So I’m writing a little tool that dissects the Bitcoin wallet.dat, mainly because I want to understand better exactly how Bitcoin works.

And I see that the outputs of transactions have a value (number of bitcoins) and a bunch of bytes that are run through the little Forth-like scripting language built in to bitcoin. E.g.:
[‘TxOut: value: 100.00 Script: DUP HASH160 6fad…ab90 EQUALVERIFY CHECKSIG’]

First: it make me a little nervous that bitcoin has a scripting language in it, even though it is a really simple scripting language (no loops, no pointers, nothing but math and crypto). It makes me nervous because it is more complicated, and complication is the enemy of security. It also makes it harder to create a second, compatible implementation. But I think I can get over that.

Looking at the code, new transactions are verified by pushing the signature an then public key on the interpreter’s stack and then running the TxOut script (did I get that right?).

Could I write code to create transactions with any valid script in the TxOut?
E.g. could I create a TxOut with a script of: OP_2DROP OP_TRUE
… to create a coin that could be spent by anybody?

And is flexibility in the types of coins created the reason it is coded this way?

The nature of Bitcoin is such that once version 0.1 was released, the core design was set in stone for the rest of its lifetime.  Because of that, I wanted to design it to support every possible transaction type I could think of.  The problem was, each thing required special support code and data fields whether it was used or not, and only covered one special case at a time.  It would have been an explosion of special cases.  The solution was script, which generalizes the problem so transacting parties can describe their transaction as a predicate that the node network evaluates.  The nodes only need to understand the transaction to the extent of evaluating whether the sender’s conditions are met.

The script is actually a predicate.  It’s just an equation that evaluates to true or false.  Predicate is a long and unfamiliar word so I called it script.

The receiver of a payment does a template match on the script.  Currently, receivers only accept two templates: direct payment and bitcoin address.  Future versions can add templates for more transaction types and nodes running that version or higher will be able to receive them.  All versions of nodes in the network can verify and process any new transactions into blocks, even though they may not know how to read them.

The design supports a tremendous variety of possible transaction types that I designed years ago.  Escrow transactions, bonded contracts, third party arbitration, multi-party signature, etc.  If Bitcoin catches on in a big way, these are things we’ll want to explore in the future, but they all had to be designed at the beginning to make sure they would be possible later.

I don’t believe a second, compatible implementation of Bitcoin will ever be a good idea.  So much of the design depends on all nodes getting exactly identical results in lockstep that a second implementation would be a menace to the network.  The MIT license is compatible with all other licenses and commercial uses, so there is no need to rewrite it from a licensing standpoint.

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