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.

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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 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 IOS secure shell (SSH) server side.

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

Information About X.509v3 Certificates for SSH 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 IOS 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 IOS 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 IOS SSH server end.

How to Configure X.509v3 Certificates for SSH Authentication

Configuring IOS SSH Server to Use Digital Certificates for Sever Authentication

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. ip ssh server algorithm hostkey {x509v3-ssh-rsa [ssh-rsa ] | ssh-rsa [x509v3-ssh-rsa ]}
  4. ip ssh server certificate profile
  5. server
  6. trustpoint sign PKI-trustpoint-name
  7. ocsp-response include
  8. end

DETAILED STEPS

  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 enters privileged EXEC mode.

Configuring IOS SSH Server to Verify User's Digital Certificate for User Authentication

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. ip ssh server algorithm authentication {publickey | keyboard | password }
  4. ip ssh server algorithm publickey {x509v3-ssh-rsa [ssh-rsa ] | ssh-rsa [x509v3-ssh-rsa ]}
  5. ip ssh server certificate profile
  6. user
  7. trustpoint verify PKI-trustpoint-name
  8. ocsp-response required
  9. end

DETAILED STEPS

  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 IOS 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 IOS 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 enters privileged EXEC mode.

Verifying Configuration for Server and User Authentication Using Digital Certificates

SUMMARY STEPS

  1. enable
  2. show ip ssh

DETAILED STEPS

Step 1

enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Example:


Device> enable
Step 2

show ip ssh

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.

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

Configuration Examples for X.509v3 Certificates for SSH Authentication

Example: Configuring IOS SSH Server to Use Digital Certificates for Sever 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)# exit

Example: Configuring IOS 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

Security commands

SSH authentication

“Secure Shell-Configuring User Authentication Methods” chapter in Secure Shell Configuration Guide

Public key infrastructure (PKI) trustpoint

“Configuring and Managing a Cisco IOS Certificate Server for PKI Deployment” chapter in Public Key Infrastructure Configuration Guide

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Feature 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 Information for X.509v3 Certificates for SSH Authentication

Feature Name

Releases

Feature Information

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.

The following commands were introduced or modified: ip ssh server algorithm hostkey , ip ssh server algorithm authentication , and ip ssh server certificate profile .