Hash script data

Question

I want to understand a bit better the hash-script-data functionality from cardano-cli. I checked the source code but had no luck in understanding how this hash is made.

From the examples, if we run:

cardano-cli transaction hash-script-data --script-data-value 42

We get the following hash:

9e1199a988ba72ffd6e9c269cadb3b53b5f360ff99f112d9b2ee30c4d74ad88b

However if we try to do a simple SHA256 hash of a 42 value we get:

73475cb40a568e8da8a045ced110137e159f890ac4da883b6b17dc651b3a8049

Also tried in the Plutus object format:

{
  "constructor": 0,
  "fields": [{
    "int": 42
  }]
}

but none of those match the hash generated by cardano-cli.

Also tried innumerous other alternatives but obviously I'm missing something, anyone knows what's the process for generating this hash?

Thank you!

Answer 1

The missing step would be converting the ScriptData into CBOR prior to hashing.

In the case of ScriptDataNumber 42 the CBOR Hex is "182a" which is a bit simplified for Numbers vs more complex ScriptData. The CBOR Jump Table shows that 0x18 indicates unsigned integer (one-byte uint8_t follows) followed by 2a which is 42 in hex.

Or using Haskell:

let plutusData = PlutusCore.Data.I 42
print $ Base16.encode $ BSL.toStrict $ Codec.Serialise.serialise plutusData
-- "182a"
print $ Cardano.Api.hashScriptData $ Cardano.Api.Shelley.fromPlutusData plutusData
-- "9e1199a988ba72ffd6e9c269cadb3b53b5f360ff99f112d9b2ee30c4d74ad88b"

and using libsodiumjs as example to hash:

var libsodiumWrappers = require("libsodium-wrappers");
var _sodium = libsodiumWrappers;

(async () => {
  await _sodium.ready;
  const sodium = _sodium;
  var h = sodium.crypto_generichash(32, sodium.from_hex('182a'));
  console.log(sodium.to_hex(h));
})();