How to configure RSA key size less than 2048 bits ?

Solution Verified - Updated -

Environment

  • Red Hat Enterprise Linux 8.x

Issue

  • How to configure crypto policy RSA key size to 1024 ?

Resolution

  • Standard DEFAULT crypto policy requires 2048 key size.

                 LEGACY         DEFAULT          FIPS            FUTURE
RSA          min.1024-bit     min.2048-bit    min.2048-bit       MIN.3072 bit

  • In the DEFAULT mode the RSA keys are accepted if they are at least 2048 bits long.

  • If application requires 1024 key size then must need to break the policy and create the custom pmod file as example below.

    [root@localhost ~]# cat /etc/crypto-policies/policies/modules/RSA1024.pmod
min_rsa_size = 1024

  • If the custom policy should only allow less than 2048 RSA bit keys for Openssh then below entry should be added :

    [root@localhost ~]# cat /etc/crypto-policies/policies/modules/RSA1024.pmod
min_rsa_size@openssh = 1024

  • Apply the custom policy :

    [root@localhost ~]# update-crypto-policies --show
DEFAULT

[root@localhost ~]# update-crypto-policies --set DEFAULT:RSA1024
Setting system policy to DEFAULT:RSA1024

Note: System-wide crypto policies are applied on application start-up.
It is recommended to restart the system for the change of policies
to fully take place.

[root@localhost ~]# shutdown -r now
(log in again after a while)
[root@localhost ~]# update-crypto-policies --show
DEFAULT:RSA1024

Note: It is recommended to update your application to accept 2048 size because 1024 keys size is no longer considered secure due to vulnerabilities.

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