Touch ID authentication concept

I hope this question is appropriate here. If not, please let me know and I'll ask it somewhere else. I'm currently developing an iOS app and in which I encrypt some user data with a user-chosen key. The data can be stored locally or in iCloud or both. I also want to allow a quicker access such as 4-digit pin code or drawn pattern. In both cases I can store an encrypted key in the iOS keychain and decrypt it with the 4-digit code or the pattern string. Basically, I don't store the master key in plaintext to the keychain. The problem is Touch ID. Currently I'm storing the master key in plaintext to the keychain because I can't figure out how not to. If anyone could give me an idea, concept, pseudo code or whatever, I would really appreciate it.