Openssl tutorial

General OpenSSL Commands

These commands allow you to generate CSRs, Certificates, Private Keys and do other miscellaneous tasks.

• 
  
  

  # Generate keys
  Generating private key Generating Public key from private key	openssl genrsa -out key.pem
  	openssl rsa -in key.pem -out key.pub -pubout
  	# Encrypt and Decrypt a file (using public key to encrypt)
  	echo --pass-- > pass.txt
  	openssl rsautl -in pass.txt -out pass.enc -pubin -inkey key.pub -encrypt
  	openssl rsautl -in pass.enc -out pass.dec -inkey key.pem -decrypt
  	cat pass.dec
  	# Compress, Encrypt, Decyrpt, Uncompress a file (using password in pass.txt)
  	echo content > file.txt
  	gzip file.txt
  	openssl bf -in file.txt.gz -out file.enc -pass file:pass.txt -e
  	openssl bf -in file.enc -out file.dec.gz -pass file:pass.dec -d
  	gzip -d file.dec.gz
  	cat file.dec

  
  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 (see How to Create and Install an Apache Self Signed Certificate for more info)

  openssl req -x509 -sha256 -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. You can also check CSRs and check certificates using our online tools.

• 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. If you are trying to verify that an SSL certificate is installed correctly, be sure to check out the SSL Checker.

• 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. Use our SSL Converter to convert certificates without messing with OpenSSL.

• 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

Create a Private Key

Use this command to create a password-protected, 2048-bit private key (domain.key):

openssl genrsa -des3 -out domain.key 2048

Enter a password when prompted to complete the process.

Verify a Private Key

Use this command to check that a private key (domain.key) is a valid key:

openssl rsa -check -in domain.key

 Creating Digital Signatures

At last, we can produce a digital signature and verify it. Signing a large file directly with a public key algorithm is inefficient, so we should first compute the digest value of the information to be signed. This can be accomplished using the following command:

openssl dgst -<hash_algorithm> -out <digest> <input_file>

In this example, <hash_algorithm> is whichever algorithm you choose to compute the digest value. The <input_file> is the file containing the data you want to hash while “digest” is the file that will contain the results of the hash application.

The next step is to compute the signature of the digest value as follows:

openssl pkeyutl -sign -in <digest> -out <sign.txt> -inkey <key>

Finally, you can check the validity of a signature like so:

openssl pkeyutl -verify -in <signature> -out <sign.txt> -inkey <key> -pubin
 

Here, “signature” is the filename of your signature, and “key.pem” is the file with the public key. To confirm the verification for yourself, you can compute the digest value of the input file and compare it to the digest value produced from the verification of the digital signature.

Certificate Signing Requests

openssl req 
-newkey rsa:2048 -nodes -keyout domain.key 
-out domain.csr

To view a certificate’s content in plain text, use:

openssl x509 -text -noout -in domain.crt

Encrypt a file using a supplied password:

$ openssl enc -aes-256-cbc -salt -in file.txt -out file.txt.enc -k PASS

Decrypt a file using a supplied password:

$ openssl enc -aes-256-cbc -d -in file.txt.enc -out file.txt -k PASS