FAQ and security

No. Your wallet keys are generated or imported on your device. We do not hold a copy enabling us to dispose of your BTC and do not maintain a withdrawable customer balance.

Actual key and control rights determine this. KAI-Pay uses Spark via the Breez SDK. Spark requires the Spark operators to cooperate for payments. Spark also provides a unilateral exit that works without the Spark operators; what it requires is explained under “What if the service closes?”.

Not automatically. Each additional service requires separate assessment. The authorisation of a linked purchase provider (“Can I buy bitcoin?”) also applies only to that provider, not to KAI-Pay.

No. We do not have a copy of your 12 words and never ask for them. Never share them with anyone – neither with us nor with third parties.

To restore your wallet in KAI-Pay: yes, provided the Spark operators cooperate. KAI-Pay derives the wallet from the 12 words alone and does not use Lightning channels of its own for you whose data would need a separate backup. A passkey wallet also shows you the 12 words; they work without the passkey.

You set up your PIN and fingerprint unlock again on the new device. The 12 words alone are not enough for a unilateral exit without the Spark operators; for that you additionally save an emergency package (see “What if the service closes?”). Test recovery before you transfer funds to the wallet.

Because only the 12 words contain your wallet. There is no account with us and no copy of your keys that we could give back to you. In the Android app “KAI-Pay Beta”, “Restore wallet” with the 12 words is therefore the only way. In the browser you can also open a passkey wallet via “Open with passkey”; it has 12 words too.

The emergency package is not a restore: it only serves the unilateral exit without the Spark operators (see “What if the service closes?”).

As there is no account, there is no log-out in the usual sense. Instead you lock the wallet: the “Log out” button at the top right (also in the menu: “Log out (lock wallet)”). This removes the keys from memory; afterwards only your PIN, fingerprint or passkey opens the wallet again. It also locks automatically after 90 seconds in the background or after 5 minutes without use.

To remove the wallet from the device entirely, use Settings → “Remove wallet from this device”. After that, only the 12 words give you access to your funds again.

Usually not technically after settlement; we cannot reverse completed payments either. Legal claims against recipients or providers may remain.

Check the existing transaction first to avoid duplication. If the outcome of a payment is unclear, the app shows “Outcome not yet clear – being checked”. The activity list shows the payment's status: “Completed”, “Pending” or “Failed”. If an earlier payment is still pending, the app points this out before you send again.

KAI-Pay currently charges nothing for the wallet itself. Payments may incur third-party fees, for example for the Bitcoin network, Lightning routing or Spark.

When you send, the app shows the amount, fee and total before you confirm. If receiving via a Lightning invoice incurs a fee, the app shows it together with the invoice. When receiving to a Bitcoin address, a fee may apply for crediting the funds; the app does not show it in advance. A displayed equivalent in national currency is an approximation and does not guarantee future BTC value.

No. A source you set up yourself, such as KAI, can only propose payments to you. The wallet checks the signature, allowed recipients and your budget; you see recipient, amount and fee before every payment, and nothing is paid until you confirm. The purpose of a proposal is stated by the source; check it yourself.

Step by step in the point-of-sale guide: set up (also on the same device), take payments in national currency or sats, with articles, several tills, daily and monthly closing, CSV export, PDF receipts and refunds.

No. The point of sale is a payment terminal and not an electronic recording system with a certified technical security device (TSE) within the meaning of the German Cash Register Security Ordinance. Its receipt is proof of payment, not a till receipt. Keep recording your sales in your till; you remain responsible for recording, receipt and tax obligations (terms of use).

Yes. Payment is always in bitcoin (sats); a display in national currency does not mean a fiat account. The wallet shows the equivalent in the reference currency of your choice at the rate of the Breez service and states when it was fetched; if the rate is older than 15 minutes, it stops converting. You can choose from 34 currencies of countries where bitcoin and Lightning are demonstrably used in everyday life (at least 50 places accepting Lightning according to BTC Map) or that are among the 20 countries with the highest crypto adoption (Chainalysis Global Crypto Adoption Index 2026).

The point of sale charges in EUR, USD, GBP, CHF, CAD, AUD or JPY at the rate of mempool.space. The rate is fixed for the invoice; if the point of sale was set up via your wallet’s point-of-sale link, wallet and receipt show exactly this amount. The point of sale only charges in national currency with a rate fetched at most 5 minutes ago and at most 30 minutes old according to mempool.space.

