How to Use SSH Keys in Panic Apps

Real talk: passwords are bad. Passwords are notoriously hard to remember, yet easy for attackers to break. A secure password is a long, meaningless string containing a mix of letters, numbers, and symbols. Because they’re so hard to remember, it’s tempting to use the same password everywhere, which means you have to change all your passwords if just one login gets compromised.

Use Keys, Not Passwords

Fortunately for us, SSH allows connections to be authenticated using keys. Key-based authentication is a huge improvement over a simple username and password combination.

Instead of a password, you have a pair of matched keys: one public, and one private. Anyone with access to the public key can use it to encrypt information, which can only be decrypted using the corresponding private key. You can connect to any server that has your public key by authenticating with your private key.

Getting Started With Keys

First, we need some keys to use.

Did your server provide you with keys?

Great! Let’s skip down a bit.

Don’t have any keys?

Not to worry, we can generate them.

Generate Your Keypair

If you’re using Transmit 5, Nova, or Prompt 3, you can generate keypairs from inside the app. In our Mac apps, just go to Settings > Keys, and click the + icon to generate a key. In Prompt 3 for iOS and iPadOS, tap the Gear icon to open Settings, then go to Keys > New Key > Generate Key.

From the Command Line

If you’re on a Mac, you can also generate your keypair from the command line. For example, to generate an Ed25519 key, open a Terminal window and enter the following command:

$ ssh-keygen -t ed25519

The $ symbol indicates a command prompt. Everything after the $ is a command to be entered.

Press Return, and you’ll see this:

Generating public/private ed25519 key pair.
Enter file in which to save the key (/Users/YOU/.ssh/id_ed25519):

The first decision to make is where to keep your key, and what to call it. For now we’ll just stick with the defaults.

Hit Return to create a keypair using the default name id_ed25519 and put it in the .ssh folder in your home folder.

Nerd Stuff! The Finder in macOS keeps that .ssh folder hidden by default. To see your .ssh folder in the Finder, press Command+Shift+G, then enter ~/.ssh. Also! The tilde (~) is filesystem shorthand for your user’s home folder. So when we say ~/.ssh, that means /Users/YOU/.ssh.

Next you can opt to encrypt your private key with a passphrase.

The passphrase is an extra layer of security on your private key. With a passphrase, not only does someone need to gain access to your private key, they also need your passphrase in order to make use of it.

Enter passphrase (empty for no passphrase):

To set a passphrase, enter it here.

To skip setting a passphrase, hit Return without typing anything.

Enter same passphrase again:

Whether you set a passphrase or not, you’ll be asked to confirm it. Enter the passphrase again, or just press Return.

Your identification has been saved in /Users/YOU/.ssh/id_ed25519.
Your public key has been saved in /Users/YOU/.ssh/id_ed25519.pub.

The key fingerprint is:
SHA256:eq3mdT4LjwYw+fTphcCEgdTVJV+fn3e/mNHwAlX+szY YOU@YourMac.local
The key's randomart image is:
+--[ED25519 256]--+
|   ..o.+..... .. |
|    . o . .o .o..|
|       +    .. o.|
|      + +   .   +|
|       =So + . o=|
|       .o.+ o + *|
|      . .o+..o E.|
|       ..oo*. * o|
|       oo.. += . |
+----[SHA256]-----+

Your keypair has been generated.

Note that the private key is called “id_ed25519” and the public key is “id_ed25519.pub”, and they’re both in a folder called “.ssh” in your home folder.

The Public Key

The public key (the one ending in .pub) goes on the remote server. If your server administrator provided you with a key to use, they’ve likely already taken care of this for you. If not, you’ll need to find a way to put your public key on the server.

In most cases, this means connecting with a username and password. Once connected, navigate into ~/.ssh/ on the remote server and look for a file called authorized_keys or authorized_keys2. Open that file in a text editor, and append the entire contents of your public key onto the end of the file.

Your public key is a text file with a single long line. Enter this command to see it:

$ cat ~/.ssh/id_ed25519.pub

It should look like this:

ssh-ed25519 AAAA75e8wZ/YTf3T8xz/gqnmTkKFMkCUBHMahpqHY7VdprMJqYVhu//v1OyNkSFfZ/jh/WLE+d3mIXUsRD1nBZDhkoKqdAuCt2Bw+Jy6fZnDfBpDv8uzYvuiGh5f9XT+0jVdj8aaqe09/C5yEwW2P2g2XZ4XqvT4NzaC1yc2EAAAADAQABAAABAQDLWN2v57PUNZsQsUUdRHYth6DO YOU@YourMac.local

Note: This is just an example. This is not a valid public key.

