How to create self-signed certificates

A digital certificate or identity certificate is an electronic document which uses a digital signature to bind a public key with an identity, information such as the name of a person or an organization, their address, and so forth. The certificate can be used to verify that a public key belongs to an individual.

In a typical public key infrastructure (PKI) scheme, the signature will be of a certificate authority (CA). However, there are situations where it is not possible use a CA, so the only solutions is to use a self-signed certificate, an identity certificate that is signed by the same entity whose identity it certifies.

The standard used by Cisco is X.509, an ITU-T standard for a public key infrastructure (PKI) and Privilege Management Infrastructure (PMI). X.509 specifies, amongst other things, standard formats for public key certificates, certificate revocation lists, attribute certificates, and a certification path validation algorithm.

The most common use of certificates is for HTTPS-based web sites. A web browser validates that an SSL web server is authentic, so that the user can feel secure that his/her interaction with the web site has no eavesdroppers and that the web site is who it claims to be. Other uses are VPN lan2lan, GetVPN and so on.

There are two different approaches to create a self-signed certificate: automatic or manual.

To automatically create an rsa key pairs and a certificate, enable the https server:

Ciscozine(config)#ip http secure-server
% Generating 1024 bit RSA keys, keys will be non-exportable...
[OK] (elapsed time was 3 seconds)

Ciscozine(config)#
*May 23 21:35:01.855: %SSH-5-ENABLED: SSH 1.99 has been enabled
Ciscozine(config)#
*May 23 21:35:02.035: %PKI-4-NOAUTOSAVE: Configuration was modified.  Issue "write memory" to save new certificate
Ciscozine(config)#

Where is the certificate?

Ciscozine#dir nvram:
Directory of nvram:/

  124  -rw-          85                      startup-config
  125  ----           0                      private-config
  126  -rw-          85                      underlying-config
    1  ----          57                      persistent-data

129016 bytes total (127855 bytes free)
Ciscozine#

Remember: Save the configuration to save the certificate!

Ciscozine#wr
Building configuration...
[OK]
Ciscozine#

Ciscozine#dir nvram:
Directory of nvram:/

  122  -rw-        1069                      startup-config
  123  ----        1914                      private-config
  124  -rw-        1069                      underlying-config
    1  ----          57                      persistent-data
    2  -rw-         559                      IOS-Self-Sig#1.cer

129016 bytes total (123933 bytes free)
Ciscozine#

The second method requires three steps: create an rsa key pairs, create a self signed trust point and enroll the certificate.

Create an RSA keys:

Ciscozine(config)#crypto key generate rsa label ciscozine-rsa modulus 2048
The name for the keys will be: ciscozine-rsa

% The key modulus size is 2048 bits
% Generating 2048 bit RSA keys, keys will be non-exportable...
[OK] (elapsed time was 10 seconds)

Ciscozine(config)#

Create a local PKI:

Ciscozine(config)#crypto pki trustpoint my-ciscozine-ca
Ciscozine(ca-trustpoint)#enrollment selfsigned
Ciscozine(ca-trustpoint)#subject-name O=Test,CN=www.ciscozine.com
Ciscozine(ca-trustpoint)#rsakeypair ciscozine-rsa

Remembers: The “subject-name” is the name of the entity whose public key the certificate identifies. For instance, “O” identify the “Organization” and “CN” the Common name

Obtain the certificate from the local certificate authority:

Ciscozine(config)#crypto pki enroll my-ciscozine-ca
% Include the router serial number in the subject name? [yes/no]: no
% Include an IP address in the subject name? [no]: no
Generate Self Signed Router Certificate? [yes/no]: yes

Router Self Signed Certificate successfully created

Ciscozine(config)#

Ciscozine#dir nvram:
Directory of nvram:/

  124  -rw-          85                      startup-config
  125  ----           0                      private-config
  126  -rw-          85                      underlying-config
    1  ----          57                      persistent-data

129016 bytes total (127855 bytes free)
Ciscozine#

Remember: You must save the configuration to save the certificate!

Ciscozine#wr
Building configuration...
[OK]
Ciscozine#

Ciscozine#dir nvram:
Directory of nvram:/

  118  -rw-        1019                      startup-config
  119  ----        5468                      private-config
  120  -rw-        1019                      underlying-config
    1  ----          57                      persistent-data
    2  -rw-         860                      Ciscozine#1.cer

129016 bytes total (120429 bytes free)
Ciscozine#

To see more information about the certificates and the rsa keys are, you can use these commands:

• show crypto pki certificates
• show crypto pki trustpoints
• show crypto key mypubkey rsa

References:

• http://www.cisco.com/en/US/docs/ios/12_3t/12_3t14/feature/guide/gtpsscer.html
• http://en.wikipedia.org/wiki/X.509
• http://en.wikipedia.org/wiki/Public_key_certificate