leo/examples/basic_bank

A simple-interest yielding bank account in Leo.

Summary

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.

Bugs

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_to_field
• record ownership
• loops and bounded iteration
• mappings
• finalize

Running the Program

Leo provides users with a command line interface for compiling and running Leo programs. Users may either specify input values via the command line or provide an input file in inputs/.

Configuring Accounts

The .env file contains a private key. 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 values. See ./run.sh for an example of how to run the program as different parties.

The Aleo SDK provides an interface for generating new accounts. To generate a new account, navigate to aleo.tools.

Providing inputs via the command line.

1. Run

leo run <function_name> <input_1> <input_2> ...

See ./run.sh for an example.

Using an input file.

1. Modify inputs/auction.in with the desired inputs.
2. Run

leo run <function_name>

For example,

leo run issue
leo run deposit
leo run withdraw