hkdf using sha256

[blackheart01]

I'm trying to generate an hkdf sha256 key from a master key and a salt. According to https://wiki.openssl.org/index.php/EVP_Key_Derivation it seems openssl has this functionality. Unfortunately, looking through the source code for spacemonkeygo/openssl, I can't find any "EVP_KDF" functions.

I've written my own HKDF function that produces the expected output:

func HKDF(master, salt []byte) ([]byte, error) {
	hmac, err := openssl.NewHMAC(salt, openssl.EVP_SHA256)
	if err != nil {
		return nil, err
	}

	_, err = hmac.Write(master)
	if err != nil {
		return nil, err
	}

	prk, err := hmac.Final()
	if err != nil {
		return nil, err
	}
	hmac.Close()

	hmac, err = openssl.NewHMAC(prk, openssl.EVP_SHA256)
	if err != nil {
		return nil, err
	}

	_, err = hmac.Write([]byte{1})
	if err != nil {
		return nil, err
	}

	key, err := hmac.Final()
	if err != nil {
		return nil, err
	}
	hmac.Close()
	
	return key, nil
}

This function works, but is only a proof of concept right now. Before I commit to making the function production ready, I thought I'd ask if anyone could point me to a spacemonkeygo/openssl function that already does this.

Thanks!