#
pezkuwi_sdk_docs/reference_docs/
extrinsic_encoding.rs

    1//! # Constructing and Signing Extrinsics
    2//!
    3//! Extrinsics are payloads that are stored in blocks which are responsible for altering the state
    4//! of a blockchain via the [_state transition
    5//! function_][crate::reference_docs::blockchain_state_machines].
    6//!
    7//! Substrate is configurable enough that extrinsics can take any format. In practice, runtimes
    8//! tend to use our [`sp_runtime::generic::UncheckedExtrinsic`] type to represent extrinsics,
    9//! because it's generic enough to cater for most (if not all) use cases. In Pezkuwi, this is
    10//! configured [here](https://github.com/pezkuwichain/pezkuwi-fellows/tree/main/runtimes/blob/94b2798b69ba6779764e20a50f056e48db78ebef/relay/pezkuwi/src/lib.rs#L1478)
    11//! at the time of writing.
    12//!
    13//! What follows is a description of how extrinsics based on this
    14//! [`sp_runtime::generic::UncheckedExtrinsic`] type are encoded into bytes. Specifically, we are
    15//! looking at how extrinsics with a format version of 5 are encoded. This version is itself a part
    16//! of the payload, and if it changes, it indicates that something about the encoding may have
    17//! changed.
    18//!
    19//! # Encoding an Extrinsic
    20//!
    21//! At a high level, all extrinsics compatible with [`sp_runtime::generic::UncheckedExtrinsic`]
    22//! are formed from concatenating some details together, as in the following pseudo-code:
    23//!
    24//! ```text
    25//! extrinsic_bytes = concat(
    26//!     compact_encoded_length,
    27//!     version_and_extrinsic_type,
    28//! 	maybe_extension_data,
    29//!     call_data
    30//! )
    31//! ```
    32//!
    33//! For clarity, the actual implementation in Substrate looks like this:
    34#![doc = docify::embed!("../../substrate/primitives/runtime/src/generic/unchecked_extrinsic.rs", unchecked_extrinsic_encode_impl)]
    35//!
    36//! Let's look at how each of these details is constructed:
    37//!
    38//! ## compact_encoded_length
    39//!
    40//! This is a [SCALE compact encoded][frame::deps::codec::Compact] integer which is equal to the
    41//! length, in bytes, of the rest of the extrinsic details.
    42//!
    43//! To obtain this value, we must encode and concatenate together the rest of the extrinsic details
    44//! first, and then obtain the byte length of these. We can then compact encode that length, and
    45//! prepend it to the rest of the details.
    46//!
    47//! ## version_and_maybe_signature
    48//!
    49//! If the extrinsic is _unsigned_, then `version_and_maybe_signature` will be just one byte
    50//! denoting the _transaction protocol version_, which is 4 (or `0b0000_0100`).
    51//!
    52//! If the extrinsic is _signed_ (all extrinsics submitted from users must be signed), then
    53//! `version_and_maybe_signature` is obtained by concatenating some details together, ie:
    54//!
    55//! ```text
    56//! version_and_maybe_signature = concat(
    57//!     version_and_signed,
    58//!     from_address,
    59//!     signature,
    60//!     transaction_extensions_extra,
    61//! )
    62//! ```
    63//!
    64//! Each of the details to be concatenated together is explained below:
    65//!
    66//! ## version_and_extrinsic_type
    67//!
    68//! This byte has 2 components:
    69//! - the 2 most significant bits represent the extrinsic type:
    70//!     - bare - `0b00`
    71//!     - signed - `0b10`
    72//!     - general - `0b01`
    73//! - the 6 least significant bits represent the extrinsic format version (currently 5)
    74//!
    75//! ### Bare extrinsics
    76//!
    77//! If the extrinsic is _bare_, then `version_and_extrinsic_type` will be just the _transaction
    78//! protocol version_, which is 5 (or `0b0000_0101`). Bare extrinsics do not carry any other
    79//! extension data, so `maybe_extension_data` would not be included in the payload and the
    80//! `version_and_extrinsic_type` would always be followed by the encoded call bytes.
