CIP-0046? | Merkelised Plutus Scripts

CIP-XXXX/README.md

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+---
+CIP: ?
+Title: Merkelised Plutus Scripts
+Authors: Las Safin <[email protected]>
+Status: Draft
+Type: Standards
+Created: 2022-11-29
+License: <CC-BY-4.0>
+---
+
+## Abstract
+
+Currently, the hash of a script is simply the hash of its [serialisation](
+https://github.com/input-output-hk/plutus/blob/a645d1ee0dd5efcd7a7da24678461e07396ad26e/plutus-ledger-api/src/PlutusLedgerApi/Common/SerialisedScript.hs#L88).
+This CIP proposes changing this such that the hash of a script (term)
+is a function of its immediate children's hashes, forming a Merkle Tree from the AST.
+This allows one to shallowly verify a script's hash, and is useful on Cardano,
+because it allows scripts to **check that a script hash is an instantiation of a parameterised script**.
+
+In addition, a `blake2b_224` built-in function must be added.
+
+This is inspired by [BIP-144](https://github.com/bitcoin/bips/blob/master/bip-0114.mediawiki),
+but the motivations are very different.
+
+## Motivation
+
+Given some core logic expressible as a script, it is common to have parameters in
+the form of constants, e.g. fees, references to other scripts, magical numbers.
+
+These parameters can either be put in a datum somewhere, or can be put into the
+script itself, either by inlining them, or applying the unapplied script to the constants.
+
+On-chain it is currently hard to check that one script is an applied form
+of another script. In cases where that is necessary, datums are instead used.
+
+By Merkelising the hashing, we make this possible,
+which unlocks checking that a script is an application of another script to some parameter.
+
+Example reasons to apply the parameters to the script:
+- Staking validators currently don't support datums, and all staking validators
+  share a single rewards account. Allowing checking applied parameters
+  makes staking validators much more powerful. (More about this below)
+- Constants can be included in reference script, leading to less CPU and memory used,
+  since they don't have to be parsed from the adjacent datum (somewhat cheap)
+  or the script context (very expensive).
+- A script address + datum can't fit in an address,
+  if you want that you also need this (or need to change what an address is).
+
+## Specification
+
+The hash of a script will be derived directly from the AST, rather than its serialisation.
+Currently, its formed by hashing the serialisation prefixed with a byte that represents its version,  e.g. 0x02 for Plutus V2.
+
+The hash of a script becomes the hash of the prefix version annotation prepended to the hash of the term.
+
+[`Term`](https://github.com/input-output-hk/plutus/blob/a645d1ee0dd5efcd7a7da24678461e07396ad26e/plutus-core/untyped-plutus-core/src/UntypedPlutusCore/Core/Type.hs#L69)
+currently has 8 constructors. On-chain, annotations are always the unit type,
+and are hence ignored for this algorithm. Each case/constructor is generally handled by
+hashing the concatenation of a prefix (single byte corresponding to the
+constructor index) along with the hashes of the arguments passed to the constructor.
+
+Similar code can be found [in Plutarch](https://github.com/Plutonomicon/plutarch-plutus/blob/95e40b42a1190191d0a07e3e4e938b72e6f75268/Plutarch/Internal.hs#L100) (for a slightly different AST).
+
+To avoid giving a single script two hashes,
+this system must be used (exclusively) since at least a version after Plutus V2.
+
+The algorithm for checking a script hash against a supplied script (of a new version)
+in the ledger will change slightly: rather than hashing the supplied serialised
+script directly, the decoding of the serialised script must be hashed.
+(NB: the hashing and decoding can be fused to avoid intermediary structures.)
+
+To allow computing the hash in scripts, we must support `blake2b-224` in Plutus scripts
+as it's what is currently used. This algorithm used might change in the future, but that is
+not relevant for this CIP.
+
+### Hashing `Error`
+
+Since there are no children, the hash of the `Error` term is the
+hash of the prefix byte for the `Error` constructor.
+You could theoretically choose any random number as the hash,
+but it has to be proven to be random, hence hashing the prefix byte
+is the best option.
+
+In pseudocode: `hash prefix`
+
+### Hashing `Builtin`, `Var`
+
+The hash of a `Builtin` is the hash of the prefix prepended to the base-256 encoded
+(i.e. serialised to bytestring) index of the built-in function.
+Because there are less than 256 built-ins, this is currently the same
+as hashing the prefix byte prepended to the byte containing the index of
+the built-in.
+
+`Var` is handled the exact same way (with a different prefix),
+but it's in this case feasible for the index to be more than 255.
+
+In pseudocode: `hash $ prefix <> serialiseBase256 index`
+
+### Hashing `Apply`, `Force`, `Delay`
+
+These are hashed by hashing the result of prepending the prefix
+byte to the concatenation of the hashes of the children.
+
+In pseudocode: `hash $ foldl' (<>) prefix (hash <$> children)`
+
+### Hashing `LamAbs`
+
+This works the exact same way as above, notably, the _name_ is excluded
+as it's a constant in the de-Bruijn encoding.
+
+In pseudocode: `hash $ prefix <> hash body`
+
+### Hashing `Constant`
+
+The universe of types used on-chain is always `DefaultUni`.
+Each possible data type is handled differently, with each having
+a different prefix. The total number of prefixes does not exceed
+255. If it did, the prefix would have to be increased to two bytes.
+
+In addition:
+Negative integers and non-negative integers have separate prefixes.
+False and True also have separate prefixes.
