Cryptosystem

From Nintendo Switch Brew
Revision as of 10:08, 8 January 2018 by SciresM (talk | contribs) (Reverted edits by SciresM (talk) to last revision by Q)
Jump to navigation Jump to search

BootROM

The bootrom initializes two keyslots in the hardware engine:

  • the SBK (Secure Boot Key) in keyslot 14
  • the SSK (Secure Storage Key) in keyslot 15.

Reads from both of these keyslots are disabled by the bootROM. The SBK is stored in FUSE_PRIVATE_KEY, which are locked to read out only FFs after the bootrom finishes.

SBK is unique per console, and not shared among consoles as originally believed.

The SSK is derived on boot via the SBK, the 32-bit console-unique "Device Key", and hardware information stored in fuses.

Pseudocode for the derivation is as follows: 

 void generateSSK() {
     char keyBuffer[0x10]; // Used to store keydata
     uint hwInfoBuffer[4]; // Used to store info about hardware from fuses
     uint deviceKey = getDeviceKey(); // Reads 32-bit device key from FUSE_PRIVATE_KEY4.
     for (int i = 0; i < 4; i++) { // Keybuffer = deviceKey || deviceKey || deviceKey || deviceKey
         ((uint *)keyBuffer)[i] = deviceKey;
     }
     
     encryptWithSBK(keyBuffer); // keyBuffer = AES-ECB(SBK, deviceKey || {...})
     
     // Set up Hardware info buffer
     uint vendor_code = *((uint *)0x7000FA00) & 0x0000000F; // FUSE_VENDOR_CODE
     uint fab_code    = *((uint *)0x7000FA04) & 0x0000003F; // FUSE_FAB_CODE
     uint lot_code_0  = *((uint *)0x7000FA08) & 0xFFFFFFFF; // FUSE_LOT_CODE_0
     uint lot_code_1  = *((uint *)0x7000FA0C) & 0x0FFFFFFF; // FUSE_LOT_CODE_1
     uint wafer_id    = *((uint *)0x7000FA10) & 0x0000003F; // FUSE_WAFER_ID
     uint x_coord     = *((uint *)0x7000FA14) & 0x000001FF; // FUSE_X_COORDINATE
     uint y_coord     = *((uint *)0x7000FA18) & 0x000001FF; // FUSE_Y_COORDINATE
     uint unk_hw_fuse = *((uint *)0x7000FA20) & 0x0000003F; // Unknown cached fuse.
     
     // HARDWARE_INFO_BUFFER = unk_hw_fuse || Y_COORD || X_COORD || WAFER_ID || LOT_CODE || FAB_CODE || VENDOR_ID
     hwInfoBuffer[0] = (lot_code_1 << 30) | (wafer_id << 24) | (x_coord << 15) | (y_coord << 6) | unk_hw_fuse;
     hwInfoBuffer[1] = (lot_code_0 << 26) | (lot_code_1 >> 2);
     hwInfoBuffer[2] = (fab_code << 26) | (lot_code_0 >> 6);
     hwInfoBuffer[3] = vendor_code;
     
     for (int i = 0; i < 0x10; i++) { // keyBuffer = XOR(AES-ECB(SBK, deviceKey || {...}), HARDWARE_INFO_BUFFER)
         keyBuffer[i] ^= ((char *)hwInfoBuffer)[i];
     }
     
     encryptWithSBK(keyBuffer); // keyBuffer = AES-ECB(SBK, XOR(AES-ECB(SBK, deviceKey || {...}), HARDWARE_INFO_BUFFER))
     
     setKeyslot(KEYSLOT_SSK, keyBuffer); // SSK = keyBuffer.
 }

Falcon coprocessor

The falcon processor (TSEC) stores a special console-unique key (that will be referred to as the "tsec key").

This is presumably stored in fuses that only microcode authenticated by NVidia has access to.

The tsec key is the source of all per-console entropy, because SSK is not used on retail.

Package1

Key table during package1

Keyslot Name Set by Per-console Per-firmware
11 Package1Key Package1 No Yes
14 SecureBootKey Bootrom No No
15 SecureStorageKey Bootrom Yes No

[1.0.0-3.0.2] Key table after package1

Keyslot Name Set by Per-console Per-firmware
12 MasterKey Package1 No Yes, on security updates
13 PerConsoleKey Package1 Yes No

[4.0.0]+ Key table after package1

Keyslot Name Set by Per-console Per-firmware
12 MasterKey Package1 No Yes, on security updates
13 PerConsoleKey_40 Package1 Yes No
14 MasterKey_40 Package1 No Yes, on security updates
15 PerConsoleKey Package1 Yes No


Key generation

Note: aes_unwrap(wrapped_key, wrap_key) is just another name for a single AES-128 block decryption.

If bit0 of 0x7000FB94 is clear, it will initialize keys like this (probably used for internal development units only): 

 // Final keys:
 package1_key    /* slot11 */ = aes_unwrap(f5b1eadb.., sbk)
 master_key      /* slot12 */ = aes_unwrap(bct->pubkey[0] == 0x11 ? simpleseed_dev0 : simpleseed_dev1, aes_unwrap(5ff9c2d9.., sbk))
 per_console_key /* slot13 */ = aes_unwrap(4f025f0e..., aes_unwrap(6e4a9592.., ssk))

[4.0.0+] Above method was removed.

