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Cryptographic Security & DRM Subsystem Analysis

The Korg Kronos integrates a multi-layered digital rights management (DRM) and integrity verification subsystem. This chapter provides a rigorous cryptanalysis of the partition encryption parameters, hardware coprocessor protocols, dynamic system call interception, and kernel-space anti-tamper checks.

Partition-Level Image Encryption (cryptoloop)

The Host's read-only system partitions are stored on the CompactFlash card as loopback filesystem images. To prevent offline binary analysis or modification, these images are encrypted using kernel-space loopback encryption:

System Partition Image Mounting Path Contents Description
Eva.img /korg/Eva Userspace GUI application binary and interface assets
Mod.img /korg/Mod Kernel driver modules, including OA.ko and KorgUsbAudioDriver.ko
WaveMotion.img /korg/rw/PCM/WaveMotion WaveMotion PCM samples and sound bank multi-sample files

Cipher and Key Derivation Parameters

  • Symmetric Cipher: AES-256 in Cipher Block Chaining (CBC) mode (lo_encrypt_type=18 / LO_CRYPT_CRYPTOAPI, lo_crypt_name=aes).
  • Effective Key Length: 124-bit. Due to an implementation truncation bug, only the first 31 characters of the 32-character hex key string are utilized, leaving the final nibble unmapped.
  • Key Assignment: The 31-character ASCII hex keys are identical across all Kronos production hardware:
    • Eva.img: 342ee59d549c7d329d835537be0540d
    • Mod.img: a336a15cd841ec8926b99e7c3884eaa
    • WaveMotion.img: 3e72c0e59fc017a9eb7d7e1168a4cdb

Hardware Coprocessor Protocol (Atmel AT88SC0204CA)

The symmetric partition keys are not stored in plaintext. They are encrypted within a local root file (/.pairFact3) and decrypted at boot time.

The decryption key is derived through an interactive challenge-response handshake with a secure EEPROM chip—an Atmel AT88SC0204CA CryptoMemory—integrated into the NKS4 coprocessor board:

loadmod.ko / GetPubIdMod.ko
      │
      ├─── stgNV2AC_sync_cmd (type 0xE0) ─────► [ OmapNKS4Module.ko ]
      └─── stgNV2AC_sync_read_cmd (type 0xE1)          │
                                                       ▼ (USB Bulk OUT)
                                            [ Coprocessor (ESP32-P4) ]
                                                       │
                                                       ▼ (I2C Bus Interconnect)
                                            [ Atmel AT88SC0204CA Chip ]

The Atmel communication path is multiplexed over the same USB endpoint infrastructure used for the NV2AC audio codec control (see NV2AC Virtual Codec Control Protocol). The coprocessor routes incoming type 0xE0/0xE1 register commands based on the target register address.

Complete Multi-Stage Key Derivation Flow

The dynamic decryption of the partition-level AES keys from /.pairFact3 involves a multi-stage, hardware-bound cryptographic chain executed by loadmod.ko. Under static analysis, Korg's proprietary kernel symbols are identified, mapping to specific steps in the key-derivation pipeline:

