Usage help: ecdsa parameters

[focusaurus]

I wonder if you can help me parse the last 2 fields in an ecdsa ssh private key. I have working code to handle the sequence, integer, and octet string, but the final 2 fields I can't figure out how to parse with der-parser. Here's the ASN.1 structure:

ECPrivateKey ::= SEQUENCE {
version INTEGER { ecPrivkeyVer1(1) } (ecPrivkeyVer1),
privateKey OCTET STRING,
parameters [0] ECDomainParameters {{ SECGCurveNames }} OPTIONAL, publicKey [1] BIT STRING OPTIONAL
}

And here's my code, with the first 2 fields of the sequence working but last 2 wrong.

fn der_read_opt_oid(i:&[u8]) -> IResult<&[u8],DerObject,u32> {
    parse_der_optional!(i, parse_der_oid)
}

fn der_read_opt_bitstring(i:&[u8]) -> IResult<&[u8],DerObject,u32> {
    parse_der_optional!(i, parse_der_bitstring)
}

fn parse_oid(i: &[u8]) -> IResult<&[u8], DerObject> {
    parse_der_explicit(i, DerTag::Oid as u8, parse_der_oid)
}

fn parse_bits(i: &[u8]) -> IResult<&[u8], DerObject> {
    parse_der_explicit(i, DerTag::BitString as u8, parse_der_bitstring)
}

fn ecdsa_private(input: &[u8]) -> Algorithm {
    match parse_der_sequence_defined!(
        input,
        parse_der_integer,
        parse_der_octetstring,
        der_read_opt_oid,
        der_read_opt_bitstring,
    ) {
        IResult::Done(_unparsed_suffix, der) => {
            assert_eq!(_unparsed_suffix.len(), 0);
            Algorithm::Ecdsa(0)
        }
        IResult::Error(error) => {
            eprintln!("Error {}", error);
            Algorithm::Unknown
        }
        IResult::Incomplete(_needed) => {
            eprintln!("Incomplete {:?}", _needed);
            Algorithm::Unknown
        }
    }
}

[focusaurus]

OK I found another programmer to pair on it and we were able to use der_read_element_content_as based on the examples in the x509 parser and got this working.