Board member Alberto Yepez and CFO David Thompson will serve as interim co-CEOs while the company conducts a search for a successor.
The move comes as the Plano, Texas-based Entrust attempts to broaden its position in the security software market away from Public Key Infrastructure (PKI) security software, which is used to authenticate users for private transactions. Although this market has been growing around 50 percent annually, analysts have been looking for a growth rate of around 70 to 80 percent.
"The PKI market has been a little slower to take off for wide scale deployments," said Howard Smith, an analyst with First Analysis Securities. "It probably makes sense to position yourself with a little less reliance on that market and broader position on security mission."
PKI security software is encryption-based technology that allows companies to perform secure e-commerce. PKI systems issue and manage digital certificates, which serve as electronic IDs online.
Ryan had been CEO of Entrust since the company was founded in 1996. He said only that he was leaving to seek "new challenges."
In a statement, Entrust Chairman James Thomson said it is "an opportune time for the company to seek new leadership with the specific skills needed to take Entrust to the next level."
Shares of Entrust fell $2.62, or 18.5 percent, to $11.50 in afternoon trading.