CA Tools is a set of scripts that creates and manages a self-signed certificate authority. We use this to issue certificates for our network equipment.
The CA and certificates generated by CA Tools are limited in a number of ways to mitigate the potential damage if the CA is compromised:
- The CA's basic constraints include a pathlen of 0. This means nobody can issue intermediate CAs.
- The CA has a name constraint that is limited to one DNS name or entry. This means you can restrict the CA to only issue certificates that are children of a given domain or subdomain.
Certificates generated by CA Tools always use Subject Alternative Names, even if the certificate only has one name. This means the certificates will be trusted by Chrome 58 and newer. All CA Tools certificates use SHA-256.
WARNING: Name constraints are not supported in macOS as of version 10.12. If you mark the CA as trusted for SSL in Keychain Access, Chrome, Safari, and anything else linked against CommonCrypto (curl, etc.) will validate certificates that don't match the CA's name constraints even though the name constraint extension is marked as critical. Firefox, which uses its own trust store and TLS implementation will properly validate name constraints on macOS.
CA Tools requires Ruby compiled with libyaml, as well as Bash, and the OpenSSL command line tool.
First, create a config.yml file. This contains your organization's info (the distinguished name).
C: US
ST: New York
L: New York
O: ACME Widgets
OU: ACME Widgets IT
emailAddress: acme@example.comThen create the certificate authority:
$ ruby mkca.rb "ACME Widgets Certificate Authority" .corp.example.com
This will create ca.key and ca.crt. Keep them safe. The second argument to mkca.rb is the name constraint. The leading dot means that ca.crt will be able to sign any number of subdomains of corp.example.com, though not corp.example.com itself. You can find more info in RFC 5280.
Ca.key contains a 2048 bit RSA key pair. Ca.crt is valid for five years. You must tell your operating system or browser to trust ca.crt in order to trust any certificates you generate.
Once you have created your CA, you can create a certificate as follows:
$ ruby mkcert.rb 2048 switch1.corp.example.com
This will generate switch1.corp.example.com.key and switch1.corp.example.com.crt. The second argument to mkcert.rb is the key size. You can also specify multiple hosts in one certificate:
$ ruby mkcert.rb 2048 switch1.corp.example.com switch2.corp.example.com
The first host will be used as the common name for the certificate. It will also be used for the filenames of the key and the certificate.
Copyright Recurse Center 2018
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