Ancast image
Ancast images are encrypted and signed binaries for the Espresso and Starbuck processors to execute.
The Espresso contains a secure boot ROM that runs upon a PowerPC hard reset, which will only boot signed code. This code comes in the form of an ancast image. Before resetting the PPC, the currently running ARM code (either IOSU or vWii IOS) must load the ancast image to the physical address 0x08000000 (Wii U images) or 0x01330000 (vWii images) for the boot ROM to verify and decrypt. The Cafe OS kernel, vWii System Menu, and BC-NAND/BC-WFS are all in the form of ancast images; WiiMode images are stored inside a DOL in Data0, while Text0 contains a standard boot stub that was probably automatically added by Nintendo's compiling software.
The Starbuck's boot chain (boot0 and boot1) only boots signed code in the form of an ancast image as well. On the ARM side, the IOS-MCP module is responsible for launching cafe2wii and relaunching IOSU (warm boot). IOS-MCP loads ARM ancast images into MEM1 (0x01000000), verifies and decrypts them before executing with full privileges (all memory protection is disabled and must be re-enabled by the binary itself).
Ancast images consist of a signature and related information, known as the ancast header, and the AES-encrypted code, known as the ancast body. ARM and PPC ancast images both use AES-128-CBC for encryption, but differ in signature algorithms. For this reason, their headers are also different.
"Ancast" is an unofficial acronym for "The princess is in another castle", which is a pun introduced during fail0verflow's initial hacking efforts. It doesn't transmit any actual meaning besides that.
ARM ancast image
Header
| Start | Length | Description |
|---|---|---|
| 0x00 | 0x04 | Magic (0xEFA282D9) |
| 0x04 | 0x04 | NULL |
| 0x08 | 0x04 | Signature offset (0x20) |
| 0x0C | 0x04 | NULL |
| 0x10 | 0x10 | NULL |
| 0x20 | 0x04 | Signature type (0x02) |
| 0x24 | 0x100 | Ancast image signature |
| 0x124 | 0x7C | Padding (must be NULL) |
| 0x1A0 | 0x02 | Must be NULL |
| 0x1A2 | 0x01 | Must be NULL |
| 0x1A3 | 0x01 | Must be NULL |
| 0x1A4 | 0x04 | Target device (0x21 for NAND, 0x22 for SD) |
| 0x1A8 | 0x04 | Console type (0x01 for Development, 0x02 for Production) |
| 0x1AC | 0x04 | Ancast image body size |
| 0x1B0 | 0x14 | Ancast image body hash |
| 0x1C4 | 0x04 | Version (always 0x02 for fw.img and c2w.img, matches Title ID version for boot1) |
| 0x1C8 | 0x38 | Padding (must be NULL) |
Signature type 0x02 is RSA.
Body
The image's body is composed of AES-128-CBC encrypted (with the Starbuck WiiU, vWii or boot1 ancast keys) data.
PPC ancast image
Header
| Start | Length | Description |
|---|---|---|
| 0x00 | 0x04 | Magic (0xEFA282D9) |
| 0x04 | 0x04 | NULL |
| 0x08 | 0x04 | Signature offset (0x20) |
| 0x0C | 0x04 | NULL |
| 0x10 | 0x10 | NULL |
| 0x20 | 0x04 | Signature type (0x01) |
| 0x24 | 0x38 | Ancast image signature |
| 0x5C | 0x44 | Padding (must be NULL) |
| 0xA0 | 0x02 | Must be NULL |
| 0xA2 | 0x01 | Must be NULL |
| 0xA3 | 0x01 | Must be NULL |
| 0xA4 | 0x04 | Target device (0x11 on Wii U images, 0x13 on vWii images, 0x12 on an unknown vWii image, 0x14 see below) |
| 0xA8 | 0x04 | Console type (0x01 for Development, 0x02 for Production) |
| 0xAC | 0x04 | Ancast image body size |
| 0xB0 | 0x14 | Ancast image body hash |
| 0xC4 | 0x3C | Padding (must be NULL) |
Signature type 0x01 is ECDSA. The Espresso boot ROM uses SHA-1 as the cryptographic hash function and hardcoded ECC-P224 public keys for signature verification.
Ancast type 0x12 is dependent on the Espresso's HID1 special-purpose register having bit28 set, as well as the PPC being in vWii mode.
Ancast type 0x14 is implied to only able to be used on Wii Us with a PpcPvr value where the upper u16 != 0x7001, and the lower u16 >0x100. The prerequisite flags can be triggered on retail units by first writing 0x2 to the lower nibble of LT_PIMEMCOMPAT, and then asserting SRESET just after a normal (0x11) Ancast image has finished verifying on the Espresso. A second uploaded Ancast image (written to RAM while SRESET is held asserted) will only verify if the type is 0x14. 0x14-type Ancast images will always use retail ECC keys to verify, even on unfused units.
Body
The image's body is composed of AES-128-CBC encrypted (with the Espresso WiiU or vWii ancast keys) data. It starts 0x100 bytes after the start of the header.