"Some make the challah in the shape of a key.
Some attach a piece of dough in the shape of a key. Breslov Customs for Pesach (page 57) says this is the minhag of the family of Reb Elazar Kenig shlita and of Manistritch.
Sefer Hatoda’ah mentions making the image of a key with sesame seeds on top of the challah. These first three customs can, perhaps, be seen from the wording of the Ohev Yisroel in one place where he says we put the image of a key on the challah.
Some place an actual key in the challah. Perhaps this is done because of the wording in many places of indenting the challah with a key."