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 > Only if the passphrase for the symmetrically encrypted
 > file is harder to crack the key passphrase.

 You're saying that, assuming the passphrases are equally
 good, (1) cracking the private key (given the public key
 and the encrypted private key) is really just as
 difficult as (2) cracking a symmetric encryption?
Let me try to clarify.
If you use the same passphrase for your key as for the
symmetric encryption then they are equally secure.
The private key is actually a large number. If you have
that number you can decrypt anything encrypted to that key
(namely the file)
However the private key is symmetrically encrypted to the
passphrase.
The file is also symmetrically encrypted to the passphrase.
If you have the private key separate from the file they
have to recreate the private key, not the passphrase.
Therefore instead of centuries it would take eons unless
they decided to go at the file instead of the key, in which
case it would take the origional centuries but they would
only have access to that one file instead of everything you
have access to.
 I'm surprised: I would not have expected that, since
 there is a mathematical relationship between the "real"
 (unencrypted) private key and the public key; in other
 words, there is just more information to play with in
 case (1).
They are mathematically related but to derive the private
from the public would take several billion times the age of
the universe.
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