4 minutes estimated reading time.

Generate OpenSSL RSA Key Pair from the Command Line

Frank Rietta — 01/27/2012 (Last Updated: 10/22/2019)

While Encrypting a File with a Password from the Command Line using OpenSSL is very useful in its own right, the real power of the OpenSSL library is its ability to support the use of public key cryptograph for encrypting or validating data in an unattended manner (where the password is not required to encrypt) is done with public keys.

The Commands to Run

Generate a 2048 bit RSA Key

You can generate a public and private RSA key pair like this:

openssl genrsa -des3 -out private.pem 2048

That generates a 2048-bit RSA key pair, encrypts them with a password you provide and writes them to a file. You need to next extract the public key file. You will use this, for instance, on your web server to encrypt content so that it can only be read with the private key.

Export the RSA Public Key to a File

This is a command that is

openssl rsa -in private.pem -outform PEM -pubout -out public.pem

The -pubout flag is really important. Be sure to include it.

Next open the public.pem and ensure that it starts with -----BEGIN PUBLIC KEY-----. This is how you know that this file is the public key of the pair and not a private key.

To check the file from the command line you can use the less command, like this:

less public.pem

Do Not Run This, it Exports the Private Key

A previous version of the post gave this example in error.

openssl rsa -in private.pem -out private_unencrypted.pem -outform PEM

The error is that the -pubout was dropped from the end of the command. That changes the meaning of the command from that of exporting the public key to exporting the private key outside of its encrypted wrapper. Inspecting the output file, in this case private_unencrypted.pem clearly shows that the key is a RSA private key as it starts with -----BEGIN RSA PRIVATE KEY-----.

Visually Inspect Your Key Files

It is important to visually inspect you private and public key files to make sure that they are what you expect. OpenSSL will clearly explain the nature of the key block with a -----BEGIN RSA PRIVATE KEY----- or -----BEGIN PUBLIC KEY-----.

You can use less to inspect each of your two files in turn:

less private.pem to verify that it starts with a -----BEGIN RSA PRIVATE KEY-----
less public.pem to verify that it starts with a -----BEGIN PUBLIC KEY-----

The next section shows a full example of what each key file should look like.

The Generated Key Files

The generated files are base64-encoded encryption keys in plain text format. If you select a password for your private key, its file will be encrypted with your password. Be sure to remember this password or the key pair becomes useless.

The private.pem file looks something like this:

The public key, public.pem, file looks like:

Protecting Your Keys

Depending on the nature of the information you will protect, it’s important to keep the private key backed up and secret. The public key can be distributed anywhere or embedded in your web application scripts, such as in your PHP, Ruby, or other scripts. Again, backup your keys!

Remember, if the key goes away the data encrypted to it is gone. Keeping a printed copy of the key material in a sealed envelope in a bank safety deposit box is a good way to protect important keys against loss due to fire or hard drive failure.

Oh, and one last thing.

If you, dear reader, were planning any funny business with the private key that I have just published here. Know that they were made especially for this series of blog posts. I do not use them for anything else.

Found an issue?

Let us Know