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Multi-witness transactions

Overview​

note

This guide assumes that you have completed the Exploring Cardano Wallets guide. You will need one UTxO sitting at each of the wallets (payment1.addr and payment2.addr) to complete this guide.

This guide also assumes that you have cardano-node running in the background and connected to the testnet network.

Recap​

Let's recap what we did so far. Our goal in the previous guide was to draw 1000 tADA from the testnet faucet and send 250 tAda from payment1 to payment2.

Make sure we are in the correct folder.

$ pwd
$HOME/cardano

We drew 1000 tAda from the Testnet Faucet into our payment1 wallet.

$ cardano-cli query utxo \
--testnet-magic 1097911063 \
--address $(cat keys/payment1.addr)
                           TxHash                                 TxIx        Amount
--------------------------------------------------------------------------------------
264c0aa805652e3607c5ea2b1e8a9f3bf9c3bc8d4d938e1a9035f352083ba703     0        1000000000 lovelace

We currently have 749.825831 tAda in our payment1 wallet and 250 tAda in our payment2 wallet.

Let's see how we can spend it all at once!

Use case​

There are many possible reasons why you would want to have multiple wallets sending their ada in a single transaction. One is, you own two wallets (payment1 and payment2) and you want to spend it on something that...

  • costs more than you have in any of your two wallets,
  • but both amounts combined would cover the costs.

Let's say you are at the bike store and you see a nice bike with a price tag of 1100 tAda on it. You only have 999 tAda (plus change) left.

The bike store owner - a devious blockchain enthusiast - is willing to give you a 10% discount, if you manage to pay him in a single transaction

There has to be no change, buddy! --Bike Store Owner

So we need to make sure to spend all our tAda from our two wallets in a single transaction.

note

He can easily verify if we spent all our money by checking if the transaction has more than one output.

There are ways to optimize the amount you spend. We will leave this for you to figure out yourself.

Technical Flow​

This scenario is pretty straight forward and looks like this.

img

As you can see in the diagram above, we will build and submit a multi-witness transaction, having two inputs and one output.

note

We can't do this with cardano-wallet, or any other wallet like Daedalus or Yoroi because we will need both signing-keys from payment1 and payment2 to sign the transaction.

Time to code​

note

As mentioned above, this guide assumes you completed the Exploring Cardano Wallets guide.
We also assume you paid 174169 Lovelace in transaction fees and that your current balances are:

  • payment1: 749825831 Lovelace
  • payment2: 250000000 Lovelace

Create a store-owner wallet​

If you don't already have a third wallet to use for this guide, let's create one where we can transfer all our funds to.

Make sure you are inside the keys directory like so: cd $HOME/cardano/keys

Generate a payment key-pair using cardano-cli:

cardano-cli address key-gen \
--verification-key-file $HOME/cardano/keys/store-owner.vkey \
--signing-key-file $HOME/cardano/keys/store-owner.skey

Then generate a wallet address for the testnet network:

cardano-cli address build \
--payment-verification-key-file $HOME/cardano/keys/store-owner.vkey \
--out-file $HOME/cardano/keys/store-owner.addr \
--testnet-magic 1097911063

Check your keys directory. It should look something like this:

$HOME/cardano/keys/
├── payment1.addr
├── payment1.skey
├── payment1.vkey
├── payment2.addr
├── payment2.skey
├── payment2.vkey
├── store-owner.addr
├── store-owner.skey
└── store-owner.vkey

0 directories, 9 files

Calculate the transaction fee​

Lets create a directory to store our transactions for this guide and enter it:

mkdir -p $HOME/cardano/multi-witness-sample && cd $_;

We want to send all our tAda sitting at the two UTxO we verified before and send it to the store-owner.addr. That means we will have two inputs.

What about the outputs? Well, the devious store-owner wants us to spend it all, so there will be one output to the store-owner and zero outputs to us. Remember? "...no change, buddy!"

Lets build that transaction.

cardano-cli transaction build-raw \
--tx-in b73b7503576412219241731230b5b7dd3b64eed62ccfc3ce69eb86822f1db251#0 \
--tx-in b73b7503576412219241731230b5b7dd3b64eed62ccfc3ce69eb86822f1db251#1 \
--tx-out $(cat ../keys/store-owner.addr)+0 \
--fee 0 \
--out-file tx2.draft

The last thing we need to do is to calculate the fees for tx2.draft. Notice the --tx-in-count and --witness-count.

cardano-cli transaction calculate-min-fee \
--tx-body-file tx2.draft \
--tx-in-count 2 \
--tx-out-count 1 \
--witness-count 2 \
--testnet-magic 1097911063 \
--protocol-params-file ../protocol.json 
179581 Lovelace

We can calculate the amount the store-owner will receive, if both UTxO are spent during the transaction:

  749825831 (payment1)
+ 250000000 (payment2)
  ---------
  999825831
-    179581 (fee)
  ---------
  999646250 (store-owner)
  =========

Build, sign and submit transaction​

We know the output amount as well as the fee. We can finally build, sign and submit our tx2.draft transaction.

We have to use payment1.skey and payment2.skey to sign our transaction.

cardano-cli transaction build-raw \
--tx-in b73b7503576412219241731230b5b7dd3b64eed62ccfc3ce69eb86822f1db251#0 \
--tx-in b73b7503576412219241731230b5b7dd3b64eed62ccfc3ce69eb86822f1db251#1 \
--tx-out $(cat ../keys/store-owner.addr)+999646250 \
--fee 179581 \
--out-file tx2.draft 

cardano-cli transaction sign \
--tx-body-file tx2.draft \
--signing-key-file ../keys/payment1.skey \
--signing-key-file ../keys/payment2.skey \
--testnet-magic 1097911063 \
--out-file tx2.signed

cardano-cli transaction submit \
--tx-file tx2.signed \
--testnet-magic 1097911063
Transaction successfully submitted

Verify multi-witness transactions​

The devious store-owner will now verify that everything went according to his plan.

cardano-cli query utxo \
--testnet-magic 1097911063 \
--address $(cat $HOME/cardano/keys/store-owner.addr)
                           TxHash                                 TxIx        Amount
--------------------------------------------------------------------------------------
258abd628eef7d6ff0f7b4e6866b4f7c21065f4d6b5e49b51e2ac4ff035ad06f     0        999646250 lovelace

Verify 258abd628eef7d6ff0f7b4e6866b4f7c21065f4d6b5e49b51e2ac4ff035ad06f on the cardano testnet explorer

Success!

He can see that the transaction has one output to his wallet. No other outputs, hence you must have spent all of your tAda.

Congratulations, you are now able to submit multi-witness transactions on Cardano. This should help you bring integrations to your existing or new upcoming applications. 🎉🎉🎉