Add support for ECDSA public key serialization (compressed/uncompressed)

[conr2d]

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• This is not a support question.

Motivation

An ECDSA public key can be represented in either compressed (33 bytes) or uncompressed (65 bytes) form. While functions such as sp_io::crypto::secp256k1_ecdsa_recover() and sp_io::crypto::secp256k1_ecdsa_recover_compressed() facilitate public key recovery, there is currently no method available for converting a public key between these forms.

Request

Introduce a method to serialize an ECDSA public key into either compressed or uncompressed form.

Solution

Add a function to sp_io::crypto with the following signature:

fn secp256k1_pubkey_serialize(pubkey: &[u8], compressed: bool) -> Result<Vec<u8>, ()>;

This function will accept a public key and a boolean flag indicating whether the output should be in compressed form, returning the serialized public key accordingly.

Are you willing to help with this request?

Yes!

[burdges]

I'm unsure how this comes up, but you should probably invoke an external rust crate like https://docs.rs/libsecp256k1 or https://docs.rs/secp256k1/ or https://docs.rs/k256 directly.

[conr2d]

• ecdsa_verify
• ecdsa_verify_prehased
• ecdsa_batch_verify
• secp256k1_ecdsa_recover
• secp256k1_ecdsa_recover_compressed

All of the above host functions can be implemented using a combination of verify_prehased, recovery_prehased, and serialize. I’m not fully aware of the rationale behind providing these specific functions, and I understand that the existing host functions cannot be removed due to the need to verify old blocks. However, it would have been preferable if these three functions had been provided from the beginning.