SLSK

Location

SLSK files can be found in SLB2 as personalized and in SPKGs as non-personalized.

Structure

Offset	Size	Name	Description
0x0	0x4	Magic	Always 0x64B2C8E5.
0x4	0x4	Header Size	Offset to code. Usually 0x2C0.
0x8	0x4	Version String Size	Size of Version String. 0 on FW 0.931, 0x10 on other.
0xC	0x4	Dynamic revoke block size	Size of dynamic key revoke block. Usually 0.
0x10	0x4	Body Size	Code size. Usually 0xCE00.
0x14	0x2	Encryption Key Revision	AES-128-CBC Key revision. Possible values are 0 to 5.
0x16	0x2	Signature Public Key Revision	RSA Public Key Revision. Possible values are 0 to 15.
0x18	0x8	Unknown	Usually zeroes.
0x20	0x20	Body Digest	SHA256 hash of decrypted Body.
0x40	Variable (0 or 0x10)	Version String	Version in ASCII. Not present on FW 0.931. Example: 0000360000000000.
Variable (0x50 on FW 0.940-3.73, 0x40 on FW 0.931)	0x90	Static key revoke block	Allows any Cmep Key Ring Base to be completely disabled.
Variable	Variable (usually 0)	Dynamic key revoke block	Allows specific permissions to be revoked from specified keys.
Variable (0xE0 on FW 0.940-3.73, 0xD0 on FW 0.931)	0xC0	Metadata	Personalized. 6 seeds of size 0x20. Contains keyslot fail keys up to certain point (FW <=3.69), non-fail keys afterwards (FW 3.70+).
Variable (0x1A0 on FW 0.940-3.73, 0x190 on FW 0.931)	0x20	HMAC	Personalized. HMAC-SHA256 of encrypted Body.
Variable (0x1C0 on FW 0.940-3.73, 0x1B0 on FW 0.931)	0x100	Header Signature	Personalized. RSA signature of Header + Metadata + HMAC.
Variable (0x2C0 on FW 0.940-3.73, 0x2B0 on FW 0.931)	Variable (usually 0xCE00)	Body	Personalized. Executable code.
Variable (usually 0xD0C0)	0x100	Footer Signature	RSA signature of Header + Metadata + HMAC + Body. (Does this really exist?, This could also be random padding)
Variable (usually 0xD1C0)	0x140	Random Padding	Must be filled to multiple of 0x200 bytes.

Footer

The last 0x340 bytes of each SLSK are not personalized and not used in any way.

Bootrom SLSK loading process

Secret debug mode

Before the SLSK is loaded, there is a check for some secret mode. Note these two ports are used in regular syscon SPI-like communications. However, usually these two pins are used as part of the SPI-like protocol for signaling. But the bootrom does not use the SPI registers at all. It uses some registers that are never seen outside of the bootrom. Even though it is logically separate from the SPI ports, it could be physically connected to the same pins although this is unconfirmed. Note that when the secret handshake passes and we are in secret mode, the MBR is read from the gamecard instead (with gamecard auth not enabled, so a regular SD card would work). Additionally, the personalization removal is done using keyslot 0x207 instead of 0x206 (see below) although it is not currently known if 0x207 is per-console. All the HMAC and signature checks are still performed, so this secret mode cannot be used to run unsigned code. However, Glitching would still work when in the secret debug mode.

int is_debug_mode(void) {
    int res = 0;
    gpio_set_port_mode(0, 3, GPIO_MODE_OUT);
    if (gpio_port_read(0, 4)) {
        // this sets a bit in some Cmep-only hardware
        // note this same reg is used to enable Cmep reset from arm
        *(uint32_t *)0xE0020000 |= 0x10;
        // theory: mux on syscon SPI ports to connect to Cmep directly

        // compute a challenge using true random numbers
        uint32_t challenge[4];
        challenge[0] = trng_read32();
        challenge[1] = trng_read32();
        challenge[2] = challenge[0];
        challenge[3] = challenge[1];

        // send challenge
        *(uint32_t *)0xE0000020 = challenge[0];
        *(uint32_t *)0xE0000024 = challenge[1];
        gpio_port_set(0, 3);

        // poll
        while (!gpio_port_read(0, 4));

        // get response
        uint32_t response[2];
        response[0] = *(uint32_t *)0xE0000028;
        response[1] = *(uint32_t *)0xE000002C;

        // clear regs
        *(uint32_t *)0xE0000028 = -1;
        *(uint32_t *)0xE000002C = -1;
        *(uint32_t *)0xE0000060 = -1; // maybe cached of 0xE0000020?
        *(uint32_t *)0xE0000064 = -1; // maybe cached of 0xE0000024?