    81//!
    82//! ### Signed extrinsics
    83//!
    84//! If the extrinsic is _signed_ (all extrinsics submitted from users used to be signed up until
    85//! version 4), then `version_and_extrinsic_type` is obtained by having a MSB of `1` on the
    86//! _transaction protocol version_ byte (which translates to `0b1000_0101`).
    87//!
    88//! Additionally, _signed_ extrinsics also carry with them address and signature information encoded
    89//! as follows:
    90//!
    91//! #### from_address
    92//!
    93//! This is the [SCALE encoded][frame::deps::codec] address of the sender of the extrinsic. The
    94//! address is the first generic parameter of [`sp_runtime::generic::UncheckedExtrinsic`], and so
    95//! can vary from chain to chain.
    96//!
    97//! The address type used on the Pezkuwi relay chain is [`sp_runtime::MultiAddress<AccountId32>`],
    98//! where `AccountId32` is defined [here][`sp_core::crypto::AccountId32`]. When constructing a
    99//! signed extrinsic to be submitted to a Pezkuwi node, you'll always use the
    100//! [`sp_runtime::MultiAddress::Id`] variant to wrap your `AccountId32`.
    101//!
    102//! #### signature
    103//!
    104//! This is the [SCALE encoded][frame::deps::codec] signature. The signature type is configured via
    105//! the third generic parameter of [`sp_runtime::generic::UncheckedExtrinsic`], which determines the
    106//! shape of the signature and signing algorithm that should be used.
    107//!
    108//! The signature is obtained by signing the _signed payload_ bytes (see below on how this is
    109//! constructed) using the private key associated with the address and correct algorithm.
    110//!
    111//! The signature type used on the Pezkuwi relay chain is [`sp_runtime::MultiSignature`]; the
    112//! variants there are the types of signature that can be provided.
    113//!
    114//! ### General extrinsics
    115//!
    116//! If the extrinsic is _general_ (it doesn't carry a signature in the payload, only extension
    117//! data), then `version_and_extrinsic_type` is obtained by logical OR between the general
    118//! transaction type bits and the _transaction protocol version_ byte (which translates to
    119//! `0b0100_0101`).
    120//!
    121//! ### transaction_extensions_extra
    122//!
    123//! This is the concatenation of the [SCALE encoded][frame::deps::codec] bytes representing first a
    124//! single byte describing the extension version (this is bumped whenever a change occurs in the
    125//! transaction extension pipeline) followed by the bytes of each of the [_transaction
    126//! extensions_][sp_runtime::traits::TransactionExtension], and are configured by the fourth generic
    127//! parameter of [`sp_runtime::generic::UncheckedExtrinsic`]. Learn more about transaction
    128//! extensions [here][crate::reference_docs::transaction_extensions].
    129//!
    130//! When it comes to constructing an extrinsic, each transaction extension has two things that we
    131//! are interested in here:
    132//!
    133//! - The actual SCALE encoding of the transaction extension type itself; this is what will form our
    134//!   `transaction_extensions_extra` bytes.
    135//! - An `Implicit` type. This is SCALE encoded into the `transaction_extensions_implicit` data (see
    136//!   below).
    137//!
    138//! Either (or both) of these can encode to zero bytes.
    139//!
    140//! Each chain configures the set of transaction extensions that it uses in its runtime
    141//! configuration. At the time of writing, Pezkuwi configures them
    142//! [here](https://github.com/pezkuwichain/pezkuwi-fellows/tree/main/runtimes/blob/1dc04eb954eadf8aadb5d83990b89662dbb5a074/relay/pezkuwi/src/lib.rs#L1432C25-L1432C25).
    143//! Some of the common transaction extensions are defined
    144//! [here][frame::deps::frame_system#transaction-extensions].
    145//!
    146//! Information about exactly which transaction extensions are present on a chain and in what order
    147//! is also a part of the metadata for the chain. For V15 metadata, it can be [found
    148//! here][frame::deps::frame_support::__private::metadata::v15::ExtrinsicMetadata].
    149//!
    150//! ## call_data
    151//!
