The BIP70 payment protocol is an interactive protocol for sending payment requests and receiving payments.

The protocol involves several steps:

  • Customer clicks a BIP21 bitcoin: URI extended with an additional r parameter (described in BIP72). The URI handler opens the user’s wallet in response

  • The wallet contacts the merchant’s server and asks for a payment request signed by the merchant’s SSL certificate. Upon receipt and validation of the signed payment request, the payment details are automatically filled out on the wallet’s Send Transaction screen. Optionally, the user is notified that they’re paying the owner of the domain corresponding to signing certificate (e.g. “example.com”)

  • The customer reviews the payment details and clicks the Send Transaction button. The transaction is generated and broadcast to the Bitcoin network. A copy of the transaction is sent to the merchant, who then replies with an acknowledgment that the payment was received (optionally rebroadcasting the transaction from their own nodes)

The protocol had several advantages over plain BIP21 URIs:

  • The merchant could request payment to any script—not just one of the popular address types—making the proposal forward compatible with new address formats

  • The customer’s wallet automatically sent their own script to be used in case a refund needed to be issued

  • Payment requests had an expiration date after which point the quoted price would no longer apply

  • It gave the spending transaction directly to the merchant, allowing them to broadcast it. This could allow a user to pay using Bluetooth or NFC even if they didn’t currently have a connection to the Internet

Its main disadvantage was that it required that spenders support the SSL certificate system, which includes many algorithms not otherwise used in Bitcoin and a complex Public Key Infrastructure (PKI) system. In practice, this meant software needed to include a library such as OpenSSL as a dependency. Bugs in that library could then be used to exploit Bitcoin programs, such as the OpenSSL heartbleed vulnerability that potentially allowed attackers to access private keys from Bitcoin Core in 2014.

Ultimately, BIP70 never saw widespread adoption and almost all merchants and wallets that did once support it have either ended support or plan to end their support in the future.

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