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Converting OpenSSH and PuTTY Style Keys  -  KB Article #2007

Related Articles -- 1053, 1712, 1805

Serv-U 9 and above support the Public Key Authentication method of user authentication, allowing users to authenticate with a key pair instead of a password. This eliminates the viability of dictionary attacks against user accounts, and also eases the process of automating transfer jobs with Linux/UNIX shell scripts.

While the data stored in Public/Private Key pairs is standardized, the storage format is not. Two common formats are available - OpenSSH and PuTTY style keys. Programs that rely on PuTTY cannot use OpenSSH style keys, and vice versa. Serv-U uses OpenSSH style keys only, and does not support PuTTY. It can be necessary to convert key formats in order to support specifc clients. Steps are included below.

OpenSSH to PuTTY Conversion

Converting OpenSSH style keys requires the PuTTYgen application. Once this application is downloaded, start the program, then follow these steps:

  • Click "Conversions" and then "Import"

  • Select your OpenSSH private key (e.g., "user17_sftpkey.key")
  • If there should be a passphrase used to secure this key, enter it in the "Key passphrase" and "Confirm passphrase" fields, then go to File and click "Save private key" to save the key to disk in PuTTY format (as a .ppk file)

PuTTY to OpenSSH Conversion

Converting PuTTY style keys to OpenSSH also requires the PuTTYgen application. Once this application is downloaded, start the program, then follow these steps:

  • Click the "Load" button for "Load an existing private key file

  • Click "Conversions" then "Export OpenSSH key"

  • Provide a name in the format mykeyname.key - the file name can be anything but it should end in .key

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