Hodler.Swift

Hodler.Swift is a plugin to BitcoinCore, that makes it possible to lock bitcoins until some time in the future. It relies on CHECKSEQUENCEVERIFY and Relative time-locks. It may be used with other forks of Bitcoin that support them. UnstooppableWallet opts in this plugin and enables it for Bitcoin as an experimental feature.

How it works

To lock funds we create P2SH output where redeem script has OP_CSV OpCode that ensures that the input has a proper Sequence Number(nSequence) field and that it enables a relative time-lock.

In this sample transaction the second input unlocks such an output. It has a signature, public key and the following redeem script in it's scriptSig:

OP_PUSHBYTES_3 070040 OP_CSV OP_DROP OP_DUP OP_HASH160 OP_PUSHBYTES_20 853316620ed93e4ade18f8218f9aa15dc36c768e OP_EQUALVERIFY OP_CHECKSIG

• OP_PUSHBYTES_3 070040 OP_CSV OP_DROP part ensures that needed amount of time is passed. Specifically 07 part of 070040 bytes says that it's locked for 1 hour. See here and here for how it's evaluated.
• OP_DUP OP_HASH160 OP_PUSHBYTES_20 853316620ed93e4ade18f8218f9aa15dc36c768e OP_EQUALVERIFY OP_CHECKSIG part is the same locking script as of P2PKH output, that ensures the spender is the owner of the private key matching the public key hashed to 853316620ed93e4ade18f8218f9aa15dc36c768e.

Detection of incoming time-locked funds

When you have such an P2SH output, you only have an address and a hash of a redeem script in the output. If you are not aware of incoming time-locked funds in advance, there's no way you can detect that a particular output is yours. For this reason, we add an extra OP_RETURN output beside that P2SH output as a hint. That output tells us

• ID of the plugin (1 byte): BitcoinCore.Swift can handle multiple plugins like this one.
• Time-lock period (2 bytes)
• Hash of the receiver's public key (20 bytes)

For example, this is a hint output for the input above. It has following data:

OP_RETURN OP_PUSHNUM_1 OP_PUSHBYTES_2 0700 OP_PUSHBYTES_20 853316620ed93e4ade18f8218f9aa15dc36c768e

Limitations

Locked time periods

This plugin can lock coins for 1 hour, 1 month, half a year and 1 year. This is a limitation arising from the need of restoring those outputs using Simplified Payment Verification (SPV) Bloom Filters. Since each lock time generates different P2SH addresses, it wouldn't be possible to restore those outputs without knowing the exact lock time period in advance. So we generate 4 different addresses for each public key and use them in the bloom filters.

Prerequisites

• Xcode 10.0+
• Swift 5+
• iOS 13+

Installation

Swift Package Manager

dependencies: [
    .package(url: "https://github.com/horizontalsystems/Hodler.Swift.git", .upToNextMajor(from: "1.0.0"))
]

License

The Hodler toolkit is open source and available under the terms of the MIT License.