The wallet and the point of sale are available in German, English, Spanish, Portuguese, French, Arabic and Vietnamese. We chose German, English, Spanish and Portuguese because that is where bitcoin via Lightning is most widely accepted in everyday life: each of these four languages accounts for at least 5% of all places accepting Lightning worldwide, together for about 85% (BTC Map, as of October 2026). French, Arabic and Vietnamese were added because we see great potential there: they reach many people in Africa, the Middle East and Southeast Asia. Arabic is shown from right to left; amounts use Latin digits, as in invoices and QR codes.

The website is available in the same seven languages. Legal texts (legal notice, privacy policy, terms, risk notice), the privacy details in “Who sees what?” and details of external providers are available in German and English; in the other languages we show the English version of them. PDF receipts in Arabic and Vietnamese are produced in English for now.

Yes – in the point of sale after “Paid” and in the wallet in the payment details (“Receipt as PDF”), with a separate refund receipt for refunds. The PDF is created on your device from the stored data; created again, the same payment gives the same file (same language and time zone). It is proof of payment, not a VAT invoice and not a till receipt.

“Find merchants” shows places that accept bitcoin via Lightning according to BTC Map (data from OpenStreetMap). An entry is not a contractual partnership with KAI-Pay, and details may be outdated – “Report outdated” leads to BTC Map. Entries marked “Confirmed by KAI-Pay” were checked by us: the place existed and accepted Lightning on the stated date. If the entry changes on OpenStreetMap afterwards or the check is older than 12 months, the mark disappears.

Searches only run when you tap. BTC Map always learns your IP address and the time; with “Near me” also only an area rounded to a few kilometres – the wallet computes the distance on your device –, with the place search the search term. In the Android app, “Near me” only asks for your approximate location (Android 12 and later); on older devices the place search remains.

“Donate” lists organisations recognised as charitable in their home country. KAI-Pay compiles the list itself; it implies no partnership with the organisations and no recommendation. The donation goes directly from your wallet to the organisation – KAI-Pay holds nothing and receives nothing from it. Payable in the wallet are only Lightning addresses on the organisation’s own domain; before every donation the wallet checks – also in the send flow itself – that the address still matches the reviewed entry and stops otherwise. Organisations without a Lightning address on their own domain are reached via their donation page.

Only the organisation can issue a donation receipt, usually not for anonymous Lightning donations. Donations to organisations outside the EU and the EEA are not deductible in Germany under § 10b EStG. This is not tax advice.

L402 is an open standard that lets online services charge small amounts via Lightning – instead of an account, subscription or credit card. Your benefits: you pay only for what you use, from just a few sats, in seconds and worldwide, and through the payment you give the service neither your name nor card details. In detail with benefits and how it works: Use cases explained.

Yes, for L402 services in the common aperture format. Under “Send”, paste the service’s L402 challenge – the content of the header WWW-Authenticate: L402 macaroon="…", invoice="…", as shown by curl, for example. The wallet only pays the included invoice and only if it matches the access (macaroon); this protects against a swapped invoice, not against an entirely forged challenge – so check where it comes from. L402 payments are limited to 1,000 sat (excluding fees), and the wallet does not pay for an access again as long as the payment is in the history on this device. Afterwards the wallet shows the access proof L402 <macaroon>:<preimage> to copy; you send it to the service yourself as the Authorization header. If the connection drops, you can find the proof in the payment under “Activity” – on this device, not after a restore with your 12 words or on another device.

The wallet does not call the service itself and reports nothing back to it; whether and for how long you get access is up to the service. Treat the proof like a password. Exception: the “Timestamp” button for registered companies gets the challenge and the proof directly from KAI Oracle (kai-oracle.com) and submits there. This timestamp is a separate paid service of Formsys GmbH (KAI Oracle) and not part of the free wallet. The wallet computes the file’s fingerprint (SHA-256) on your device and sends only the fingerprint, not the file.

In the Android app “KAI-Pay Beta” (from the version with NFC): if you hold the unlocked phone to an NFC tag with a payment link (lightning: or bitcoin:), Android reads it and opens KAI-Pay under “Send” with that destination and the note “Read via NFC” – even if the app was closed. If a process is open, it stays unchanged; the app then shows “New payment link received” to open it. Nothing is paid until you have checked and confirmed recipient, amount and fee – just like with a QR code. The app ignores other content; the same tag only counts once within a few seconds. Reading sends nothing; only “Check” contacts the recipient’s server where needed – as with a QR code. NFC can be switched off in the Android settings; without NFC, the QR code remains.

