Fix for some failed logins

Source/Framework/Cryptography/SRP6.cs

@@ -328,11 +328,13 @@ namespace Framework.Cryptography
             BigInteger x = new(xBytes, true, true);
             if ((xBytes[0] & 0x80) != 0)
             {
-                byte[] fix = new byte[65];
+                byte[] fix = [1, ..new byte[64]];
-                fix[64] = 1;
                 x -= new BigInteger(fix, true);
             }
 
+            if (x.Sign == -1)
+                return x += N - 1;
+
             return x % (N - 1);
         }