Skip to content

Instantly share code, notes, and snippets.

Embed
What would you like to do?
Using OpenSSL to create keys for Mac OS X.

Creating Keys

This is a brief guide to creating a public/private key pair that can be used for OpenSSL. While the "easy" version will work, I find it convenient to generate a single PEM bundle and then export the private/public key from that as needed. This document also covers how to add and remove a password from your private key and how to make sure that keychain will automatically unlock it when you sign in.

Just make it work

Generate an ssh key-pair:

$ ssh-keygen

If you just pound enter through the setup procedure then you will end up with a new key pair created in the default location: /Users/yourname/.ssh/. There will be two files:

  • id_rsa This is your private key, you must keep it secret and never allow anybody else to gain control of it. Treat this key like a password, keep it safe and make a backup copy. You can add it to keychain using ssh-add -K ~/.ssh/id_rsa.

  • id_rsa.pub This is your public key, you can share it freely. This part of the key is used during authentication to encode a message which can only be decoded with the private key. It cannot be used to derive the private key so there is no risk in sharing it.

When a server administrator asks for a copy of your public key, send them a copy of the id_rsa.pub file. They'll be able to add it to your user account's list of authorized keys and that will enable you to log in without typing a password.

Doing it the hard way

This method involves creating the keys as a bundle, exporting the public key and manually setting the permssions on all of the paths. You'll also have to configure OpenSSH to use your new bundle for authentication.

A summary of the steps follows:

$ mkdir ~/.ssh
$ chmod 0700 ~/.ssh
$ cd ~/.ssh
$ openssl genpkey -algorith RSA -aes-256-cbc -outform PEM -out yourname.pem \
-pkey_opt rsa_keygen_bits:4096
$ chmod 0400 yourname.pem
$ ssh-keygen -y -f yourname.pem > yourname.pub
$ chmod 0444 yourname.pub

Rational

I prefer to generate a certificate using OpenSSL directly, then export the private or public-key in the necessary format as needed. The benefits to this appraoch are three- fold:

  1. This is a process similar to the one you'd use to generate certificates used for other tasks like S/MIME or to become part of a signed certificate for HTTPS.
  2. There is a single certificate file from which you can derive the private or public key in whichever format you need. It's much easier to manage one key than two, or even several if you require the key in different formats.
  3. You gain control over the key length, encryption method, and algorithm so that you can consciously decide to use weaker keys for old/slow hardware (e.g. the Raspberry Pi media server in your closet) and strong but slower keys where appropriate.

Default software and Mac OS X

In order to generate the key I prefer to use OpenSSL directly rather than the ssh-keygen tool. While it is possible to provide flags to ssh-keygen using OpenSSL gives us access to options that are not avaiable in the standard Mac OS X version of SSH but doesn't require us to build the SSH client from scratch.

Update OpenSSL

Unfortunately the version of OpenSSL that ships with Mac OS x is rather dated and so it's missing some of the features of the latest versions. One of those features is the genpkey command which is the new recommended way to generate keys. Assuming you have Homebrew installed (see: https://brew.sh) you can install an up-to-date version of OpenSSL with:

$ brew install openssl
$ brew link openssl --force

Many packages that you install with homebrew are likely to depend on OpenSSL anyway so this is not a terrible idea even if you don't care about using OpenSSL directly.

Updating OpenSSH

If you're interested in rebuilding openssh you should link against LibreSSL so that passwords can be installed in your keychain.

$ brew install openssh --with-libressl

This is a relatively new option and caution should be taken because compatibility may not be perfect. LibreSSL is not intended to be a 1:1 replacement for OpenSSL.

It appears that just building OpenSSH will not have it request key information from the Mac OS X keychain, nor will it automatically start SSH-Agent so there may be some trouble-shooting steps required if you prefer to go this path. I do not build a new version of SSH.

