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ICP transfer

View this samples code on GitHub.

Overview​

ICP transfer is a canister that can transfer ICP from its account to other accounts. It is an example of a canister that uses the ledger canister. Sample code is available in Motoko and Rust.

[!NOTE] The ICP ledger supports the ICRC1 standard, which is the recommended standard for token transfers. You can read more about the differences here and find an example of how to transfer ICRC1 tokens from a canister in Motoko and Rust.

Architecture​

The sample code revolves around one core transfer function which takes as input the amount of ICP to transfer, the account (and optionally the subaccount) to which to transfer ICP and returns either success or an error in case e.g. the ICP transfer canister doesn’t have enough ICP to do the transfer. In case of success, a unique identifier of the transaction is returned. This identifier will be stored in the memo of the transaction in the ledger.

This sample will use the Motoko variant.

Prerequisites​

This example requires an installation of:

  • Install the IC SDK.
  • Download and install git.

How to get there​

The following steps will guide you through the process of setting up the token transfer canister for your own project.

[!TIP] If you just want to interact with this example, follow steps 4-6 and 8-11 below.

Step 1: Create a new dfx project and navigate into the project's directory.​

dfx dfx new --type=motoko icp_transfer --no-frontend
cd icp_transfer

Step 2: Determine ledger file locations​

[!NOTE] You can read more about how to setup the ICP ledger locally here.

Go to the releases overview and copy the latest replica binary revision. At the time of writing, this is d87954601e4b22972899e9957e800406a0a6b929.

The URL for the ledger Wasm module is https://download.dfinity.systems/ic/<REVISION>/canisters/ledger-canister.wasm.gz, so with the above revision it would be https://download.dfinity.systems/ic/d87954601e4b22972899e9957e800406a0a6b929/canisters/ledger-canister.wasm.gz.

The URL for the ledger.did file is https://raw.githubusercontent.com/dfinity/ic/<REVISION>/rs/rosetta-api/icp_ledger/ledger.did, so with the above revision it would be https://raw.githubusercontent.com/dfinity/ic/d87954601e4b22972899e9957e800406a0a6b929/rs/rosetta-api/icp_ledger/ledger.did.

[OPTIONAL] If you want to make sure you have the latest ICP ledger files, you can run the following script. Please ensure that you have jq installed as the script relies on it.

curl -o download_latest_icp_ledger.sh "https://raw.githubusercontent.com/dfinity/ic/00a4ab409e6236d4082cee4a47544a2d87b7190d/rs/rosetta-api/scripts/download_latest_icp_ledger.sh"
chmod +x download_latest_icp_ledger.sh
./download_latest_icp_ledger.sh

Step 3: Configure the dfx.json file to use the ledger :​

Replace its contents with this but adapt the URLs to be the ones you determined in step 2:

[!IMPORTANT] Don't forget to add the icp_ledger_canister as a dependency for icp_transfer_backend, otherwise the build will fail.

{
    "canisters": {
        "icp_transfer_backend": {
            "main": "src/icp_transfer_backend/main.mo",
            "type": "motoko",
            "dependencies": ["icp_ledger_canister"]
        },
        "icp_ledger_canister": {
            "type": "custom",
            "candid": "https://raw.githubusercontent.com/dfinity/ic/d87954601e4b22972899e9957e800406a0a6b929/rs/rosetta-api/icp_ledger/ledger.did",
            "wasm": "https://download.dfinity.systems/ic/d87954601e4b22972899e9957e800406a0a6b929/canisters/ledger-canister.wasm.gz",
            "remote": {
                "id": {
                    "ic": "ryjl3-tyaaa-aaaaa-aaaba-cai"
                }
            }
        }
    },
    "defaults": {
        "build": {
            "args": "",
            "packtool": ""
        }
    },
    "output_env_file": ".env",
    "version": 1
}

Step 4: Start a local replica:​

dfx start --background --clean

Step 5: Deploy the ledger canister to your network:​

[!IMPORTANT] Transfers from the minting account will create Mint transactions. Transfers to the minting account will create Burn transactions.

Take a moment to read the details of the call made below. Not only are you deploying the ICP ledger canister, you are also:

  • Deploying the canister to the same canister ID as the mainnet ledger canister. This is to make it easier to switch between local and mainnet deployments and set in dfx.json using specified_id.
  • Setting the minting account to the anonymous principal (2vxsx-fae)
  • Minting 100 ICP tokens to the DEFAULT_ACCOUNT_ID (1 ICP is equal to 10^8 e8s, hence the name).
  • Setting the transfer fee to 0.0001 ICP.
  • Naming the token Local ICP / LICP
dfx deploy icp_ledger_canister --argument "(variant {
    Init = record {
      minting_account = \"$(dfx ledger --identity anonymous account-id)\";
      initial_values = vec {
        record {
          \"$(dfx ledger --identity default account-id)\";
          record {
            e8s = 10_000_000_000 : nat64;
          };
        };
      };
      send_whitelist = vec {};
      transfer_fee = opt record {
        e8s = 10_000 : nat64;
      };
      token_symbol = opt \"LICP\";
      token_name = opt \"Local ICP\";
    }
  })
"

