Published On: August 3ʳᵈ, 2020 08:22

Security Configuration Guide, Cisco IOS XE Amsterdam 17.3.x (Catalyst 9500 Switches)

X.509v3 Certificates for SSH Authentication

The X.509v3 Certificates for SSH Authentication feature uses the X.509v3 digital certificates in server and user authentication at the secure shell (SSH) server side.

This module describes how to configure server and user certificate profiles for a digital certificate.

Prerequisites for X.509v3 Certificates for SSH Authentication

  • The X.509v3 Certificates for SSH Authentication feature introduces the ip ssh server algorithm authentication command to replace the ip ssh server authenticate user command. If you use the ip ssh server authenticate user command, the following deprecation message is displayed.

    Warning: SSH command accepted but this CLI will be deprecated soon. 
    Please move to new CLI “ip ssh server algorithm authentication”. 
    Please configure “default ip ssh server authenticate user” to make the CLI ineffective.
    
    • Use the default ip ssh server authenticate user command to remove the ip ssh server authenticate user command from effect. The IOS secure shell (SSH) server then starts using the ip ssh server algorithm authentication command.

Restrictions for X.509v3 Certificates for SSH Authentication

  • The X.509v3 Certificates for SSH Authentication feature implementation is applicable only on the Cisco IOS XE secure shell (SSH) server side.

  • The SSH server supports only the x509v3-ssh-rsa algorithm-based certificate for server and user authentication.

Information About X.509v3 Certificates for SSH Authentication

The following section provides information about digital certificates, and server and user authentication.

Digital Certificates

The validity of the authentication depends upon the strength of the linkage between the public signing key and the identity of the signer. Digital certificates in the X.509v3 format (RFC5280) are used to provide identity management. A chain of signatures by a trusted root certification authority and its intermediate certificate authorities binds a given public signing key to a given digital identity.

Public key infrastructure (PKI) trustpoint helps manage the digital certificates. The association between the certificate and the trustpoint helps track the certificate. The trustpoint contains information about the certificate authority (CA), different identity parameters, and the digital certificate. Multiple trustpoints can be created to associate with different certificates.

Server and User Authentication using X.509v3

For server authentication, the Cisco IOS XE secure shell (SSH) server sends its own certificate to the SSH client for verification. This server certificate is associated with the trustpoint configured in the server certificate profile (ssh-server-cert-profile-server configuration mode).

For user authentication, the SSH client sends the user's certificate to the SSH server for verification. The SSH server validates the incoming user certificate using public key infrastructure (PKI) trustpoints configured in the server certificate profile (ssh-server-cert-profile-user configuration mode).

By default, certificate-based authentication is enabled for server and user at the SSH server end.

How to Configure X.509v3 Certificates for SSH Authentication

The following section provides information about how to configure X.509v3 Certificates for SSH Authentication.

Configuring the SSH Server to Use Digital Certificates for Server Authentication

To configure the SSH server to use digital certificates for server authentication, perform this procedure:

Procedure
  Command or Action Purpose
Step 1

enable

Example:
Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:
Device# configure terminal

Enters global configuration mode.

Step 3

ip ssh server algorithm hostkey {x509v3-ssh-rsa [ssh-rsa] | ssh-rsa [x509v3-ssh-rsa]}

Example:
Device(config)# ip ssh server algorithm hostkey x509v3-ssh-rsa

Defines the order of host key algorithms. Only the configured algorithm is negotiated with the secure shell (SSH) client.

Note 

The IOS SSH server must have at least one configured host key algorithm:

  • ssh-rsa – public key based authentication

  • x509v3-ssh-rsa – certificate-based authentication

Step 4

ip ssh server certificate profile

Example:
Device(config)# ip ssh server certificate profile

Configures server certificate profile and user certificate profile and enters SSH certificate profile configuration mode.

Step 5

server

Example:
Device(ssh-server-cert-profile)# server

Configures server certificate profile and enters SSH server certificate profile server configuration mode.

Step 6

trustpoint sign PKI-trustpoint-name

Example:
Device(ssh-server-cert-profile-server)# trustpoint sign trust1

Attaches the public key infrastructure (PKI) trustpoint to the server certificate profile. The SSH server uses the certificate associated with this PKI trustpoint for server authentication.

Step 7

ocsp-response include

Example:
Device(ssh-server-cert-profile-server)# ocsp-response include

(Optional) Sends the Online Certificate Status Protocol (OCSP) response or OCSP stapling along with the server certificate.

Note 

By default the no form of this command is configured and no OCSP response is sent along with the server certificate.

Step 8

end

Example:
Device(ssh-server-cert-profile-server)# end

Exits SSH server certificate profile server configuration mode and returns to privileged EXEC mode.

