Openssl Generate Pem Key From Modulus

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. Oct 04, 2005  $ openssl x509 -noout -text -in server.crt $ openssl rsa -noout -text -in server.key The `modulus' and the `public exponent' portions in the key and the Certificate must match. But since the public exponent is usually 65537 and it's bothering comparing long modulus.

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:

Openssl Generate Pem Key From Modulus 10

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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< Cryptography

Download and install the OpenSSL runtimes. If you are running Windows, grab the Cygwin package.

OpenSSL can generate several kinds of public/private keypairs.RSA is the most common kind of keypair generation.[1]

Other popular ways of generating RSA public key / private key pairs include PuTTYgen and ssh-keygen.[2][3]

Generate an RSA keypair with a 2048 bit private key[edit]

Openssl Convert To Pem

Execute command: 'openssl genpkey -algorithm RSA -out private_key.pem -pkeyopt rsa_keygen_bits:2048'[4] (previously “openssl genrsa -out private_key.pem 2048”)


Openssl Generate Pem Key From Modulus Chart

Make sure to prevent other users from reading your key by executing chmod go-r private_key.pem afterward.

Extracting the public key from an RSA keypair[edit]


Execute command: 'openssl rsa -pubout -in private_key.pem -out public_key.pem'


A new file is created, public_key.pem, with the public key.

It is relatively easy to do some cryptographic calculations to calculate the public key from the prime1 and prime2 values in the public key file.However, OpenSSL has already pre-calculated the public key and stored it in the private key file.So this command doesn't actually do any cryptographic calculation -- it merely copies the public key bytes out of the file and writes the Base64 PEM encoded version of those bytes into the output public key file.[5]

Viewing the key elements[edit]

Execute command: 'openssl rsa -text -in private_key.pem'

All parts of private_key.pem are printed to the screen. This includes the modulus (also referred to as public key and n), public exponent (also referred to as e and exponent; default value is 0x010001), private exponent, and primes used to create keys (prime1, also called p, and prime2, also called q), a few other variables used to perform RSA operations faster, and the Base64 PEM encoded version of all that data.[6](The Base64 PEM encoded version of all that data is identical to the private_key.pem file).

Password-less login[edit]

Often a person will set up an automated backup process that periodically backs up all the content on one 'working' computer onto some other 'backup' computer.

Because that person wants this process to run every night, even if no human is anywhere near either one of these computers, using a 'password-protected' private key won't work -- that person wants the backup to proceed right away, not wait until some human walks by and types in the password to unlock the private key.Many of these people generate 'a private key with no password'.[7]Some of these people, instead, generate a private key with a password,and then somehow type in that password to 'unlock' the private key every time the server reboots so that automated toolscan make use of the password-protected keys.[8][3]

Further reading[edit]

  1. Key Generation
  2. Michael Stahnke.'Pro OpenSSH'.p. 247.
  3. ab' Documentation: SSH Key Overview'
  4. 'genpkey(1) - Linux man page'
  5. 'Public – Private key encryption using OpenSSL'
  6. 'OpenSSL 1024 bit RSA Private Key Breakdown'
  7. 'DreamHost: Personal Backup'.
  8. Troy Johnson.'Using Rsync and SSH: Keys, Validating, and Automation'.
  • Internet_Technologies/SSH describes how to use 'ssh-keygen' and 'ssh-copy-id' on your local machine so you can quickly and securely ssh from your local machine to a remote host.
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