We are occasionally asked if PuTTY could acquire the ability to use foreign private key formats. The PuTTY suite supports these key formats, in the sense that PuTTYgen can load and save them and convert them into PuTTY's own PPK key format; but a conversion step is required, because PuTTY itself does not include the foreign key import code.
I'm unwilling to do this, because I designed the PPK format rather carefully with some specific design goals in mind, and PuTTY in its current form actually depends on those design goals. The advantages of the PuTTY key format are:
I think OpenSSH will read a
.pub file for this
purpose if it appears alongside the private key file, but this is a
source of confusion as often as convenience (I've seen people
replace a private key file and leave an out-of-date
.pub alongside it, and then be very confused by the
resulting SSH authentication process!).
OpenSSH's approach of keeping the public key encrypted might also provide some security against this type of attack, but it's unclear that it provides proper protection: encryption designed for confidentiality often leaves ways in which the encrypted data can be usefully modified by an attacker. For real integrity protection you want a real dedicated MAC, which is designed to do precisely that.
So in order to support non-PPK key formats in PuTTY, we would have
to revamp the public-key authentication code considerably, to make
it a lot more flexible about when it asked the user for a
passphrase. This would also lead to confusing subtle differences
in operation depending on the key type in use; supporting OpenSSH's
.pub files would also introduce the same failure mode
as OpenSSH exhibits when the two files get out of sync; and for all
of this coding effort on our part and potential UI-level chaos, we
would be providing users with the ability to decrease the security
of their private keys without even necessarily being aware of it.
Therefore, our current intention is not to implement this feature, and our recommendation is that people convert foreign private key formats to PPK in order to use them with PuTTY. This is not too difficult now that we have a Unix command-line port of PuTTYgen, which should make it feasible to automate such conversion.