Add descriptions for unavailable keys (#708)

* Describe unavailable key types

* Cleanup

Sources/Packages/Resources/Localizable.xcstrings

@@ -5833,6 +5833,17 @@
         }
       }
     },
+    "create_secret_key_type_macOS_update_required_label" : {
+      "extractionState" : "manual",
+      "localizations" : {
+        "en" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Unavailable on this version of macOS"
+          }
+        }
+      }
+    },
     "create_secret_mldsa_warning" : {
       "extractionState" : "manual",
       "localizations" : {

Sources/Packages/Sources/SecretKit/Erasers/AnySecretStore.swift

@@ -64,7 +64,7 @@ public final class AnySecretStoreModifiable: AnySecretStore, SecretStoreModifiab
     private let _create: @Sendable (String, Attributes) async throws -> AnySecret
     private let _delete: @Sendable (AnySecret) async throws -> Void
     private let _update: @Sendable (AnySecret, String, Attributes) async throws -> Void
-    private let _supportedKeyTypes: @Sendable () -> [KeyType]
+    private let _supportedKeyTypes: @Sendable () -> KeyAvailability
 
     public init<SecretStoreType>(_ secretStore: SecretStoreType) where SecretStoreType: SecretStoreModifiable {
         _create = { AnySecret(try await secretStore.create(name: $0, attributes: $1)) }
@@ -87,7 +87,7 @@ public final class AnySecretStoreModifiable: AnySecretStore, SecretStoreModifiab
         try await _update(secret, name, attributes)
     }
 
-    public var supportedKeyTypes: [KeyType] {
+    public var supportedKeyTypes: KeyAvailability {
         _supportedKeyTypes()
     }
 

Sources/Packages/Sources/SecretKit/Types/SecretStore.swift

@@ -62,10 +62,37 @@ public protocol SecretStoreModifiable<SecretType>: SecretStore {
     ///   - attributes: The new attributes for the secret.
     func update(secret: SecretType, name: String, attributes: Attributes) async throws
 
-    var supportedKeyTypes: [KeyType] { get }
+    var supportedKeyTypes: KeyAvailability { get }
 
 }
 
+public struct KeyAvailability: Sendable {
+
+    public let available: [KeyType]
+    public let unavailable: [UnavailableKeyType]
+
+    public init(available: [KeyType], unavailable: [UnavailableKeyType]) {
+        self.available = available
+        self.unavailable = unavailable
+    }
+
+    public struct UnavailableKeyType: Sendable {
+        public let keyType: KeyType
+        public let reason: Reason
+
+        public init(keyType: KeyType, reason: Reason) {
+            self.keyType = keyType
+            self.reason = reason
+        }
+
+        public enum Reason: Sendable {
+            case macOSUpdateRequired
+        }
+    }
+
+}
+
+
 extension NSNotification.Name {
 
     // Distributed notification that keys were modified out of process (ie, that the management tool added/removed secrets)

Sources/Packages/Sources/SecureEnclaveSecretKit/SecureEnclaveStore.swift

@@ -186,17 +186,22 @@ extension SecureEnclave {
             await reloadSecrets()
         }
         
-        public var supportedKeyTypes: [KeyType] {
-            if #available(macOS 26, *) {
-                [
-                    .ecdsa256,
-                    .mldsa65,
-                    .mldsa87,
-                ]
+        public let supportedKeyTypes: KeyAvailability = {
+            let macOS26Keys: [KeyType] = [.mldsa65, .mldsa87]
+            let isAtLeastMacOS26 = if #available(macOS 26, *) {
+                true
             } else {
-                [.ecdsa256]
+                false
             }
-        }
+            return KeyAvailability(
+                available: [
+                    .ecdsa256,
+                ] + (isAtLeastMacOS26 ? macOS26Keys : []),
+                unavailable: (isAtLeastMacOS26 ? [] : macOS26Keys).map {
+                    KeyAvailability.UnavailableKeyType(keyType: $0, reason: .macOSUpdateRequired)
+                }
+            )
+        }()
     }
 
 }