Cisco IPS 4200 Series Sensors

PuTTYgen Generation of SSH Authorized Keys and RSA Authentication on Cisco Secure IDS Configuration Example

Document ID: 61864

Updated: Mar 24, 2008



This document explains how to use the Key generator for PuTTY (PuTTYgen) to generate Secure Shell (SSH) authorized keys and RSA authentication for use on Cisco Secure Intrusion Detection System (IDS). The primary issue when you establish SSH authorized keys is that only the older RSA1 key format is acceptable. This means that you need to tell your key generator to create an RSA1 key, and you must restrict the SSH client to use the SSH1 protocol.



There are no specific requirements for this document.

Components Used

The information in this document is based on these software and hardware versions:

  • Recent PuTTY - February 7, 2004

  • Cisco Secure IDS

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.


Refer to the Cisco Technical Tips Conventions for more information on document conventions.


This section presents you with the information to configure the features this document describes.

Note: Use the Command Lookup Tool (registered customers only) to find additional information on the commands this document uses.

Configure PuTTYgen

Complete these steps to configure PuTTYgen.

  1. Launch PuTTYgen.

  2. Click the SSH1 key type and set the number of bits in the generated key to 2048 in the Parameters group at the bottom of the dialog box.

  3. Click Generate and follow the instructions.

    The key information is displayed in the upper section of the dialog box.

  4. Clear the Key Comment edit box.

  5. Select all the text in Public key for pasting into authorized_keys file and press Ctrl-C.

  6. Type a passphrase in the Key passphrase and Confirm passphrase edit boxes.

  7. Click Save private key.

  8. Save the PuTTY private key file into a directory private to your Windows login (in the Documents and Settings/(userid)/My Documents subtree in Windows 2000/XP).

  9. Launch PuTTY.

  10. Create a new PuTTY session as seen here:

    • Session:

    • IP Address: IP address of the IDS sensor

    • Protocol: SSH

    • Port: 22

    • Connection:

    • Auto-login username: cisco (can also be the login you use on the Sensor)

    • Connection/SSH:

    • Preferred SSH version: 1 only

    • Connection/SSH/Auth:

    • Private key file for authentication: Browse to the .PPK file stored in step 8.

    • Session: (back to the top)

    • Saved sessions: (enter the sensor name, click Save)

  11. Click Open and use password authentication to connect to the Sensor CLI, since the public key is not on the Sensor yet.

  12. Enter the configure terminal CLI command and press Enter.

  13. Enter the ssh authorized-key mykey CLI command, but do not press Enter at this time. Make sure and type a space at the end.

  14. Right-click in the PuTTY terminal window.

    The clipboard material copied in step 5 is typed into the CLI.

  15. Press Enter.

  16. Enter the exit command and press Enter.

  17. Confirm the authorized key is entered properly. Enter the show ssh authorized-keys mykey command and press Enter.

  18. Enter the exit command to quit the IDS CLI and press Enter.


RSA Authentication

Complete these steps.

  1. Launch PuTTY.

  2. Locate the Saved Session created in step 10 and double-click on it. A PuTTY terminal window opens and this text appears:

    Sent username "cisco" 
    Trying public key authentication. 
    Passphrase for key "": 
  3. Type the private key passphrase you created in step 6 and press Enter.

    You are automatically logged in.


There is currently no specific troubleshooting information available for this configuration.

Related Information

Updated: Mar 24, 2008
Document ID: 61864