decodeDer

    pub fn decodeDer(decoder: *der.Decoder) !BitString {
        const ele = try decoder.element(asn1_tag.toExpected());
        const bytes = decoder.view(ele);

        if (bytes.len < 1) return error.InvalidBitString;
        const padding = bytes[0];
        if (padding >= 8) return error.InvalidBitString;
        const right_padding: u3 = @intCast(padding);

        // DER requires that unused bits be zero.
        if (@ctz(bytes[bytes.len - 1]) < right_padding) return error.InvalidBitString;

        return BitString{ .bytes = bytes[1..], .right_padding = right_padding };
    }