        // end handshake
        gpio_port_clear(0, 3);

        // compute expected result
        uint32_t expected[4];
        if (bigmac_aes256_ecb_encrypt(expected, challenge, sizeof(challenge), g_debug_challenge_key) == 0) {
            // check result
            if (memcmp_timingsafe(expected, response, 8) == 0) {
                res = 1;
            }
        }
        memset(g_debug_challenge_key, 0, sizeof(g_debug_challenge_key));
        memset(challenge, 0, sizeof(challenge));
        memset(response, 0, sizeof(response));
        memset(expected, 0, sizeof(expected));
    } else {
        memset(g_debug_challenge_key, 0, sizeof(g_debug_challenge_key));
    }
    return res;
}

See also: Ernie_Firmware#Kermit_Bootrom_JIG_Mode.

Personalization Removal

First, personalization layer is removed. It uses AES-128-CBC with a derived key and decrypts data at Metadata offset (0xE0 on FW 0.940-3.73, 0xD0 on FW 0.931) for size of (body_size + 0xC0 (Metadata) + 0x20 (HMAC) + 0x100 (Header Signature)).

There are two possible paths to derive the key used to remove personalization. Normally, the key is derived using keyslot 0x206. There is however an alternative path, triggered in secret debug mode, when instead keyslot 0x207 is used and with a different seed.

Once personalization is removed, the source keyslots are locked down. Keyslots 0x9, 0x206, 0x207 are locked down completely (leaving only 0xA0 protection). However, keyslot 0x8 allows encryption, leaving Update Manager SM add personalization layer during firmware update without having to derive the keys itself.

HMAC and RSA verification

A key is derived from keyslot 0x344 and put into keyslot 0x20. This key is then immediately used to calculate HMAC-SHA256 over SLSK header excluding the SLSK Header Signature (usually 0x00 to 0x1C0 or to 0x1B0).

2 bytes are read from keyring slot 0x603. This is the bitmask of allowed RSA public keys (0xFFFF on 1.692). If the bitmask is zero, a hardcoded RSA modulus is used. Otherwise, it checks SLSK Signature Public Key Revision against the mask and if it is allowed, it gets the RSA modulus from keyring RSA storage starting at keyslot 0x700.

After calculating RSA powmod, it checks the padding over the SLSK Signature and compares previously calculated HMAC-SHA256 against the contents.

Finally, it protects keyslots 0x700 to 0x77F to disable reading out the modulus.

Metadata decryption and encrypted body verification

Using keyslot (0x208+enc_key_revision) and the SLSK Metadata first seed (0x20 bytes), the SLSK Body decryption key is derived and put into keyslot 0xA. Then, 5 more keys are derived in the same way, using SLSK Metadata seeds. These 5 keys are put into keyslots 0xB, 0xC, 0xD, 0xE and 0xF.

Once done, keyslots 0x208, 0x209, 0x20A, 0x20B, 0x20C, 0x20D (all possible keys bound to SLSK Encryption Key Revisions) are protected.

SLSK HMAC is decrypted using keyslot 0xA. This is HMAC-SHA256 of the encrypted Body segment. HMAC-SHA256 is calculated over the encrypted Body segment using keyslot 0x20, then keyslot 0x20 is protected. Finally, the calculated HMAC over the encrypted Body is compared to the HMAC decrypted from the SLSK header.

Protecting the keys

Some keys are protected, depending on the bit flags buffer that we call Static key revoke block. However, on FW 3.68 SLSK, Static key revoke block is all zeroes so no keys should be protected by this function (?).

Body decryption

Body is decrypted using keyslot 0xA as key and a hardcoded IV. Then, the key is protected. The decrypted Body is the executable code segment.

One could guess that at this step, SHA256 of the decrypted Body is calculated and compared to the SLSK Header SHA256, but Team Molecule has not confirmed this theory, like in Prototype DEM-3000H First Loader showcased by Yifan Lu where the SHA256 integrity check is not present.

Remainder area clearing

The remainder (0x1C000 - body_size) after the decrypted body is cleared with DMAC. DMAC registers are also cleared.