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OpenSSL: Generating an RSA Key From the Command Line

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原文：https://rietta.com/blog/2012/01/27/openssl-generating-rsa-key-from-command/

OpenSSL: Generating an RSA Key From the Command Line

JAN 27TH, 2012 ♦ POSTED
BY FRANK RIETTA

Last Updated on March 18, 2014. Thanks to a tip from Chase Schultz, a security researcher in
San Francisco, it came to my attention that the instructions in this post contained an error and if you followed them exactly you ended up exporting copy of the private key rather than the public key.

I have updated this post to correct the error related to the command used to export the public key. The

-pubout

flag
had been editorially dropped in error when this blog was converted to Markdown format from
the old Blogger site. The example has been corrected and additional information about how to visually inspect the generated key file to ensure that it is a public key and not a private key has been added.

If you discover an error in the content on this post or any post on this blog, please contact me privately and it will be addressed ASAP. If the discovery is security related, please follow Rietta’s
Responsible Disclosure contact procedure.

This post is one of the most visited on this blog and is a maintained post that is periodically updated to be most useful to you. For more, see the list
of all maintained posts.

This post is part of our ongoing Encryption Series that
provides in-depth coverage of OpenSSL. To learn more about encryption key generation, management, and use please see the posts in the Encryption category. Our tips
and tricks are immediately applicable with examples that you can use right away. If you like this article, you may be interested in the Raspberry Pi crypto key
management project.

A few of weeks ago, I posted about how to Encrypt a File with a Password from the Command Line using OpenSSL. While 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 a

-----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-----

-----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:

private.pem

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

public.pem

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.

But! How do I use this Key?

Don’t worry, I will write about how to use your freshly minted RSA keys in a future blog post. In the mean time checkout the PHP functionality at http://us2.php.net/openssl_seal. Hint,
you can use your new public key with openssl_seal.

I hope this helps!

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.

Invitation to the Web Application Topics Newsletter

This post is part of the Web Application Topics series. If you are interested in having future
issues sent directly to you by e-mail, please sign up for free, today. For back issues, see the Web
Application Topics category on this blog.

Revision History

As a maintained post, this document is updated from time to time.

March 18, 2014: Corrected error related to the command used to export the public RSA key. The

-pubout

flag
had been editorially dropped in error when this blog was converted to Markdown format from Blogger. The example has been corrected and additional content about checking the type of the key added.

January 3, 2014: Added the video version to visually demonstrate the process of generating a military-grade RSA key pair in Mac OS X Mavericks.

October 6, 2013: Promoted to a Maintained
Post status

September 3, 2013: Reformatted as part of the migration to the new blog.

January 27, 2012: Originally posted on The Rietta Blog, hosted on the Blogger platform

Posted by Frank
Rietta Jan 27th, 2012 Encryption, Maintained, Security, Web
Application Topics

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标签： security rsa openssl

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