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A Basic Bank using Leo


This program implements a bank that issues tokens to users and allows users to deposit tokens to accrue simple interest on their deposits.

User Flow

  1. The bank issues users tokens via the issue function.
  2. A user deposits tokens via the deposit function.
  3. Upon a user's request to withdraw, the bank calculates the appropriate amount of compound interest and pays the user the principal and interest via the withdraw function.

Note that the program can be easily extended to include addition features such as a transfer function, which would allow users to transfer tokens to other users.


You may have already guessed that this program has a few bugs. We list some of them below:

  • withdraw can only be invoked by the bank. A malicious bank could lock users' tokens by not invoking withdraw.
  • withdraw fails if the sum of the interest and principal is greater than the user's balance.
  • User's can increase their principal by depositing tokens multiple times, including immediately before withdrawl.
  • Integer division rounds down; if the calculated interest is too small, then it will be rounded down to zero.

Can you find any others?

Language Features and Concepts

  • record declarations
  • assert_eq
  • core functions, e.g. BHP256::hash
  • record ownership
  • loops and bounded iteration
  • mappings
  • finalize

How to Run

Follow the Leo Installation Instructions.

This basic bank program can be run using the following bash script. Locally, it will execute Leo program functions to issue, deposit, and withdraw tokens between a bank and a user.

cd basic_bank

The .env file contains a private key and address. This is the account that will be used to sign transactions and is checked for record ownership. When executing programs as different parties, be sure to set the private_key field in .env to the appropriate value. You can check out how we've set things up in ./ for a full example of how to run the program as different parties.


Issue Tokens

We will be playing the role of two parties.

The private key and address of the bank.
private_key: APrivateKey1zkpHtqVWT6fSHgUMNxsuVf7eaR6id2cj7TieKY1Z8CP5rCD
address: aleo1t0uer3jgtsgmx5tq6x6f9ecu8tr57rzzfnc2dgmcqldceal0ls9qf6st7a

The private key and address of the user.
private_key: APrivateKey1zkp75cpr5NNQpVWc5mfsD9Uf2wg6XvHknf82iwB636q3rtc
address: aleo1zeklp6dd8e764spe74xez6f8w27dlua3w7hl4z2uln03re52egpsv46ngg

Let's make some bank transactions. We'll take the role of the bank and issue 100 tokens to the user. We swap the private key into .env and run the issue transition function. The inputs are simply the recipient of the issuance and the amount.

echo "
" > .env

leo run issue aleo1zeklp6dd8e764spe74xez6f8w27dlua3w7hl4z2uln03re52egpsv46ngg 100u64

Deposit Tokens

Now, let's have the user deposit 50 of their tokens with the bank. We'll take the role of the user and call the deposit function, having the user use the output record that was issued to them by the bank. The inputs are the output record from the issue transition and the amount the user wishes to deposit.

echo "
" > .env

leo run deposit "{
owner: aleo1zeklp6dd8e764spe74xez6f8w27dlua3w7hl4z2uln03re52egpsv46ngg.private,
amount: 100u64.private,
_nonce: 4668394794828730542675887906815309351994017139223602571716627453741502624516group.public
}" 50u64

You'll see that the output contains a new private record belonging to the user with 50 credits, and a finalize deposit function taking the arguments (bank address, amount) that will update a public mapping with 50 credits. This information is queryable on-chain.


With the 50 token deposit, let's say 15 periods of time pass with compounding interest at a rate of 12.34% on the principal amount.

You can run the calculation yourself, it comes out to 266 tokens accrued using those numbers.

Withdraw Tokens

Now, let's have the bank withdraw all tokens after 15 periods. Let's switch to the bank role, and call the withdraw transition function. The inputs are the recipient's address, amount, rate, and periods.

echo "
" > .env

leo run withdraw aleo1zeklp6dd8e764spe74xez6f8w27dlua3w7hl4z2uln03re52egpsv46ngg 50u64 1234u64 15u64

You'll see here the withdrawal function creates a new private record for the user containing all 266 withdrawn tokens, and then calls the finalize withdraw function with arguments (address, amount), which will update the public balance of the bank back to 0. The public mapping will be queryable on-chain.