How can I convert from cborHex to VRFKeyHash type from the Cardano Serialization Lib?

Question

I need to generate a VRF Key Hash in this format: vrf_vk13hg4gg5fg67399nuz2ldk89kqw9h379cfgtkpvd83ra89f908kcqv2cw3j

When I generate it using cardano-cli, I get this file:

{
    "type": "VrfVerificationKey_PraosVRF",
    "description": "VRF Verification Key",
    "cborHex": "5820d3e6e69edf63e50ce0d12bc8fd891d6d00a09cb484a55f99caf0767bd9d75488"
}

What I'm trying to do is to convert this to VRFKeyHash. I'm using this function (I'm reading the value directly from the file, that's why I'm parsing it):

const fromHex=(hex)=>Buffer.from(hex,"hex");
const vrfKeyHash = CardanoWasm.VRFKeyHash.from_bytes(fromHex(JSON.parse(vrfKeyFile).cborHex));

But when I do this, I get the following error:

Deserialization failed in VRFKeyHash because: Invalid cbor: expected tuple 'hash length' of length 32 but got length Len(34).

What is it that I'm doing wrong?

Answer 1

Verification keys can be represented in different ways. The one that you want is a representation where the key hash is converted with a prefix vrf_ to the bech32 format. This can be done via the following repo (1). Given your specific given vrf verification key file you first have to convert it to a vrf key hash. This can be done via the cardano-cli with

cardano-cli node key-hash-VRF --verification-key-file fileName.vkey 

Which returns 80bdf88001bcfb0723f93aff337b299d1502f1d1884afe65342b7b8dfd18c2e6 for your vkey file. Then one can convert this hash to bech32 format with prefix vrf_ as follows

cabal run exe:bech32 -- vrf_ <<< 80bdf88001bcfb0723f93aff337b299d1502f1d1884afe65342b7b8dfd18c2e6

which returns vrf_1sz7l3qqphnaswgle8tlnx7efn52s9uw33p90uef59dacmlgcctnqut3ndw. To reverse the bech32 transformation you can run the following

cabal run exe:bech32 -- <<< vrf_vk13hg4gg5fg67399nuz2ldk89kqw9h379cfgtkpvd83ra89f908kcqv2cw3j

which return 8dd154228946bd12967c12bedb1cb6038b78f8b84a1760b1a788fa72a4af3db0, the vrf key hash of the example bech32 address you gave.

Answer 2

To convert from cborHex to VRFKeyHash I did the following, assuming there's a file with this format...

{
    "type": "VrfVerificationKey_PraosVRF",
    "description": "VRF Verification Key",
    "cborHex": "5820d3e6e69edf63e50ce0d12bc8fd891d6d00a09cb484a55f99caf0767bd9d75488"
}

...which can be generated using cardano-cli or cardanocli-js:

const vrfKeyFile = readFileSync(vrfKeyAccount.vkey, 'utf-8');
CardanoWasm.VRFKeyHash.from_bytes(Buffer.from(JSON.parse(vrfKeyFile).cborHex.substring(4), "hex"));

You need to get rid of the first 2 bytes ("5820" prefix) in order for it to work.