Imagine you get hold of someone's private SSH key, which is encrypted. Now imagine you also get hold of the unencrypted key (e.g., because some SSH client stores it after decryption).
Can you determine the encryption passphrase using this information?
(I am asking because the passphrase to one of my own keys, which I did not have to type for months now, stopped working. I might recover the key from the registry, but I also want to know why the passphrase that I believe I remember correctly stopped working.)