How to calculate "Data Hash" before submit to the ledger in Hyperledger Fabric (SDK Node)?

Question

I'm researching Hyperledger Fabric v2.2 with SDK Node v2.2 and reach a trouble that I don't known how to calculate the "data hash" before submit it to the ledger, I need it for verify between client data and ledger data.

My submit transaction chaincode function:

file: chaincode.js
...
async addNewDataToLedger(ctx, dataId, data){
    return await ctx.stub.putState(dataId, Buffer.from(data));
}

Then in my application, I have a JSON data such as

const dataSample = {a : 1, b : 2};
const transaction = contract.createTransaction('addNewDataToLedger');
const result = await transaction.submit('number1', JSON.stringify(dataSample));

After submit it to the ledger. I will receiver a "Data Hash" such as e2c07a0541f0b90ef0cea2174365719abeda50fd024468f9d7639b76cb1878fb

I guess it hash my Buffer.from(data) and something secret, because I realized that, "Data Hash" is always different even though the same input data.

And if data_hash is encrypt by data: {...} section like below block, therefore how to verify the data of client with data in the ledger is matched?

{
  header: {
    number: {}, 
    previous_hash: "e4617a6446d30628b723206d3f0a0e61308e08dc5eef502bb4c87d228c1c4d10", 
    data_hash: "38afae3941a400149c8508f7d8e4a26bf938cf60dd3a6f4be602ec829f7115fd"
  }, 
  data: { //IS 'data_hash' HASHED THIS SECTION OBJECT?
    data: [...]
  }, 
  metadata: {
    metadata: [...]
  }
}

I searched everywhere but couldn't find what "Data hash" is encoded by, and how to verify data integrity, hope you can help me, many thanks!!

Answer 1

The block data_hash is not the hash over a single data key or even a single transaction.

The block data_hash is a SHA 256 hash over all the transactions in the block, as seen by the code here.