ScriptPubKey

You might not know that as far as the Blockchain is concerned, there is no such thing as a Bitcoin Address. Internally, the Bitcoin protocol identifies the recipient of Bitcoin by a ScriptPubKey.

A ScriptPubKey may look like this: OP_DUP OP_HASH160 14836dbe7f38c5ac3d49e8d790af808a4ee9edcf OP_EQUALVERIFY OP_CHECKSIG

It is a short script that explains what conditions must be met to claim ownership of bitcoins. We will go into the types of operations in a ScriptPubKey as we move through the lessons of this book.

We are able to generate the ScriptPubKey from the Bitcoin Address. This is a step that all bitcoin clients do to translate the “human friendly” Bitcoin Address to the Blockchain readable address.

var publicKeyHash = new KeyId("14836dbe7f38c5ac3d49e8d790af808a4ee9edcf");

var testNetAddress = publicKeyHash.GetAddress(Network.TestNet);
var mainNetAddress = publicKeyHash.GetAddress(Network.Main);

Console.WriteLine(mainNetAddress.ScriptPubKey); // OP_DUP OP_HASH160 14836dbe7f38c5ac3d49e8d790af808a4ee9edcf OP_EQUALVERIFY OP_CHECKSIG
Console.WriteLine(testNetAddress.ScriptPubKey); // OP_DUP OP_HASH160 14836dbe7f38c5ac3d49e8d790af808a4ee9edcf OP_EQUALVERIFY OP_CHECKSIG

Notice the ScriptPubKey for testnet and mainnet address is the same? Notice the ScriptPubKey contains the hash of the public key? We will not go into the details yet, but note that the ScriptPubKey appears to have nothing to do with the Bitcoin Address, but it does show the hash of the public key.

Bitcoin Addresses are composed of a version byte which identifies the network where to use the address and the hash of a public key. So we can go backwards and generate a bitcoin address from the ScriptPubKey and the network identifier.

var paymentScript = publicKeyHash.ScriptPubKey;
var sameMainNetAddress = paymentScript.GetDestinationAddress(Network.Main);
Console.WriteLine(mainNetAddress == sameMainNetAddress); // True

It is also possible to retrieve the hash from the ScriptPubKey and generate a Bitcoin Address from it:

var samePublicKeyHash = (KeyId) paymentScript.GetDestination();
Console.WriteLine(publicKeyHash == samePublicKeyHash); // True
var sameMainNetAddress2 = new BitcoinPubKeyAddress(samePublicKeyHash, Network.Main);
Console.WriteLine(mainNetAddress == sameMainNetAddress2); // True

Note: A ScriptPubKey does not necessarily contain the hashed public key(s) permitted to spend the bitcoin.

So now you understand the relationship between a Private Key, a Public Key, a Public Key Hash, a Bitcoin Address and a ScriptPubKey.

In the remainder of this book, we will exclusively use ScriptPubKey. A Bitcoin Address is only a user interface concept.