parseDer

        pub fn parseDer(pub_key: []const u8) ParseDerError!struct { modulus: []const u8, exponent: []const u8 } {
            const pub_key_seq = try der.Element.parse(pub_key, 0);
            if (pub_key_seq.identifier.tag != .sequence) return error.CertificateFieldHasWrongDataType;
            const modulus_elem = try der.Element.parse(pub_key, pub_key_seq.slice.start);
            if (modulus_elem.identifier.tag != .integer) return error.CertificateFieldHasWrongDataType;
            const exponent_elem = try der.Element.parse(pub_key, modulus_elem.slice.end);
            if (exponent_elem.identifier.tag != .integer) return error.CertificateFieldHasWrongDataType;
            // Skip over meaningless zeroes in the modulus.
            const modulus_raw = pub_key[modulus_elem.slice.start..modulus_elem.slice.end];
            const modulus_offset = for (modulus_raw, 0..) |byte, i| {
                if (byte != 0) break i;
            } else modulus_raw.len;
            return .{
                .modulus = modulus_raw[modulus_offset..],
                .exponent = pub_key[exponent_elem.slice.start..exponent_elem.slice.end],
            };
        }