How to Use the Command 'cfssl' (with Examples)

Cloudflare’s cfssl is a powerful toolset for working with Public Key Infrastructure (PKI) and Transport Layer Security (TLS). It offers a variety of options for managing, verifying, and generating certificates. This valuable toolkit is used widely in IT security for ensuring secure communications over networks.

Use Case 1: Show Certificate Information of a Host

Code:

cfssl certinfo -domain www.google.com

Motivation: Understanding the certificate information of a host is crucial for security auditing and verification. Organizations and individuals often need to ensure that the certificates presented by a host match the expected details, are correctly configured, and are not expired. By obtaining this information directly from the host, one can ascertain the security status and trustworthiness of the server in question.

Explanation:

• cfssl: This is the command-line tool from Cloudflare that we’re using to retrieve SSL/TLS information.
• certinfo: This subcommand is employed to fetch and display certificate information.
• -domain: This option specifies the domain of the host from which we want to obtain certificate details.
• www.google.com: This is the target domain whose SSL/TLS certificate information we wish to explore.

Example Output: The output will show detailed information about the certificate including the subject, issuer, expiration dates, public key details, and more. It’s similar to what one might see when examining a certificate in a web browser, but in a detailed CLI format.

Use Case 2: Decode Certificate Information from a File

Code:

cfssl certinfo -cert path/to/certificate.pem

Motivation: When dealing with multiple certificates stored locally, it’s important to verify their contents without relying on external tools or access. Decoding certificates from files can help in configuration management, certificate auditing, and ensuring compliance with security policies.

Explanation:

• certinfo: This subcommand is again used to extract and display certificate information.
• -cert: This option indicates that the source of the certificate data is a file rather than a domain.
• path/to/certificate.pem: The file path points to the certificate file which we want to decode and explore.

Example Output: The command will return a structured JSON output showing the certificate details such as subject name, issuer, validity period, and other relevant metadata directly from the file. This allows users to easily check the certificate’s integrity and relevancy.

Use Case 3: Scan Host(s) for SSL/TLS Issues

Code:

cfssl scan host1 host2 ...

Motivation: Periodically scanning hosts for SSL/TLS issues is a proactive security measure. Such scans help in identifying potential misconfigurations, outdated protocols, or vulnerabilities that could be exploited. Enterprises often use these scans to maintain robust security postures and comply with security best practices.

Explanation:

• cfssl: The command-line tool being used to perform the scan.
• scan: This subcommand initiates a thorough check on listed hosts for SSL/TLS vulnerabilities or issues.
• host1 host2 ...: These are the target hosts that will be analyzed for their SSL/TLS setups. It allows for multiple hosts to be specified, enabling batch assessments.

Example Output: The output provides a detailed report highlighting any issues found, including deprecated protocol usage, invalid certificates, and potentially insecure configurations. Each host’s SSL/TLS setup will be analyzed and presented, aiding in swift remediation.

Use Case 4: Display Help for a Subcommand

Code:

cfssl genkey|gencsr|certinfo|sign|gencrl|ocspdump|ocsprefresh|ocspsign|ocspserve|scan|bundle|crl|print-defaults|revoke|gencert|serve|version|selfsign|info -h

Motivation: Learning the full capabilities of cfssl and understanding how to correctly and effectively use its subcommands is key for maximizing its utility. The help option provides users with usage details, options, and descriptions, supporting effective tool use without needing external documentation.

Explanation:

• cfssl: The main command for Cloudflare’s toolkit.
• genkey|gencsr|certinfo|sign|gencrl|ocspdump|ocsprefresh|ocspsign|ocspserve|scan|bundle|crl|print-defaults|revoke|gencert|serve|version|selfsign|info: These are the various subcommands available in cfssl, and each has a specific function related to certificate management.
• -h: This option displays help information for the specified subcommand, offering guidance on usage, flags, and examples.

Example Output: The output provides detailed command usage, options available with the subcommand, and a description of what the subcommand accomplishes. It’s invaluable for both new users and seasoned experts looking for a quick reminder.

Conclusion:

cfssl is an essential tool for anyone managing SSL/TLS and PKI environments. From verifying certificates directly from hosts to checking configuration of certificates stored in files, and scanning for vulnerabilities, cfssl enriches the command-line toolkit with capabilities for ensuring secure communications. Understanding how to use cfssl efficiently opens up a myriad of possibilities for maintaining robust, secure, and trustworthy digital environments.