The Dynamic Key-Derivation Algorithm

  1. Coprocessor Characterization (sdflkjsvnd2g): The Host reads the 7-byte unique hardware identifier (Public ID) from Atmel channel 0x19 via a type 0xE1 command. It converts the ID to big-endian order and computes its square, yielding a 14-byte value (ID_BE²). This value is passed to sdflkjsvnd2g(atmel_bytes, 0, buf), which invokes __gmpz_powm (RSA modular exponentiation) using Korg's public modulus and exponent. The resulting bignum array is used to initialize the primary Blowfish key-schedule arrays (P-array and S-boxes).
  2. Hardware Challenge Verification (fFfFfFfFfFfF1G / fFfFfFfFfFfF1H): The Host issues a random challenge value to Atmel channel 0x50 using a type 0xE0 command. The coprocessor signs the challenge using its internal cryptographic engine. The Host reads the response from channel 0x50 and executes a validation check via fFfFfFfFfFfF1G and fFfFfFfFfFfF1H. A response byte containing 0xFF represents an authentication failure and aborts system initialization.
  3. Symmetric Key Recovery (fFfFfFfFfFfF13): The Host executes three synchronous volume-write register reads to retrieve the 24-byte symmetric key material from the coprocessor:
    • fFfFfFfFfFfF13(channel=0x08, volume=8, buf+0x00): Retrieves Part 1 (8 bytes).
    • fFfFfFfFfFfF13(channel=0x08, volume=8, buf+0x08): Retrieves Part 2 (8 bytes).
    • fFfFfFfFfFfF13(channel=0x10, volume=8, buf+0x10): Retrieves Part 3 (8 bytes).
  4. Decryption of /.pairFact3 (moancjsd82): The Host invokes moancjsd82(key=24_response_bytes, data=.pairFact, len=80). This routine initializes a fresh Blowfish-CFB-64 key-schedule using the 24 bytes of symmetric key material retrieved in Step 3. It decrypts the 80-byte /.pairFact3 file in memory, yielding three 16-byte binary key blocks (48 bytes total).
  5. Hash Signature Verification (md5): The Host calculates the MD5 hash of the decrypted 48-byte key block and compares it against the reference signature to verify integrity.
  6. Buffer Allocation (aaaaaaaaa199): If the MD5 hash is verified, loadmod.ko calls aaaaaaaaa199(result) to copy the 48-byte key buffer to a secure location in Host BSS memory (BSS+0x1200).
  7. Loopback Mount Hex-Encoding: At mount time, the system hex-encodes the keys in BSS space to produce three 31-character ASCII hex passphrases, passing them to the loopback mounting system.

Cryptanalysis of the Key-Recovery Dependency Loop

A classic cryptographic dependency loop prevents deriving the 24-byte symmetric key offline from the known plaintext (the 31-character AES keys) and the encrypted ciphertext (/.pairFact3 file):

  1. Blowfish-CFB-64 Dependencies: The /.pairFact3 file is decrypted using Blowfish-CFB-64. The decryption process requires both a Blowfish key-schedule and an initialization vector (IV).
  2. State-Dependent Initialization Vector: Under Korg's implementation, the 24-byte payload retrieved from the volume-writes constitutes the Blowfish key-schedule key. The IV is dynamically generated as the intermediate Blowfish state after the key-schedule is completed with the 24-byte key.
  3. The Dependency Loop: To decrypt /.pairFact3, the 24-byte key must be known. To derive the 24-byte key by reversing the Blowfish-CFB-64 cipher using known plaintext-ciphertext pairs, the IV must be known. However, the IV cannot be calculated without already knowing the 24-byte key.
\[\text{24-Byte Key} \longrightarrow \text{Key-Schedule Setup} \longrightarrow \text{Dynamic IV} \longrightarrow \text{Blowfish-CFB-64 Decryption}\]

This dependency loop makes brute-forcing the key space mathematically difficult. The IV's dependency on the 24-byte key space requires brute-forcing up to \(2^{64}\) states. Consequently, recovering the 24-byte symmetric key requires capturing the three volume-write response payloads ([0xB2, 0x00, channel, 0x08]) using a physical USB bus analyzer on stock hardware.

Dynamic System Call Interception (loadmod.ko)

Following successful hardware verification, loadmod.ko modifies the Host's kernel system call table (sys_call_table) in Ring 0 to intercept file operations:

Intercepted Syscall Replacement Handler Function Security Operational Role
sys_mount Intercepts mount calls Detects mount requests targeting /korg/Eva, /korg/Mod, or /korg/WaveMotion, automatically mounting the target image via loopback using the derived AES keys
sys_umount Intercepts unmount calls Ensures clean teardown of the encrypted loopback loop device mappings
sys_oldumount Intercepts legacy unmount calls Mandates standard loopback unmounting for compatibility
sys_ioctl Intercepts system I/O control calls Transparently routes loopback status queries

To write to the sys_call_table, loadmod.ko requires write permissions on the kernel text page. Therefore, the Host kernel must be compiled with CONFIG_DEBUG_RODATA=n. If write-protection is enabled on kernel pages, the syscall overwrite faults, causing a kernel panic.

