SSH Config and crypto key generate RSA command

SSH Config and crypto key generate RSA command

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 ip domain-name commands).

You will be unable to complete the crypto key generate rsa command 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

router (Config)# crypto key generate rsa

how many bits in the modulus [512] :1024

router (Config)# ip ssh version2

router (Config)# CTRL Z



Secure Shell (SSH) may generate an additional RSA key pair if you generate a key pair on a router having no RSA keys. The additional key pair is used only by SSH and will have a name such as {router_FQDN }.server.

For example, if a router name is “,” the key name is “”

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.

Router 360 bits 512 bits 1024 bits 2048 bits (maximum)
Cisco 2500 11 seconds 20 seconds 4 minutes, 38 seconds More than 1 hour
Cisco 4700 Less than 1 second 1 second 4 seconds 50 seconds

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.

Router(config)# crypto key generate rsa general-keys
The name for the keys will be:
Choose the size of the key modulus in the range of 360 to 2048 for your General Purpose Keys. Choosing a key modulus greater than 512 may take a few minutes.
How many bits in the modulus[512]? <return>
Generating RSA keys.... [OK].

Syntax Description : Optional Strings to embed with SSH Crypto key

general-keys (Optional) Specifies that a general-purpose key pair will be generated, which is the default.
usage-keys (Optional) Specifies that two RSA special-usage key pairs, one encryption pair and one signature pair, will be generated.
signature (Optional) Specifies that the RSA public key generated will be a signature special usage key.
encryption (Optional) Specifies that the RSA public key generated will be an encryption special usage key.
label key-label (Optional) Specifies the name that is used for an RSA key pair when they are being exported.If a key label is not specified, the fully qualified domain name (FQDN) of the router is used.
exportable (Optional) Specifies that the RSA key pair can be exported to another Cisco device, such as a router.
modulus modulus-size (Optional) Specifies the IP size of the key modulus.By default, the modulus of a certification authority (CA) key is 1024 bits. The recommended modulus for a CA key is 2048 bits. The range of a CA key modulus is from 350 to 4096 bits.

Note    Effective with Cisco IOS XE Release 2.4 and Cisco IOS Release 15.1(1)T, the maximum key size was expanded to 4096 bits for private key operations. The maximum for private key operations prior to these releases was 2048 bits.
storage devicename: (Optional) Specifies the key storage location. The name of the storage device is followed by a colon (:).
redundancy (Optional) Specifies that the key should be synchronized to the standby CA.
on devicename : (Optional) Specifies that the RSA key pair will be created on the specified device, including a Universal Serial Bus (USB) token, local disk, or NVRAM. The name of the device is followed by a colon (:).Keys created on a USB token must be 2048 bits or less.

Command Default:Related Commands

Command Description
copy Copies any file from a source to a destination, use the copy command in privileged EXEC mode.
crypto key storage Sets the default storage location for RSA key pairs.
debug crypto engine Displays debug messages about crypto engines.
hostname Specifies or modifies the hostname for the network server.
ip domain-name Defines a default domain name to complete unqualified hostnames (names without a dotted-decimal domain name).
show crypto key mypubkey rsa Displays the RSA public keys of your router.
show crypto pki certificates Displays information about your PKI certificate, certification authority, and any registration authority certificates.


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