A former encryption research firm employee has been sentenced to death for selling over 150,000 items of classified documents to overseas intelligence agencies for 10 years, causing "unfathomable losses" to multiple departments' coded communications.

With China's rising economic power and advancing technological and military development, the country is facing increasing challenges to fight foreign espionage agencies' attempts to steal State secrets by hooking up with insiders, observers said.

Huang Yu, 42, worked for an encryption research firm from 1997 to 2004. He earned $700,000 from foreign agencies for selling classified files including over 2,000 State secrets, 90 of them top secrets, the China Central Television (CCTV) reported Tuesday. The report did not mention which country's intelligence agency Huang worked for.

Huang's acts "greatly threatened the code communication security of core departments in the Party and military," an anonymous political commissar with the Chengdu branch of the State security agency told the CCTV.

"Had it happened during wartime, it could have cost lives," the official was quoted as saying.

Huang, who kept a copy of everything he handled during work, was dismissed from the firm in 2004 for his poor performance after being transferred three times.

Sensing his layoff, Huang offered to sell information of a military encryption device online to an overseas spy agency in 2002, and was hired since then for a monthly salary of $5,000, according to the report.

Huang continued to work for the spy agency after he was dismissed by his former employer. He asked his wife, who worked as a data administrator at another secrecy-related firm, to copy files and take them home, and also secretly copied classified files from his brother-in-law's computer who worked at the same firm as Huang.

Huang often requested updated information on the firm from his former colleagues. 

Police arrested Huang in 2011 and seized his illegal gains. Huang's wife and brother-in-law were sentenced to five and three years in prison, respectively, and 29 employees at Huang's former employer were also punished, CCTV reported.

Song Zhongping, a Beijing-based military expert, said that with China's growing military strength, many countries attempt to pry into China's military capability.

"Some neighboring counties with territorial disputes, such as Japan, may acquire China's military tactics by recruiting spies to prepare for war," Song told the Global Times, adding that those countries could find loopholes in China's military strategy and come up with counter measures. 

Rarely used sentence

 Most spies were sentenced to less than 20 years in prison, with only a few receiving the death penalty after 1949.

Among them, Liu Liankun, a former major general of the People's Liberation Army, was sentenced to death in 1999 for selling classified documents to Taiwan between 1992 and 1999. Liu's case was dubbed one of the biggest espionage cases after 1949, reported people.cn.

Experts attributed Huang's death sentence to the huge impact caused by his activities. 

"China had to develop a new military code, but more importantly, Huang put China's overall military capabilities in danger because foreign countries could have been easily monitoring China's military information for years," Song said.

Experts said that the case reflects major problems in the confidential system of many classified departments. 

Public awareness

The case was just one of the espionage cases revealed around April 15, China's first National Security Education Day, which also saw many exhibitions, lectures and posters in government organizations, schools, companies and residential communities.

China's espionage cases have been exposed more frequently only recently, as the country, facing national security pressure, has to arouse national security awareness, Zhao Hongrui, dean of the School of Law at Harbin Institute of Technology, who also helped frame China's National Security Law, told the Global Times. The National People's Congress passed the National Security Law on July 1, 2015.

Zhao said that more espionage cases will be revealed regularly in the future, although most of them would remain confidential for security reasons.

Experts said that more espionage cases in recent years targeted wider fields, such as the economy and environment, and foreign spy agencies also acquired the help of more ordinary people, apart from military officials and researchers.

"Some foreign spy agencies would collect water and air samples to analyze missile adaptability in China," Zhao said.

Media reports said taxi drivers, students, and even restaurant owners attracted by commissions offered by some foreign spy agencies took photos of military facilities or stole commercial secrets.