Code Breaker (Alphabet Soup Series, #2)

It had been seven years since Mark and Holly Hill had left the military and the top-secret team that had used blackmail and other means to convince some very powerful people to make the "correct" decisions. After all those years, memories of those other means still disturbed their sleep. Fortunately, with time, the nightmares had become less frequent, until...
As a civilian, Mark had been writing security software for his employer. What Mark didn't realize was that someone was using his software to deliver classified material to foreign nationals. The realization came when two government agents knocked on his door and spirited him away in the middle of the night.
Reluctantly, Mark agreed to help discover the identity of the spy who was using his software. Only then would the government officials ignore the fact that he was using military-grade encryption for a civilian application. If he worked fast enough, Mark might even be able to protect Holly's secret.