Chăm luyện

Last updated 1 November 2026.

The short version

Chăm luyện has no ads, no analytics and no tracking. A grown-up can buy a plan in the app with their Apple account. Without an account, nothing about you leaves the device except downloads of voice files. An account, which is made when a grown-up buys a plan in the app or by us on request, adds the few things listed below and nothing more.

What goes over the internet

Voice files. One voice comes with the app. The others are downloaded from the app's own file server (packs.khangazun.com), when you picks one, a couple of minutes after the app opens, when Settings is opened, and when the app goes to the background. Each download asks for a sound file by its name; it carries no information about you, their progress or their name.

What the file server sees. Like any website, the server, run by Cloudflare, receives your device's IP address and the usual details of a web request when a file is downloaded. Cloudflare handles that under its own privacy policy. We receive nothing from these downloads.

Checked before use. Every downloaded voice is checked against a fingerprint built into the app, and is discarded if it does not match.

Your account, only when you have one. An account is made in one of three ways. When a grown-up buys a plan in the app, the purchase makes it: no phone number, no name and no password are asked for, and the same Apple ID gets it back with Restore Purchases on another device. Or we make it on request, when a grown-up writes to us by Zalo or email: that account has a phone number and a password. Or, from version 1.5, a grown-up makes it in the app with a phone number and a password, with no SMS code; our staff confirm the number when the grown-up contacts us, and a self-made account whose number was never confirmed, that never had a plan and was not used for 30 days is deleted automatically. Our server (Cloudflare Workers and a Cloudflare D1 database) then holds, for an account with a phone number, the phone number, the account holder's name if given and a salted hash of the password (never the password itself); for every account, the plan records (package, start and end dates; for a purchase also Apple's transaction numbers and a random number Apple attaches to the purchase), the devices that are signed in (when each one signed in and was last used, and, from version 1.3, the device's model, its system version and the app's version and build number, which only our staff see, to help you when you write to us; the app does not send a device identifier, a serial number or the name you gave the device, and does not share any of this with anyone) and the date of the last sign-in. It is used only to sign in, to know whether a plan covers today, and to help you when you write to us. It is not sold, not shared for advertising, and not used for analytics or tracking. Earlier versions of the app could also create an account with a phone number and a six-digit SMS code, and still can until they are updated. To stop automated abuse of that sign-up code, the server also counts how many sign-up requests come from one internet address in a day: it keeps only a one-way hash of the address (never the address itself) with the time, and deletes it after 24 hours.

Plans bought in the app. On iPhone, iPad and Mac a grown-up can buy a plan (1 month, 6 months or 1 year) with their Apple account; Apple takes the payment and we never see any card or payment details. The app sends Apple's signed receipt for the purchase to our server, which checks Apple's signature and then adds the period to the account; we keep the transaction number with the plan record. A plan can be restored with Restore Purchases in the app while the account still exists; deleting the account removes the plan records, and a purchase cannot then be moved to another account.

The national board, only with an account that has a plan. The app sends your best score in each break game, and the total number of stars collected in each grade, together with the name typed in the app. Other users see that name, the scores and stars and the last four digits of the account's phone number, so two users with the same name can be told apart. Names are checked on our server, and an unsuitable one is shown as "Bạn nhỏ". From version 1.8 the board also has a "This week" view (Vietnam weeks, starting Monday): the app sends the best score of each game reached this week and the number of questions answered right this week, and our server keeps each week's figures for five weeks. Other users see this week's scores and stars on that view, never the count of right answers. The app also shows everyone a line such as "this week the children answered 1,240 questions right and earned 380 stars": these are totals over all users, with no name and no number of users. Nothing else about your progress is sent.

Study reminders, only if a grown-up turns them on. After a few lessons the app can offer a daily reminder, behind a question for a grown-up. A reminder is a notification made and shown by the device itself at the chosen time; nothing is sent to us or to anyone else, and the grown-up can turn it off in the app or in the device's settings.

The buddy picture on the board. By default the board shows only a built-in buddy look, which carries no personal information. If a grown-up chooses to give the buddy a picture the user drew or a photo, a small copy of it (about 96 pixels) is sent to our server and shown next to the user's name on the national board, so other users who open the board can see it. The picture is checked automatically by an AI image-checking service on Cloudflare before it is shown, and our staff can block it; until it passes, or if it is blocked, other users see a built-in buddy or a red cross instead. Please do not use a photo of a face you would not want other users to see. Changing the buddy picture replaces the copy, and deleting the account removes it.

Game ideas you send. A grown-up with an account that has a plan can send us an idea for a new game from the game picker: a short text (up to 300 characters) and, if they choose, one picture, which the app shrinks before sending. The text and the picture are sent to our server and read by our staff, together with the account's phone number so we can tell which family sent it. Please do not put names, addresses or photos of people in an idea. They are used only to decide which games to make, are not shown to anyone else, and are deleted when the account is deleted.

