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Encryption details
When a NAX0 file is recreated, the header and raw content in NAX0 are completely different except for the magicnum and size-field. This is the case when a title is redownloaded/reinstalled, and when "save/" files are recreated after manual deletion.
When a NAX0 file is recreated, the header and raw content in NAX0 are completely different except for the magicnum and size-field. This is the case when a title is redownloaded/reinstalled, and when "save/" files are recreated after manual deletion.
[[Error_codes|Error]] 0x250E02 is returned by NCM when attempting to access a NcaId where the NAX0 header was manually modified, for example: offset 0x0, 0x10, 0x28, 0x38, each with size 0x1.
[[Error_codes|Error]] 0x250E02 is returned by NCM when attempting to access a NcaId where the NAX0 header HMAC validation fails.
{| class="wikitable" border="1"
{| class="wikitable" border="1"
| 0x0
| 0x0
| 0x20
| 0x20
| Data appears to be random.
| Header HMAC
|-
|-
| 0x20
| 0x20
| 0x28
| 0x28
| 0x20
| 0x20
| Data appears to be random.
| Encrypted AES-XTS Keys
|-
|-
| 0x48
| 0x48
| 0x8?
| 0x8
| Size of the file relative to offset 0x4000.
| Size of the file relative to offset 0x4000.
|-
| 0x50
| 0x30
| Padding
|}
|}
= Encryption =
The actual content uses AES-XTS with the same non-standard tweak (endianness swap) as usual, and sector size 0x4000.
Key derivation is as follows:
First, FS retrieves the 16-byte console-unique seed that NS registered (data originates from the ns_systemseed:/ [[Flash_Filesystem#System_Savegames|system savegame]]). Next, FS calculates encryptedSeedKeys = <0x20 hardcoded keydata depending on if this NAX0 is a save or an NCA> XORed with the system seed, calculates kek = [[SPL_services#GenerateAesKek|GenerateAesKek]](<hardcoded keydata>, 0, 0), and then calculates decryptedSeedKeys by doing decryptedSeedKeys[i] = [[SPL_services#GenerateAesKey|GenerateAesKey]](kek, encryptedSeedKeys[i]); for both keys.
Once the decryptedSeedKeys are calculated, FS calculates naxSpecificKeys = hmac-sha256(key=decryptedSeedKeys[0], data=<NAX0 path, relative to /Nintendo/Contents>); (example path: '/registered/000000FF/cafebabecafebabecafebabecafebabe.nca'). Then, FS does the following decryptions in place: aes128_ecb_decrypt(key=naxSpecificKeys[0], data=Header + 0x28); aes128_ecb_decrypt(key=naxSpecificKeys[1], data=Header + 0x38); Finally, hmac-sha256(key=Header[0x20:0x60], data=decryptedSeedKeys[1]) is calculated, and compared using a constant-time memcmp to the hmac stored at Header + 0x0. If it matches, decryption proceeds using the two decrypted keys at Header + 0x28 as an AES-XTS keypair.