Encryption/Decryption of Animal Crossing: New Leaf's Bell/Money values.

ACNL.cs

        private static uint DecryptACNLMoney(ulong money)
        {
            // Unpack 64-bit value into (u32, u16, u8, u8) values.
            var enc = (uint)(money & 0xFFFFFFFF);
            var adjust = (ushort)((money >> 32) & 0xFFFF);
            var shift_val = (byte)((money >> 48) & 0xFF);
            var chk = (byte)((money >> 56) & 0xFF);

            // Validate 8-bit checksum
            if ((((enc >> 0) + (enc >> 8) + (enc >> 16) + (enc >> 24) + 0xBA) & 0xFF) != chk) return 0;
            var left_shift = (byte)((0x1C - shift_val) & 0xFF);
            var right_shift = 0x20 - left_shift;
            // Handle error case: Invalid shift value.
            if (left_shift >= 0x20)
            {
                return 0 + (enc << right_shift) - (adjust + 0x8F187432);
            }
            // This case should occur for all game-generated values.
            return (enc << left_shift) + (enc >> right_shift) - (adjust + 0x8F187432);
        }

        private static ulong EncryptACNLMoney(uint dec)
        {
            // Make a new RNG
            var r = new Random(Guid.NewGuid().GetHashCode());
            // Generate random adjustment, shift values.
            var adjust = (ushort) r.Next(0x10000);
            var shift_val = (byte)r.Next(0x1A);

            // Encipher value
            var enc = dec + adjust + 0x8F187432;
            enc = (enc >> (0x1C - shift_val)) + (enc << (shift_val + 4));
            // Calculate Checksum
            var chk = (byte) (((enc >> 0) + (enc >> 8) + (enc >> 16) + (enc >> 24) + 0xBA) & 0xFF);
            // Pack result
            return ((ulong)enc << 0) | ((ulong)adjust << 32) | ((ulong)shift_val << 48) | ((ulong)chk << 56);
        }