Puttygen VS ssh keygen

They both store a "RSA key pair for version 2 of the SSH protocol" and can be converted interchangeably however regarding the actual stored format difference:


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but this is a source of confusion as often as convenience (I've seen people replace a private key file and leave an out-of-date .pub alongside it, and then be very confused by the resulting SSH authentication process!).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

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