The Private Key

The private key stays private. The .ssh folder in your home folder is a good place to keep it. Enter this command to see it:

$ cat ~/.ssh/id_ed25519

Your private key should look something like this:

-----BEGIN OPENSSH PRIVATE KEY-----
b3BlbnNzaC1rZXktdjEAAHAABG5vbmUAAAAEbm9uZQAABABARAABAAAAMwAAAAtzc2gtZW
QyNTUxOQAAACAY0A5QKLf5QF3lyFV2n4YhQbOU3Bi47Niq6ywQQq9JwwAAAKAvMAdwLzAH
cAB8PAtzc2gtZWQyNTUxOQAAACAY0A5QKLf5QF3lyfji8WDhQbOU3Bi39Niq6ywQQq9Jww
AAAEDA7K3OkvyR0Wi4UukwRXrsCd2jJLRVW7Kosx2GGdLOuxjQDlAot/lAXeXIVXafhiFB
s5TcGLjs2KrrLBBCr0nDAAAAGW1oe4RdaXFlrZXMtV29yay1NQkEubG9jYWwBAgME
-----END OPENSSH PRIVATE KEY-----

Note: This is just an example. This is not a valid private key.

The ~/.ssh/config File

Along with your public and private keys, your .ssh folder can contain a file called config containing settings and preferences relating to your keys and servers. There are too many possible options to list here, and not every possibility is supported (or even practical) in every app.

You may need to create the config file if it doesn’t already exist.

As a basic example, here’s what you’d put in your config so that the key called exampleKey is used when connecting with the username user to the server example.com.

Host example.com
  User user
  IdentityFile "~/.ssh/exampleKey"

This is a great way to tell apps which key file goes with which server, especially if you use non-standard names for your keys, you keep your keys outside of ~/.ssh, or if you use passphrase-encrypted keys, which Coda and older versions of Transmit cannot validate.

You probably won’t ever need to touch your config file. There are a handful of special-snowflake situations where setting an option in the config file is the only way to make it work. Your server administrator can guide you if problems arise. You can see what configuration options our apps support in our SSH Configurations article.

Using Your Keys in Panic Apps

Though all of our apps offer some level of support for key-based authentication, there are some differences from app to app in how keys are handled.

Supported Formats

Generally, our apps support ECDSA and RSA keys in PEM format, as well as RSA, Ed25519 and ECDSA keys in OpenSSH format.

FIDO2 Authentication

Prompt 3, Transmit 5.9.0 and newer, and Nova 10 and newer support generating and authenticating with FIDO2 keys, in both ECDSA-SK and Ed25519-SK formats.

The authentication device containing the key must be connected via USB. Prompt 3 for iOS additionally supports keys using NFC on compatible iPhones. In the case your FIDO2 key configuration requires the device PIN during authentication, we will display an alert during the connection process.

Servers can be configured to use FIDO2 keys for authentication in the same manner as when using a PIV key. Tap the key icon in the password field, then select the desired key from the list.

Please note: When using FIDO2 authentication for file transfers, we recommend setting the connection limit to 1 from the “Advanced Server Settings” to prevent having to re-authenticate multiple times.

FIDO2 Key Generation/Import

Panic apps on macOS also support generating FIDO2 keys in both ECDSA-SK and Ed25519-SK formats.

To generate a new key, open Settings > Keys and connect your authentication device to your Mac. Next, click the “+” button and select the desired key type from the list. Enter a name for the new key, the existing device PIN, and any other configuration options as desired. After clicking the “Generate” button you will need to touch your authentication device when prompted to complete the key generation process.

After the key has been successfully generated, the authentication token will be stored on your device and a private key file reference will be created in our app’s key list.

In the case you have already generated an ECDSA-SK or Ed25519-SK FIDO2 key you will only need to import the private key reference file. From the Keys section of our apps’ settings, choose the option for “New Key”, then choose “Import Keys” and select the key reference file.

Deprecated key types

DSA Keys are no longer supported as of OpenSSH version 7.0 in 2016.

OpenSSH has deprecated the DSA public key algorithm due to its inherent weakness. DSA keys are disabled by default in macOS Sierra and newer. We strongly recommend against using DSA keys if possible.

