Western sources claim that China had provided Pakistan with fissile material in exchange for centrifuge technology assistance. Zia-ul-Haq hoped to exploit the close relationship with the Chinese further in order to protect Pakistan from potential preventative attacks… [T]he impact of Israeli attack on Osirak and the crash of the centrifuges in 1981 forced Zia-ul- Haq to realize that the nuclear program was vulnerable not just to preventive strikes but also to natural calamities. Zia-ul- Haq then dispatched Lieutenant-General Syed Zamin Naqvi and A.Q. Khan to request bomb-grade fissile materials and bomb designs. Their visit bore fruit as Pakistan then received the Chinese CHIC-4 weapon design along with 50 kilograms of HEU in 1981, material sufficient for two bombs. A.Q. Khan confirmed in a purported 2004 letter to his wife, “The Chinese gave us drawings of the nuclear weapon, gave us 50 kg of enriched uranium, gave us 10 tons of UF6 (natural) and 5 tons of UF6(3%).”
According to A.Q. Khan’s accounts, the Chinese nuclear material was kept in storage until 1985. When Pakistan acquired its own uranium enrichment capability and wanted to return the fissile material, China responded that “the HEU loaned earlier was now considered as a gift … in gratitude” for Pakistan’s help with Chinese centrifuges. It was then that KRL “promptly fabricated hemispheres for two weapons and added them to Pakistan’s arsenal.”
Khan, Feroz Hassan. Eating Grass: The Making of the Pakistani Bomb. Stanford University Press; Stanford. 2012. p. 188 (typographical inconsistencies in original)
Chinese Gave The CHIC-4 Nuclear Bomb Design To The Pakistanis – An Ex-Pakistani General Admits
Brigadier General (retired) Feroz Khan reveals, of course, a known secret that Chinese gave the CHIC-4 nuclear bomb design on the basis of which Pakistanis have designed their nuclear bomb. It is a known fact that despite being able to manufacture fissile weapons-grade uranium (from the stolen Dutch company URENCO centrifuge designs) Pakistanis were having a lot of problems in building a workable nuclear bomb that can be delivered via an aircraft or a missile. This is where the Chinese helped. Of course, it is a different story that they had problems in making missiles too and then traded their nuclear designs with North Koreans in exchange for missile designs.
For a country that has struggled to nudge its capital-investment ratio to 15% of GDP—compared with around 30% for India and 28% for Bangladesh in recent years—this gush of Chinese money comes as a godsend. Not only does it promise to energise the economy and fix such problems as chronic power shortages; it represents a strategic insurance policy against India. China has long been Pakistan’s chief arms supplier, and has quietly provided diplomatic cover and technical aid for its nuclear programme. As Chinese officials are fond of saying, China is an “all-weather friend”—unlike America, which has lavished some $78bn in economic and military aid on Pakistan since independence, but periodically gets stingy when Islamabad fails to curb terrorists.
This became the universal design because of the speed at which a critical explosive mass can be assembled. The Hiroshima bomb developed a critical mass more primitively by shooting one subcritical piece into another using chemical explosives. The Chinese use of implosion employed the same technique used in the plutonium bomb dropped on Nagasaki. The precise design of this Chinese weapon is known because in late 2001 or early 2002, it was sold by the Pakistani proliferator A. Q. Khan to the Libyans, who subsequently turned it over to the C.I.A. The question is: Did Mr. Khan also sell it to Iran?
https://www.nytimes.com/2017/10/02/opinion/iran-nuclear-deal-trump.html