Create self-signed certificates in Keychain Access on Mac
You can create a self-signed certificate using Certificate Assistant in Keychain Access. Self-signed certificates don’t provide the guarantees of a certificate signed by a certificate authority but they can be useful if the person signing it is trusted.
In the Keychain Access app on your Mac, choose Keychain Access > Certificate Assistant > Create a Certificate.
Enter a name for the certificate.
Choose an identity type, then choose the type of certificate.
For an explanation of certificate types, click Learn More.
To manually specify the information in the certificate, such as key pairs, extensions and encryption, click “Let me override defaults”, then follow the instructions. If you have questions while creating your certificate, click Learn More.
Note: You can create RSA keys up to 4096 bits. RSA keys smaller than 2048 bits are no longer supported.
Review the certificate, then click Done.