Mastering the OpenSSL x509 Command (with examples)

OpenSSL is a robust tool that serves a variety of functions related to cryptography and secure communications. A significant part of its functionality revolves around managing X.509 certificates, which are crucial for establishing secure SSL/TLS connections. The OpenSSL x509 command is specifically designed to help users inspect, convert, and manipulate these certificates. This article illustrates how to leverage the x509 command through several practical examples.

Use case 1: Displaying Certificate Information

Code:

openssl x509 -in filename.crt -noout -text

Motivation:

Understanding the details of an X.509 certificate is essential for anyone working with SSL/TLS. Details such as issuer, subject, validity period, and public key information are fundamental to establishing authenticity and trust in digital communications. Using this command, you can efficiently extract and review all pertinent certificate information.

Explanation:

• openssl x509: Invokes the x509 command within the OpenSSL toolkit, used specifically for certificate management tasks.
• -in filename.crt: Specifies the input file from which the certificate information will be read. Replace filename.crt with your actual certificate file.
• -noout: Suppresses the output of the encoded certificate to avoid cluttering the terminal with unneeded data.
• -text: Formats the output to display the certificate information in a readable text form, detailing fields such as issuer, subject, and validity.

Example Output:

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number: 123456789
        Signature Algorithm: sha256WithRSAEncryption
        Issuer: C = US, O = Example Corp, CN = Example CA
        Validity
            Not Before: Jan 1 00:00:00 2023 GMT
            Not After : Dec 31 23:59:59 2023 GMT
        Subject: C = US, ST = California, L = San Francisco, O = Example Corp, CN = www.example.com
        ...

Use case 2: Display a Certificate’s Expiration Date

Code:

openssl x509 -enddate -noout -in filename.pem

Motivation:

Knowing the expiration date of a certificate is crucial for ensuring uninterrupted secure communications. Certificates need to be renewed or replaced promptly before they expire to avoid service disruptions or security vulnerabilities.

Explanation:

• openssl x509: Calls the x509 command to perform tasks related to certificate handling.
• -enddate: Specifically extracts the expiration date of the certificate, allowing users to focus solely on the validity period.
• -noout: Suppresses unnecessary output, limiting the result to just the expiration information.
• -in filename.pem: Specifies the input certificate file, with .pem indicating the format of the file.

Example Output:

notAfter=Dec 31 23:59:59 2023 GMT

Use case 3: Converting a Certificate Between DER and PEM Formats

Code:

openssl x509 -inform der -outform pem -in original_certificate_file -out converted_certificate_file

Motivation:

Different systems and applications may require certificates in different formats – PEM and DER being the most common. Converting certificates between these formats ensures compatibility with a wide range of software.

Explanation:

• openssl x509: Initiates the x509 command for certificate processing.
• -inform der: Specifies that the input file is in DER (Distinguished Encoding Rules) binary format.
• -outform pem: Indicates that the output should be in PEM (Privacy-Enhanced Mail) text format, which is more readable and includes additional headers and footers.
• -in original_certificate_file: Identifies the source certificate file to be converted.
• -out converted_certificate_file: Defines the destination file where the converted certificate will be stored.

Example Output:

There is no standard output printed to the console. Instead, a new file converted_certificate_file is created containing the certificate in PEM format.

Use case 4: Storing a Certificate’s Public Key in a File

Code:

openssl x509 -in certificate_file -noout -pubkey -out output_file

Motivation:

Extracting the public key from a certificate can be necessary for many cryptographic operations, such as setting up secure communications or verifying signatures. Storing the public key separately simplifies these processes, making it easily accessible when needed.

Explanation:

• openssl x509: Employs the x509 tool to interact with certificates.
• -in certificate_file: Determines the source file from which the certificate and associated public key are read.
• -noout: Prevents additional output aside from the desired public key to keep the result clear and focused.
• -pubkey: Specific flag to extract just the public key from the certificate.
• -out output_file: Directs the extracted public key to be saved in the specified output file.

Example Output:

Again, there is no direct console output. Instead, the public key is saved in output_file, facilitating later use for encryption processes or certificate validation tasks.

Conclusion:

Understanding how to use the openssl x509 command effectively can greatly enhance your ability to manage and manipulate X.509 certificates. Whether you’re extracting detailed information, checking expiration dates, converting between formats, or accessing public keys, these command examples illustrate essential skills for maintaining secure digital communications.