API key protection

src/isnd/Helpers/ApiKeyProtector.cs

@@ -0,0 +1,50 @@
+using System;
+using System.Security.Cryptography;
+using System.Text;
+using Microsoft.AspNetCore.DataProtection;
+
+namespace isnd.Helpers
+{
+    public static class ApiKeyProtector
+    {
+        private const byte LegacyDelta = 145;
+
+        public static string UnprotectWithFallback(IDataProtector protector, string protectedValue)
+        {
+            if (string.IsNullOrWhiteSpace(protectedValue))
+            {
+                return protectedValue;
+            }
+
+            try
+            {
+                return protector.Unprotect(protectedValue);
+            }
+            catch (CryptographicException)
+            {
+                return UnprotectLegacy(protectedValue);
+            }
+            catch (FormatException)
+            {
+                return UnprotectLegacy(protectedValue);
+            }
+        }
+
+        public static string UnprotectLegacy(string protectedValue)
+        {
+            if (string.IsNullOrWhiteSpace(protectedValue))
+            {
+                return protectedValue;
+            }
+
+            var bytes = Convert.FromBase64String(protectedValue);
+            var unprotectedBytes = new byte[bytes.Length];
+            for (var index = 0; index < bytes.Length; index++)
+            {
+                unprotectedBytes[index] = (byte)(bytes[index] ^ LegacyDelta);
+            }
+
+            return Encoding.UTF8.GetString(unprotectedBytes);
+        }
+    }
+}