PuTTY wish key-formats-natively

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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:

• Public half of key is stored in plaintext. OpenSSH's private key format encrypts the entire key file, so that the client has to ask you for your passphrase before it can do anything with the key at all. In particular, this means it has to ask for your passphrase before it can even offer the public key to the server for authentication. PuTTY's format stores the public key in plaintext and only encrypts the private half, which means that it can automatically send the public key to the server and determine whether the server is willing to accept authentications with that key, and it will only ever ask for a passphrase if it really needs to.

  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but this is a source of confusion as often as convenience (I've seen people replace a private key file and leave an out-of-date .pub alongside it, and then be very confused by the resulting SSH authentication process!).

• Key is fully tamperproofed. Key formats which store the public key in plaintext can be vulnerable to a tampering attack, in which the public half of the key is modified in such a way that signatures made with the doctored key leak information about the private half. For this reason, PuTTY's key format contains a MAC (Message Authentication Code), keyed off the passphrase, and covering the public and private halves of the key. Thus, we provide the convenience of having the public key available in plaintext but we also instantly detect any attempt at a tampering attack, giving a combination of security and convenience which I do not believe is found in any other key format. As a side benefit, the MAC also covers the key's comment, preventing any possible mischief that might be possible if someone were to swap two keys and interchange the comments.

  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.

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