    152//! This is the main payload of the extrinsic, which is used to determine how the chain's state is
    153//! altered. This is defined by the second generic parameter of
    154//! [`sp_runtime::generic::UncheckedExtrinsic`].
    155//!
    156//! A call can be anything that implements [`Encode`][frame::deps::codec::Encode]. In FRAME-based
    157//! runtimes, a call is represented as an enum of enums, where the outer enum represents the FRAME
    158//! pallet being called, and the inner enum represents the call being made within that pallet, and
    159//! any arguments to it. Read more about the call enum
    160//! [here][crate::reference_docs::frame_runtime_types].
    161//!
    162//! FRAME `Call` enums are automatically generated, and end up looking something like this:
    163#![doc = docify::embed!("./src/reference_docs/extrinsic_encoding.rs", call_data)]
    164//!
    165//! In pseudo-code, this `Call` enum encodes equivalently to:
    166//!
    167//! ```text
    168//! call_data = concat(
    169//!     pallet_index,
    170//!     call_index,
    171//!     call_args
    172//! )
    173//! ```
    174//!
    175//! - `pallet_index` is a single byte denoting the index of the pallet that we are calling into, and
    176//!   is what the tag of the outermost enum will encode to.
    177//! - `call_index` is a single byte denoting the index of the call that we are making the pallet,
    178//!   and is what the tag of the inner enum will encode to.
    179//! - `call_args` are the SCALE encoded bytes for each of the arguments that the call expects, and
    180//!   are typically provided as values to the inner enum.
    181//!
    182//! Information about the pallets that exist for a chain (including their indexes), the calls
    183//! available in each pallet (including their indexes), and the arguments required for each call can
    184//! be found in the metadata for the chain. For V15 metadata, this information [is
    185//! here][frame::deps::frame_support::__private::metadata::v15::PalletMetadata].
    186//!
    187//! # The Signed Payload Format
    188//!
    189//! All _signed_ extrinsics submitted to a node from the outside world (also known as
    190//! _transactions_) need to be _signed_. The data that needs to be signed for some extrinsic is
    191//! called the _signed payload_, and its shape is described by the following pseudo-code:
    192//!
    193//! ```text
    194//! signed_payload = blake2_256(
    195//! 	concat(
    196//!     	call_data,
    197//!     	transaction_extensions_extra,
    198//!     	transaction_extensions_implicit,
    199//! 	)
    200//! )
    201//! ```
    202//!
    203//! The bytes representing `call_data` and `transaction_extensions_extra` can be obtained as
    204//! descibed above. `transaction_extensions_implicit` is constructed by SCALE encoding the
    205//! ["implicit" data][sp_runtime::traits::TransactionExtension::Implicit] for each transaction
    206//! extension that the chain is using, in order.
    207//!
    208//! Once we've concatenated those together, we hash the result using a Blake2 256bit hasher.
    209//!
    210//! The [`sp_runtime::generic::SignedPayload`] type takes care of assembling the correct payload for
    211//! us, given `call_data` and a tuple of transaction extensions.
    212//!
    213//! # The General Transaction Format
    214//!
    215//! A General transaction does not have a signature method hardcoded in the check logic of the
    216//! extrinsic, such as a traditionally signed transaction. Instead, general transactions should have
    217//! one or more extensions in the transaction extension pipeline that auhtorize origins in some way,
    218//! one of which could be the traditional signature check that happens for all signed transactions
    219//! in the [Checkable](sp_runtime::traits::Checkable) implementation of
    220//! [UncheckedExtrinsic](sp_runtime::generic::UncheckedExtrinsic). Therefore, it is up to each
    221//! extension to define the format of the payload it will try to check and authorize the right
    222//! origin type. For an example, look into the [authorization example pallet
    223//! extensions](pallet_example_authorization_tx_extension::extensions)
    224//!
    225//! # Example Encoding
    226//!
