Workaround to allow use of ed25519 keys on Azure

* Allow passing a dummy RSA key to Azure to satisfy its obtuse
requirements (recommend deleting the corresponding private key)
* Then `ssh_authorized_key` can be used to provide Fedora CoreOS
or Flatcar Linux with a modern ed25519 public key to set in the
authorized_keys via Ignition

azure/fedora-coreos/kubernetes/controllers.tf

@@ -1,3 +1,11 @@
+locals {
+  # Typhoon ssh_authorized_key supports RSA or a newer formats (e.g. ed25519).
+  # However, Azure requires an older RSA key to pass validations. To use a
+  # newer key format, pass a dummy RSA key as the azure_authorized_key and
+  # delete the associated private key so it's never used.
+  azure_authorized_key = var.azure_authorized_key == "" ? var.ssh_authorized_key : var.azure_authorized_key
+}
+
 # Discrete DNS records for each controller's private IPv4 for etcd usage
 resource "azurerm_dns_a_record" "etcds" {
   count               = var.controller_count
@@ -55,7 +63,7 @@ resource "azurerm_linux_virtual_machine" "controllers" {
   admin_username = "core"
   admin_ssh_key {
     username   = "core"
-    public_key = var.ssh_authorized_key
+    public_key = local.azure_authorized_key
   }
 
   lifecycle {

azure/fedora-coreos/kubernetes/variables.tf

@@ -82,6 +82,12 @@ variable "ssh_authorized_key" {
   description = "SSH public key for user 'core'"
 }
 
+variable "azure_authorized_key" {
+  type        = string
+  description = "Optionally, pass a dummy RSA key to satisfy Azure validations (then use an ed25519 key set above)"
+  default     = ""
+}
+
 variable "networking" {
   type        = string
   description = "Choice of networking provider (flannel, calico, or cilium)"

azure/fedora-coreos/kubernetes/workers.tf

@@ -17,6 +17,7 @@ module "workers" {
   # configuration
   kubeconfig            = module.bootstrap.kubeconfig-kubelet
   ssh_authorized_key    = var.ssh_authorized_key
+  azure_authorized_key  = var.azure_authorized_key
   service_cidr          = var.service_cidr
   cluster_domain_suffix = var.cluster_domain_suffix
   snippets              = var.worker_snippets

azure/fedora-coreos/kubernetes/workers/variables.tf

@@ -73,6 +73,12 @@ variable "ssh_authorized_key" {
   description = "SSH public key for user 'core'"
 }
 
+variable "azure_authorized_key" {
+  type        = string
+  description = "Optionally, pass a dummy RSA key to satisfy Azure validations (then use an ed25519 key set above)"
+  default     = ""
+}
+
 variable "service_cidr" {
   type        = string
   description = <<EOD

azure/fedora-coreos/kubernetes/workers/workers.tf

@@ -1,3 +1,7 @@
+locals {
+  azure_authorized_key = var.azure_authorized_key == "" ? var.ssh_authorized_key : var.azure_authorized_key
+}
+
 # Workers scale set
 resource "azurerm_linux_virtual_machine_scale_set" "workers" {
   resource_group_name = var.resource_group_name
@@ -22,7 +26,7 @@ resource "azurerm_linux_virtual_machine_scale_set" "workers" {
   admin_username = "core"
   admin_ssh_key {
     username   = "core"
-    public_key = var.ssh_authorized_key
+    public_key = var.azure_authorized_key
   }
 
   # network