Bitcoin script types

Bitcoin doesn't make a distinction between Bitcoin script coming from different sources, like scriptPubKey in transaction output or witness and scriptSig in transaction input. There are many other possible script containers for Bitcoin script: redeem script, witness script, taproot leaf scripts of different versions. In fact, any "script" of [bitcoin::Script] type can be used for inputs and outputs. What is a valid script for in one context will not be a valid script for some other. That would mean that in principle with existing [bitcoin::Script] type every input script can be used as an output script, leading to potentially harmful code coming from an unaware developer.

While all [bitcoin::Script]s have the same parsing rules converting byte string into a set of instructions (i.e. the same syntax), there are multiple ways how the consensus meaning of these instructions will be interpreted under different contexts (different semantics). Moreover, the scripts may be nested - or to be committed into some other Bitcoin script – in a nested structures like in several layers, like redeemScript inside of scriptSig used for P2SH, or tapScript within witnessScript coming from witness field for Taproot. These nested layers do distinguish on the information they contain, since some of them only commit to the hashes of the nested scripts ([bitcoin::ScriptHash], [WitnessProgram]) or public keys ([bitcoin::PubkeyHash], [bitcoin::WPubkeyHash]), while other contain the full source of the script.

The present type system represents a solution to the problem: it distinguish different logical types by introducing Script wrapper types. It defines [LockScript] as bottom layer of a script, containing no other script commitments (in form of their hashes). It also defines types above on it: [PubkeyScript] (for whatever is there in scriptPubkey field of a [bitcoin::TxOut]), [SigScript] (for whatever comes from scriptSig field of [bitcoin::TxIn]), [RedeemScript] and [WitnessScript]. For taproot, we define [LeafScript] as a top level of specific script branch (see [bitcoin::util::psbt::TapTree]) and [crate::TapScript] as a type specific for the current 0xC0 tapleaf version semantics, defined in BIP-342.

There are conversion functions, which, for instance, can analyse [PubkeyScript] and if it is a custom script or P2PK return a [LockScript] type - or otherwise fail with error. These conversions functions reside in [convert] module. So with this type system one is always sure which semantic information it does contain.

Type conversion

LockScript -+-> (PubkeyScript + RedeemScript) -+-> SigScript
|                                  +-> WitnessScript
+-> PubkeyScript
|
TapScript ----> LeafScript

PubkeyScript --?--> LockScript