practices, technology, or legal requirements. Any changes will be posted on this page with an updated revision date. We encourage you to review this policy periodically. Continued use of the platform after changes are posted constitutes acceptance of the updated policy.
Learn More About ICP
For more information about the Internet Computer Protocol and its privacy model:
If you are located in the European Economic Area (EEA), United Kingdom, or Switzerland, the General Data Protection Regulation (GDPR) provides you with specific rights regarding your personal data. Although Sudoku Challenge collects minimal personal data, the following applies to any data processed (including cookies set by third-party services):
Legal Basis: We process data based on legitimate interest (security and running the game) and consent (analytics cookies, and advertising cookies if ads are ever added). You may withdraw consent at any time via Cookie settings at the bottom of every page, or in your browser.
Right of Access: You have the right to request information about what data we hold about you.
Right to Erasure: You may request deletion of your data. Since game data is stored on-chain under a cryptographic sign-in ID (not linked to your real identity), it is already pseudonymous.
Right to Portability: You can request your game data in a machine-readable format.
Right to Object: You may object to processing based on legitimate interest, including analytics tracking.
Data Transfers: Game data is stored on the Internet Computer, a decentralized network with nodes globally distributed. Third-party services (Google) may transfer data outside the EEA under Standard Contractual Clauses.
Complaints: You have the right to lodge a complaint with your local data protection authority.
Under the California Consumer Privacy Act (CCPA) and California Privacy Rights Act (CPRA), California residents have specific privacy rights:
Right to Know: You may request what personal information we collect, use, and disclose. As noted above, we collect minimal data — no email, name, or address.
Right to Delete: You may request deletion of your personal information.
Right to Opt-Out of Sale: We do not sell personal information. We never have and never will.
Non-Discrimination: We will not discriminate against you for exercising your CCPA rights.
Advertising: Third-party advertising partners may use cookies for interest-based advertising. This may constitute "sharing" under CPRA. You can opt out via Google Ads Settings or the "Do Not Sell or Share" mechanism in your browser.
To make a request, contact us at support@sudokuchallenge.com. We will verify your identity and respond within 45 days.
Payment Processing
When you subscribe to Sudoku Challenge Premium, payment is processed by Stripe, Inc. — a PCI-DSS Level 1 certified payment processor. We do not store, process, or have access to your full credit card number, CVV, or bank account details.
What Stripe receives: Your payment card details, billing address, email address (for receipts), and IP address.
What we receive from Stripe: Your subscription status (active/cancelled), card brand and last 4 digits, country of the card, and payment amount. We use this solely to manage your subscription.
What we store: Your subscription status and Stripe customer ID, linked to your Internet Identity sign-in ID. We do not store any payment card details.
Payment methods accepted include credit/debit cards (Visa, Mastercard, Amex), Google Pay, and Apple Pay. All payment data is encrypted in transit using TLS.
Account & Data Retention Policy
Avatar Retention
To optimize performance and reduce long-term storage costs, user-uploaded avatars are retained only while an account remains active.
If an account shows no activity for 6 months, the uploaded avatar will be permanently deleted.
The account will automatically revert to the default system avatar.
Users may upload a new avatar at any time after returning.
Active Premium subscribers are exempt from avatar deletion regardless of activity.
Account Retention
To maintain a clean and efficient platform, inactive accounts are periodically removed.
If an account remains inactive for 24 months (2 years), it may be permanently deleted.
Deletion includes all associated data such as profile information, statistics, history, and settings.
Deleted usernames become unavailable and cannot be reclaimed, even if the user returns later.
Users may create a new account using the same Internet Identity, but they must choose a new username.
Active Premium subscribers are exempt from account deletion regardless of activity — as long as the subscription remains active, no data will be removed.
Reactivation
Any login or activity resets the inactivity timer.
Users who return before the deletion window expires will retain their account and data (except avatars removed after 6 months of inactivity).
Subscription Data
Subscription data is retained for the duration of your subscription plus 90 days after cancellation (for dispute resolution). You may request deletion of your account and associated data at any time by contacting support@sudokuchallenge.com.
International Data Transfers
Sudoku Challenge operates on the Internet Computer Protocol, a decentralized network with data centers in multiple countries. Your game data may be replicated across nodes globally. Third-party services (Google, Stripe) may process data in the United States and other jurisdictions. These providers maintain appropriate data protection safeguards including Standard Contractual Clauses (SCCs) and compliance with applicable data protection laws.
Questions About Your Privacy?
If you have any questions, concerns, or requests regarding this privacy policy or your data, please don't hesitate to reach out.
In short: We don't collect personal information. Your game data lives on a decentralized blockchain. Google Analytics runs only if you accept analytics cookies, and you can change that choice any time under Cookie settings. Payment processing is handled securely by Stripe. We never sell your data.