Create .ppk Files for plink/pscp on Windows Without Opening puttygen

I was writing a deploy script that needed to run plink/pscp against a server automatically from a Windows machine and ran into the same old problem: it needs a private key in PuTTY's .ppk format and will not accept a regular OpenSSH file. Normally I would open puttygen and convert it, but when automation needs to generate a new key within the script, opening a GUI each time is not practical.

I tried running puttygen in batch mode and immediately hit a wall.

puttygen id_ed25519 -O private -o out.ppk
# เด้ง dialog: unrecognised option "-O"

Practical options

There are two ways — a shortcut if you have WSL, and a do-it-yourself approach if you want to avoid dependencies.

The WSL shortcut:

sudo apt install putty-tools
puttygen id_ed25519 -O private -o out.ppk

Write the PPK v3 file yourself

PPK is a straightforward text format. The trickiest part is the MAC at the end of the file, which must be calculated correctly or plink will reject the key.

PPK v3 MAC = HMAC-SHA-256, calculated by concatenating the SSH-string representations of 5 items in order: the algorithm name, the encryption value ("none" if no passphrase is set), the comment, the public blob, and the private blob. An SSH-string is "a 4-byte big-endian length + the data itself", and the HMAC key is an empty string when the key is unencrypted.

import base64, hashlib, hmac, struct, sys

def ssh_string(b: bytes) -> bytes:
    return struct.pack('>I', len(b)) + b

def ed25519_ppk(openssh_priv_path, out_path, comment='imported'):
    # ---- ดึง seed 32 ไบต์ + public 32 ไบต์ จากไฟล์ OpenSSH ที่ decode แล้ว ----
    # (ในของจริงผม parse ตัว "-----BEGIN OPENSSH PRIVATE KEY-----" ออกมาเป็น pub/seed)
    pub, seed = load_openssh_ed25519(openssh_priv_path)   # bytes(32), bytes(32)
    alg = b'ssh-ed25519'
    public_blob  = ssh_string(alg) + ssh_string(pub)
    private_blob = ssh_string(seed)                        # ed25519: seed 32 ไบต์แรก

    pub_b64  = base64.b64encode(public_blob).decode()
    priv_b64 = base64.b64encode(private_blob).decode()

    mac_data = (ssh_string(alg) + ssh_string(b'none') +
                ssh_string(comment.encode()) +
                ssh_string(public_blob) + ssh_string(private_blob))
    mac = hmac.new(b'', mac_data, hashlib.sha256).hexdigest()

    def wrap(b64, per=64):
        return [b64[i:i+per] for i in range(0, len(b64), per)]

    lines = [f'PuTTY-User-Key-File-3: {alg.decode()}',
             'Encryption: none', f'Comment: {comment}',
             f'Public-Lines: {len(wrap(pub_b64))}', *wrap(pub_b64),
             f'Private-Lines: {len(wrap(priv_b64))}', *wrap(priv_b64),
             f'Private-MAC: {mac}']
    open(out_path, 'w', newline='\r\n').write('\n'.join(lines) + '\n')
    print('wrote', out_path)

Test that it actually works

Do not assume the file is correct until plink actually authenticates:

plink -i out.ppk -batch user@host "echo OK; id"
# ต้องได้ OK + uid ของ user กลับมา ไม่ใช่ prompt ถามรหัส

If plink reports "Unable to load key file", the MAC or blob is wrong — go back and check that all 5 SSH-strings are concatenated in order and that the HMAC key really is an empty string.

The problem is not the key itself, but puttygen on Windows, which cannot be automated. Once I understood that PPK v3 just needs a MAC calculated with the right formula, I could generate everything within the script. Automated deployments that need to carry a key no longer need a GUI step in the middle. If you work on Windows→Linux automation, feel free to adapt this to your needs. If I have left anything out or made a typo, I will come back and fix it later.