If successful, the output should be:

Deployed canisters.
URLs:
  Backend canister via Candid interface:
    icp_ledger_canister: http://127.0.0.1:4943/?canisterId=bnz7o-iuaaa-aaaaa-qaaaa-cai&id=ryjl3-tyaaa-aaaaa-aaaba-cai

Step 6: Verify that the ledger canister is healthy and working as expected by using the command:​

dfx canister call icp_ledger_canister account_balance '(record { account = '$(python3 -c 'print("vec{" + ";".join([str(b) for b in bytes.fromhex("'$(dfx ledger --identity default account-id)'")]) + "}")')'})'

The output should be:

(record { e8s = 10_000_000_000 : nat64 })

Step 7: Prepare the ICP transfer canister:​

Replace the contents of the src/icp_transfer_backend/main.mo file with the following:

import IcpLedger "canister:icp_ledger_canister";
import Debug "mo:base/Debug";
import Result "mo:base/Result";
import Option "mo:base/Option";
import Blob "mo:base/Blob";
import Error "mo:base/Error";
import Array "mo:base/Array";
import Principal "mo:base/Principal";

actor {
  type Tokens = {
    e8s : Nat64;
  };

  type TransferArgs = {
    amount : Tokens;
    toPrincipal : Principal;
    toSubaccount : ?Blob;
  };

  public shared ({ caller }) func transfer(args : TransferArgs) : async Result.Result<IcpLedger.BlockIndex, Text> {
    Debug.print(
      "Transferring "
      # debug_show (args.amount)
      # " tokens to principal "
      # debug_show (args.toPrincipal)
      # " subaccount "
      # debug_show (args.toSubaccount)
    );

    let transferArgs : IcpLedger.TransferArgs = {
      // can be used to distinguish between transactions
      memo = 0;
      // the amount we want to transfer
      amount = args.amount;
      // the ICP ledger charges 10_000 e8s for a transfer
      fee = { e8s = 10_000 };
      // we are transferring from the canisters default subaccount, therefore we don't need to specify it
      from_subaccount = null;
      // we take the principal and subaccount from the arguments and convert them into an account identifier
      to = Blob.toArray(Principal.toLedgerAccount(args.toPrincipal, args.toSubaccount));
      // a timestamp indicating when the transaction was created by the caller; if it is not specified by the caller then this is set to the current ICP time
      created_at_time = null;
    };

    try {
      // initiate the transfer
      let transferResult = await IcpLedger.transfer(transferArgs);

      // check if the transfer was successfull
      switch (transferResult) {
        case (#Err(transferError)) {
          return #err("Couldn't transfer funds:\n" # debug_show (transferError));
        };
        case (#Ok(blockIndex)) { return #ok blockIndex };
      };
    } catch (error : Error) {
      // catch any errors that might occur during the transfer
      return #err("Reject message: " # Error.message(error));
    };
  };
};

Step 8: Deploy the token transfer canister:​

dfx deploy icp_transfer_backend

Step 9: Determine out the address of your canister:​

TOKENS_TRANSFER_ACCOUNT_ID="$(dfx ledger account-id --of-canister icp_transfer_backend)"
TOKENS_TRANSFER_ACCOUNT_ID_BYTES="$(python3 -c 'print("vec{" + ";".join([str(b) for b in bytes.fromhex("'$TOKENS_TRANSFER_ACCOUNT_ID'")]) + "}")')"

Step 10: Transfer funds to your canister:​

[!IMPORTANT] Make sure that you are using the default dfx account that we minted tokens to in step 7 for the following steps.

Make the following call to transfer funds to the canister:

dfx canister --identity default call icp_ledger_canister transfer "(record { to = ${TOKENS_TRANSFER_ACCOUNT_ID_BYTES}; memo = 1; amount = record { e8s = 2_00_000_000 }; fee = record { e8s = 10_000 }; })"

If successful, the output should be:

(variant { Ok = 1 : nat64 })

Step 11: Transfer funds from the canister:​

Now that the canister owns ICP on the ledger, you can transfer funds from the canister to another account, in this case back to the default account:

dfx canister call icp_transfer_backend transfer "(record { amount = record { e8s = 100_000_000 }; toPrincipal = principal \"$(dfx identity --identity default get-principal)\"})"

Security considerations and best practices​

If you base your application on this example, we recommend you familiarize yourself with and adhere to the security best practices for developing on the Internet Computer. This example may not implement all the best practices.

For example, the following aspects are particularly relevant for this app: