This report raises some questions, and while I’m no currency expert, I’m not sure they’re valid question (seems they’ve already been answered):

The Swiss federal criminal police, in a report released Monday, expresses serious doubt that North Korea is capable of manufacturing the fake bills, which it said were superior to real ones.

The Swiss report includes color enlargements that show the differences between genuine bills and counterfeit supernotes. The supernotes are identical to U.S. banknotes except for added distinguishing marks, which can be detected only with a magnifying glass. In addition, under ultraviolet or infrared light, stripes appear or the serial numbers disappear on the supernotes.

The Bundeskriminalpolizei didn’t hazard a guess as to who’s been manufacturing the supernotes, but said experts agreed that the counterfeits weren’t the work of an individual but of a government or governmental organization.

The U.S. Secret Service, the lead federal agency in combating counterfeiters, declined to provide details or respond to the Swiss report. But spokesman Eric Zahren said the agency stood by its allegations against Pyongyang.

“Our investigation has identified definitive connections between these highly deceptive counterfeit notes and the North Koreans,” Zahren said. “Our investigation has revealed that the supernotes continue to be produced and distributed by sources operating out of North Korea.”

The Swiss report says the Secret Service has refused to provide any information about its investigations. It notes that if the United States produced concrete evidence to back up its allegations, “it would have a basis for going to war.” Under international law, counterfeiting another country’s currency is considered a cause for war.

[. . .]

North Korea’s capacity for printing banknotes is extremely limited, because its banknote printing press dates from the 1970s. Its own currency is of “such poor quality that one automatically wonders whether this country would even be in a position to manufacture the high-quality `supernotes,’ ” the report says.

For years, analysts have wondered why the supernotes - which are detectable only with sophisticated, expensive technology - appear to have been produced in quantities less than it would cost to acquire the sophisticated machinery needed to make them. The paper and ink used to make U.S. currency are made through exclusive contract and aren’t available on the open marketplace. The machinery involved is highly regulated.

[. . .]

Using its printing presses dating back to the 1970’s, North Korea is today printing its own currency in such poor quality that one automatically wonders whether this country would even be in a position to manufacture the high-quality `supernotes.’

Yet that North Korean press is the exact type used by the U.S. money-making agencies, and I’ve not seen elsewhere that the age of that press is an issue:

…North Korea invested US$10 million in an intaglio printing press, the same type used by the United States Bureau of Engraving and Printing.

It is also perhaps not correct to say that North Korea does not have access to the sorts of inks needed for such an undertaking:

They purchased the necessary presses and new variable printing dye technology about the same time as other Nations did. They bought an ink-type that changes from green to magenta while the US one goes from green to black. As a personal note in this, beyond the fact that a dark magenta can do a quick tomfoolery on the eye and look black, as noted elsewhere, a chemical analysis of the ink and its structure should also allow a nefarious Nation with spare time and manpower on its hands to figure out how to do a slight rework so that it goes from green to black.

And North Korea’s involvement in supernotes is from multiple sources for decades:

Asia Delta clerks found an additional $78,000 in Supernotes in their vaults. They traced the bills to accounts opened by North Korea’s Zokwang Trading Co. and another firm that Pyongyang managed in Macao.

U.S. Secret Service agents learned that the phony cash had arrived in the Portuguese enclave from the North Korean trade missions in Guangzhou and Zhuhai, China. Subsequently, Japanese police arrested four Japanese businessmen as they tried to spend bogus U.S. money they said they had obtained from a bank in Zhuhai.

Then, in March 1996, Thai police in Bangkok captured a former Japanese Red Army courier carrying a huge sum in intaglio-printed Supernotes, according to a Newsweek Asia edition cover story. The Japanese and several accomplices had been living for long periods in asylum in North Korea.

As OFK has noted, the conspiracy theorists will never tire of the possibilities:

[Experts] have been wondering for some time now what the CIA is actually doing in its secret printing works. There is a machine in this plant, which is located in a well-known city north of Washington, which is exactly of the type required for printing these super counterfeit bills. The CIA could use it to procure funds which would be subject to no controls from Congress, for undercover operations in international crisis regions. The arch enemy in Pyongyang could then be conveniently blamed for the counterfeit money operation. There is no proof to support this theory, but some counterarguments. . .