I will approach the question from practical side. All the examples are not real, but for illustration purpose only.
I have a message, the secret Bitcoin key, which I want to store safely.
To do this I come up with 4 keys which look independent, somewhat private, and can be recovered.
Key 1 is a set of last words from page 100 and up in my favorite book to match the message length:
help knows the at sortes state the citizens color science costs unfairness
Key 2 is the Merkle Root from Bitcoin blockchain block number 121047, which could be my 2 kids' birthdays 121 and 047 (for the day number within year easily available at blockchain.info)
Key 3 is the name of the file of my main wedding photograph, stored in my albums, mail, and shared with friends.
Key4 is another Bitcoin address generated solely for the purpose of being a key, two copies will be stored in home safe and safety deposit box in the bank
The keys are not re-used anywhere. KEY3 is replicated to match the length of the message. The message and all keys are XORed (byte by byte) to get the secure code.
The question is: How safe is the resulting code? Can I store it online? Can security hold if 1 or 2 keys (but no more) are compromised?
I understand that there might be no correct answer, but would appreciate any thoughts if I miss some vulnerabilities. Should I have more keys? What kind of keys would help? Can I mix in more plain text keys?
PS. I can try to figure out how to post the HTML doing the job if anyone is interested.