In some examples, a system for authenticating users can include a processor to train a first predictive application based on a first set of user engagements with advertisements, wherein the first predictive application is associated with a first advertising identifier. The processor can also train a second predictive application based on a second set of user engagements with the advertisements, wherein the second predictive application is associated with a second advertising identifier. Additionally, the processor can compare the first predictive application and the second predictive application and authenticate a user in response to detecting a similarity of the first predictive application and the second predictive application is below a threshold value, wherein authenticating the user enables the user to access a resource or service.