By default the Cisco ASA firewall has a self signed certificate that is regenerated every time you reboot it. This can be an issue when you are using SSL VPN as the web browser of your user will give a warning every time it sees an untrusted certificate.

• October 2, 2015
• Posted by: Syed Shujaat
• Category: Cisco, Networking Solutions

Use this command to generate RSA key pairs for your Cisco device (such as a router). keys are generated in pairs–one public RSA key and one private RSA key.

If your router already has RSA keys when you issue this command, you will be warned and prompted to replace the existing keys with new keys.

NOTE: Before issuing this command, ensure that your router has a hostname and IP domain name configured (with the hostname and ipdomain-name commands).

• In this chapter from IKEv2 IPsec Virtual Private Networks: Understanding and Deploying IKEv2, IPsec VPNs, and FlexVPN in Cisco IOS, authors Graham Bartlett and Amjad Inamdar introduce a number of designs where IKEv2 is used. Each design will use a simple deployment of two routers with the focus on the configuration of IKEv2.
• Oct 31, 2013 Key pairs are RSA keys. RSA keys can be used for SSH or SSL. SCEP enrollment supports the certification of RSA keys. For the purposes of generating keys, the maximum key modulus for RSA keys is 2048. The default size is 1024.
• Try re-generating the SSH key. Something like: crypto key generate rsa general-keys modulus 4096. Failing that try rebooting the ASA. Failing that, make sure you are using an up to date version of PuTTY, and enable stronger keys on the ASA with (your software might be too old for this): sh key-exchange group dh-group14-sha1.
• Generating a RSA Key Pair for CISCO ASA 5520. To generate a RSA key pair, perform the following steps: Step 1. Generate the types of key pairs needed for your PKI implementation. To do so, perform the following steps, as applicable. If you do not use additional keywords, this command generates one general purpose RSA key pair.

Cisco Asa Rsa Key Generation Example

You will be unable to complete the cryptokeygeneratersacommand without a hostname and IP domain name. (This situation is not true when you generate only a named key pair.)

Here are the steps to Enable SSH and Crypto Key setup : 2 config must requried for SSH

1 Setup Local VTY line User ID and password

router (Config) # Line VTY 0 15

router (Config-line)# login local

router (Config-line)# Exit

!!! create local login ID/Pass

router (Config)# username [loginid] password [cisco]

router (Config)# username loginid1 password cisco1

2. router (Config)# ip domain-name example.com

router (Config)# crypto key generate rsa

Cisco Rsa Vpn

how many bits in the modulus [512] :1024

router (Config)# ip ssh version2

router (Config)# CTRL Z

This command is not saved in the router configuration; however, the RSA keys generated by this command are saved in the private configuration in NVRAM (which is never displayed to the user or backed up to another device) the next time the configuration is written to NVRAM.

Modulus Length

When you generate RSA keys, you will be prompted to enter a modulus length. The longer the modulus, the stronger the security. However, a longer modules take longer to generate (see the table below for sample times) and takes longer to use.

The size of Key Modulus range from 360 to 2048. Choosing modulus greater than 512 will take longer time.

Cisco IOS software does not support a modulus greater than 4096 bits. A length of less than 512 bits is normally not recommended. In certain situations, the shorter modulus may not function properly with IKE, so we recommend using a minimum modulus of 2048 bits.

Syntax Description : Optional Strings to embed with SSH Crypto key