    227//! Using [`sp_runtime::generic::UncheckedExtrinsic`], we can construct and encode an extrinsic as
    228//! follows:
    229#![doc = docify::embed!("./src/reference_docs/extrinsic_encoding.rs", encoding_example)]
    230
    231#[docify::export]
    232pub mod call_data {
    233	use codec::{Decode, Encode};
    234	use sp_runtime::{traits::Dispatchable, DispatchResultWithInfo};
    235
    236	// The outer enum composes calls within
    237	// different pallets together. We have two
    238	// pallets, "PalletA" and "PalletB".
    239	#[derive(Encode, Decode, Clone)]
    240	pub enum Call {
    241		#[codec(index = 0)]
    242		PalletA(PalletACall),
    243		#[codec(index = 7)]
    244		PalletB(PalletBCall),
    245	}
    246
    247	// An inner enum represents the calls within
    248	// a specific pallet. "PalletA" has one call,
    249	// "Foo".
    250	#[derive(Encode, Decode, Clone)]
    251	pub enum PalletACall {
    252		#[codec(index = 0)]
    253		Foo(String),
    254	}
    255
    256	#[derive(Encode, Decode, Clone)]
    257	pub enum PalletBCall {
    258		#[codec(index = 0)]
    259		Bar(String),
    260	}
    261
    262	impl Dispatchable for Call {
    263		type RuntimeOrigin = ();
    264		type Config = ();
    265		type Info = ();
    266		type PostInfo = ();
    267		fn dispatch(self, _origin: Self::RuntimeOrigin) -> DispatchResultWithInfo<Self::PostInfo> {
    268			Ok(())
    269		}
    270	}
    271}
    272
    273#[docify::export]
    274pub mod encoding_example {
    275	use super::call_data::{Call, PalletACall};
    276	use crate::reference_docs::transaction_extensions::transaction_extensions_example;
    277	use codec::Encode;
    278	use sp_core::crypto::AccountId32;
    279	use sp_keyring::sr25519::Keyring;
    280	use sp_runtime::{
    281		generic::{SignedPayload, UncheckedExtrinsic},
    282		MultiAddress, MultiSignature,
    283	};
    284
    285	// Define some transaction extensions to use. We'll use a couple of examples
    286	// from the transaction extensions reference doc.
    287	type TransactionExtensions = (
    288		transaction_extensions_example::AddToPayload,
    289		transaction_extensions_example::AddToSignaturePayload,
    290	);
    291
    292	// We'll use `UncheckedExtrinsic` to encode our extrinsic for us. We set
    293	// the address and signature type to those used on Pezkuwi, use our custom
    294	// `Call` type, and use our custom set of `TransactionExtensions`.
    295	type Extrinsic = UncheckedExtrinsic<
    296		MultiAddress<AccountId32, ()>,
    297		Call,
    298		MultiSignature,
    299		TransactionExtensions,
    300	>;
    301
    302	pub fn encode_demo_extrinsic() -> Vec<u8> {
    303		// The "from" address will be our Alice dev account.
    304		let from_address = MultiAddress::<AccountId32, ()>::Id(Keyring::Alice.to_account_id());
    305
    306		// We provide some values for our expected transaction extensions.
    307		let transaction_extensions = (
    308			transaction_extensions_example::AddToPayload(1),
    309			transaction_extensions_example::AddToSignaturePayload,
    310		);
    311
    312		// Construct our call data:
    313		let call_data = Call::PalletA(PalletACall::Foo("Hello".to_string()));
    314
    315		// The signed payload. This takes care of encoding the call_data,
    316		// transaction_extensions_extra and transaction_extensions_implicit, and hashing
    317		// the result if it's > 256 bytes:
    318		let signed_payload = SignedPayload::new(call_data.clone(), transaction_extensions.clone());
    319
    320		// Sign the signed payload with our Alice dev account's private key,
    321		// and wrap the signature into the expected type:
    322		let signature = {
    323			let sig = Keyring::Alice.sign(&signed_payload.encode());
    324			MultiSignature::Sr25519(sig)
    325		};
    326
    327		// Now, we can build and encode our extrinsic:
    328		let ext = Extrinsic::new_signed(call_data, from_address, signature, transaction_extensions);
    329
    330		let encoded_ext = ext.encode();
    331		encoded_ext
    332	}
    333}