Master fixing 560 (#728)

* Remove those credential files that cannot be decrypted.

* Adding comments, handler for IOException.

GoogleCloudExtension/GoogleCloudExtension/Accounts/WindowsCredentialsStore.cs

@@ -17,6 +17,7 @@
 using GoogleCloudExtension.GCloud;
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.IO;
 using System.Linq;
 using System.Security.Cryptography;
@@ -75,6 +76,7 @@ public IEnumerable<WindowsInstanceCredentials> GetCredentialsForInstance(Instanc
                 result = Directory.EnumerateFiles(instanceStoragePath)
                     .Where(x => Path.GetExtension(x) == PasswordFileExtension)
                     .Select(x => LoadEncryptedCredentials(x))
+                    .Where(x => x != null)
                     .OrderBy(x => x.User);
             }
             _credentialsForInstance[instancePath] = result;
@@ -125,13 +127,43 @@ public string GetStoragePathForInstance(Instance instance)
             return Path.Combine(s_credentialsStoreRoot, instancePath);
         }
 
+        /// <summary>
+        /// Attempts to load and decrypt the credentials stored in <paramref name="path"/> and returns an
+        /// <seealso cref="WindowsInstanceCredentials"/> instance with the information stored in the file. If
+        /// the file cannot be loaded or decrypted it will return null.
+        /// 
+        /// Note: The function will attempt to delete the file if it cannot be decrypted, this typically means that
+        /// the user's key is no longer valid. The function does not attempt to delete the file in case of a <see cref="IOException"/>
+        /// since that will probably also throw again.
+        /// </summary>
         private WindowsInstanceCredentials LoadEncryptedCredentials(string path)
         {
-            var userName = GetUserName(path);
-            var encryptedPassword = File.ReadAllBytes(path);
-            var passwordBytes = ProtectedData.Unprotect(encryptedPassword, null, DataProtectionScope.CurrentUser);
+            try
+            {
+                var userName = GetUserName(path);
+                var encryptedPassword = File.ReadAllBytes(path);
+                var passwordBytes = ProtectedData.Unprotect(encryptedPassword, null, DataProtectionScope.CurrentUser);
 
-            return new WindowsInstanceCredentials { User = userName, Password = Encoding.UTF8.GetString(passwordBytes) };
+                return new WindowsInstanceCredentials { User = userName, Password = Encoding.UTF8.GetString(passwordBytes) };
+            }
+            catch (CryptographicException)
+            {
+                Debug.WriteLine($"Failed to decrypt credentials from: {path}");
+                try
+                {
+                    File.Delete(path);
+                }
+                catch (IOException)
+                {
+                    Debug.WriteLine($"Failed cleaning corrupted credentials {path}");
+                }
+                return null;
+            }
+            catch (IOException)
+            {
+                Debug.WriteLine($"Failed to load credentials from: {path}");
+                return null;
+            }
         }
 
         private void SaveEncryptedCredentials(string path, WindowsInstanceCredentials credentials)