Textastic should do the same as your command line. Textastic uses libssh2 instead of OpenSSH, but that should not matter.

Here are a few things we could try:

1. Please send me a screenshot of your connection's settings in Textastic. Maybe I can spot a problem.

2. You could generate a new public/private key file pair and send the two files to me. I could then try to add it to my server and connect to it from Textastic to see if your keys work in general. I can then send you screenshots of my configuration.

You can send me the key files (e.g. id_rsa and id_rsa.pub) by email to support@textasticapp.com

3. It that still doesn't help, you could add the public key to your server and give me the login details. I can then try to connect to your server from Textastic on my device.

Can you successfully connect to your server from your Windows PC using the user name and the generated key file? Maybe with PuTTY or ssh in a WSL terminal?

I also think that the host name at the end of the line of the public key does not matter. 

Are you 100% sure that the user name is correct? Maybe there is an invisible character like space before or after the name? Please try to delete the user name and enter it again. 

Another idea: can you have a look at the SSH server logs?

Ok, I tried it with the following command line on my Mac:

ssh-keygen -m PEM -t rsa -b 4096

I saved the key as ~/ssh/id_rsa_test without a passphrase.

I transferred the key file "id_rsa_test" using AirDrop to my iPad and selected the Files app as the AirDrop target. I put the file into "On my iPad/Textastic/ssh/".

I added the contents of "id_rsa_test.pub" to ~/.ssh/authorized_keys on the server for the user I'm trying to log in as.

On my iPad in Textastic, I enabled public key auth for the connection and used "/ssh/id_rsa_test" as the private key file path.

I could then successfully connect to my server using the SSH terminal and using the SFTP connection.

Here is what you can try: go to the File Transfer screen, edit the connection and enable "Debug Log". Then try to connect to the server using SFTP and have a look at the log file.

Please have a look at this topic in the manual for details: https://www.textasticapp.com/v9/manual/remote_servers/sftp_ftps_ftp.html#public-key-authentication

It also has sections about the supported key file formats and key types.

It's not necessary anymore to use the PEM format. You can use the default key format.

Are you sure "ssh-keygen -m PEM -t rsa -b 409" is the command line you used? When I do that, I get the following error:

Invalid RSA key length: minimum is 1024 bits

You can also use more modern key types like ECDSA or ED25519 in the latest version of Textastic.

Textastic tries to open the files that were open the last time the app was used and it tries to restore the external folders at launch.

If a file was previously opened using the "Open…" command or a folder added using the "Add External Folder…" command, it is possible that the file provider hangs for some reason and opening the file or folder times out, resulting in a crash.

If the app crashes directly at launch, you can try the following:

• go to the Settings app
• scroll down to Textastic
• enable "Reset Recent Files"

I'm working on this feature right now and hope to include it in the next major update. 

Thanks a lot for your feedback! It's always good to hear what new users are missing.