Register + mount the GPG secrets engine at gpg/ (#87)

Complete the deploy of the [vault-plugin-secrets-gpg](https://git.unkin.net/unkin/vault-plugin-secrets-gpg) engine. Puppet ([puppet-prod #480](unkin/puppet-prod#480)) installs the `openbao-plugin-secrets-gpg` RPM onto the OpenBao nodes; this registers that binary in the plugin catalog and enables the secrets engine so `gpg/` is actually usable.

- Add a `gpg_secret_backend` module using the standard `hashicorp/vault` provider (already required at 5.6.0): `vault_plugin` (catalog register with a pinned sha256) + `vault_mount` (enable at the mount path).
- Wire it through `vault_cluster` (new `gpg_secret_backend` variable + module block) and the config discovery (`config.hcl` group + syd1 terragrunt input), mirroring `litellm_secret_backend`.
- Add `config/gpg_secret_backend/gpg.yaml` mounting at `gpg/` and pinning the released v0.1.0 binary sha256 (`0e92d740…a7b20`, extracted from the published RPM). Puppet installs the RPM floating, so this sha must be bumped in lockstep on any plugin upgrade or OpenBao rejects the binary.

Validated locally with `tofu validate` + `tofu fmt`. Granting non-root access to `gpg/*` (auth roles + policies) is a follow-up scoped to whoever consumes the engine (e.g. passv from CI).

Reviewed-on: #87
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

modules/vault_cluster/main.tf

@@ -346,6 +346,34 @@ module "plugin" {
   plugin_version = each.value.version
 }
 
+module "gpg_secret_backend" {
+  source = "./modules/gpg_secret_backend"
+
+  for_each = var.gpg_secret_backend
+
+  path        = each.key
+  plugin      = each.value.plugin
+  description = each.value.description
+
+  depends_on = [module.plugin]
+}
+
+module "gpg_key" {
+  source = "./modules/gpg_key"
+
+  for_each = var.gpg_key
+
+  backend                = each.value.backend
+  name                   = each.value.name
+  algorithm              = each.value.algorithm
+  identity               = each.value.identity
+  exportable             = each.value.exportable
+  deletion_allowed       = each.value.deletion_allowed
+  min_decryption_version = each.value.min_decryption_version
+
+  depends_on = [module.gpg_secret_backend]
+}
+
 module "vault_policy" {
   source = "./modules/vault_policy"
 

modules/vault_cluster/modules/gpg_key/main.tf

@@ -0,0 +1,12 @@
+# Manages an OpenPGP key inside a gpg secrets engine mount, via the
+# gpgvaultsecret provider. The private key never leaves Vault; consumers use the
+# exported public_key to encrypt and delegate decryption back to the engine.
+resource "gpg_key" "this" {
+  backend                = var.backend
+  name                   = var.name
+  algorithm              = var.algorithm
+  identity               = var.identity
+  exportable             = var.exportable
+  deletion_allowed       = var.deletion_allowed
+  min_decryption_version = var.min_decryption_version
+}

modules/vault_cluster/modules/gpg_key/terraform.tf

@@ -0,0 +1,9 @@
+terraform {
+  required_version = ">= 1.10"
+  required_providers {
+    gpg = {
+      source  = "artifactapi.k8s.syd1.au.unkin.net/terraform-unkin/gpgvaultsecret"
+      version = "0.1.0"
+    }
+  }
+}

modules/vault_cluster/modules/gpg_key/variables.tf

@@ -0,0 +1,39 @@
+variable "backend" {
+  description = "Mount path of the gpg secrets engine (e.g. \"gpg\")"
+  type        = string
+}
+
+variable "name" {
+  description = "Name of the key"
+  type        = string
+}
+
+variable "algorithm" {
+  description = "Key algorithm: rsa-2048, rsa-3072, rsa-4096 or ed25519"
+  type        = string
+  default     = "rsa-3072"
+}
+
+variable "identity" {
+  description = "OpenPGP User ID (defaults to the key name)"
+  type        = string
+  default     = null
+}
+
+variable "exportable" {
+  description = "Allow exporting the private key (enable-only)"
+  type        = bool
+  default     = false
+}
+
+variable "deletion_allowed" {
+  description = "Whether the key may be deleted"
+  type        = bool
+  default     = false
+}
+
+variable "min_decryption_version" {
+  description = "Minimum key version usable for decryption/verification"
+  type        = number
+  default     = null
+}

modules/vault_cluster/modules/gpg_secret_backend/main.tf

@@ -0,0 +1,8 @@
+# Mounts the gpg secrets engine. The plugin is registered ("imported") in the
+# catalog separately via the config/plugins/ discovery and the plugin module;
+# this module just enables a mount of the already-registered plugin type.
+resource "vault_mount" "this" {
+  path        = var.path
+  type        = var.plugin
+  description = var.description
+}

modules/vault_cluster/modules/gpg_secret_backend/terraform.tf

@@ -0,0 +1,9 @@
+terraform {
+  required_version = ">= 1.10"
+  required_providers {
+    vault = {
+      source  = "hashicorp/vault"
+      version = "5.6.0"
+    }
+  }
+}

modules/vault_cluster/modules/gpg_secret_backend/variables.tf

@@ -0,0 +1,16 @@
+variable "path" {
+  description = "Mount path of the GPG secrets engine (e.g. \"gpg\")"
+  type        = string
+}
+
+variable "plugin" {
+  description = "Registered plugin name to mount (the catalog name = mount type)"
+  type        = string
+  default     = "vault-plugin-secrets-gpg"
+}
+
+variable "description" {
+  description = "Human-friendly description of the mount"
+  type        = string
+  default     = null
+}

modules/vault_cluster/variables.tf

@@ -327,6 +327,29 @@ variable "plugins" {
   default = {}
 }
 
+variable "gpg_secret_backend" {
+  description = "Map of GPG/OpenPGP secret engines to mount (path => registered plugin + description). The plugin is registered separately via config/plugins."
+  type = map(object({
+    plugin      = optional(string, "vault-plugin-secrets-gpg")
+    description = optional(string)
+  }))
+  default = {}
+}
+
+variable "gpg_key" {
+  description = "Map of OpenPGP keys to manage in a gpg engine mount"
+  type = map(object({
+    name                   = string
+    backend                = string
+    algorithm              = optional(string, "rsa-3072")
+    identity               = optional(string)
+    exportable             = optional(bool, false)
+    deletion_allowed       = optional(bool, false)
+    min_decryption_version = optional(number)
+  }))
+  default = {}
+}
+
 variable "policy_auth_map" {
   description = "Map of auth mounts -> auth roles -> policy names"
   type        = map(map(list(string)))