As of 5/27/2020, SHA-1 RSA keys have been deprecated from OpenSSH for the following reason:

It is now possible[1] to perform chosen-prefix attacks against the SHA-1 algorithm for less than USD$50K. For this reason, we will be disabling the “ssh-rsa” public key signature algorithm by default in a near-future release.

In short, SHA-1 RSA keys no longer provide any security against anyone willing to spend a modest amount of money (for a government or corporation) to hack you. Attacks on these keys will only become easier over time.

It is highly recommended that you no longer use SHA-1 RSA keys for any reason. You should switch to a more secure key type as soon as possible.

PuTTY/PPK

Keys in the PuTTY format (PPK) are not supported. If you have a PuTTY key, you can convert it to OpenSSH/PEM by following these instructions under the Dealing with Private Keys in Other Formats section.

The Present

We’re using an SSH library based on libssh2 and OpenSSL. This library, used in Nova, Transmit 5, Prompt, Coda 2, Transmit iOS, and Code Editor, currently supports the following:

KexAlgorithms

ecdh-sha2-nistp256
ecdh-sha2-nistp384
ecdh-sha2-nistp521
curve25519-sha256
curve25519-sha256@libssh.org
diffie-hellman-group-exchange-sha1
diffie-hellman-group14-sha1
diffie-hellman-group1-sha1
diffie-hellman-group-exchange-sha256
diffie-hellman-group14-sha256
diffie-hellman-group16-sha512
diffie-hellman-group18-sha512

Ciphers

chacha20-poly1305@openssh.com
aes128-ctr
aes192-ctr
aes256-ctr
aes128-cbc
aes192-cbc
aes256-cbc
blowfish-cbc
arcfour
arcfour128
cast128-cbc
3des-cbc

MACs

sha2-512-etm@openssh.com
sha2-512
sha2-256-etm@openssh.com
sha2-256
sha1
sha1 96
ripemd160

Beyond what libssh2 includes, we’ve added support for EtM and ChaCha20Poly1305. We’ll continue to refine and improve this library, and push our changes to the upstream libssh2 project.

Legacy releases

Transmit 4 and Coda 1 used the OpenSSH library built-in to Mac OS X. This means key support in Transmit 4 and Coda 1 is limited to what the OS-provided library supports.

Host Key Verification

The first time you connect to a server, we keep a local copy of the key the server uses to identify itself. On future connections, we can use this stored key to verify that the server we’re connecting to now is the same one we’ve connected to before. Without host key verification, we’d be vulnerable to man-in-the-middle attacks.

If an app warns that the host key has changed, it means this server’s key looks different from the key we stored the first time we connected to this server. If this is unexpected, you should reject the changed key, cease connecting to this server, and contact your server administrator.

In Transmit 5, Nova, Prompt, and Code Editor, the host key fingerprint is displayed the first time you connect to a new server.

In Coda, Transmit iOS, and older versions of Transmit, the host key is blindly accepted on first connection. This is generally fine, but it’s something to be aware of if you’re on an untrusted local network.

To view the host key fingerprint used in Nova, Prompt for macOS, or Transmit, open the file ~/.ssh/known_hosts and find the line that corresponds to your server. If you need to reset the host key for a server, just remove the entire line for that server from the known_hosts file.

In Code Editor and Prompt for iOS, you can view the fingerprint at any time from the server settings.

Advanced Features

Prompt, as well as the terminals in Nova and Code Editor support agent forwarding. Coda, Transmit, and Transmit iOS do not.

Port forwarding, X11 forwarding, and ProxyCommand are not currently supported.

App-Specific Notes

Transmit 5

In the latest version of Transmit we’ve added the ability to store keys right in Transmit itself. Additionally, Transmit 5 still supports keys defined in in your config file.

For a more comprehensive overview of the many ways Transmit 5 can be configured to use key-based authentication please see Transmit 5 SFTP Authentication.

Prompt

When creating a new server connection, tap the key icon next to the password field to choose a private key. If the key is encrypted with a passphrase, you can enter it when choosing the key. If you do not enter the passphrase, you will be prompted for it whenever you connect to this server.

Important! If you want to use a key with a passphrase for agent forwarding, you must enter the passphrase when adding the key to the server connection.

You can view, import, and create keys in the Keys pane of Prompt’s Settings.

To add a key for use in Prompt, open the Settings pane, tap Keys, then tap the + button at the top right of the Keys pane. You can choose to either Generate a new key, or Import an existing key.

