Overview
OpenSSL is a powerful tool that allows you to create and manage SSL/TLS certificates, private keys, and public keys. It is widely used in the Linux community and is often used in conjunction with web servers like Apache and Nginx. In this article, we will explore the various OpenSSL commands that allow you to check whether a certificate is valid and verify its authenticity.
Checking Certificate Information
To check the information contained within a certificate, you can use the openssl x509 command. This command allows you to view the certificate’s subject, issuer, validity dates, public key, and signature algorithm, among other details. To use the command, simply specify the path to the certificate file:
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This will display the certificate information in a human-readable format, making it easy to read and understand.
Verifying Certificate Authenticity
In addition to checking the certificate information, you can also use OpenSSL to verify the certificate’s authenticity. This is done by checking the certificate chain, which is a series of certificates that link the end-user certificate to a trusted root certificate. To verify the certificate chain, you can use the openssl verify command:
This command will check whether the certificate chain is valid and whether the end-user certificate is signed by a trusted root certificate. If the certificate chain is valid, the command will return OK. If the certificate chain is not valid, the command will return an error message.
Checking Certificate Revocation
Certificate revocation is the process of invalidating a certificate before its expiration date. This can happen if the private key is compromised, the certificate is no longer needed, or the certificate authority (CA) issuing the certificate goes out of business. To check whether a certificate has been revoked, you can use the openssl crl command:
This command will display the Certificate Revocation List (CRL), which is a list of certificates that have been revoked by the CA. If the end-user certificate is on the CRL, it has been revoked and should not be trusted.
Checking Certificate Expiration
Certificates have a finite lifespan and must be renewed periodically to ensure their continued validity. To check the expiration date of a certificate, you can use the openssl x509 command:
This command will display the start and end dates of the certificate. If the end date has passed, the certificate is no longer valid and should not be trusted.
Checking Certificate Chain
The certificate chain is a series of certificates that link the end-user certificate to a trusted root certificate. To check the certificate chain, you can use the openssl verify command with the -show_chain option:
This command will display the entire certificate chain, including the end-user certificate, intermediate certificates, and the trusted root certificate. If any certificate in the chain is invalid or untrusted, the command will display an error message.
Additional Tips and Tricks
Here are some additional tips and tricks to help you manage your SSL/TLS certificates using OpenSSL:
Converting Certificate Formats
Sometimes, you may need to convert a certificate from one format to another, such as from PEM to DER. To do this, you can use the openssl x509 command with the -outform option:
This will convert the certificate from PEM format to DER format and save it to a new file.
Creating a Self-Signed Certificate
If you need to create a self-signed certificate for testing or development purposes, you can use the openssl req command:
This command will generate a new RSA private key and a self-signed X.509 certificate and save them to the specified files.
Using OpenSSL with Apache
If you are using Apache as your web server, you can use OpenSSL to generate a new SSL/TLS certificate and configure Apache to use it. First, generate a new private key and certificate signing request (CSR) using the openssl req command:
Then, submit the CSR to a certificate authority to obtain a signed certificate. Once you have the signed certificate, you can configure Apache to use it by adding the following lines to your Apache configuration file:
SSLEngine on
SSLCertificateFile /path/to/certificate.crt
SSLCertificateKeyFile /path/to/private.key
FAQs for openssl commands to check certificate
What is OpenSSL?
OpenSSL is an open-source software library for secure communication, which implements the Secure Sockets Layer (SSL) and Transport Layer Security (TLS) protocols. It also includes a set of command-line tools that can be used for cryptographic tasks, such as generating and verifying digital signatures, encrypting and decrypting data, and managing digital certificates.
How can I check if a certificate is valid?
You can use the OpenSSL command-line tool to check the validity of a certificate. The most common way is to run the following command: openssl x509 -in cert.pem -noout -text, where cert.pem is the name of the certificate file. This will display the detailed information about the certificate, including the dates when it was issued and when it expires. You can also run the following command to check if the certificate matches a specific domain: openssl x509 -noout -in cert.pem -subject -nameopt RFC2253 | grep "DNS:".
How can I check the chain of trust of a certificate?
To check the chain of trust of a certificate, you can use the openssl verify command. This command will check if the certificate has been issued by a trusted certificate authority (CA) and if the entire chain of trust is valid. For example, you can run the command openssl verify -verbose -CAfile cacert.pem cert.pem, where cert.pem is the name of the certificate file and cacert.pem is the file that contains the trusted CA certificates. If the certificate is valid and the chain of trust is complete, the command will display a message that says “OK”.
How can I check the validity of a Certificate Revocation List (CRL)?
To check the validity of a CRL, you can use the openssl crl command. This command can be used to check if the CRL has expired, if it has been signed by a trusted CA, and if the revoked certificates are included in the CRL. For example, you can run the command openssl crl -in crl.pem -CAfile cacert.pem -noout -text, where crl.pem is the name of the CRL file and cacert.pem is the file that contains the trusted CA certificates. This will display the detailed information about the CRL, including the dates when it was issued and when it expires, as well as the list of revoked certificates.
How can I check the encryption strength of a certificate?
To check the encryption strength of a certificate, you can use the openssl x509 command. This command can be used to display the public key size and the encryption algorithm used by the certificate. For example, you can run the command openssl x509 -noout -text -in cert.pem | grep "Public-Key:", where cert.pem is the name of the certificate file. This will display the public key size in bits. You can also run the command openssl x509 -noout -text -in cert.pem | grep "Signature Algorithm:" to check the encryption algorithm used by the certificate.