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

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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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bool success = false;

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

Chilkat.SshKey key = new 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.
string keyStr = key.LoadText("qa_data/putty_private_key.ppk");
if (key.LastMethodSuccess == false) {
    Debug.WriteLine(key.LastErrorText);
    return;
}

success = key.FromPuttyPrivateKey(keyStr);
if (success == false) {
    Debug.WriteLine(key.LastErrorText);
    return;
}

//  Export to an unencrypted OpenSSH key.
bool bEncrypt = false;
string unencryptedKeyStr = key.ToOpenSshPrivateKey(bEncrypt);
if (key.LastMethodSuccess == false) {
    Debug.WriteLine(key.LastErrorText);
    return;
}

success = key.SaveText(unencryptedKeyStr,"qa_output/unencrypted_openssh.pem");
if (success == false) {
    Debug.WriteLine(key.LastErrorText);
    return;
}

//  Export to an encrypted OpenSSH key.  The Password property supplies the passphrase used to
//  encrypt the output.
bEncrypt = true;
key.Password = "myExportPassword";
string encryptedKeyStr = key.ToOpenSshPrivateKey(bEncrypt);
if (key.LastMethodSuccess == false) {
    Debug.WriteLine(key.LastErrorText);
    return;
}

success = key.SaveText(encryptedKeyStr,"qa_output/encrypted_openssh.pem");
if (success == false) {
    Debug.WriteLine(key.LastErrorText);
    return;
}

Debug.WriteLine("Done!");