The Binding of Isaac: Rebirth — Red Key and Cracked Key Guide

The Red Key and Cracked Key are Repentance's most important progression tools after defeating Mother. Both create red room doors on walls, but they differ in acquisition, durability, and optimal use cases. Understanding which key to use — and where — prevents wasted Home visits and failed Tainted unlock attempts.

Red Key: Permanent Red Room Creator

The Red Key is an active item obtained from Mom's bedroom chest on your first Home visit. It creates permanent red room doors on any wall outline, persisting for the entire run. On Home, the critical use is the faint red outline below Mom's room — using Red Key here opens the Tainted character closet. Red Key also creates Ultra Secret rooms when used on valid wall positions on any floor, enabling access to angel pool items and special encounters.

Cracked Key: Single-Use Trinket Transform

On repeat Home visits when Mom's chest no longer contains Red Key, drop any trinket in a boss room or item room before picking up Dad's Note at the end of the Ascent Mausoleum. During the Ascent climb, the trinket transforms into a Cracked Key card in your consumable slot. Cracked Key works identically to Red Key for one use — open the Tainted closet, create one red room, or access one Ultra Secret position — then consumes itself.

Optimal Usage Priority

• First Home visit — Always use Red Key on the Tainted closet outline before experimenting elsewhere.
• Tainted unlock farming — Save Cracked Keys exclusively for closet access when Red Key is unavailable.
• Ultra Secret hunting — Use Red Key on calculated wall positions per Ultra Secret room rules. See secret room guide.
• Never waste on random walls — Red rooms cost a key use. Plan positions before activating.

Soul of Cain cards also create red rooms and function as Cracked Key equivalents found in rune drops. Tainted Cain and Crystal Key trinket provide additional red room access methods documented in our Tainted characters guide.