Configuring the SSH Server to Verify Digital Certificates for User Authentication

To configure the SSH Server to use digital certificates for user authentication, perform this procedure:

Procedure
  Command or Action Purpose
Step 1

enable

Example:
Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:
Device# configure terminal

Enters global configuration mode.

Step 3

ip ssh server algorithm authentication {publickey | keyboard | password}

Example:
Device(config)# ip ssh server algorithm authentication publickey

Defines the order of user authentication algorithms. Only the configured algorithm is negotiated with the secure shell (SSH) client.

Note 

The SSH server must have at least one configured user authentication algorithm.

Note 

To use the certificate method for user authentication, the publickey keyword must be configured.

Note 

The ip ssh server algorithm authentication command replaces the ip ssh server authenticate user command.

Step 4

ip ssh server algorithm publickey {x509v3-ssh-rsa [ssh-rsa] | ssh-rsa [x509v3-ssh-rsa]}

Example:
Device(config)# ip ssh server algorithm publickey x509v3-ssh-rsa

Defines the order of public key algorithms. Only the configured algorithm is accepted by the SSH client for user authentication.

Note 

The SSH client must have at least one configured public key algorithm:

  • ssh-rsa – public-key-based authentication

  • x509v3-ssh-rsa – certificate-based authentication

Step 5

ip ssh server certificate profile

Example:
Device(config)# ip ssh server certificate profile

Configures server certificate profile and user certificate profile and enters SSH certificate profile configuration mode.

Step 6

user

Example:
Device(ssh-server-cert-profile)# user

Configures user certificate profile and enters SSH server certificate profile user configuration mode.

Step 7

trustpoint verify PKI-trustpoint-name

Example:
Device(ssh-server-cert-profile-user)# trustpoint verify trust2

Configures the public key infrastructure (PKI) trustpoint that is used to verify the incoming user certificate.

Note 

Configure multiple trustpoints by executing the same command multiple times. A maximum of 10 trustpoints can be configured.

Step 8

ocsp-response required

Example:
Device(ssh-server-cert-profile-user)# ocsp-response required

(Optional) Mandates the presence of the Online Certificate Status Protocol (OCSP) response with the incoming user certificate.

Note 

By default the no form of this command is configured and the user certificate is accepted without an OCSP response.

Step 9

end

Example:
Device(ssh-server-cert-profile-user)# end

Exits SSH server certificate profile user configuration mode and returns to privileged EXEC mode.

Verifying Configuration for Server and User Authentication Using Digital Certificates

To verify configuration for server and user Authentication using digital certificates, perform this procedure:

Procedure

  Command or Action Purpose
Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

show ip ssh

Example:

Device# show ip ssh

SSH Enabled - version 1.99
Authentication methods:publickey,keyboard-interactive,password
Authentication Publickey Algorithms:x509v3-ssh-rsa,ssh-rsa
Hostkey Algorithms:x509v3-ssh-rsa,ssh-rsa
Authentication timeout: 120 secs; Authentication retries: 3
Minimum expected Diffie Hellman key size : 1024 bits


Displays the currently configured authentication methods. To confirm the use of certificate-based authentication, ensure that the x509v3-ssh-rsa algorithm is the configured host key algorithm.

Configuration Examples for X.509v3 Certificates for SSH Authentication

The following section provides examples for user and server authentication using digital certificates.

Example: Configuring the SSH Server to Use Digital Certificates for Server Authentication

This example shows how to configure the SSH Server to use digital certificates for server authentication.


Device> enable
Device# configure terminal
Device(config)# ip ssh server algorithm hostkey x509v3-ssh-rsa 
Device(config)# ip ssh server certificate profile
Device(ssh-server-cert-profile)# server
Device(ssh-server-cert-profile-server)# trustpoint sign trust1
Device(ssh-server-cert-profile-server)# end


Example: Configuring the SSH Server to Verify Digital Certificates for User Authentication

This example shows how to configure the SSH server to verify user's digital certificate for user authentication.


Device> enable
Device# configure terminal
Device(config)# ip ssh server algorithm authentication publickey
Device(config)# ip ssh server algorithm publickey x509v3-ssh-rsa 
Device(config)# ip ssh server certificate profile
Device(ssh-server-cert-profile)# user
Device(ssh-server-cert-profile-user)# trustpoint verify trust2
Device(ssh-server-cert-profile-user)# end


Additional References for X.509v3 Certificates for SSH Authentication

Related Documents

Related Topic

Document Title

Cisco IOS commands

Cisco IOS Master Command List, All Releases

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Feature History and Information for X.509v3 Certificates for SSH Authentication

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 1. Feature History and Information for X.509v3 Certificates for SSH Authentication

Feature Name

Releases

Feature Information

X.509v3 Certificates for SSH Authentication

Cisco IOS XE Everest 16.5.1a

The X.509v3 Certificates for SSH Authentication feature uses the X.509v3 digital certificates in server and user authentication at the SSH server side.