Obtaining and Starting PuTTYgen
PuTTYgen originates from PuTTY and is also part of the PuTTY installation package. It can be downloaded here: https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html
When you run PuTTYgen you will see a window where you have two choices: Generate, to generate a new public/private key pair, or Load to load in an existing private key.
Generating a New Key
- First, you need to select which type of key you want to generate, and also select the strength of the key.
- Then press the Generate button, to actually generate the key.
- Once you have generated the key, select a comment field and a passphrase.
- Now you’re ready to save the private key to disk; press the Save private key button.
Your key pair is now ready for use. You may also want to copy the public key to your server, either by copying it out of the Public key for pasting into authorized_keys file box, or by using the Save public key button. However, you don’t need to do this immediately; if you want, you can load the private key back into PuTTYgen later and the public key will be available for copying and pasting again.
Selecting the Type of Key
Before generating a key pair using PuTTYgen, you need to select which type of key you need. PuTTYgen currently supports these types of key:
- An RSA key for use with the SSH-2 protocol.
- A DSA key for use with the SSH-2 protocol.
- An ECDSA (elliptic curve DSA) key for use with the SSH-2 protocol.
- An Ed25519 key (another elliptic curve algorithm) for use with the SSH-2 protocol.
- An RSA key for use with the SSH-1 protocol.
The SSH-2 protocol supports more than one key type. The types supported by WinSCP are RSA, DSA, ECDSA, and Ed25519.
The SSH-1 protocol only supports RSA keys; if you will be connecting using the SSH-1 protocol, you must select the last key type or your key will be completely useless.
Selecting the Size (Strength) of the Key
The Number of bits input box allows you to choose the strength of the key PuTTYgen will generate.
For RSA, 2048 bits should currently be sufficient for most purposes.
For ECDSA, only 256, 384, and 521 bits are supported. (ECDSA offers equivalent security to RSA with smaller key sizes.)
For Ed25519, the only valid size is 256 bits.
The Generate Button
Once you have chosen the type of key you want, and the strength of the key, press the Generate button and PuTTYgen will begin the process of actually generating the key.
First, a progress bar will appear and PuTTYgen will ask you to move the mouse around to generate randomness. Wave the mouse in circles over the blank area in the PuTTYgen window, and the progress bar will gradually fill up as PuTTYgen collects enough randomness. You don’t need to wave the mouse in particularly imaginative patterns (although it can’t hurt); PuTTYgen will collect enough randomness just from the fine detail of exactly how far the mouse has moved each time Windows samples its position.
When the progress bar reaches the end, PuTTYgen will begin creating the key. The progress bar will reset to the start, and gradually move up again to track the progress of the key generation. It will not move evenly, and may occasionally slow down to a stop; this is unfortunately unavoidable, because key generation is a random process and it is impossible to reliably predict how long it will take.
When the key generation is complete, a new set of controls will appear in the window to indicate this.
The Key Fingerprint Box
The Key fingerprint box shows you a fingerprint value for the generated key. This is derived cryptographically from the public key value, so it doesn’t need to be kept secret; it is supposed to be more manageable for human beings than the public key itself.
The fingerprint value is intended to be cryptographically secure, in the sense that it is computationally infeasible for someone to invent a second key with the same fingerprint, or to find a key with a particular fingerprint.
Setting a Passphrase for Your Key
The Key passphrase and Confirm passphrase boxes allow you to choose a passphrase for your key. The passphrase will be used to encrypt the key on disk, so you will not be able to use the key without first entering the passphrase.
When you save the key, PuTTYgen will check that the Key passphrase and Confirm passphrase boxes both contain exactly the same passphrase, and will refuse to save the key otherwise.
If you leave the passphrase fields blank, the key will be saved unencrypted. You should not do this without good reason; if you do, your private key file on disk will be all an attacker needs to gain access to any machine configured to accept that key. If you want to be able to passwordless log in without having to type a passphrase every time, you should consider using Pageant so that your decrypted key is only held in memory rather than on disk.
Under special circumstances you may genuinely need to use a key with no passphrase; for example, if you need to run an automated batch script that needs to make an SSH connection, you can’t be there to type the passphrase. In this case we recommend you generate a special key for each specific batch script (or whatever) that needs one, and on the server side you should arrange that each key is restricted so that it can only be used for that specific purpose. The documentation for your SSH server should explain how to do this (it will probably vary between servers).
Choosing a good passphrase is difficult. Just as you shouldn’t use a dictionary word as a password because it’s easy for an attacker to run through a whole dictionary, you should not use a song lyric, quotation or other well-known sentence as a passphrase. If you want your passphrase to make grammatical sense, this cuts down the possibilities a lot and you should use a longer one as a result.
Do not forget your passphrase. There is no way to recover it.
Saving Your Private Key to a Disk File
Once you have generated a key, set a comment field and set a passphrase, you are ready to save your private key to disk.
Press the Save private key button. PuTTYgen will put up a dialog box asking you where to save the file. Select a directory, type in a file name, and press Save.
Saving Your Public Key to a Disk File
RFC 4716 specifies a standard format for storing SSH-2 public keys on disk. Some SSH servers (such as ssh.com’s) require a public key in this format in order to accept authentication with the corresponding private key. (Others, such as OpenSSH, use a different format)
To save your public key in the SSH-2 standard format, press the Save public key button in PuTTYgen. PuTTYgen will put up a dialog box asking you where to save the file. Select a directory, type in a file name, and press Save.
You will then probably want to copy the public key file to your SSH server machine.
If you use this option with an SSH-1 key, the file PuTTYgen saves will contain exactly the same text that appears in the Public key for pasting box. This is the only existing standard for SSH-1 public keys.
The text is copy of PuTTY User Manual or was inspired by it.