When a key is generated using PuttyGen, the format of the key is usually different from the OpenSSH format used on Linux and you will get an error like the one below. This is why after generating a key, you need to convert it to OpenSSH Format
Converting PuttyGen keys to OpenSSH Format
Open your PuttyGen software
Then click on the Load button
Navigate to where you have your private key stored and select it (if you store your key with a passphrase, it will ask for the code)
When you load your key, you should get a screen similar to the one below
Click on the conversions menu and select the export OpenSSH Key.
This will create a new private key in OpenSSH format which is a different format from the PuttyGen generated key as shown below
After doing this, the converted key can be sent to the user to connect from their Linux system to Hosted~FTP~
Importing your key to Linux for Hosted~FTP~ connection
Download your private key onto your Linux System.
Move your private key into the /.ssh folder by running the command mv filepath movepath as shown below
Navigate into the /.ssh folder and set the permission for the private key to 700 by running the command “chmod 700 filename”
Finally, you can connect to the Hosted~FTP~ by running the following command “sftp -o IdentityFile=privatekey path” username@host.
If the key was created with a password, it will ask you to enter the password as shown in the image below.
Similar Link:
Using PKI keys to connect to Hosted~FTP~ on Linux