1. A multitask execution system for executing a plurality of tasks in parallel, comprising: an encryption processor configured to generate a key stream unique to each task, by using key data, a task ID for identifying each task, and an output value, the number of the key data being smaller than the number of the plurality of tasks, the output value being output from a monotonic incremental counter when each task is generated, a value of the monotonic incremental counter configured to continue to be increased without being decreased, and to encrypt data stored in a protected area in a memory space for each task by using the generated key stream.