Create your own certificate authority in Keychain Access on Mac
In the Keychain Access app on your Mac, choose Keychain Access > Certificate Assistant > Create a Certificate Authority.
Enter a name for the certificate authority.
Choose an identity type, then choose the type of user certificate to be issued by the certificate authority.
To change the way the key pairs are generated and to manually specify the values in the certificate, select “Let me override defaults.”
To make this the default certificate authority, select “Make this CA the default.”
Note: You can create RSA keys up to 4096 bits. RSA keys smaller than 2048 bits are no longer supported.
Enter your email address in the “Email from” field, then click Create.
Review the certificate authority.