One of the most versatile SSL tools is OpenSSL which is an open source implementation of the SSL protocol. There are versions of OpenSSL for nearly every platform, including Windows, Linux, and Mac OS X. OpenSSL is commonly used to create the CSR and private key for many different platforms, including Apache. However, it also has hundreds of different functions that allow you to view the details of a CSR or certificate, compare an MD5 hash of the certificate and private key (to make sure they match), verify that a certificate is installed properly on any website, and convert the certificate to a different format.
The most common OpenSSL commands and their usage:
General OpenSSL Commands
These commands allow you to generate CSRs, Certificates, Private Keys and do other miscellaneous tasks.
Generate a new private key and Certificate Signing Request
openssl req -out CSR.csr -new -newkey rsa:2048 -nodes -keyout privateKey.key
Generate a self-signed certificate
openssl req -x509 -nodes -days 365 -newkey rsa:2048 -keyout privateKey.key -out certificate.crt
Generate a certificate signing request (CSR) for an existing private key
openssl req -out CSR.csr -key privateKey.key -new
Generate a certificate signing request based on an existing certificate
openssl x509 -x509toreq -in certificate.crt -out CSR.csr -signkey privateKey.key
Remove a passphrase from a private key
openssl rsa -in privateKey.pem -out newPrivateKey.pem
Checking Using OpenSSL
If you need to check the information within a Certificate, CSR or Private Key, use these commands.
Check a Certificate Signing Request (CSR)
openssl req -text -noout -verify -in CSR.csr
Check a private key
openssl rsa -in privateKey.key -check
Check a certificate
openssl x509 -in certificate.crt -text -noout
Check a PKCS#12 file (.pfx or .p12)
openssl pkcs12 -info -in keyStore.p12
Debugging Using OpenSSL
If you are receiving an error that the private doesn’t match the certificate or that a certificate that you installed to a site is not trusted, try one of these commands.
Check an MD5 hash of the public key to ensure that it matches with what is in a CSR or private key
openssl x509 -noout -modulus -in certificate.crt | openssl md5
openssl rsa -noout -modulus -in privateKey.key | openssl md5
openssl req -noout -modulus -in CSR.csr | openssl md5
Check an SSL connection. All the certificates (including Intermediates) should be displayed
openssl s_client -connect www.paypal.com:443
Converting Using OpenSSL
These commands allow you to convert certificates and keys to different formats to make them compatible with specific types of servers or software. For example, you can convert a normal PEM file that would work with Apache to a PFX (PKCS#12) file and use it with Tomcat or IIS.
Convert a DER file (.crt .cer .der) to PEM
openssl x509 -inform der -in certificate.cer -out certificate.pem
Convert a PEM file to DER
openssl x509 -outform der -in certificate.pem -out certificate.der
Convert a PKCS#12 file (.pfx .p12) containing a private key and certificates to PEM
openssl pkcs12 -in keyStore.pfx -out keyStore.pem -nodes
You can add -nocerts to only output the private key or add -nokeys to only output the certificates.
Convert a PEM certificate file and a private key to PKCS#12 (.pfx .p12)
openssl pkcs12 -export -out certificate.pfx -inkey privateKey.key -in certificate.crt -certfile CACert.crt