The solution is a 32 characters long plain-text (the private key).
As far as I know, there is no 32 character private key. A
mini private key would have 30 characters.
every character is represented as 1 byte and considering a private key is 32 bytes you can simply convert a 32 char long string to a 32 byte long byte array and use it as a private key.
for instance this sentence ("Hi this is satoshi nakamoto comb") is 32 byte long and can be represented as "10f0c810a0c56a1282574ce4bf9d3e843afc5be688c277ab75e6da1e6f95d756" in base16 or "Kwne8vNfRRiq8d6CB6YpCtbK6EKCRyoCjPGomLshKgubBWwiDQWD" in base58 WIF.