Peer to peer fast bitcoin payments. Transfer bitcoin free and fast - online or fully offline. No central server. No channels. No hops. No routing. No account. Self custody always. It works on Bitcoin today. No changes. No BIPs. No hacks. It just works.
A Kagikai instrument is a Bitcoin UTXO you can hand to someone - whether you're online or off. Two keys change hands. No broadcast. No fee.
Your device generates keys B and C locally - only the public keys go to the creator. The creator builds a Taproot address, funds it once, and is permanently locked out. You hold the spending threshold from day one.
Keys B and C are NaCl-sealed to the recipient's public key. Delivered via QR, NFC, HTTPS, or deep link online - or QR code and manual confirmation with zero internet. Done in under five seconds.
The receiver signs a nonce with Key B and publishes the proof to Nostr. The sender verifies it - and only then deletes their copy. The transfer is irreversible. No server. No intermediary.
Keys B and C aggregate via MuSig2 into a single Taproot keypair. Settlement is a standard 64-byte Schnorr signature - indistinguishable on-chain from any ordinary Bitcoin payment.
The final holder broadcasts a single sweep transaction to any address they control. Only the second on-chain event in the instrument's entire lifetime - regardless of how many transfers happened in between.
Keys live in the device's secure hardware enclave - Android Keystore (AES-256-GCM, hardware security chip; on StrongBox devices keys are designed to never leave the Titan M2) and iOS Secure Enclave (keys are generated and stored inside the Secure Enclave and are designed to be non-extractable under normal operating conditions). No cloud. No server. No custody.
Every prior Bitcoin instrument keeps the creator in the spending path. You trust them not to use their key.
Kagikai inverts the threshold and puts control in the hands of the holder. In a 2-of-3 scheme the holder holds both Key B and Key C - that's the full spending threshold. The creator holds only Key A - one key, below the threshold of two, which is then securely deleted before the instrument is funded. The creator is designed to be unable to spend - the threshold requirement is enforced by every Bitcoin node on the network.
The same instrument works across all connectivity situations. There's no special mode to switch into - Kagikai adapts to whatever transport is available.
When connected, payments flow via HTTPS, QR code scan, NFC tap, or app deep link. Verification hits the live mempool for instant confirmation.
No signal? Instruments transfer over QR code and verify in app or against a cached local bounded UTXO snapshot. Zero bytes of internet required.
Kagikai instruments are created at set satoshi values. You don't transfer a single instrument and get change back - you carry the right mix.
A 10,000 sat instrument is a 10,000 sat instrument. It doesn't split. It doesn't make change. When you hand it over, the full amount moves to the new holder - instantly, with no fee.
The user generates a ladder of denominations, and holders combine instruments to hit the right amount. If you don't have a combination that works, you can always sweep back into the funding wallet (bitcoin tx fee applies) and create a new set of smaller or larger instrument combinations to achieve the right amount.When interacting with a merchant, the merchant keeps a change float so you don't have to sweep to reorganize - they return the difference as smaller instruments.
*As the price of bitcoin increases, smaller denominations can be added to later updates to keep pace with the decline of fiat value. At this time 1,000 sats is the smallest economically viable unit for sweeping individually as on-chain tx fees will consume much of the instrument's value. The use of MuSig2 for batched multi-instrument sweeps makes the tx fee more economically viable.
Three complementary protocols, each designed for a different job.
Lightning handles arbitrary-amount routed payments. Ark handles no-setup receiving with a server. Kagikai handles peer to peer Bitcoin transfers, offline payments, and zero-fee instrument handover - the denominated instrument use case the protocol has always supported.
Online or offline - Kagikai instruments work anywhere.
Tap-to-pay at the counter, web checkout, or a market stall with no signal. The same QR or NFC workflow works in every environment.
Send bitcoin as a pre-funded instrument at fixed denominations. Fast delivery and free transfer - ideal for gifts, tips, and rewards.
An instrument that has changed hands 47 times carries exactly the same value as one that changed hands once. Unlimited hops, zero cumulative fee.
Peer payments in areas with poor mobile signal. Verification runs against a cached UTXO snapshot - no internet required.
MBI-PRP supports HTTPS, NFC, QR, and deep link transports. No account or prior arrangement needed - works in any existing checkout stack.
Accept bitcoin at the counter with zero payment processing fees. No middleman. No monthly subscription. No percentage cut. Every sat goes straight to you.
KMS runs on your own hardware — a spare laptop, a mini PC behind the counter, or a VPS you control. No sign-up with a payment processor. No custody risk. Funds land directly in your wallet, not someone else's account.
Your merchant key is encrypted at rest with AES-256-GCM and only lives in memory while KMS is running. No cloud. No third-party access. You hold the keys at all times.
Full technical specification: threshold inversion, buyer-generated key protocol, bounded UTXO set, MBI-PRP payment protocol, adversarial security analysis, MuSig2 and Taproot implementation, and applications to transferable value instruments.
Be among the first to test Kagikai. Sign up and we'll reach out when the beta is ready.
Longtime Bitcoiner and open source advocate with a career spanning design, full stack development, and leading digital teams. A Linux user since the early days, with deep roots in the open internet and the cypherpunk tradition of building tools that put users in control.
Kagikai does not collect user data, does not require an account, and holds no Bitcoin on anyone's behalf. The core cryptographic method has been published as a formal research paper and tested with real transactions on the Bitcoin mainnet.
For interviews, commentary, or coverage of Kagikai and the MBI protocol, reach out directly. Happy to provide technical background, demo access, or high-resolution assets.
Note: Kagikai is a cryptographic protocol for transferring control of Bitcoin UTXOs. It does not constitute a payment service, money transmission system, or financial product. Businesses and merchants integrating Kagikai-based instruments into commercial operations may have independent obligations under applicable AML, KYC, consumer protection, and payment services laws in their jurisdiction. Users and operators are responsible for ensuring compliance with all local regulations. Nothing on this page constitutes financial, legal, or regulatory advice.
