A .ppk file is PuTTY’s native private-key format. On Linux, the standard OpenSSH client may not read it directly, so install the PuTTY tools, convert the key to OpenSSH format, protect the converted file, and connect with ssh -i. The server must contain the matching public key in the target account’s ~/.ssh/authorized_keys.
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What a PPK file is—and why Linux may need a conversion
PPK means PuTTY Private Key. It is the native private-key format used by PuTTY and related tools. OpenSSH, the SSH client normally used from a Linux terminal, may require a private key in an OpenSSH-compatible format instead. A format error does not necessarily mean the key is damaged; it often means the client and file formats do not match.
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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Converting a PPK creates an OpenSSH private-key file for the Linux client. The conversion does not by itself authorize an account. Authentication succeeds only when the server has the corresponding public key in the authorized_keys file of the account you are trying to log into.
Keep the original PPK as a protected backup. Treat both it and the converted private key as secrets: never paste either into a public issue, chat, ticket, or server configuration file.
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What you need before connecting
- A Linux computer with an SSH client and permission to install the distribution’s PuTTY tools package.
- The PPK file and, if it is protected, its passphrase.
- The server hostname or IP address and the SSH port if it is not the default SSH port.
- The exact username of the account whose public key is authorized on the server.
- An existing way to reach the server, such as a console or another login, if you still need to install the public key.
Do not assume that a key authorized for one account works for another. A key installed for alice is not automatically valid for a login as bob.
Prepare the server with the matching public key
Public-key authentication uses a key pair. You keep the private half on your computer; the server stores the public half. The public key must be a complete OpenSSH public-key line in the target account’s ~/.ssh/authorized_keys file.
Extract the OpenSSH public-key line
From a system with the PuTTY tools installed, run:
puttygen keyfile.ppk -LCopy the complete OpenSSH public-key output as one line. Do not copy a proprietary display, a fingerprint line, or a partial selection from another PuTTYgen field.
Install it for the intended account
- Use an existing login or a server console to access the target account.
- Create the account’s
~/.sshdirectory if it does not exist. - Open or create
~/.ssh/authorized_keysand paste the entire public-key line on its own line. - Save the file without wrapping, truncating, or altering the key text.
- Check that the home directory,
.sshdirectory, andauthorized_keysfile are owned by the target account and have restrictive permissions.
PuTTY documentation records this hardening command:
chmod go-w $HOME $HOME/.ssh $HOME/.ssh/authorized_keysThat removes group and other write permission from the relevant paths. The private key on your client is commonly protected with mode 600, which allows only its owner to read and write it.
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Convert the PPK on Linux for OpenSSH
1. Install the PuTTY tools
On Debian- and Ubuntu-based systems, install the package named putty-tools:
sudo apt install putty-toolsOther distributions use different package managers and package names. Install the distribution’s package that provides puttygen; the exact installation command is distribution-specific.
2. Convert the private key
Run puttygen with the PPK as input and request an OpenSSH private-key output:
puttygen keyfile.ppk -O private-openssh -o ~/.ssh/keyfileThe -O private-openssh option selects the OpenSSH private-key format. The -o option writes the converted key to the path you specify. Choose a filename that identifies the key without exposing its contents.
If the PPK has a passphrase, puttygen may ask for it during conversion. A successful conversion produces the output file; it does not change or delete the original PPK.
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3. Protect the converted file
chmod 600 ~/.ssh/keyfileUse the actual path and filename you selected. If the file is readable by other users, the SSH client may reject it or the private key may be exposed to those users. Keep the file in a protected location under your home directory and do not upload it to the server.
4. Connect with the converted key
ssh -i ~/.ssh/keyfile [email protected]Replace username with the account where you installed the public key and replace the hostname with the server’s real address. The client uses the private key named by -i and offers the matching public-key identity to the server.
5. Specify a non-default SSH port when necessary
ssh -i ~/.ssh/keyfile -p PORT [email protected]Replace PORT with the server’s configured SSH port. A correct key cannot authenticate if the command reaches the wrong service or endpoint.
6. Turn on client diagnostics when a connection fails
ssh -v -i ~/.ssh/keyfile [email protected]The -v option provides client-side diagnostic messages. Use it to confirm which hostname, port, and identity file the client is attempting. Do not publish diagnostic output if it contains private host details or other sensitive information.
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When you run ssh -i, the client proves possession of the private key. The server checks whether the corresponding public key appears in the authorized_keys file for the account named in the connection command. The private key itself never belongs in that server file.
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Three values must line up:
- Key pair: the converted private key must correspond to the public key you installed.
- Account: the username in the SSH command must be the account whose home directory contains the authorized key.
- Endpoint: the hostname or IP address and SSH port must identify the intended server.
Ownership and permissions are part of the server-side check. A home directory or .ssh path that is writable by unrelated users can cause the server to reject an otherwise correct key. Verify the complete path, not only the final file.
Using the PPK directly with PuTTY on Windows
If you are connecting from Windows with PuTTY, conversion is unnecessary because PuTTY understands its native format. In PuTTY’s SSH authentication credentials panel, select the PPK private-key file. PuTTY’s documentation states: “This key must be in PuTTY’s native format (*.PPK).”
The Windows workflow is therefore different from the Linux OpenSSH workflow:
- Open PuTTY and enter the server hostname, username, and SSH port in the session settings.
