Add a storage manager, and make .gitignore ignore emacs backups.

.gitignore

@@ -9,3 +9,4 @@ DerivedData/
 .netrc
 WalletSdk.xcodeproj
 build/
+*~

.swiftlint.yml

@@ -1,5 +1,6 @@
 excluded:
   - .build
 disabled_rules:
+  - colon
   - cyclomatic_complexity
   - todo

Sources/WalletSdk/PlatformContext.swift

@@ -0,0 +1,25 @@
+// File: PlatformContext
+//
+//    Store and retrieve sensitive data.  Data is stored in the Application Support directory of the app, encrypted in place
+// via the .completeFileProtection option, and marked as excluded from backups so it will not be included in iCloud backps.
+
+//
+// Imports
+//
+
+import Foundation
+
+//
+// Code
+//
+
+// Class: SpruceKitPlatformContext
+//    A container for platform-specific subsystems.
+
+class SpruceKitPlatformContext: NSObject {
+   let storageMgr = StorageManager() // Secure storage.
+}
+
+//
+// Copyright © 2024, Spruce Systems, Inc.
+//

Sources/WalletSdk/StorageManager.swift

@@ -0,0 +1,146 @@
+// File: Storage Manager
+//
+//    Store and retrieve sensitive data.  Data is stored in the Application Support directory of the app, encrypted in place
+// via the .completeFileProtection option, and marked as excluded from backups so it will not be included in iCloud backps.
+
+//
+// Imports
+//
+
+import Foundation
+
+//
+// Code
+//
+
+// Class: StorageManager
+//    Store and retrieve sensitive data.
+
+class StorageManager: NSObject {
+   // Local-Method: path()
+   //    Get the path to the application support dir, appending the given file name to it.  We use the application support
+   // directory because its contents are not shared.
+   //
+   // Arguments:
+   //    file - the name of the file
+   //
+   // Returns:
+   //    An URL for the named file in the app's Application Support directory.
+
+   private func path(file: String) -> URL? {
+      do {
+         //    Get the applications support dir, and tack the name of the thing we're storing on the end of it.  This does
+         // imply that `file` should be a valid filename.
+
+         let asdir = try FileManager.default.url(for:            .applicationSupportDirectory,
+                                                 in:             .userDomainMask,
+                                                 appropriateFor: nil,  // Ignored
+                                                 create:         true) // May not exist, make if necessary.
+
+         return asdir.appendingPathComponent(file)
+      } catch { // Did the attempt to get the application support dir fail?
+         print("Failed to get/create the application support dir.")
+         return nil
+      }
+   }
+
+   // Method: add()
+   //    Store a value for a specified key, encrypted in place.
+   //
+   // Arguments:
+   //     key   - the name of the file
+   //     value - the data to store
+   //
+   // Returns:
+   //    A boolean indicating success.
+
+   func add(key: String, value: Data) -> Bool {
+      guard let file = path(file: key) else { return false }
+
+      do {
+         try value.write(to: file, options: .completeFileProtection)
+      } catch {
+         print("Failed to write the data for '\(key)'.")
+         return false
+      }
+
+      return true
+   }
+
+   // Method: get()
+   //    Get a value for the specified key.
+   //
+   // Arguments:
+   //    key - the name associated with the data
+   //
+   // Returns:
+   //    Optional data potentially containing the value associated with the key; may be `nil`.
+
+   func get(key: String) -> Data? {
+      guard let file = path(file: key) else { return nil }
+
+      do {
+         let d = try Data(contentsOf: file)
+         return d
+      } catch {
+         print("Failed to read '\(file)'.")
+      }
+
+      return nil
+   }
+
+   // Method: remove()
+   //    Remove a key/value pair. Removing a nonexistent key/value pair is not an error.
+   //
+   // Arguments:
+   //    key - the name of the file
+   //
+   // Returns:
+   //    A boolean indicating success; at present, there is no failure path, but this may change in the future.
+
+   func remove(key: String) -> Bool {
+      guard let file = path(file: key) else { return true }
+
+      do {
+         try FileManager.default.removeItem(at: file)
+      } catch {
+         // It's fine if the file isn't there.
+      }
+
+      return true
+   }
+
+   // Method: sys_test()
+   //    Check to see if everything works.
+
+   func sys_test() {
+      let key   = "test_key"
+      let value = Data("Some random string of text. 😎".utf8)
+
+      if !add(key: key, value: value) {
+         print("\(self.classForCoder):\(#function): Failed add() key/value pair.")
+         return
+      }
+
+      guard let payload = get(key: key) else {
+         print("\(self.classForCoder):\(#function): Failed get() value for key.")
+         return
+      }
+
+      if !(payload == value) {
+         print("\(self.classForCoder):\(#function): Mismatch between stored & retrieved value.")
+         return
+      }
+
+      if !remove(key: key) {
+         print("\(self.classForCoder):\(#function): Failed to delete key/value pair.")
+         return
+      }
+
+      print("\(self.classForCoder):\(#function): Completed successfully.")
+   }
+}
+
+//
+// Copyright © 2024, Spruce Systems, Inc.
+//