SSH - RSA key length question

[lmmartins]

Regarding this issue found for compliance purposes:

Threat:

The SSH protocol (Secure Shell) is a method for secure remote login from one computer to another.
The SSH Server is using a small Public Key.
Best practices require that RSA digital signatures be 2048 or more bits long to provide adequate security. Key lengths of 1024 are acceptable through 2013, but since 2011 they are considered deprecated.
For more information, please refer to NIST Special Publication 800-131A (http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-131Ar1.pdf (http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-131Ar1.pdf)).
Only server keys that are not part of a certificate are reported in this QID. OpenSSH certificates using short keys are reported in QID 38733. X.509 certificates using short keys are reported in QID 38171.

Impact:

A man-in-the-middle attacker can exploit this vulnerability to record the communication to decrypt the session key and even the messages.

Solution:

DSA keys and RSA keys shorter than 2048 bits are considered vulnerable. It is recommended to install a RSA public key length of at least 2048 bits or greater, or to switch to ECDSA or EdDSA.

What is the specific command on the Fortigate equipment, at global level, to change this length to 2048 ?

We try to explorer the command: config system global, but can't find the right settings.

Can you please, help us ?

Thanks in advance.

[jintrah_FTNT]

Hi Luis,

You would need to run firmware 6.4.7 or later, there is an enhancement wherein RSA key bits generated have been increased 2048, https://docs.fortinet.com/document/fortigate/6.4.7/fortios-release-notes/743723/new-features-or-enha...

Best regards,

Jin

[lmmartins]

Hello Jintrah

Thanks for the feedback, but we already have the Firmware v7.2.0 build 1157 installed.

Best regards

[jintrah_FTNT]

Most welcome!

Yes running 7.2 would mean a RSA key length of 2048 to be already in use.

Best regards,

Jin

[lmmartins]

Yes, we know.

But we have also the RSA key length of 1024 active, and the question is : is it possible to remove this RSA key length of 1024, and leave only the RSA key length of 2048 ?

Sorry I forgot to mention.