GENEVA: Nuclear talks between Iran and a US-led grouping of six world powers are “hard … very hard,” a senior US administration official said here Wednesday as the two sides resumed attempts to strike a deal in the crisis over whether the Islamic Republic seeks nuclear weapons.

The official said an agreement is still possible but “we will have to see because it is hard. It is very hard.” The euphoria of the new era of hopefully straight talk and cooperation which new Iranian President Hassan Rouhani ushered in is running up against the realpolitik of the Iranian nuclear crisis.

The devil is in the details of exactly what is agreed but politics risks sabotaging the sort of nitty-gritty haggling needed to get the details straight. Iranian Deputy Foreign Minister Abbas Araqchi told reporters in Geneva that “lost trust should be revived” before the two sides could get down to work, after a meeting here two weeks ago at which Iran felt a text agreed between Iran and the United States was revised at the last minute due to pressure from France.

Iran is trying to get US-led sanctions against it lifted. Oil and banking sanctions have crippled Iran’s ability to sell its oil and do business abroad, devastating the Iranian economy.

Iranian Foreign Minister Javad Zarif, his country’s chief nuclear negotiator, said here the goal was to get down on Thursday to drawing up a final text. He said the talks depended on “finding a way to agree on the questions still separating us.”

Here is a look at what apparently are those questions. It is a synthesis of unnamed sources with inside information on the nuclear story and public reports. Diplomats have given few details on the talks either on or off the record as they try to keep negotiations secret.

The most important matter is Iran’s insistence that it has the right to enrich uranium, the process that makes fuel for civilian power reactors but also the explosive core of atom bombs, The United States has for years insisted that the Non-Proliferation Treaty does not give signatory states the right to enrich, even if the NPT mandates the right to a peaceful nuclear program. A US softening on this may have been what led France to protest at the last Geneva meeting that essential requirements were not being met in a draft agreement.

The six negotiating nations – Britain, Germany and France plus the United States, Russia and China — may accept a re-statement of Iran’s right as an NPT nation to a civilian nuclear program as long as it is in accord with monitoring done by the International Atomic Energy Agency, which is currently not the case. The IAEA charges Iran with not answering questions about possible military nuclear work, for instance.

Iran in return would drop its insistence that its right to enrich be explicitly acknowledged, a change Iranian Foreign Minister Javad Zarif indicated in comments this week. The result is that the current state of affairs will be acknowledged, namely that each side is interpreting the NPT according to its own desires.

Another issue is the heavy water reactor Iran is building at Arak. This reactor could produce plutonium, like uranium a possible atomic bomb material. France wants Iran to stop construction. The United States and Iran were ready to let this go in a first phase, and deal with Arak in a second phase on finding a comprehensive and final solution.

The irony is that Arak is in an unfinished state and that the Iranians apparently have problems in finishing the reactor. A reliable source said Iran cannot obtain all the components it needs, including, significantly, the reprocessing technology needed to take plutonium out of fuel rods. It would thus not be a loss for Iran to freeze work at Arak but Zarif has said Iran will not allow itself to be dictated to. Stopping all construction on Arak, even more shutting the project down, does pose a problem for the Iranians.

Another key issue is stockpiles of enriched uranium which Iran has amassed. These include almost 200 kilograms of uranium enriched up to 20 percent and over 7,000 kilograms of uranium enriched up to 5 percent. The more advanced refinement of 20 percent is a considerable step towards weapon-grade uranium of over 90 percent. The lower enrichment level of up to 5 percent is sufficient for power reactor fuel. Iran has said it will not ship these stockpiles, which if refined further would be enough material for about five atomic bombs, out of the country.

The fear is that the stockpiles could be used for a so-called break-out to make the fissile material for a bomb. Iran could not break-out if the uranium were not there.

An ancillary problem is how many and how sophisticated are the centrifuges Iran has turning to enrich. Iran has a total of some 19,000 centrifuges installed but only about 10,000 actually processing nuclear fuel. One source said that if Iran does not reduce this number, a pledge not to install more centrifuges would ring hollow since it has 9,000 ready to bring online. Among these are some 1,000 advanced centrifuges installed but not yet “nuclear.”

Any agreement, even a first-phase one, would surely have Iran pledge not to use these centrifuges, called IR-2m’s since they are 3-5 times as powerful as the bulk of Iran’s centrifuges, the IR-1’s. A source said that Iran had technical difficulties with the IR-2m’s which it had not yet solved, and so it would not really be conceding anything by not using them for enrichment.

There are other wrinkles to resolve in any agreement, such as increased verification measures. Real concessions are required. As diplomats here stress, the proof is in the pudding. We shall see.

Michael Adler is a Public Policy Scholar at the Wilson Center in Washington where he writes about non-proliferation and the Middle East, including Iran.