No access to encryption key. How to proceed further

I don't have the encryption key with me. In the runtime, its protected and is just having an option to replace the values. The developer is no longer working with us so would not be able to get the data from him. All the credentials are encrypted and I'm not aware of few of the credentials. How to decrypt the values now.  And while trying to run in my local, I'm getting an error  "You need to increment your key size. The minimum allowed key size is: 16, but your key size is: 10 ". Upon checking the jar file has a key of size 10 and not 16.