Synthesis Engine Tamper Protection ("Cripple Check")

To prevent launching the synthesis engine if loadmod.ko is bypassed, the Host kernel implements a cross-module validation check:

  1. During initialization, loadmod.ko writes a magic registration value 0x22FB39CC into kernel memory using a modified register_cdrom() kernel interface.
  2. When OA.ko initializes its real-time DSP scheduler, it calls cdrom_find_device() to verify that this magic registration value is present.
  3. Synthesis Behavior Options:
    • Verification Success: OA.ko scales all digital audio output samples by a factor of {1.0, 1.0, 1.0, 1.0}.
    • Verification Failure: OA.ko scales all digital audio output samples by a factor of {-1.0, -1.0, -1.0, -1.0}. This phase inversion distorts the physical output, rendering the analog signal unusable.

Dynamic Self-Check and Anti-Tamper Mechanics

loadmod.ko implements several closely coupled anti-analysis checks. A failure in any early verification phase corrupts the internal state of subsequent stages, causing a cascading failure that obfuscates the original source of the error.

1. Relocated Memory Hash Self-Test (aaaaaaaaa6)

When loaded, loadmod.ko hashes its relocated .text and .data memory spaces in RAM. The resulting 16-byte MD5 hash is compared against a reference table scattered across its .data segment:

Target Reference Offsets: [0x260, 0x19b, 0x22f, 0xd3, 0xec, 0x94, 0x25f, 0x113, 
                           0x3a1, 0x31a, 0x33c, 0x370, 0x304, 0x3cc, 0x276, 0x20e]

Because this routine hashes the relocated binary image in RAM, the check is highly relocation-dependent. If the module is loaded at memory addresses that differ from the original physical Kronos memory mapping, the relocated offset bytes change the hash, causing the self-test to fail.

2. Pseudo-Random Number Generator String Obfuscation

All system strings (including log messages, filesystem paths, and device names) are stored in the .data segment as obfuscated blocks. The driver decrypts these strings at runtime using a LCG PRNG located at BSS+0x11e0:

\[X_{n+1} = (0x0BB38435 \times X_n + 0x3619636B) \pmod{2^{32}}\]

Because the PRNG state is seeded by the MD5 self-test routine, any relocation mismatch or binary tampering corrupts the PRNG seed. As a result, all subsequent string decryptions yield garbage data, causing subsequent file opens (e.g., loading .pairFact3) to fail silently.

Alternative Operational Architecture Configurations

To run the synthesis engine on custom x86 boards or virtualized environments, two alternative operational configurations are defined:

                           [ System Boot Selection ]
                                      │
           ┌──────────────────────────┴──────────────────────────┐
           ▼                                                     ▼
 [ Config A: Decrypted Bypass ]                        [ Config B: Coprocessor Emulated ]
           │                                                     │
           ├─► Decrypt images offline                            ├─► Keep original loopback images
           ├─► Mount partitions directly                         ├─► Deploy stock loadmod.ko (unpatched)
           ├─► Pre-seed 0x22FB39CC in kernel                     ├─► ESP32 emulates AT88SC0204CA I2C
           └─► ESP32 returns generic ACKs                        └─► ESP32 returns 24 Blowfish key bytes

Configuration Alpha: Decrypted Partition Bypass Mode

This configuration bypasses the entire DRM and coprocessor challenge-response pipeline. It is the recommended mode for rapid systems development:

  1. Offline Decryption: Decrypt the loopback images (Eva.img, Mod.img, WaveMotion.img) using the known AES keys and mount their contents directly.
  2. Kernel Patching: Deploy a custom Host kernel that pre-initializes the magic value 0x22FB39CC within the register_cdrom address space, satisfying the synthesis engine's check.
  3. Coprocessor Requirements: The ESP32-P4 only needs to return a generic type 0xE1 acknowledgment ([0x00, 0x00, 0x00, 0xE1]) to avoid blocking volume write calls.

