Given a private key as a string, how will I generate a public key?

[naddika]

Given a private key as a string, how will I generate a public key?

[davecgh]

I'm not familiar with the Cosmos SDK, but looking at the github repo, I do see it listed as a direct dependency in their go.mod, so it appears that it does use it.

For your question, it depends on what format the private key string is in. Assuming it's just the hex-encoded bytes of the private key and is already known to be good, you'd just do the typical hex.DecodeString to decode the string into raw bytes and then call secp256k1.PrivKeyFromBytes to convert it to a private key that you can then convert to a public key via the PubKey method it exposes. From there, you have the public key and can serialize it if you want or otherwise use it however your want.

There is an example in the secp256k1/ecdsa package of doing decoding a hard-coded private key string, using it to sign a message, getting the public key for it, and then verifying the signature+message with the public key:

https://pkg.go.dev/github.com/decred/dcrd/dcrec/secp256k1/[email protected]/ecdsa#example-Sign

Here is a trivial example that does only what you asked and then prints out the serialized compressed public key:

package main

import (
	"encoding/hex"
	"fmt"

	"github.com/decred/dcrd/dcrec/secp256k1/v4"
)

func main() {
	privKeyString := "0000000000000000000000000000000000000000000000000000000000000001"
	privKeyBytes, err := hex.DecodeString(privKeyString)
	if err != nil {
		fmt.Println(err)
		return
	}
	privKey := secp256k1.PrivKeyFromBytes(privKeyBytes)
	pubKey := privKey.PubKey()
	fmt.Printf("Serialized Public Key: %x\n", pubKey.SerializeCompressed())
}

Output:

Serialized Public Key: 0279be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798