Reports about the national board. Next to every row on the national board, except the user's own, a small flag lets a grown-up report that row, behind a short question meant for adults, or hide it on that device only. A report holds the game, an identifier of the row (a one-way code made from the board row, not the phone number), the reason chosen (the name, the picture or something else), an optional note of up to 300 characters, the app version and the platform. It is sent to our server and read by our staff, together with the reporting account so we can tell which family sent it and stop repeats. Please do not put names or addresses in the note. When two different grown-ups report the same picture, the app shows the built-in buddy look instead until our staff have looked at it; staff can remove a picture, block it, or delete a name's scores. Reports are used only to keep the board safe, are not shown to anyone else, and are deleted when the account is deleted. A row hidden on a device is remembered only on that device.

Question reports. A grown-up with an account that has a plan can tell us that a question looks wrong, with the small flag at the top of a lesson (after a short question meant for adults). The report holds the question's code, the question text and answer choices as the user saw them, the answer the app counts as right, the answer the user had picked, the reason chosen (for example a wrong answer or a typing mistake), an optional note of up to 300 characters, the app version and the platform. It is sent to our server and read by our staff, together with the account's phone number so we can tell which family sent it. Please do not put names or addresses in the note. Reports are used only to correct questions, are not shown to anyone else, and are deleted when the account is deleted.

Đấu 1-1 (1-vs-1 quiz), only for a signed-in account (from version 1.9). Đấu 1-1 is a quick quiz against another student of the same grade. During a match the app keeps a network connection open to our server: the server sends each question, receives the answers with the time taken, scores them, and measures the connection delay so the match is fair. The opponent sees only the name entered in the app (filtered as on the leaderboard), the last four digits of the phone number, the companion picture, the score in the match, whether the current question has been answered and, after each question, right or wrong. There is no chat and no typed text, voice or photos; the two players can only send each other five fixed reaction pictures before and after a match, and the server does not store them. When no student of the same grade is waiting, and always outside 06:00 to 21:30 Vietnam time, the opponent is a computer player run by the server, shown with a sample name and picture. The server stores: each match (the two accounts, grade, kind of match, list of questions, scores, result and times) for 90 days; the questions you have seen and whether you got them right, so they do not repeat, for 60 days; the number of matches per day, to count the 3 free matches without a plan, for 3 days; the Đấu 1-1 points per grade shown on the Đấu 1-1 leaderboard (all time and this week) until you delete your duel history or your account; and the list of students you have blocked. On the result screen you can report a student (a reason and an optional note of up to 300 characters, with that student's shown name) or block them; reports reach our staff, and you will not meet a blocked student again. In Settings, the Đấu 1-1 section lists blocked students (unblock), and "Xoá lịch sử đấu" deletes your matches, seen questions and Đấu 1-1 points (only the day's match count is kept until its 3 days end, so the free limit stays right). The daily challenge and the Đấu 1-1 badges stay on the device. Matches give no stars or play time, and nothing else about your learning is sent.

What is stored, and where

Your profile, on the device: the name typed in, the buddy, stars, progress and settings, kept in the app's own storage. A small copy of the progress is also kept in the device's Keychain (iPhone, iPad and Mac), so that installing the app again does not lose it; it stays on the device and is not sent to us. On Android the system's own app backup can restore the same files after a reinstall. If your device backs itself up, for example to iCloud, the backup can include the profile, as it does other apps' data.

Downloaded voices are kept in a separate folder that is left out of backups, since they can be downloaded again.

Deleting the app removes the files; the small Keychain copy stays on the device for a reinstall.

Deleting an account. A signed-in grown-up can delete the account in Settings with the password. That removes the phone number, name, plan records, signed-in devices and board scores (including Đấu 1-1 matches and points) from our server. Our back-office activity log may keep a short record of staff actions (who, what, when) that refers to the account.

Feedback email

If a grown-up taps Góp ý cho người làm ứng dụng in Settings, the app opens a new email in your mail app with a short template: the app version, the device model and system version, the chosen accent, whether the voice is on, and the number of stars. Nothing is sent until you press Send, and you can change or delete any of it first.

Age and accounts

Chăm luyện is meant for pupils in grades 1 to 9 (about ages 6 to 15), learning with a grown-up at home. It asks only for a first name, so the buddy can greet the child; that name stays on the device unless a signed-in account is on the national board or plays Đấu 1-1, in which case the name, scores and, if chosen, the buddy picture are shown to other users, as described above. Accounts and plans are created and managed by adults. The app has no ads and no tracking, and uses no advertising ID or hardware identifier of the device. Everything the user hears was written and recorded in advance; no AI talks to you.

Contact

Questions about this policy, or anything else: support@khangazun.com.