Add-AzVMSshPublicKey to vmConfig fails when calling New-AzVM - windows

Question

I'm following the windows quickstart for creating a VM in azure powershell
I'm stuck here:
# Configure the SSH key
$sshPublicKey = cat ~/.ssh/id_rsa.pub
Add-AzVMSshPublicKey `
-VM $vmconfig `
-KeyData $sshPublicKey `
-Path "/home/azureuser/.ssh/authorized_keys"
First of all I think the following code is wrong, as cat returns System.String[] and running this verbatim results in
Add-AzVMSshPublicKey : Cannot convert 'System.Object[]' to the type 'System.String'
So... I instead use Get-Content "./path/to/file" -raw which just returns a string and the command runs without errors
Now when I run
New-AzVM `
-ResourceGroupName $resourceGroupName `
-Location $location -VM $vmConfig
I get the following error, meaning the keyData I set earlier wasn't set correctly.
New-AzVM : The value of parameter linuxConfiguration.ssh.publicKeys.keyData is invalid.
I've found the issue - So Azure key vault gives me a PEM public key in the form
-----BEGIN PUBLIC KEY-----
MIIBojANBgkqhkiG9w0BAQEFAAO...
...
...
...
...0CS94AFAgMBAAE=
-----END PUBLIC KEY-----
Whereas the VM is expecting it in OpenSSH format
ssh-rsa ..........
I've tried to convert it with
ssh-keygen -i -m PKCS8 -f ./key.pem
but nothing gets output
UPDATE
Aaaand it's a powershell issue

Answer 1

First, the tutorial is a guide to create Linux VM via Azure PowerShell, not Windows. Second, the command cat just outputs the content of the file. And command $sshPublicKey = cat ~/.ssh/id_rsa.pub creates a variable in string:
The parameter -KeyData of the command Add-AzVMSshPublicKey also expect a string:
So there is no problem with the PowerShell command and all the commands work fine on my side. And the error shows the value of the key data is invalid, what you need to do is to make sure if the SSH public key is no problem.

Answer 2

To get this key from an Azure Key vault,
Get-AzKeyVaultKey -OutFile *filename* returns a public key in PEM Form
-----BEGIN PUBLIC KEY------
....
-----END PUBLIC KEY-----
The vm requires the key data to be a one-liner in OpenSSH format
ssh-rsa ....... mykeylabel
But Powershell's ssh-keygen, unlike its UNIX counterpart, cannot convert between these formats as it has an open bug