Generate New Key

To generate a new key, tap the + button on the Keys pane of Prompt’s settings and choose Generate New Key. Choose a descriptive name for your key, and optionally set a passphrase. Choose your key type, and size. Then tap “Generate” to create your keypair. Once it’s finished generating, tap Copy Public Key to put the public key on your pasteboard. We’ll use it in the next step.

Now that you have your keypair, you’ll want to put the public key on the remote server. Usually this means connecting with a username and password one last time. Once connected, navigate into ~/.ssh/ on the remote server and look for a file called authorized_keys or authorized_keys2. Open that file in a text editor, and paste the public key onto the end of the file.

Copy from Clipboard

To import a key from the iOS Clipboard, first select and copy the entire contents of the private key to the pasteboard. After the private key is on the Clipboard, go to Prompt’s Settings, tap Keys, then tap the + button and choose Copy from Clipboard. If your key is in a valid and supported format – and if it’s the private key, not the public key – Prompt will import the key for you.

Import from iTunes

Use iTunes File Sharing to import your private key. Note that Prompt does not support importing arbitrary files via iTunes File Sharing; this only works for keys.

Agent Forwarding

To enable agent forwarding in Prompt, toggle the Agent Forwarding switch in the Server settings. If your key uses a passphrase, you’ll need to have entered it when you added the key to the server entry.

~/.ssh/config

In Prompt for Mac it is also possible to use SSH keys stored on your computer in addition to those saved within the app itself. External keys will be used when they are stored in ~/.ssh/ and specified in your ~/.ssh/config file, or after they have been added to the system ssh-agent.

Coda

In the Server pane of Coda’s Site configuration sheet, there is a button with a key icon to the right of the password field. This button opens a file picker that allows you to choose a private key to use when connecting to this server. Coda automatically attempts to use any keys it finds in your .ssh folder.

When choosing a key via this button, Coda will attempt to verify the format of the key to make sure that it’s valid and supported.

If your key is encrypted with a passphrase, Coda’s key-chooser will be unable to verify it. See the config file workaround below.

If you’ve specified an encrypted key for use with this server in your config file, you can leave the key button alone and put the passphrase in Coda’s password field.

The Terminal, Source Control, and MySQL functions in Coda also support keys, but you will need to add your key to the config file.

Transmit 4

When connecting to an SFTP server, there is a button with a key icon to the right of the password field. This button works in much the same way as the same button in Coda: it opens a file picker that allows you to choose a private key for use when connecting to this server. Transmit will automatically attempt to use any keys it finds in your .ssh folder.

When choosing a key via this button, Transmit will attempt to verify the format of the key to make sure that it’s valid and supported.

If your key is encrypted with a passphrase, Transmit’s key-chooser will be unable to verify it. See the config file workaround below.

If you’ve specified an encrypted key for use with this server in your config file, you can leave the key button alone and put the passphrase in Transmit’s password field.

Code Editor

When creating a remote server connection in a new Site, tap the key icon next to the password field to choose a private key. If the key is encrypted with a passphrase, you can enter it when choosing the key. If you do not enter the passphrase, you will be prompted for it whenever you connect to this server.

Important! If you want to use a key with a passphrase for agent forwarding, you must enter the passphrase when adding the key to the server connection.

You can view, import, and create keys in the Keys pane of Code Editor’s Settings.

To add a key for use in Code Editor, open the Settings pane, tap Keys, then tap the + button at the top right of the Keys pane. You can choose to either Generate a new key, or Import an existing key.

Generate New Key

To generate a new key, tap the + button on the Keys pane of Code Editor’s settings and choose Generate New Key. Choose a descriptive name for your key, and optionally set a passphrase. Choose your key type, and size. Then tap “Generate” to create your keypair. Once it’s finished generating, tap Copy Public Key to put the public key on your pasteboard. We’ll use it in the next step.

Now that you have your keypair, you’ll want to put the public key on the remote server. Usually this means this means connecting with a username and password one last time. Once connected, navigate into ~/.ssh/ on the remote server and look for a file called authorized_keys or authorized_keys2. Open that file in a text editor, and paste the public key onto the end of the file.

Import From Pasteboard

To import a key from the iOS Pasteboard, first select and copy the entire contents of the private key to the pasteboard. After the private key is on the pasteboard, go to Code Editor’s Settings, tap Keys, then tap the + button and choose Import from Pasteboard. If your key is in a valid and supported format – and if it’s the private key, not the public key – Coda will import the key for you.

