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The Bitcoin Canister

Overview

The Bitcoin canister is the core component of the Bitcoin integration project. It enables other canisters deployed on the Internet Computer to use Bitcoin and interact with the Bitcoin network.

To this end, it provides a low-level API with a small set of functions, which serve as the foundation to build powerful Bitcoin libraries and other development tools, and Bitcoin smart contracts running on the Internet Computer.

API

The Bitcoin canister exposes the following functions:

  • get_utxos: The function returns the unspent transaction outputs (UTXOs) of a given Bitcoin address.

  • get_balance: The function returns the balance of a given Bitcoin address.

  • send_transaction: The function sends the given transaction to the Bitcoin network.

The full interface description can be found here, expressed in Candid syntax.

More details about the functions are provided below.

Get Unspent Transaction Outputs of a Bitcoin Address

Given a base58-encoded address as part of a GetUtxosRequest, the function returns all UTXOs associated with the provided address.

type Satoshi = nat64;

type OutPoint = record {
  txid : blob;
  vout : nat32
};

type Utxo = record {
  outpoint: OutPoint;
  value: Satoshi;
  height: nat32;
  confirmations: nat32;
};

type GetUtxosRequest = record {
  address : text;
  min_confirmations: opt nat32;
  offset: opt nat32;
};

type GetUtxosError = variant {
  MalformedAddress;
  // More error types to be added here.
};

get_utxos: (GetUtxosRequest) -> (variant {
  Ok : record {
    utxos: vec Utxo;
    total_count: nat32;
  };
  Err : opt GetUtxosError;
});

If the call fails, e.g., because the address is malformed, a GetUtxosError is returned, indicating the reason for the failed call.

The optional min_confirmations parameter can be used to limit the returned UTXOs to those with at least the provided number of confirmations. If this parameter is not used, the default value is 0.

The optional offset parameter can be used to specify a starting offset in the list of UTXOs. This parameter is useful for addresses with many UTXOs.
Note that there is no guarantee that the set of UTXOs will remain unchanged between function calls with different offsets, i.e., every call will return the UTXOs starting from the provided offset based on the current view. If this parameter is not used, the default value is 0.

Get the Balance of a Bitcoin Address

Given a base58-encoded address as part of a GetBalanceRequest , the function returns the current balance of this address in Satoshi (100,000,000 Satoshi = 1 Bitcoin).

type GetBalanceRequest = record {
  address : text;
  min_confirmations: opt nat32;
};

type GetBalanceError = variant {
  MalformedAddress;
  // More error types to be added here.
};

get_balance: (GetBalanceRequest) -> (variant {
  Ok : Satoshi;
  Err: opt GetBalanceError;
});

If the call fails, e.g., because the address is malformed, a GetBalanceError is returned, indicating the reason for the failed call.

The optional min_confirmations parameter can be used to limit the set of considered UTXOs for the calculation of the balance to those with at least the provided number of confirmations.

Send a Bitcoin Transaction

Given a SendTransactionRequest containing the the raw bytes of a Bitcoin transaction, the transaction is forwarded to the Bitcoin network if it passes a set of validity checks.

type SendTransactionRequest = record {
  transaction: blob;
};

type SendTransactionError = variant {
  MalformedTransaction;
  // More error types to be added here.
};

send_transaction: (SendTransactionRequest) -> (variant {
  Ok : null;
  Err : opt SendTransactionError;
});

The following validity checks are performed:

  • The transaction is well-formed.

  • The transaction only consumes unspent outputs.

  • All signatures are correct.

  • There is a positive transaction fee.

  • The transaction does not create dust, i.e., an output that holds a smaller Bitcoin amount than it costs to spend the Bitcoin in the output.

Note
 The Bitcoin canister provided as part of the developer preview only checks that the transaction is well-formed.

If at least one of these checks fails, a SendTransactionError is returned, indicating the reason for the failed call.

The Bitcoin canister caches the transaction and periodically forwards the transaction until the transaction appears in a block or the transaction times out after 24 hours, at which point the transaction is removed from the cache.

Note
 The Bitcoin canister provided as part of the developer preview does not cache transactions.