+
+#### Hashing non-negative integers
+
+The serialisation according to [CIP-58](https://github.com/cardano-foundation/CIPs/blob/a1b9ff0190ad9f3e51ae23e85c7a8f29583278f0/CIP-%3F/README.md#representation-of-builtininteger-as-builtinbytestring-and-conversions),
+prefixed with the two-byte prefix, is hashed.
+
+In pseudocode: `hash $ prefix <> prefix' <> serialiseCIP58 n`
+
+#### Hashing negative integers
+
+The same algorithm as above is used, but the number hashed is `1 - n`.
+
+In pseudocode: `hash $ prefix <> prefix' <> serialiseCIP58 (1 - n)`
+
+#### Hashing bytestrings
+
+The bytestring is hashed as-is.
+We use the blake2b-256 hash here, such that we can usefully check that
+the script refers to a bytestring that we know only the hash of.
+
+In pseudocode: `hash $ prefix <> blake2b_256 bs`
+
+#### Hashing strings
+
+The flat-encoding is hashed.
+
+In pseudocode: `hash $ prefix <> flat x`
+
+#### Hashing lists, pairs
+
+Lists and pairs are hashed like a Merkle tree,
+much the same way that terms are.
+The children have a known type, and are hashed according to how that
+type should be hashed, i.e. with the correct algorithm and prefix.
+
+In pseudocode: `hash $ foldl' (<>) prefix (hash <$> children)`
+
+#### Hashing `()`, `False`, `True`
+
+Each has its own separate prefix, like `Error`, hence:
+
+In pseudocode: `hash prefix`
+
+#### Hashing `Data`
+
+The `CBOR` encoding is used, notably, it must be compatible with the `serialiseData`
+built-in to be useful on-chain.
+We use the blake2b-256 hash here, such that we can usefully check that
+the script refers to a datum that we know only the hash of.
+If the hashing algorithm for data changes, we must also change it here.
+
+In pseudocode: `hash $ prefix <> blake2b_256 (serialiseData d)`
+
+## Rationale
+
+Given this minor change, we can now check that one script is the application of another script.
+Concretely, given hash `script`, hash `original`, parameter `d` (as data),
+intermediate hashes `h0`, `h1`, hashing prefixes `ver_prefix`, `app_prefix`, `const_prefix`, we check:
+```
+script == blake2b_224 $ ver_prefix <> h0
+h0 == blake2b_224 $ app_prefix <> original <> h1
+h1 == blake2b_224 $ const_data_prefix <> blake2b_256 (serialiseData d)
+```
+
+We essentially open the Merkle tree commitment partially and check that the supplied path is correct.
+
+### Relation with CIP-58
+
+This CIP does not _depend_ on CIP-58, but to hash integers on-chain
+the way it's done here, CIP-58's integer-to-bytestring serialisation built-in
+must be available in Plutus.
+
+### Relation with BIP-144
+
+BIP144 uses this trick to avoid submitting the parts of the script that aren't used.
+Given that reference scripts are common in Haskell, this isn't a big win for efficiency,
+but it might be worth implementing for the sake of scripts used only once.
+This CIP however doesn't require that that be implemented.
+
+The argument for privacy doesn't apply, private smart contracts can be achieved through
+the use of non-interactive zero-knowledge probabilistic proofs.
+
+### Reference scripts
+
+Currently, different instances of the same script will need their own reference inputs
+since their hashes are different. It seems feasible to allow sharing of a single reference script,
+given the parameters and language version as witnesses, but given the complexity
+involved, it is not specified in this CIP.
+
+### Staking
+
+This makes staking validators much more powerful, since a single protocol can
+now manage many rewards accounts (by instantiating the script with a numeric identifier).
+However, it is arguably not the optimal solution due to the reference
+script problem described above. Even if the reference script problem
+is solved as described above, it seems logical to allow supplying a datum
+to a staking validator, or somehow combining the payment address and staking address for scripts,
+and using the same datum for both, while somehow solving the separate accounts problem.
+
+Given the heavy complexity of fixing staking validators, Merkelising script hashing seems much more feasible.
+
+### Alternatives
+
+#### Parameterised Reference Scripts
+
+See https://github.com/cardano-foundation/CIPs/pull/354.
+
+Seemingly, Merkelisation is a less invasive and possibly cleaner change.
+
+#### Changing how constants are hashed
+
+The hashing of constants might not have a clear best solution, but most notably,
+it is not clear how much/less to Merkelise the hashing.
+E.g., the hashing of data itself could be Merkelised. This is not done in this CIP.
+The hashing of a `Data` constant could also prepend the prefix directly to the serialisation,
+rather than to the hash of the `Data`. It is not clear what is best.
+
+##### Hashing strings, lists, pairs differently
+
+Strings are not very useful in Plutus.
+Hence, the hashing algorithm for them isn't optimised such that
+they can be easily verified.
+
+Strings have essentially no purpose on-chain, since they're only used
+for tracing, which should not be used in production.
+
+In the context of checking applied parameters, it is likely that only
+`Data`, `Integer`, `Bool`, `ByteString`, will be used as parameters,
+since they cover all useful behaviour in an efficient way.
+If you want to parameterise your script by a pair of integers,
+it is likely best to unwrap that into two separate integer parameters
+for the sake of efficiency of _running_ the script, which is likely
+to be more common that checking the parameters.
+
+Built-in lists and pairs are not commonly used as parameters, but it's plausible
+that they might still be the most efficient method in some scenarios.
+Hence, they have been included.
+They use Merkle-tree hashing since that's the simplest and most useful in this case.
+
+## Path to Active
+
+### Implementation plan
+
+Las Safin will implement this if IOG don't have time.
+
+## Copyright
+
+This CIP is licensed under CC-BY-4.0.