Creating directories

OpenSSH requires that keys be stored in ~/.ssh and that path must be restricted so that only the user can access it. It also requires that any identify files be accessible only by the user too. Permssions for ~/.ssh/config can be more relaxed but it is good practice to keep those private so as not to leak inforamtion about user names or servers you connect to.

Create the directories by running:

$ mkdir ~/.ssh

While this will create the directory you will have to modify the default permissions. Read/write/execute for the owner and no access for any other user is required. Recall, the execute flag on a directory allows you to view its content.

$ chmod 0700 ~/.ssh

You might want to create an empty ssh config file and set appropriate permissions so that you don't have to remember how to do it later when there's some problem and you are half-asleep, drunk, and responding to a PagerDuty alert.

$ touch ~/.ssh/config
$ chmod 0600 ~/.ssh/config

You can save a few copy steps if you're following this guide by changing into your ssh path for the remaining steps:

$ cd ~/.ssh

Generating keys

The first step to generating keys is to create the bundle using OpenSSL. This approach allows us to specify a few extra options when creating keys that are normally hidden by ssh-keygen:

$ openssl genpkey -algorith RSA -aes-256-cbc -outform PEM -out yourname.pem \
-pkey_opt rsa_keygen_bits:4096

The options: are

  • genpkey is the new command for generating keys, it supercedes the old genrsa method. Mac OS X's default OpenSSL does not have this command so building your own version is required.

  • -algorith rsa uses the RSA algorithm for the key and is recommended for maximum compatibility. Other options include ECDSA, which is less computationally intensive on very low-end hardware (e.g. 50 MHz ARM) and DH which has characteristics similar to RSA but is rarely used.

  • -aes-256-cbc is the cypher used to encrypt the bundle and causes the user to be prompted for a password. There are a number of available ciphers but AES-256-cbc is among the stronger options available and widely used too.

  • -outform PEM there are several output formats that you can use but PEM is widely used by open source software and tends to be the best supported. The format is also nicely encoded so that you can debug with any text editor and has the advantage of bundling the public and private key into a single file which makes them easier to move around. You can always output the public or private key from a PEM bundle that contains both.

  • -pkey_opt … can be specified multiple times and supplies options to the generation function. This can be specified multiple times to suplly several options

    • rsa_keygen_bits:4096 sets the length of the keys produced. 1024 bits is generally considered the absolute minimum for secure communication today though there is some concern that they will be broken for well-funded attackers in the near future so 2048 bits is recommended where possible. Longer keys provide greater security however there is diminishing returns as key length increases. Also, increasing the key length also increases computational costs exponentially (by the cube of the change, so 2048 is 8x more demanding than 1024-bit). You may want to use smaller keys for slower hardware or if you find yourself frequently reconnecting due to bad connections during a session for better performance.
  • -out yourname.pem defines the output file for your bundle. You should store a copy of this certificate in ~/.ssh so that it can be used to authenticate ssh sessions. The file must not be accessible to other users on the system so set the permissions accordingly. You should also store the file and the password somewhere safe (like in your password vault or on a USB drive in a safe deposit box).

When generating the key you will be prompted for a password. Make sure to use a very strong, unique, random password for this file. You won't have to type it in regularly so generate it with your password vault. In a pinch you can generate a random password using OpenSSL via: openssl rand -base64 48.

When the bundle has been generated, copy it to your~/.ssh folder and change its permissions accordingly:

$ cp ./yourname.pem ~/.ssh/yourname.pem
$ chmod 0400 ~/.ssh/yourname.pem

I prefer to make the bundle read-only for my user so I never accidentally edit it or strip the password. chmod 0600 ~/.ssh/yourname.pem would also work if you don't mind it being editable by your user.

Extracting the public key

You'll want to be able to send the public key to other people and leave it on other computers without risking your private key. The easiest way to export your public key is using the ssh-keygen method which prints it to standard out.

