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