Exit Access Card icon

As it turns out, the code is a pretty secure 1024 bit key.

An Exit Access Card is a special item needed to unlock the Exit Door in facilities in Endless modes starting from day 5. The Exit Access Card is found in a Corporate Safe somewhere in the facility, which occurs in addition to the number of safes that normally occur.

The Exit Access Card is a single-use item which is consumed when it is used to unlock the Exit Door; it is therefore not possible to carry it out of the mission. It can be sold at a Nanofabricator for 250 credits, but it must then be bought back in order to complete the mission. If the Exit Access Card is irretrievably lost, then the mission becomes unwinnable.