$ ssh-keygen -y -f yourname.pem

You can always redirect that to a file if you want to send it via email or copy it via SFTP. Generally I prefer not to keep a copy of my public keys on disk so that I am justified in always treating ~/.ssh as a secret.

Configuring OpenSSH

Remember to either edit your ~/.ssh/config to specify this bundle as the default identify file by adding the line:

IdentityFile ~/.ssh/yourname.pem

Alternatively you can specify it on a host-by-host basis by using ssh command-line options: ssh -i ~/.ssh/yourname.pem example.com -l someuser. When you are prompted for a password, remember that you should enter the one used when creating the bundle, not the log-in password for your computer or the remote system you are connecting to.

Finally, you should consider adding the key to your Mac OX X keychain using:

$ ssh-add -k ~/.ssh/yourname.pem

This will store the password in the login Keychain which is unlocked automatically whenever you sign in. Storing your password this way means you won't have to re-type the password you used when creating the bundle in order to use it.

Using ssh -i ~/.ssh/yourname.pem foo.example.com will also add your key to Keychain.

Public Keys and Github.com

It's a good idea to add your public key to github.com so that you can pull from private repositories and push changes to your public repositories. You can do this at:

https://github.com/settings/ssh

Once you've uploaded your public key, other users can download it by going to

https://github.com/yourusername.keys

For example, my public key is located here: https://github.com/colinstein.keys

You may want to create different key-pairs for different repositories or organizations and then use ~/.ssh/config and local .gitconfig files ot manage those relationships.

After generating keys in the above manner for each github account you can configure ssh by editing ~/.ssh/config and adding entries like the following for each account:

# Foo Account
Host github.com-foo
  HostName github.com
  User git
  IdentityFile ~/.ssh/foo.pem
  
# Bar Account
Host github.com-bar
  HostName github.com
  User git
  IdentityFile ~/.ssh/bar.pem

When cloing a repository you would then clone from the appropriate host:

$ git clone git@github.com-foo:somegithubuser/somerepo.git

You can also edit the existing git remote by editing the .gitconfig inside the checked out repository:

[remote "origin"]
  url = git@github.com-foo:somegithubuser/somerepo.git

Git also provides a number of ways to configure SSH via git config and git remote add foo git@github.com-foo:somegithubuser/somerepo.git. A full run through of those options is well outside the scope of this gist.

@murvinlai

This comment has been minimized.

Copy link

murvinlai commented Jun 1, 2016

The command is outdated. It should be:
openssl genpkey -algorithm RSA -aes-256-cbc -outform PEM -out yourname.pem \ -pkeyopt rsa_keygen_bits:4096

@ericdfields

This comment has been minimized.

Copy link

ericdfields commented Mar 30, 2018

what @murvinlai said

@Snesi

This comment has been minimized.

Copy link

Snesi commented May 7, 2018

Yep, missing an m in algorithm and you have to remove the _ in pkey_opt.

But thank you for the explanation, this was very helpful!

@sprive

This comment has been minimized.

Copy link

sprive commented Sep 29, 2019

The brew link commands is outdated -- you can't force link anymore. Note in the failure message, the text suggests a solution:

Warning: Refusing to link macOS-provided software: openssl
If you need to have openssl first in your PATH run:
  echo 'export PATH="/usr/local/opt/openssl/bin:$PATH"' >> ~/.bash_profile```
@brotskydotcom

This comment has been minimized.

Copy link

brotskydotcom commented Oct 5, 2019

Thanks much for this explanation!

One additional tip: if you need the public key in PEM form, for example to use with PyJWT, you can convert it via:

ssh-keygen -e -f yourname.pub -m PEM > yourname.pub.pem
@iqramali

This comment has been minimized.

Copy link

iqramali commented May 3, 2020

This also worked single liner, doesn't need line break then works well:
openssl genpkey -algorithm RSA -aes-256-cbc -outform PEM -out yourname.pem -pkeyopt rsa_keygen_bits:4096

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
You can’t perform that action at this time.