ATLANTA  A counterfeiter at a Georgia state prison ticks off the remaining days of his three-year sentence on his Facebook page. He has 91 digital “friends.” Like many of his fellow inmates, he plays the online games FarmVille and Street Wars.

He does it all on a Samsung smartphone, which he says he bought from a guard. And he used the same phone to help organize a short strike among inmates at several Georgia prisons last month.

Technology is changing life inside prisons across the country at the same rapid-fire pace it is changing life outside. A smartphone hidden under a mattress is the modern-day file inside a cake.

“This kind of thing was bound to happen,” said Martin F. Horn, a former commissioner of the New York City Department of Correction who teaches at the John Jay College of Criminal Justice. “The physical boundaries that we thought protected us no longer work.”