How to recover AccountId from ECDSA signature in ink!

Question

In ink! 4.0 ink::env crate, it provides ecdsa_recover function. It returns compressed public key &[u8, 33] as is explained here.

Assumed that blake2 is used for hashing.

#[ink(message)]
pub fn verify(&self, req: ForwardRequest, signature: [u8; 65]) -> bool {
    let mut message_hash = <ink::env::hash::Blake2x256 as ink::env::hash::HashOutput>::Type::default();
    ink::env::hash_encoded::<ink::env::hash::Blake2x256, _>(&req, &mut message_hash);

    let output = self.env().ecdsa_recover(&signature, &message_hash).expect("Failed to recover");

    let signature_account_id = // How to get AccountId from compressed public key?

    todo!();
}

(Above code before todo!() is not confirmed to work)

How can I recover AccountId from compressed public key?

Answer 1

To get the AccountId, you need to hash the compressed public key with Blake2x256

I will just update your question - the function used in the question (where the output public key is the output of ecdsa_recover) is for ink! version < 4.0.0, ink! v4's ecdsa_recover function stores the output public key in the third parameter output, see the function here.

pub fn ecdsa_recover(
    signature: &[u8; 65],
    message_hash: &[u8; 32],
    output: &mut [u8; 33]
) -> Result<()>

Then you can hash the output value with Blake and get the AccountId.