In the Utility pallet there is a function:

/// Derive a derivative account ID from the owner account and the sub-account index.
pub fn derivative_account_id(who: T::AccountId, index: u16) -> T::AccountId {
    let entropy = (b"modlpy/utilisuba", who, index).using_encoded(blake2_256);
    Decode::decode(&mut TrailingZeroInput::new(entropy.as_ref()))
        .expect("infinite length input; no invalid inputs for type; qed")

using_encoded() converts (b"modlpy/utilisuba", who, index) to a slice, encodes it, then it invokes blake2_256 on the slice.

TrailingZeroInput adds zeros to the end of this byte array.

Decode::decode() deserializes the value.

But what actually converts the deserialized value into an AccountId?

I see that AccountId is defined in the node primitives:

/// Some way of identifying an account on the chain. We intentionally make it equivalent
/// to the public key of our transaction signing scheme.
pub type AccountId = <<Signature as Verify>::Signer as IdentifyAccount>::AccountId;

1 Answer 1


frame_system::Config::AccountId requires Decode + Encode.

And in your pallet, trait Config: frame_system::Config.

So, T::AccountId has these functions.

Just treat it as a codec object.

You don't need to care about the primitives::AccountId. frame_system::Config::AccountId will guarantee it has the decode function.

And if you want to know how the decoding works: https://stackoverflow.com/questions/61232644/converting-between-accountid32-and-accountid-in-substrate

Let me give you a re-implementation as an example.


use std::marker::PhantomData;

type AccountId = u8;

trait Decode {
    fn decode(bytes: [u8; 1]) -> Self;
impl Decode for AccountId {
    fn decode(bytes: [u8; 1]) -> Self {

trait FrameSystemConfig {
    type AccountId: Decode;
trait PalletUtilityConfig: FrameSystemConfig {}

struct PalletUtility<T>(PhantomData<T>);
impl<T> PalletUtility<T> 
    T: PalletUtilityConfig
    fn bytes2account(bytes: [u8; 1]) -> T::AccountId {

struct Runtime;
impl FrameSystemConfig for Runtime {
    type AccountId = AccountId;
impl PalletUtilityConfig for Runtime {}

fn main() {
    println!("{}", <PalletUtility<Runtime>>::bytes2account([0]));
  • Still not clear how Rust goes from bytes to T::AccountId. Is there an implicit conversion?
    – Bruno
    Commented Feb 14, 2023 at 21:26
  • 1
    Updated my answer. This is just a Rust question.
    – AurevoirXavier
    Commented Feb 15, 2023 at 3:26

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