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I've found ssh-keygen ("ssh" package) comand produce another keys that puttygen command ("putty" package). If I create public and private keys with ssh-keygen some ssh-servers can not accept my keys. If I create keys with puttygen one server accept it. Why linux repositories don't propose some common solution (package) for it? I've found another package ssh- which produce another keys that must work with putty. But why there is no handy solution in SSH?

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2 Answers 2

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OpenSSH is the de facto standard implementation of the SSH protocol. If PuTTY and OpenSSH differ, PuTTY is the one that's incompatible.

If you generate a key with OpenSSH using ssh-keygen with the default options, it will work with virtually every server out there. A server that doesn't accept such a key would be antique, using a different implementation of SSH, or configured in a weird restrictive way. Keys of a non-default type may not be supported on some servers, in particular ECDSA keys make session establishment very slightly faster but are only supported by recent versions of OpenSSH.

PuTTY uses a different key file format. It comes with tools to convert between its own .ppk format and the format of OpenSSH.

This ssh- you found is a commercial product which has its own different private key format. There's no reason to use it instead of OpenSSH, it can only be less compatible, it requires paying, and there's about zero tutorial on how to use it out there.

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Most Linux distros have putty available for Linux. You could install putty on the Linux side and use puttygen to convert the .ppk files to the regular ssh style key files (called PEM files - even though they don't get a .pem in the file name).

puttygen id_dsa.ppk -O private-openssh -o id_dsa

NOTE: You can also use puttygen to import ssh style PEM files back into putty.

PuTTY's author opted for simplicity so the public and private keys which make up the underlying security used by putty/ssh 2 key authentication are stored in a single proprietary .ppk file. Typically these keys are maintained as a 2 separate files by ssh.

On Linux the key files are typically kept in the directory, .ssh.

There is a good overview of the conversion process here in this Stack Overflow question titled: Convert PEM to PPK file format.

The author of putty also discusses his rationale for using .ppk files in the putty users manual. You can read about it here in section 8.2.12.

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Do you mean my Linux have obsolete and vulnerable SSH-1 (if I don't use putty)? And is it affordable? Vulnerabilities of SSH-1 are described in wikipedia –  YarLinux May 3 '13 at 16:28
Not sure where you got that. No you should be fine. What version of Linux are you using? What Linux Distro are you using? Run this command to find out your Linux version: uname -a. Linux distro: lsb_release -a. –  slm May 3 '13 at 16:33
I use Ubuntu 12.04. Do you mean SSH-2 has different formats? I just confused by name of package and command which "ssh". –  YarLinux May 3 '13 at 16:38
I see. The ssh tool you're referring to is usually part of a package called openssh. The version of this software has nothing to do with the SSH-1 and SSH-2 you're referring to. That terminology (SSH-1 & SSH-2) is referring to the type of key file that you're working with. This file type shouldn't be an issue for you as long as you're using recent versions of openssh. –  slm May 3 '13 at 16:46
Are there three different formats of keys? OpenSSH, ssh.com and PuTTY? –  YarLinux May 3 '13 at 16:58

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