When using `curl` to make HTTPS requests, it does indeed use certificates as part of the TLS (Transport Layer Security) protocol, which provides the ‘S‘ in ‘HTTPS‘. The process of using certificates is integral to establishing a secure connection.
Here’s how it works:
1. Server Certificate: The server (e.g., the website you’re connecting to) has a digital certificate that it presents to clients (e.g., `curl`) as proof of its identity. This certificate includes the server’s public key and has been digitally signed by a trusted third-party called a Certificate Authority (CA).
2. CA Certificates: The client (in this case, `curl`) has a list of trusted CAs, and it uses this list to verify the server’s certificate. If the certificate was signed by a CA on the client’s trusted list, then the client accepts the certificate as valid.
3. Certificate Store: The list of trusted CAs is typically stored in a certificate store on your system. The location of this store can vary depending on your operating system. For example, on Ubuntu, the store is typically in `/etc/ssl/certs/`. On macOS, `curl` uses the system-wide certificate store, managed through Keychain Access.
When you run a `curl` command with HTTPS, `curl` automatically checks the server’s certificate against its list of trusted CAs. If the certificate is valid, `curl` proceeds with the connection; if not, `curl` gives an error and aborts the connection.
You can override this behavior with the `-k` or `–insecure` option, which tells `curl` to proceed with the connection even if the certificate is invalid. However, this is generally not recommended because it bypasses the protections provided by HTTPS.
In terms of where `curl` gets its certificates from, it depends on how `curl` was built and configured. Some distributions of `curl` come pre-configured with a set of trusted CA certificates. In other cases, `curl` relies on the operating system’s certificate store. You can also specify a different certificate file or directory with the `–cacert` or `–capath` options, respectively.
The `curl` command relies on a database of Certificate Authorities (CAs) to validate the certificates of the servers it connects to. Where `curl` gets this database can depend on how `curl` is installed and configured.
1. System-wide Certificate Store: On many systems, `curl` is configured to use the system-wide certificate store. This means that `curl` uses the same trusted CA database that other applications on the system use. On macOS, this is managed through the Keychain Access utility. On many Linux systems, this is usually a directory such as `/etc/ssl/certs/`.
2. Bundled Certificate Database: Some distributions of `curl`, especially standalone or Windows versions, may come with their own CA certificate bundle. This bundle is used instead of, or in addition to, the system-wide certificate store.
3. User-Specified Certificate Database: The user can specify a different certificate database using the `–cacert` option with `curl`. This allows `curl` to use a certificate database other than the system-wide store or the bundled database.
So to answer your question: By default, `curl` does use the certificates that your system (like macOS or Windows) has. But it can also use a different certificate database depending on its configuration and how it’s invoked. The `curl` command itself does not manage its own independent set of trusted CAs; it relies on external databases, whether they’re provided by the system, bundled with `curl`, or specified by the user.
`curl`’s behavior regarding certificates can vary depending on how it was compiled and the options specified when it was run.
Typically, `curl` uses the system’s certificate store, but it can also be compiled with a built-in list of trusted certificate authorities or be given a specific list at runtime.
To check how your `curl` handles certificates, you can run it with the verbose `-v` flag when making a request to a site over HTTPS:
curl -v https://www.google.com
Look for the line that starts with `CAfile`. This will tell you the location of the certificate file `curl` is using. If `curl` is using a directory of certificates rather than a single file, it’ll be shown on a line starting with `CApath`.
However, not all builds of `curl` will provide this level of detail about the certificate verification process.
If you want to explicitly specify a certificate for `curl` to use for a particular session, you can use the `–cacert` option:
This will instruct `curl` to use the given certificate file for that particular session.
Do note that the actual path and behavior may depend on your operating system and how `curl` has been installed and configured. If you’re not certain, you might want to consult the `curl` documentation or get in touch with a systems administrator.