Encoding Attestation Data

Attestation data is encoded according to the Schema the Attestation is associated with. The Schema is parsed by the client reading the Attestation, in a way that results in a series of data types that can be used to abi.decode the attestation data. In order for this to happen, the Attestation data must first be encoded using abi.encode.

The SDK offers a much easier to encode a JSON object into attestation data. However, this information outlines the low-level steps required to encode attestation data.

Modelling a Schema as a Solidity Struct

The easiest way to think of a Schema is like a Solidity struct. Let's consider a simple Schema:

(string username, string teamname, uint16 points, bool active)

This can be thought of as a struct:

struct Profile {
  string username;
  string teamname;
  uint16 points;
  bool active;
}

If we were to encode an attestation that references this schema, we could do it client side using the ethers library (or equivalent):

const encodedStruct = ethers.utils.defaultAbiCoder.encode(
  ["string", "string", "uint16", "bool"],
  ["bojo", "torries", 3, false],
);

This can later be decoded by some off-chain client, with the subgraph, or by another on-chain contract using Solidity:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract StructDecodingExample {
  struct Profile {
    string username;
    string teamname;
    uint16 points;
    bool active;
  }

  function receiveEncodedStruct(bytes memory encodedStruct) public pure returns (gamer memory) {
    Profile memory gamer;

    // Decode the struct using abi.decode
    (gamer.username, gamer.teamname, gamer.points, gamer.active) = abi.decode(
      encodedStruct,
      (string, string, uint16, bool)
    );

    return gamer;
  }
}

Encoding Nested Data

Sometimes a schema is a bit more complicated, in that it contains more than just a flat data structure, and may contain nested data structure like so:

(string username, string teamname, uint16 points, bool active, ( string gametype, string gamemode ) preferences)

Which can be thought of as this nested struct:

struct Preferences {
  string gametype;
  string gamemode;
}

struct Profile {
  string username;
  string teamname;
  uint16 points;
  bool active;
  Preferences setup;
}

Again, encoding the attestation for this schema using a client side library such as ethers would look like this:

const encodedStruct = ethers.utils.defaultAbiCoder.encode(
  ["string", "string", "uint16", "bool", "tuple(string gametype, string gamemode)"],
  ["bojo", "torries", 3, false, { gametype: "RPG", gamemode: "multiplayer" }],
);

Encoding Arrays

Encoding arrays within attestation data is also straightforward, let's use the example before, but with arrays:

(string username, string teamname, uint16 points, bool active, ( string[] gametype, string[] gamemode ) preferences)

Which can be thought of as this nested struct:

struct Preferences {
  string[] gametype;
  string[] gamemode;
}

struct Profile {
  string username;
  string teamname;
  uint16 points;
  bool active;
  Preferences setup;
}

Again, encoding the attestation for this schema using a client side library such as ethers would look like this:

const encodedStruct = ethers.utils.defaultAbiCoder.encode(
  ["string", "string", "uint16", "bool", "tuple(string[] gametype, string[] gamemode)"],
  [
    "bojo",
    "torries",
    3,
    false,
    { gametype: ["RPG", "first-person-shooter", "racing"], gamemode: ["multiplayer", "singleplayer"] },
  ],
);