Normal key generation looks like this on 1.0.0/2.0.0: 

 keyblob_key /* slot13 */ = aes_unwrap(aes_unwrap(wrapped_keyblob_key, tsec_key /* slot13 */), sbk /* slot14 */)
 cmac_key    /* slot11 */ = aes_unwrap(59c7fb6f.., keyblob_key)
 
 if aes_cmac(buf=keyblob+0x10, len=0xA0, cmac_key) != keyblob[0:0x10]:
   panic()
 
 aes_ctr_decrypt(buf=keyblob+0x20, len=0x90, iv=keyblob+0x10 key=keyblob_key)
 
 // Final keys:
 package1_key    /* slot11 */ = keyblob[0x80:0x90]
 master_key      /* slot12 */ = aes_unwrap(bct->pubkey[0] == 0x4f ? normalseed_dev : normalseed_retail, keyblob+0x20)
 per_console_key /* slot13 */ = aes_unwrap(4f025f0e.., keyblob_key)

.. and on 3.0.0, they moved keyslots around a little to generate the same per-console key as 1.0.0: 

 old_keyblob_key /* slot10 */ = aes_unwrap(aes_unwrap(df206f59.., tsec_key /* slot13 */), sbk /* slot14 */)
 keyblob_key     /* slot13 */ = aes_unwrap(aes_unwrap(wrapped_keyblob_key, tsec_key /* slot13 */), sbk /* slot14 */)
 cmac_key        /* slot11 */ = aes_unwrap(59c7fb6f.., keyblob_key)
 
 if aes_cmac(buf=keyblob+0x10, len=0xA0, cmac_key) != keyblob[0:0x10]:
   panic()
 
 aes_ctr_decrypt(buf=keyblob+0x20, len=0x90, iv=keyblob+0x10 key=keyblob_key)
 
 // Final keys:
 package1_key    /* slot11 */ = keyblob[0x80:0x90]
 master_key      /* slot12 */ = aes_unwrap(bct->pubkey[0] == 0x4f ? normalseed_dev : normalseed_retail, keyblob+0x20)
 per_console_key /* slot13 */ = aes_unwrap(4f025f0e.., old_keyblob_key)

.. and on 4.0.0 it was further moved around: 

 old_keyblob_key /* slot15 */ = aes_unwrap(aes_unwrap(df206f59.., tsec_key /* slot13 */), sbk /* slot14 */)
 keyblob_key     /* slot13 */ = aes_unwrap(aes_unwrap(wrapped_keyblob_key, tsec_key /* slot13 */), sbk /* slot14 */)
 cmac_key        /* slot11 */ = aes_unwrap(59c7fb6f.., keyblob_key)
 
 if aes_cmac(buf=keyblob+0x10, len=0xA0, cmac_key) != keyblob[0:0x10]:
   panic()
 
 aes_ctr_decrypt(buf=keyblob+0x20, len=0x90, iv=keyblob+0x10 key=keyblob_key)
 
 // Final keys:
 package1_key        /* slot11 */ = keyblob[0x80:0x90]
 master_key          /* slot12 */ = aes_unwrap(normalseed_retail, keyblob+0x20)
 new_master_key      /* slot14 */ = aes_unwrap(2dc1f48d.., keyblob+0x20)
 new_per_console_key /* slot13 */ = aes_unwrap(0c9109db.., old_keyblob_key)
 per_console_key     /* slot15 */ = aes_unwrap(4f025f0e.., old_keyblob_key)

SBK and SSK keyslots are cleared after keys have been generated.

See table above for which keys are console unique.

The key used to verify a keyblob's MAC is not the keyblob key but a key derived from it; this is likely part of an attempt to mitigate side-channel attacks as the MAC is an alterable part of the keyblob.

The bootloader only stores the hardcoded constants for the keyblob used in the current revision. Nintendo are withholding all the future hardcoded constants.

This means that if you have an attack on the bootloader, you need to re-preform it every time they move to a new keyblob.

Dumping the SBK and TSEC key of any single system should be enough to derive all key material on the system.

The key-derivation is described in more detail here.

Keyblob

The keyblob is console unique, and changes with every bootloader update.

Seeds

 normalseed_retail = d8a2410a...
 
 [1.0.0] wrapped_keyblob_key = df206f59...
 [1.0.0] simpleseed_dev0   = aff11423...
 [1.0.0] simpleseed_dev1   = 5e177ee1...
 [1.0.0] normalseed_dev    = 0542a0fd...
 
 [3.0.0] wrapped_keyblob_key = 0c25615d...  
 [3.0.0] simpleseed_dev0   = de00216a...
 [3.0.0] simpleseed_dev1   = 2db7c0a1...
 [3.0.0] normalseed_dev    = 678c5a03...
 
 [3.0.1] wrapped_keyblob_key = 337685ee...  
 [3.0.1] simpleseed_dev0   = e045f5ba...
 [3.0.1] simpleseed_dev1   = 84d92e0d...
 [3.0.1] normalseed_dev    = cd88155b...
 
 [4.0.0] wrapped_keyblob_key = 2d1f4880...

Table of used keyblobs

System version Used keyblob Used master static key encryption key in keyblob
1.0.0-2.3.0 1 1
3.0.0 2 1
3.0.1-3.0.2 3 1
4.0.0 4 1

Bootloader stage 1

It is currently unknown what key generation the stage 2 bootloader does.

Secure Monitor

The secure monitor performs some runtime cryptographic operations. See SMC for what operations it provides.

Retrieved from "https://switchbrew.org/w/index.php?title=Cryptosystem&oldid=3385"