- Open the SSH authentication credentials panel.
- Select the original
.ppkfile as the private key. - Open the session and enter the PPK passphrase when prompted, if one is set.
PuTTY can also work with Pageant, its authentication agent. Load the PPK into Pageant once, then let the agent supply the key to subsequent PuTTY connections instead of selecting the file for every session. Agent forwarding can make the agent available to a subsequent SSH hop; enable forwarding only when that multi-hop workflow requires it.
OpenSSH versus PuTTY: which workflow fits?
| Aspect | OpenSSH on Linux | PuTTY on Windows |
|---|---|---|
| Client | The system ssh command |
PuTTY |
| PPK conversion | Usually required; convert with puttygen to an OpenSSH private key |
Not required; PuTTY reads PPK directly |
| Where the key is used | A protected private-key file passed with -i |
A selected PPK file or a key loaded into Pageant |
| Passphrase handling | The SSH client prompts unless an agent such as ssh-agent is holding the key |
PuTTY prompts unless Pageant is supplying the loaded key |
| Agent forwarding | Available through an SSH-agent workflow when configured | PuTTY can forward the Pageant agent for a subsequent SSH hop; enable it only when needed |
| Server requirement | The matching public key must be in the target account’s authorized_keys, with appropriate ownership and permissions |
|
Choose the client that matches where you are working. A PPK is not inherently tied to Windows; the key can be converted for Linux, while PuTTY can continue using the original file on Windows.
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Troubleshooting common failures
“Invalid format” or “cannot load key”
The OpenSSH client is probably being given the PPK directly. Convert it with:
puttygen keyfile.ppk -O private-openssh -o ~/.ssh/keyfileThen set mode 600 and point ssh -i at the converted file. Alternatively, use PuTTY or Pageant, which natively understand PPK.
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“Permission denied (publickey)”
- Confirm that the public-key line is complete and unmodified.
- Confirm that it is in the
authorized_keysfile for the account named in the command. - Check ownership of the account’s home directory,
.sshdirectory, andauthorized_keys. - Check that those paths are not writable by unrelated users.
- Verify that the private key you converted is the pair corresponding to the installed public key.
Use ssh -v to see the client-side identity and endpoint details while you investigate.
The key works for one username but not another
Authorization is account-specific. Install the matching public-key line in the second account’s authorized_keys file, or connect using the username for which it is already installed.
The command reaches the wrong server
Check the hostname or IP address and the SSH port. A valid key is rejected when the command connects to a different machine or service from the one where you installed the public key.
A passphrase prompt appears
A passphrase-protected key is expected to prompt when the client needs to unlock it. You can enter the passphrase, or load the key into an agent such as ssh-agent on Linux or Pageant with PuTTY so the agent performs the key handling for later connections.
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The public key was copied from the wrong PuTTYgen field
Run puttygen keyfile.ppk -L and use the OpenSSH public-key output. A fingerprint or a proprietary display is not the complete line that belongs in authorized_keys.
The server file looks correct but authentication still fails
Trace the entire path from the account’s home directory to authorized_keys. Verify the account owner, restrictive permissions, exact username, exact endpoint, and that no line wrapping or truncation occurred when the key was pasted. These checks distinguish a server authorization problem from a client format problem.
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Security practices for PPK conversion and SSH login
- Protect private material: keep the PPK and converted private key out of public repositories, chat messages, issue trackers, and server configuration files.
- Use restrictive client permissions: apply
chmod 600to the converted private key and store it under a protected~/.sshpath. - Install only the public half on the server: the server needs the OpenSSH public-key line, never the private key.
- Limit agent forwarding: forwarding exposes an agent to a subsequent SSH hop, so enable it only for a workflow that requires it.
- Keep the account and endpoint explicit: record which username and server each authorized key is intended to access so a correct key is not tested against the wrong target.
- Retain a recovery path: do not remove an existing access method until the converted key has successfully authenticated and you have verified the account and server.
Command reference
| Purpose | Command |
|---|---|
| Install PuTTY tools on Debian- or Ubuntu-based Linux | sudo apt install putty-tools |
| Extract the OpenSSH public-key line | puttygen keyfile.ppk -L |
| Convert PPK to an OpenSSH private key | puttygen keyfile.ppk -O private-openssh -o ~/.ssh/keyfile |
| Protect the converted private key | chmod 600 ~/.ssh/keyfile |
| Connect on the standard SSH port | ssh -i ~/.ssh/keyfile [email protected] |
| Connect on a specified port | ssh -i ~/.ssh/keyfile -p PORT [email protected] |
| Enable client diagnostics | ssh -v -i ~/.ssh/keyfile [email protected] |
| Harden server path write permissions | chmod go-w $HOME $HOME/.ssh $HOME/.ssh/authorized_keys |
End-to-end checklist
- Identify the target username, hostname or IP address, and SSH port.
- Confirm that the server has the PPK’s matching public key in that account’s
authorized_keys. - Install the package that provides
puttygenon the Linux client. - Extract the public-key line with
puttygen keyfile.ppk -Lif server preparation is still needed. - Convert the PPK with the
private-opensshoutput option. - Set the converted key to mode
600. - Run
ssh -iwith the same username and endpoint used during server preparation. - If authentication fails, use
ssh -v, then check format, key pairing, account, endpoint, and permissions in that order.
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