The computer files were encrypted and difficult to decipher, according to Mr. Albright. At least one current IAEA official doubts that the Tinners, former associates of Khan's, were the only people in possession of the design.
"They both faced struggles in building a nuclear weapon small enough to fit atop their ballistic missiles, and these designs were for a warhead that would fit," says the study.
US and UN investigators have long known that the Khan network sold to Libya a nearly complete set of blueprints and instruction manuals for a relatively basic nuclear warhead of Chinese design.
But it was apparently not until sometime last year that US intelligence was able to crack the Tinner family's codes and discover the hidden, second design. They were apparently not the only ones surprised by its existence in the hands of known nuclear smugglers.
The Pakistanis, too, were upset, according to the Albright study, because they recognized that the design was most likely their own. Although Pakistani warheads are also derived from Chinese originals, Pakistan has gone much further in developing the sophisticated electronics and triggering mechanisms necessary for smaller designs.
The Pakistanis "were genuinely shocked; Khan may have transferred his own country's most secret and dangerous information to foreign smugglers so that they could sell it for a profit," writes Albright.
In some ways, possessing a sophisticated warhead design would not help would-be nuclear proliferators all that much.
For one thing, it might take only a crude bomb, even thought it might be as bulky as a truck, to provide the strategic deterrent power of nuclear weapons. For another, designing a bomb is not the hardest part of obtaining a bomb. Producing or obtaining the fissile material necessary for the weapon's explosive heart remains a more daunting challenge.
Still, nuclear engineering is difficult. North Korea's nuclear test of 2006 was widely considered by Western scientists to be a fizzle. Even US national labs have occasionally designed warhead duds.