Methods, systems, and computer programs for verifying a password are disclosed. For example, the password can be verified on a mobile device to control user access to the mobile device. In some implementations, a mobile device includes a user interface, a main processor, and a co-processor. The user interface receives a submitted password value from a user. The main processor calls the co-processor to provide a hash chain input value based on the submitted password value. The main processor evaluates a hash chain based on the hash chain input value provided by the co-processor. Evaluating the hash chain generates a submitted password verification value. The submitted password verification value is compared to a stored password verification value stored on the mobile device. Access to mobile device functionality may be permitted or denied based on a result of the comparison.