People use smartphones today to do everything from pay for coffee to board a plane. Now, a growing number of hotels want guests to be able to unlock their guest rooms with their phones.

“It’s a simple proposition,” said Mark Vondrasek, who leads digital initiatives for Starwood Hotels. “Why can’t I use my phone as my room key?”

Starwood will begin offering smartphone key apps at 10 hotels in its Aloft, Element and W hotel brands this week, including five in the U.S. The introduction comes after testing conducted this year at two of Starwood’s Aloft brand hotels, one in New York City and one in Cupertino, California.

In September, Hyatt Hotels and Resorts started testing mobile keys at one of its hotels in New York City, a pilot it expects to wrap up by early 2015, said Brett Cowell, vice president of strategic systems.

“We’re also focused on starting some additional pilots in other locations on the West Coast,” he said. For now, the program is invitation only, available to frequent visitors who are members of Hyatt’s loyalty program.

This quarter, Hilton Worldwide will start testing mobile keys at 10 hotels in the U.S., a project it expects to finish in early 2015. By the middle of next year, it says its loyalty program members in the U.S. visiting its Hilton, Waldorf-Astoria, Conrad and Canopy by Hilton brands will be able to use their phones as keys.

Hotels say their research tells them that the prospect of being able to skip the line at the front desk is appealing to business travelers.

But that convenience factor must be weighed against security concerns, said Harry Sverdlove, chief technology officer at the cybersecurity firm Bit9.

“The biggest challenge here is that convenience trumps security, especially in the hospitality industry,” he said.

While encryption — which hotels say they are using for mobile keys — certainly helps, the more difficult a digital system is to access and make changes to, in general, the harder it is to breach, Sverdlove said.

A system flexible enough to accommodate requests for physical keys, multiple guests per room and other considerations is a priority for hotels concerned about the user experience, but these concessions can make the system more vulnerable, he said.

“It’s not about one encryption key opening everything,” said Selva Selvaratnam, senior vice president at a unit of the lock manufacturer Assa Abloy, which is working with Starwood on its mobile key platform. “Everything is layered,” he said, and encryption takes place at several points in transmission.

Sverdlove said another security concern was more prosaic. People accustomed to using an easy-to-guess or multiple-use password for their hotel loyalty account need to keep in mind that this password unlocks more than a cache of points if they are using their phone as a key.

In addition to security concerns, hotels face other issues. They have to make sure the technology doesn’t drain either the batteries in guests’ phones or the ones embedded in the door locks.