Phone to phone: under “Receive”, the app can “Offer via NFC” an invoice or your Lightning address. Your phone then behaves like an NFC tag with exactly this payment link for up to two minutes. If someone holds their unlocked Android phone with KAI-Pay Beta against it, “Send” opens there – with the amount for an invoice; for a Lightning address the payer enters the amount –, and payment happens only after confirmation, no camera or QR code needed. During this time any phone within NFC range can read the link. Offering ends when you leave the app, with “Stop NFC” and, for an invoice, as soon as the payment arrives; meanwhile your phone does not read NFC links itself. This only works between Android phones; on the iPhone it is currently not possible for KAI-Pay. The browser-based point of sale does not have NFC yet.

Yes, as a payment plan with a reminder: under “Standing orders” you set the recipient (Lightning address or LNURL), amount (in sat or a national currency) and schedule – monthly on a day or weekly on a weekday. When a payment is due, the wallet shows it when you open it; nothing is paid until you confirm, with a fresh invoice from the recipient. For a national currency, the wallet converts at the current rate (at most 15 minutes old) when you tap “Pay now” and shows amount, rate and sat before you confirm. On this device the wallet remembers every paid or skipped due date and does not offer it again; if the outcome of a payment is unclear, it shows the due date as unresolved under “Standing orders” until the activity resolves it. It does not catch up on due dates before you created the plan or during a pause. Plans and records exist only on this device – if you create the same standing order on several devices, pay on only one. KAI-Pay does not pay automatically without confirmation: your keys are only on your device, so no server can pay for you. If you like, “Add to calendar” creates a reminder for your own calendar; the file is a copy, pausing or deleting in the wallet does not change your calendar.

KAI-Pay itself does not sell bitcoin. Under “Buy bitcoin”, the wallet opens the external website of Strike. The provider of the bitcoin purchase there is Zap (Strike) Europe Limited (Malta). It holds an authorisation as a crypto-asset service provider under MiCA granted by the Malta Financial Services Authority (MFSA) and is listed in the corresponding ESMA register. The authorisation concerns Strike; it is not an authorisation or official recommendation of KAI-Pay or Formsys GmbH and guarantees neither a particular bitcoin value nor the availability of the service or the repayment of funds used. Bitcoin holdings are not covered by any statutory deposit guarantee.

KAI-Pay does not accept your purchase order, does not process the purchase, holds neither purchase funds nor the purchased bitcoin and receives no remuneration for the link. The link itself does not forward a private key, invoice or wallet address to Strike. The purchase contract is concluded solely with the provider named at Strike. Account, identity check, purchase costs, holding or withdrawal periods and payout are governed by its terms; we remain responsible for our own services (terms, section 15). We give no advice and make no promise that Strike is suitable or the cheapest for you; check the final price and payout fees before buying.

How the bitcoin reaches your wallet: first back up your 12 words. Then copy your Lightning address under “Buy bitcoin” and, after the purchase, paste it at Strike as the recipient when sending – you do not need an invoice or a bitcoin address for this. Compare the address character by character and check the amount before sending. Without a Lightning address of your own, a fresh Lightning invoice for the amount Strike offers you to send also works; if it has expired, create a new one instead of buying again. Only the incoming payment in KAI-Pay counts, not a message from Strike or returning to the app – if Strike shows an error message, do not send again, first check for the incoming payment. Successfully executed bitcoin payments usually cannot be reversed.

The purchase is not anonymous: Strike knows your identity and can link the invoice or address you enter to your account (privacy policy 2.18). Never enter your 12 words or private keys at Strike or any other provider – they are not needed for buying or payout. Bitcoin is subject to strong price fluctuations; substantial losses up to a total loss are possible. No investment or tax advice.

If the provider has a disruption, the link target changes or for security reasons, we may switch the link off; the wallet shows this and otherwise keeps working unchanged.

It depends on whether you have backed up your 12 words. The app keeps your wallet data in the storage of your browser or the app on this device; your 12 words are stored there only in encrypted form. If this data is deleted – for example in the browser settings or with “Remove wallet from this device” – the wallet can only be restored with your 12 words, or with the passkey for a passkey wallet. You then only have the exit data for a unilateral exit if you saved an emergency package beforehand. Deleting an app is not a backup. The privacy policy explains what remains where after removal.

