CIA

Overview

CIA stands for CTR Importable Archive. This format allows the installation titles to the 3DS. CIA files and titles on Nintendo's CDN contain identical data. As a consiquence, valid CIA files can be generated from CDN content. This also means CIA files can contain anything, titles on Nintendo's CDN can contain.

Under normal circumstances CIA files are used where downloading a title is impracticle or not possible. Such as distributing a Download Play child, or installing forced Gamecard updates. Those CIA(s) are stored by the titles in question, in an auxiliary CFA file.

Development Units, are capable of manually installing CIA files via the Dev Menu.

An sample (developer) CIA can be downloaded here Credit: Jl12. It includes a .cia file, with everything is decrypted/extracted. It also includes some screenshots, as well as a copy of the directory where the title was installed.

Format

This is the current version of the CIA format, it was finalised in late 2010. (Older versions of the CIA format can be viewed on the Talk page)

The CIA format has a similar structure to the WAD format.

The file is represented in little-endian.

The data is aligned in 64 byte blocks (if a content ends at the middle of the block, the next content will begin from a new block).

CIA Header

START	SIZE	DESCRIPTION
0x00	0x04	Archive Header Size (Usually = 0x2020 bytes)
0x04	0x02	Type
0x06	0x02	Version
0x08	0x04	Certificate chain size
0x0C	0x04	Ticket size
0x10	0x04	TMD file size
0x14	0x04	Meta size (0 if no Meta data is present)
0x18	0x08	Content size
0x20	0x01	Magic? Must be = 0x80
0x21	0x1FFF	Magic? Must be Zero Filled

The order of the sections in the CIA file:

• certificate chain
• Ticket
• TMD file data
• APP file data
• Meta file data (Not a necessary component)

The APP data (NCCH/SRL) is encrypted, using 128-bit AES-CBC. The encryption uses the decrypted titlekey of the ticket, and the titleid padded with zeros as the IV. To get the decrypted titlekey, the titlekey stored in the ticket must be decrypted using 128-bit AES-CBC with the 3DS common key, and the same IV as mentioned previously.

Meta

The structure of this data is as follows:

START	SIZE	DESCRIPTION
0x00	0x180	Title ID dependency list - Taken from the application's ExHeader
0x180	0x280	Reserved/Unused
0x400	0x36C0	Icon Data(.ICN) - Taken from the application's ExeFS

Obviously this section is not present in TWL CIA files, or any other CIA file which does not contain a CXI.

Tools

• ctrtool - Reading/Extraction of CIA files. This can only decrypt the title-key for development CIAs, since retail CIAs use the AES hardware key-scrambler for the common-key keyslot.

Title Key Encryption

The unencrypted Title Key is used to encrypt the data in a CIA. The encrypted Title Key of a CIA can be found at offset 0x1BF in a CIA's Ticket. Each Title Key is encrypted with AES-CBC to get the encrypted Title Key.

To encrypt an unencrypted title key, you need:

• Common key (as byte array)
• Title ID (as ulong)
• (and of course the unencrypted title key you want to encrypt) (as byte array)

The title key encryption process starts by converting the ulong (Title ID) into a byte array using by retrieving the bytes of the Title ID using BitConverter.GetBytes(). If the converted bytes (title ID) are in Little Endian, reverse those bytes. (in C# it would be Array.Reverse(byte_array_from_bitconverter)) This process makes the Title Key encryption IV.

Next, after you've gotten your Title Key's IV, you can start your cryptography transformation. Using AESManaged, where:

Key = Common Key

IV = the byte array found in the conversion process above

Mode = CipherMode.CBC

Create the encryptor (AesManaged.CreateEncryptor(key, iv)) where the key and IV are both the same as above.

Then, create a CryptoStream and a MemoryStream. The Crypto stream should start with the arguments (memorystream, aes_transform_from_above, CryptoStreamMode.Write).

Write to the CryptoStream where buffer=unencrypted_titlekey, offset=0, and count=the length of the unencrypted title key.

Use FlushFinalBlock() on the CryptoStream.

Finally, then, the encrypted title key will be available from your memory stream. (to output the calculated encrypted title key as a byte array, you can use memorystream.ToArray(), for example)

Example function: (C#)

        public static byte[] EncryptMyTitleKey(byte[] commonKey, byte[] titleKey, ulong titleId)
        {
            // Make encryption IV
            byte[] titleidasbytes = new byte[0x10];
            for (int i = 0; i < 0x10; i++)
            {
                titleidasbytes[i] = 0;
            }
            byte[] bitBytes = BitConverter.GetBytes(titleId);
            if (BitConverter.IsLittleEndian)
            {
                Array.Reverse(bitBytes);
            }
            bitBytes.CopyTo(titleidasbytes, 0);
            // Encrypt
            ICryptoTransform transform = new AesManaged { Key = commonKey, IV = titleidasbytes, Mode = CipherMode.CBC }.CreateEncryptor(commonKey, titleidasbytes);
            MemoryStream memstream = new MemoryStream();
            CryptoStream cryptostream = new CryptoStream(memstream, transform, CryptoStreamMode.Write);
            cryptostream.Write(titleKey, 0, titleKey.Length);
            cryptostream.FlushFinalBlock();
            return memstream.ToArray();
        }