The first thing to do would be to generate a 2048-bit RSA key pair locally. This pair will contain both your private and public key. You can use Java key tool or some other tool, but we will be working with OpenSSL. To generate a public and private key with a certificate signing request (CSR), run the following OpenSSL command. Now we need to generate a 256-bit key for AES 256 GCM (Note: Installing Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy is a must). Newsletter keyUsages is an Array indicating what can be done with the newly generated key.

While Encrypting a File with a Password from the Command Line using OpenSSLis very useful in its own right, the real power of the OpenSSL library is itsability to support the use of public key cryptograph for encrypting orvalidating data in an unattended manner (where the password is not required toencrypt) 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 provideand writes them to a file. You need to next extract the public key file. You willuse this, for instance, on your web server to encrypt content so that it canonly 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 thepublic 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 keyto exporting the private key outside of its encrypted wrapper. Inspecting theoutput file, in this case private_unencrypted.pem clearly shows that the keyis 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 makesure that they are what you expect. OpenSSL will clearly explain the nature ofthe 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 withyour 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 tokeep the private key backed up and secret. The public key can be distributedanywhere 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 aprinted copy of the key material in a sealed envelope in a bank safety depositbox is a good way to protect important keys against loss due to fire or harddrive 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.

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OpenSSL is a giant command-line binary capable of a lot of various securityrelated utilities. Each utility is easily broken down via the first argument ofopenssl. For instance, to generate an RSA key, the command to use will beopenssl genpkey.

Generate 2048-bit AES-256 Encrypted RSA Private Key .pem

Openssl Generate Aes Gcm Keys

The following command will result in an output file of private.pem in whichwill be a private RSA key in the PEM format.

Let’s break this command down:

  • openssl: The binary that contains the code to generate an RSA key (and manyother utilities).
  • genpkey: Specifies the utility to use.
  • -algorithm RSA: Specifies to use the RSA algorithm.
  • -aes256: Specifies to use the AES-256 cipher, which is newer and moresecure than DES. Default is no cipher.
  • -out private.pem: Specifies that a file named “private.pem” should becreated with the contents of the private key. Default is STDOUT.

When executing this command, it will ask for a password to encrypt the keywith. After selecting a password, a file will be created in the currentdirector named private.pem.

Private RSA keys generated with this utility start with the text -----BEGIN PRIVATE KEY-----.

You can inspect this file with the command cat private.pem.

Openssl Generate Aes Gcm Key Free

Export Public RSA Key From Private Key

In order to export the public key from the freshly generated private RSA Key,the openssl rsautility, which is used for processing RSA keys.

Openssl Generate Aes-256-cbc Key

The command to export a public key is as follows:

This will result in a public key, due to the flag -pubout.

Inspect this file with cat public.pem:

The public key can be uploaded to other servers and services to encrypt datafor the private key to decrypt.

Openssl Generate Aes Gcm Key Download

This file will start with -----BEGIN PUBLIC KEY-----. If this file doesn’tstart with “BEGIN PUBLIC KEY”, do not upload it as a public key to any source!