[Xylakant]

The password to the key must be in the binary - either in clear or encoded form and at some point it needs to be in memory in decoded form. Otherwise the binary could not decode the key itself. You could drop the passphrase immediately after decoding the key to make it harder for the attacker, but fundamentally all info to decode the key must be somewhere on the machine itself.

Trying all strings from the binary if any of them matches is a cheap and easy operation, so try it first, if it doesn't work use a more elaborate approach.