How to Implement Base64 Outputs in Contracts

Storing information off-chain using a link to a metadata file has been the common practice for NFTs. However, there is a growing trend towards storing all information purely on-chain, which has led to the idea of providing data links that contain the data in it.

In this article, we will discuss how to build a JSON data URL for a tokenURI using base64 encoding in Solidity contracts.

Base64 Encoding with OpenZeppelin Utilities

OpenZeppelin (opens in a new tab) provides the tools required to implement base64 encoding in a Solidity contract in an easy way.

To use the base64 encoding utility, import "@openzeppelin/contracts/utils/Base64.sol" and then call Base64.encode(string) to build the base64 string.

Building the JSON Output

A data URL starts with the data: schema, followed by the mime type and the encoding of the data. This prefix sums up to data:application/json;base64,.

To build the JSON output, leverage the abi.encodePacked function to concatenate strings:

abi.encodePacked(
    "data:application/json;base64,",
    Base64.encode(dataURI)
)

Here, dataURI is the encoded JSON data that we want to include in the URL.

Sample Snippet

A complete example of a Solidity contract function that builds a JSON output using base64 encoding is shown below:

function tokenURI(uint256 tokenId)
    public
    view
    override(ERC721)
    returns (string memory)
{
    bytes memory dataURI = abi.encodePacked(
        "{",
        '"mintedFor": "',
        Strings.toHexString(uint256(uint160(mintedFor[tokenId])), 20),
        '",',
        '"attribute": "value"',
        "}"
    );
 
    return
        string(
            abi.encodePacked(
                "data:application/json;base64,",
                Base64.encode(dataURI)
            )
        );
}

In this example, mintedFor was encoded with its value as a hexadecimal string. Also a sample attribute with a value of "value" was included in the JSON output.

Example Project

For a complete example of a Solidity contract that uses base64 encoding to build a JSON data URL, you can check out the example project on GitLab (opens in a new tab).

By implementing base64 encoding in your Solidity contracts, you can provide all relevant data on-chain. One of the benefits of using a standard format like JSON and data URLs is that it can be easily handled by many applications. Moreover, using on-chain storage means that no other service needs to be maintained for storage, and the data will be available forever, as long as the blockchain exists.