Import from Local

Use this option to import a private key from the Local file storage on your iOS device. One example where this is useful is if you’ve got your private key on your Mac. Use Code Editor to connect to your Mac on the same local network, then transfer the key into Code Editor’s Local file storage. Once the key is in Code Editor’s local file storage, it can be imported for use.

Import from iTunes

Use iTunes File Sharing to import your private key. Note that Code Editor does not support importing arbitrary files via iTunes File Sharing; this only works for keys.

Agent Forwarding

To enable agent forwarding in Code Editor, toggle the Agent Forwarding switch in the Terminal pane of the Site’s settings. If your key uses a passphrase, you’ll need to have entered it when you added the key to the server entry.

Transmit iOS

When creating a new server connection, tap the key icon next to the password field to choose a private key. If the key is encrypted with a passphrase, you can enter it when choosing the key. If you do not enter the passphrase, you will be prompted for it whenever you connect to this server.

You can view, import, and create keys in the Keys pane of Transmit’s Settings.

To add a key for use in Transmit, open the Settings pane, tap Keys, then tap the + button at the top right of the Keys pane. You can choose to either Generate a new key, or Import an existing key.

Generate New Key

To generate a new key, tap the + button on the Keys pane of Transmit’s settings and choose Generate New Key. Choose a descriptive name for your key, and optionally set a passphrase. Choose your key type (we recommend RSA), and size (we recommend 2048 or 4096). Then tap “Generate” to create your keypair. Once it’s finished generating, tap Copy Public Key to put the public key on your pasteboard. We’ll use it in the next step.

Now that you have your keypair, you’ll want to put the public key on the remote server. Usually this means this means connecting with a username and password one last time. Once connected, navigate into ~/.ssh/ on the remote server and look for a file called authorized_keys or authorized_keys2. Open that file in a text editor, and paste the public key onto the end of the file.

Import From Pasteboard

To import a key from the iOS Pasteboard, first select and copy the entire contents of the private key to the pasteboard. After the private key is on the pasteboard, go to Transmit’s Settings, tap Keys, then tap the + button and choose Import from Pasteboard. If your key is in a valid and supported format – and if it’s the private key, not the public key – Transmit will import the key for you.

Import from Local

Use this option to import a private key from the Local file storage on your iOS device. One example where this is useful is if you’ve got your private key on your Mac. Use Transmit to connect to your Mac on the same local network, then transfer the key into Transmit’s Local file storage. Once the key is in Transmit’s local file storage, it can be imported for use in Transmit.

Import from iTunes

Use iTunes File Sharing to import your private key. Note that Transmit does not support importing arbitrary files via iTunes File Sharing; this only works for keys.

Troubleshooting

Why does it say my key is not in a supported format?

The most common reason you’d see this error is if you select a passphrase-encrypted key via the key chooser button in either Coda or Transmit 4 or older on macOS. Coda and Transmit want to validate the key before letting you use it, but the encryption prevents that from happening. This should no longer be an issue in Nova, Transmit 5, and Prompt.

As a workaround, add your key to the ~/.ssh/config file, skip the key button altogether, and put the passphrase in the password field.

You’ll also get this error if you use a key in an unsupported format, such as a PuTTy key. Make sure you’re using a supported key.

Why can’t I import my key from the pasteboard?

Most of the time this is a format issue. Are you sure you’re using a supported key?

Double-check that it’s the private key, not the public key. They look different (see the above sections on each), so it should be easy to tell.

One particularly nasty gotcha to watch out for involves the text substitution feature of macOS. For example, let’s say you copy and paste the contents of your private key somewhere easily accessible from your iOS device. You might notice that macOS has helpfully changed runs of hyphens (----) into em-dashes (––).

Your private key used to look like this:

-----BEGIN RSA PRIVATE KEY-----
MIIEowIBAAKCAQEAy1jdr+ez1DWbELFFHUR2LYegzv2K5DKFrP8FXOLBNoPvH5nr
seh/EqkCgYEA6iSdXnky6ilRQe2V5e1SepzFFW4MqS9tZUyLfT+c2CS/CKjv0Xj0
<snip>

But it now looks like this:

–––BEGIN RSA PRIVATE KEY–––
MIIEowIBAAKCAQEAy1jdr+ez1DWbELFFHUR2LYegzv2K5DKFrP8FXOLBNoPvH5nr
seh/EqkCgYEA6iSdXnky6ilRQe2V5e1SepzFFW4MqS9tZUyLfT+c2CS/CKjv0Xj0
<snip>

It’s a subtle difference, but it’s enough to break your key. Watch out!