I want to SFTP a file from one server to another, authorizing with a private-key.

I believe the public key is installed correctly (WinSCP lets me log in with private key), and I deliberately did not create a passphrase when generating the key-pair.... but when I try to SFTP on the command line, I'm prompted for both the non-existent passphrase and the user password.

How can I SFTP with just the unwrapped private-key?


I have two servers:

  • toServer123

  • fromServer123

I want to SFTP a file from one server to another, using a private-key login.

  1. I generate a public-private key pair using puttyGen:

    • after providing the prompted random movement, I save the public and private key
    • I leave the passphrase field blank and choose 'yes' when prompted about unencrypted private key
  2. I install the public key on toServer123:

    • I create a user paultest with password 'password123'
    • I test that I can ssh into toServer123 as user paultest
    • I create /home/paultest/.ssh/authorized_keys and add the public key
    • The public key looks like:

      cat /home/paultest/.ssh/authorized_keys #=>

      ssh-rsa verylongstringoflettersandnumbersNoNewlines paultest@toServer123

    • I chmod permissions: authorized_keys (644), .ssh (700)

  3. I install the private key on the fromServer123:

    • I create dir: /home/support_user/sftp_proc (chmod 700)

    • I upload my_private_key.ppk (chmod 600) to /sftp_proc

    • I create file text.txt in /sftp_proc which just contains "this is a test"

  4. as support_user, I try to SFTP to toServer123 from fromServer123:

    sftp -oIdentityFile=./my_private_key.ppk paultest@toServer123

What I expect:

I get logged into the server without further prompt, since I'm logging in with an unwrapped key file.

What I get:

Connecting to toServer123...

Unauthorized access to or use of this system is prohibited.
All access and use may be monitored and recorded.

Enter passphrase for key './my_private_key.ppk':

I just hit enter, and get:

paultest@toServer123's password:

Debugging steps:

  • If I provide the paultest password, the SFTP works - but I don't want to use a password, I want to log in with a private key

  • If I try to log in using winSCP and provide the private key, I am able to log in with just that - I get no passphrase or password prompts.


What am I doing wrong, if my goal is to log in without providing a password, and without being prompted for the non-existing passphrase?


my_private_key.ppk looks like:

PuTTY-User-Key-File-2: ssh-rsa
Encryption: none
Comment: rsa-key-20190528
Public-Lines: 6
Private-Lines: 14
Private-MAC: cf6c5c786f51a623b28eabe226c98dd6faa09787
  • 1
    What does the ./my_private_key.ppk look like? – Martin Prikryl May 29 at 6:33
  • @MartinPrikryl I added the format of the .ppk file to the question - does that look correct? – Paul May 29 at 9:09
  • OK, then the answer by @Romeo is correct. – Martin Prikryl May 29 at 9:12
  • 1
    @MartinPrikryl Right you are - I converted the private key and it worked! – Paul May 29 at 9:16

AFAIK sftp expect different format of key. The keys of OpenSSH should look like:


So you should convert your ppk key to RSA key. Here is how to use putty utils to do it

  • If a wrong key format is used, it would say "invalid format", and not prompt for a passphrase – Martin Prikryl May 29 at 6:58
  • 2
    OK, I get "invalid format" with OpenSSH 7.7. But if I use an older version of OpenSSH, like 5.3, I indeed get a prompt for a passphrase. +1 – Martin Prikryl May 29 at 9:13
  • 1
    You nailed it - format of the private-key was incorrect. The default private-key generated by puttyGen worked in software like WinSCP, but not in whatever version of SFTP was installed on my server. Thank you! :) – Paul May 29 at 9:16
  • @MartinPrikryl don't worry I will. I am competing the task first, In case there's any extra information I can add into the question before I close it. – Paul May 29 at 10:09
  • 1
    @MartinPrikryl, fine, I won't add anything else. it's marked answered now. Thanks for your help. Please stop messaging me. – Paul Jun 10 at 13:14

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