Flask session cookies

I am using session cookies for my flask application. They allow me to access data quickly when accessing a database is slow and may cause problems. Is it safe to store information such as a phone number in a session cookie? If not, would there be any way to encrypt the cookies? I am concerned an attacker can steal the data in the cookie even though I am using HTTPS redirects. Currently I am using session['variable'] to store my data and lookup = "select * from table where variable= "" for receiving information from the database. I am wondering if there is a more efficient/more secure way to store information so it cannot be tampered or viewed by the end user. Thanks for any responses in advance!