Is it possible to extract the ECDSA public key from Multichain's public key?

Question

The documentation at https://www.multichain.com/developers/address-key-format/ only describes how the private key is derived from ECDSA private key. But we have a scenario where we would like to use the ECDSA public key of the user (for either signing or encryption). Assuming we are able to retrieve the public key, is it possible to derive the ECDSA public key?

Answer 1

A MultiChain-formatted private key is just a way of representing the ECDSA private key in text form, and it's derived from it in any cryptographic sense. So I'm not sure what you mean by deriving an ECDSA public key from a MultiChain public key.

But perhaps you mean from a MultiChain address? If so I'm afraid the public key cannot be derived from this, since the address is the text form of a (one-way) hash of the public key.

Answer 2

If the user/address has only incoming transactions there is no information of his public key on the chain. However if there is an outgoing transaction from that address you may find it in the vin scriptSig. See here for an example how we collect public keys from transaction inputs:

https://github.com/coblo/gui-demo/blob/master/app/sync.py#L165