PowerShell

Convert a PuTTY Private Key (.ppk) to OpenSSH (.pem)

Demonstrates converting a PuTTY format private key to OpenSSH format. The .ppk is imported with FromPuttyPrivateKey and re-exported with ToOpenSshPrivateKey, both unencrypted and encrypted.

Note: The file paths are relative to the application's current working directory. Supply the paths to your own files.

Background: PuTTY and OpenSSH store the same underlying key material in different container formats, so converting between them is a re-encoding rather than a new key — the corresponding public key, and therefore the server-side authorized_keys entry, is unchanged. This matters when moving between Windows tooling built around PuTTY and Unix tooling that expects PEM. Note that the Password property serves double duty: it decrypts the key on import and encrypts it on export, so set it appropriately for each step.

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Add-Type -Path "C:\chilkat\ChilkatDotNet47-x64\ChilkatDotNet47.dll"

$success = $false

#  Demonstrates converting a PuTTY format private key (.ppk) to OpenSSH (.pem) format.

$key = New-Object Chilkat.SshKey

#  Set the password before importing an encrypted PuTTY key.  If the key is not encrypted it
#  makes no difference whether Password is set.  This should come from a secure source rather
#  than being hard-coded.
$key.Password = "myKeyPassword"

#  LoadText is a convenience method that reads any text file into a string.  It does not itself
#  load the key.
$keyStr = $key.LoadText("qa_data/putty_private_key.ppk")
if ($key.LastMethodSuccess -eq $false) {
    $($key.LastErrorText)
    exit
}

$success = $key.FromPuttyPrivateKey($keyStr)
if ($success -eq $false) {
    $($key.LastErrorText)
    exit
}

#  Export to an unencrypted OpenSSH key.
$bEncrypt = $false
$unencryptedKeyStr = $key.ToOpenSshPrivateKey($bEncrypt)
if ($key.LastMethodSuccess -eq $false) {
    $($key.LastErrorText)
    exit
}

$success = $key.SaveText($unencryptedKeyStr,"qa_output/unencrypted_openssh.pem")
if ($success -eq $false) {
    $($key.LastErrorText)
    exit
}

#  Export to an encrypted OpenSSH key.  The Password property supplies the passphrase used to
#  encrypt the output.
$bEncrypt = $true
$key.Password = "myExportPassword"
$encryptedKeyStr = $key.ToOpenSshPrivateKey($bEncrypt)
if ($key.LastMethodSuccess -eq $false) {
    $($key.LastErrorText)
    exit
}

$success = $key.SaveText($encryptedKeyStr,"qa_output/encrypted_openssh.pem")
if ($success -eq $false) {
    $($key.LastErrorText)
    exit
}

$("Done!")