Restore the wallet on a clean device with your 12 words (“Restore wallet”). If your keys may be exposed, safely move the remaining funds to a new wallet with new 12 words.

This also applies if the wallet on the lost device was protected by a PIN: the words are encrypted there, but anyone with a copy of the device data can try out a 6-digit PIN outside the app within a short time. This is ruled out only if you have set up fingerprint unlock without a PIN fallback in the web app. In the app “KAI-Pay Beta” the PIN remains mandatory. In the browser, “fingerprint” means your device’s user verification; depending on the device, the screen lock may be accepted too. Anyone who knows it can then open the wallet as well. Never send your 12 words to us or to third parties.

Then someone used your 12 words on another device to send money or to try to. The alert comes because notifications report every Lightning payment of your wallet, including from other devices. Create a new wallet with new 12 words right away and send your remaining balance there. Consider the old words burned and do not use them again. Check where your words were written down or stored.

The alert is on by default once you switch on notifications (Settings → Notifications → “Also for sent payments (security alert)”). It also reports your own payments; that is intended. It has one limit: anyone who knows your words can unsubscribe the notifications first. So the alert does not replace keeping your words safe.

Since September 2025, attackers have repeatedly planted malicious code in widely used software packages in the npm registry. Web apps such as KAI-Pay are also built from this registry. The malicious code swaps payment destinations in the browser for attacker addresses that look almost identical to the original.

On 8 October 2026 we checked every package KAI-Pay is built from, as well as the wallet and point of sale we serve: no known malicious version, no trace of the malicious code. Since then, every change to the packages and every release is checked against a public database of known malicious packages, and once a week we automatically check the pages we serve. We do not run package install scripts during the build, and we usually only adopt new package versions after a waiting period of seven days (security fixes may come sooner). If we find anything, we will inform you here.

What you should watch out for yourself: before every payment, compare the complete destination – beginning, middle and end – with its source, especially after copy and paste. Forged addresses often differ in only a few characters in the middle. Use saved contacts or scan the QR code directly. If the wallet warns that a destination resembles a contact or a destination you paid before but does not match it, only send once you have confirmed the address through a second channel. With a hardware wallet such as the BitBox, the address shown on its own display is what counts. Only open KAI-Pay via app.kai-pay.net or the official app, never enter your 12 words on a website or in a chat, and keep your browser and extensions up to date and limited to what you need.

Your keys are not held by us. KAI-Pay derives your wallet from your 12 words alone, using the open-source Breez SDK Spark; KAI-Pay servers are not involved in this. With the 12 words, the wallet can therefore in principle also be restored with other software based on this SDK, as long as the Spark operators cooperate. A passkey, by contrast, is tied to app.kai-pay.net. If that site is no longer available, only your 12 words will help, even for a passkey wallet.

Spark also provides a unilateral exit: funds can be moved back to the Bitcoin blockchain without the Spark operators' cooperation. This requires exit data that the wallet keeps on the device – the 12 words alone are not enough – and your own on-chain bitcoin to pay the network fees. You save an encrypted copy of the exit data as an emergency package under “Settings” → “Emergency exit”; it becomes out of date with every payment. If you run your own Bitcoin node, you can additionally have it check and watch the watch extract (also under “Emergency exit”). It is unencrypted, belongs only on your own devices and should then be deleted from your phone; nobody from KAI-Pay will ask for it (privacy policy 2.13). Because of time locks in the Bitcoin network, the exit can take several days to weeks. It does not succeed in every case; for small balances, the fees can exceed the balance itself. The KAI-Pay app does not currently carry out the exit itself.

Dependencies on the Bitcoin network and on providers such as the Spark operators may remain.

KAI-Pay cannot replace missing keys. Arrange secure succession or representation outside unprotected communication channels.

No, there is no anonymity guarantee. Network and transaction data may be linked. Encryption is not anonymity: even where KAI-Pay encrypts, such as the point-of-sale note, the payment, amount, time and counterparties remain visible to those involved. The privacy policy and, in the wallet, the page “Who sees what?” explain which data goes to whom. A bitcoin purchase at an external provider is not anonymous either (“Can I buy bitcoin?”).

No. Exporting your activity as a CSV file and statutory reporting by providers do not discharge your own duties. The CSV export covers the full history known to the wallet on this device; please still check it yourself.