Xbox Hard Drive Locking Mechanism

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by SpeedBump (original version: 13 August 2002)

The hard drive in the MS Xbox(tm) is a standard ide drive, which implements a rarely used security feature to restrict access to its data. This document will describe in full the security features, and the algorithms required to access the data on an xbox drive.

The IDE (ATA) commands

The ATA spec defines a feature subset which allows for the user to limit access to the drive's data behind a hardware implemented locking mechanism. There are several commands in the SECURITY feature subset, but the command of most interest is the SECURITY_UNLOCK command.

SECURITY_UNLOCK requires that the user provide one of two 32 byte passwords, either a user or master password. The Xbox uses the user password. Details on the data formats and timings for the data to be sent to the ide drive can be found in the ata specs (see [[2]]).

The password

The drive password is generated in two distinct phases. The first phase extracts a key (referred to as the HDKey) from the eeprom data on the Xbox. The HDKey is unique to each Xbox making this first phase dependant only on the Xbox eeprom of the unit. The second phase uses this HDKey to generate a password which is specific to the drive being unlocked (keyed to the model and serial numbers of the drive).

Drive Data

During the second phase, the serial and model numbers are needed. These values are available in the response data from the DEVICE_IDENTITY ata command. However, the data needs to be properly reorganized. It is read in big endian words, and needs to be byte swapped first to get the byte ordering correct. Then, starting from the end of the data (serial == 20 bytes, model == 40 bytes) ignore ASCII spaces (byte value of 0x20) at the end of the data. Zeros are *not* trimmed, *only* spaces. Do not be fooled into believing that this data is a string. On some drives this is the case, but on others there are non-ascii values in the fields.

Basic Security algorithms

There are two primary crytography routines needed when generating an XBox drive password, SHA1 and RC4.

SHA1 is a hashing algorithm. It's primary purpose is to take an input message and create a (relatively) small signature (called a digest) which is unique to the original message. One of the goals of SHA1 is to make it difficult to alter the input message in such a way as to result in the same output digest.

RC4 is a symmetric cipher. This means that the algorithm for encryption is the same as that for decryption. The purpose is to make one key work in both directions.

There is an algorithm called HMAC which uses a hashing algorithm (in this case SHA1) to generate a cryptographically "strong" signature. I'm sure there is a mathematical basis for this, but I'm not willing to try to understand it :)

The Password Algorithm

(some syntax notes, key data is shown entering functions from the side, data is shown entring from above or below, in order of presentation from left to right)

                                       RC4_key >--(second)-->--,
                                         /|\                   |
                                          |                    |
 .-<--|__eeprom_key__|-->-----------> HMAC_SHA1                |
 |                                       /|\                   |
 |                                        |                    |
 |                        .--->-----------'                    |
 |                        |                                    |
 |  eeprom_data = |__data_hash___|__enc_conf__|__enc_data__|   |
 |                        |             |           |          |
 |                        |            \|/          |          |
 |                        |        rc4_decrypt <----|---------<|
 |                        |             |           |          |
 |                       \|/            |           |          |
 |                 (must be equal)      |          \|/         |
 |                       /|\            |      rc4_decrypt <---'
 |                        |             |           |
 |                        |            \|/         \|/
 |                        |      |_confounder_|____data____|
 |                        |       /            /    |
 |                        |      /            /     |
 |                        |     /            /      |
 |                        |    /            /       |
 |                        |   \|/          /       \|/
 `--->-----------------> HMAC_SHA1        /   |__HDKey__|__|
                                /|\      /         |
                                 \______/          |
               |              model_number   serial_number
               |                      \        /
               |                       \      /
               `--->-----------------> HMAC_SHA1

This seems to be the easiest way to show the required calculations.

Basically there are several intermediate steps. First, generate the RC4_key from the eeprom_key and the data_hash (first 20 bytes of eeprom_data). Use the RC4_key to decrypt the encrypted confounder (8 bytes 20 bytes into eeprom_data) and the encrypted data (20 bytes 28 bytes into eeprom_data). Now generate an HMAC_SHA1 hash from the eeprom_key and the decrypted confounder and data. Verify that this hash matches the data_hash stored in the eeprom. If they don't match then the eeprom data is not correct. If the hashes match then the first 16 bytes of the decrypted data field is the HDKey.

Once you have the HDKey get the model number and serial number from the ide drive. Generate an HMAC_SHA1 hash from the HDKey, model and serial numbers. The resulting 20 bytes are the HD password. The remaining 12 bytes needed for the password are zeros.

Remaining Questions

The algorithm is well known, however it is dependant on the eeprom_key. It would be ideal if this key could be compiled into a driver to perform the generation and the unlocking. However noone appears to be able to answer the question of legality. Is it legal to privide the eeprom key? Either way, the drive can be unlocked. The ability to distribute the key will only help people use the drive outside the xbox (plus make it simpler to unlock the drive in the xbox).