Bull Bitcoin has announced a deep Payjoin integration with its exchange, claiming to be the first Bitcoin exchange in the world to offer Payjoin as a deposit and withdrawal option. According to a press release shared with Bitcoin Magazine, the update introduces two new concepts: Payjoin-by-default and Payjoin trading.

Founded by Francis Pouliot in Canada in 2013, Bull Bitcoin is the world's longest-running Bitcoin-only exchange. Its non-custodial platform allows customers in Canada, Europe, and Latin America to buy Bitcoin directly into their own wallets and to sell Bitcoin to their bank accounts. The company also develops BULL Wallet, a free and open-source self-custodial Bitcoin wallet, and has been self-funded since its inception.

Payjoin is a Bitcoin on-chain privacy protocol that breaks basic assumptions made by on-chain analysis firms about user behavior. One such assumption holds that one of the unspent transaction outputs (UTXOs) resulting from a Bitcoin transaction will be change returning to the sender. Another is that amounts are more often than not denominated in dollars. Payjoin protocols add an off-chain coordination layer so that the amounts sent and returned to the sender obfuscate user intent, protecting privacy on-chain.

The protocol requires neither a hard fork nor a soft fork to Bitcoin consensus. Instead, it must be adopted by user wallets and exchanges in order to scale, though it does not depend on collective action to benefit individual users. That adoption model means Payjoin's reach grows one wallet and one service at a time, and how widely other exchanges follow Bull Bitcoin's lead will shape how far its network-level privacy effects extend. Bull Bitcoin's Payjoin integration is closely based on the work of the Payjoin Foundation.

Under the update, every time a BULL Wallet user buys or sells Bitcoin on-chain through the built-in exchange, the transaction will be a Payjoin by default. This allows Bitcoin buyers to passively aggregate their wallet's UTXOs, which can reduce their Bitcoin mining fees in the future. "Many Bitcoin users do not even know that they must periodically 'merge' their coins by spending them to a new address in their Bitcoin wallet, if they want to reduce the economic costs of future Bitcoin transactions," the press release explained. Payjoin-by-default automates this process. Beyond individual savings, it helps reduce the size of Bitcoin's UTXO set, making it easier for everyone to run a Bitcoin node, while also reducing fee pressure for every Bitcoin user.

Payjoin trading is a novel concept that protects users from malicious actors seeking to gain insights into their financial habits through chain analysis. Because transaction amounts in Bitcoin are public, it is possible to reverse-engineer information such as someone's salary or trading strategies such as dollar-cost averaging. This sets Bull Bitcoin apart from most cryptocurrency exchanges, which operate custodial models in which trades are matched on internal ledgers and only deposits and withdrawals touch the blockchain. As a non-custodial exchange, every Bull Bitcoin trade leaves an on-chain trail, making this protection particularly relevant.

Widespread adoption of Payjoin across the Bitcoin network makes common input ownership and change output detection heuristics less reliable. Bull Bitcoin's Payjoin implementation reduces round numbers in the amounts sent as well as address reuse, and lowers the possibility of third-party onlookers guessing the fiat value of Bitcoin transactions. The introduction of Payjoin-by-default and Payjoin trading means that the more people use Bull Bitcoin, the more privacy the Bitcoin network gains as a whole: "This is part of our philosophy to align the incentives of our company with the interests of all Bitcoin users."

In December 2024, BULL Wallet became the world's first Bitcoin wallet to integrate the Payjoin V2 protocol. This was a highly experimental project coordinated with Dan Gould and the Payjoin Dev Kit team, who have since founded the Payjoin Foundation. Researchers have used the insights from this experimentation to improve the Payjoin experience, making it more seamless and resistant to attacks. The software powering Payjoin, for both the wallet and the exchange, is fully free and open-source.

To benefit from these features, users need only download BULL Wallet, create a Bull Bitcoin exchange account, and log into their account via the BULL mobile app. "The rest happens automatically, like magic!"

This article first appeared on Bitcoin Magazine and was written by Juan Galt.