One of the government’s star witnesses in a United Nations bribery trial is expected to face tough cross-examination today from defense attorneys seeking to undermine his credibility for using illegal drugs and lying about hiring prostitutes.

The witness, Nishan Kohli, spent yesterday on the witness stand in Federal District Court in Manhattan, testifying against a former procurement official for the world body, Sanjaya Bahel. According to Kohli’s testimony, Mr. Bahel gave members of Kohli’s family, whose business is situated in India, inside information over a four-year period to help guarantee they got almost $100 million in contracts to provide the United Nations with computers, radio equipment, and satellite-dish services.

Prosecutors say Mr. Bahel traded tips to the Kohli family in return for cash envelopes, a 10% share of some of the profits, and a sweetheart deal on an apartment in a Turtle Bay high-rise. If convicted, Mr. Bahel could be sentenced to decades in prison on charges of mail fraud, wire fraud, and receiving bribes.

Kohli, 30, pleaded guilty in December to a felony charge and agreed to cooperate with federal investigators against Mr. Bahel, 57. Kohli faces up to 10 years in prison, but he is expected to receive a reduced sentence in return for his cooperation. He is the only member of his family charged in the alleged bribery.

In trying to paint Kohli as a liar, the defense says it will show that the former businessman failed to tell his government handlers that he paid for sex with a prostitute during his marriage, behavior that prosecutors say they learned about only several nights ago. Federal Judge Thomas Griesa has also denied a prosecution motion to bar the defense from inquiring about Kohli’s use of cocaine and marijuana.

Yesterday, Kohli testified that Mr. Bahel had demanded the family pay for a secret cell phone. Kohli testified. Speaking to Mr. Bahel over the secret phone was “the first thing he did in the morning, and the last thing he did at night,” Kohli said of his father.