Configuration Beta: Coprocessor-Emulated Cryptographic Loopback Mode

This configuration enables zero-patch operations, allowing the deployment of stock, unmodified Korg recovery disks:

  1. Coprocessor Emulation: The ESP32-P4 fully emulates the Atmel AT88SC0204CA CryptoMemory chip over the I2C bus.
  2. Zero-Patch Integrity: The Host runs the stock, unmodified kernel and the unpatched loadmod.ko module.
  3. Key Delivery: The ESP32-P4 must return the exact 24-byte symmetric key matching the Host's unique /.pairFact3 file during Stage 3 volume-reads.

The .pairFact3 Reauthorization Provisioning Process

The /.pairFact3 file is device-specific and is downloaded from Korg's support website during unit registration. The file is not generated by the Host system or restore DVDs:

  1. Public ID Capture: The user reads the 24-character alphanumeric Public ID from the screen (or queries the ID using GetPubIdMod.ko).
  2. Korg Support Request: The user submits the Public ID to Korg's reauthorization portal.
  3. RSA Cipher Generation: Korg's server uses the unit's unique Atmel hardware signature (derived from the database records of its 7-byte ID) and Korg's private RSA key to generate the encrypted 80-byte .reauth file.
  4. Host Provisioning: The user downloads the .reauth file and writes it to /.pairFact3 on the root filesystem partition. Because the file is encrypted using the 24-byte symmetric coprocessor key matching that specific Atmel chip, only the matching hardware coprocessor can generate the Blowfish parameters needed to decrypt the file at boot time.

Atmel AT88SC0204CA Coprocessor Emulation Specification

To satisfy the Host's cryptographic check in Configuration Beta, the coprocessor's USB and I2C firmware must return the following responses for incoming register reads:

USB Command Packet (Bulk OUT) Coprocessor Target Response (Interrupt IN) Target Cryptographic Stage
[0xB6, 0x00, 0x19, 0x07] [0x74, 0xFF, 0x31, 0xB4, 0xA6, 0xA7, 0xD8] Public ID (used in RSA key-schedule setup)
[0xB6, 0x00, 0x50, 0x08] [0xD7, 0xFC, 0x94, 0xF5, 0x71, 0x11, 0xFA, 0xE1] Verification Signature (channel 0x50 check)
[0xB2, 0x00, 0x08, 0x08] 8-byte Device-Specific Key Payload (Part 1) Symmetric Key Bytes [0:7]
[0xB2, 0x00, 0x08, 0x08] 8-byte Device-Specific Key Payload (Part 2) Symmetric Key Bytes [8:15]
[0xB2, 0x00, 0x10, 0x08] 8-byte Device-Specific Key Payload (Part 3) Symmetric IV Bytes [16:23]
Any other type 0xE1 [0x00, 0x00, 0x00, 0xE1] Generic command acknowledgment

Multi-Byte Response Packing and Wire Format

As specified in Structural Payload Byte-Swap Rules, multi-byte response payloads returned via the Interrupt IN endpoint must be byte-reversed per 32-bit word.

However, the Host reads the payload length byte directly from byte 7 of the raw wire packet before executing the word-level byte-swap. The coprocessor's USB controller must pack the transmission frame according to the following layout:

Wire Byte 0-3: [xx, xx, xx, 0xE1]     — Header (Type 0xE1 at byte 3)
Wire Byte 4:   data[2]                — First word, byte-swapped
Wire Byte 5:   data[1]
Wire Byte 6:   data[0]
Wire Byte 7:   LENGTH (N)             — Read before swap (N = total data bytes)
Wire Byte 8:   data[6]                — Second word, byte-swapped
Wire Byte 9:   data[5]
Wire Byte 10:  data[4]
Wire Byte 11:  data[3]                — Maps to data[3] after Host byte-swap

Channel 0x19 Wire Transmission Example

To transmit the 7-byte Public ID (74 FF 31 B4 A6 A7 D8), the coprocessor transmits a 12-byte payload over the USB wire:

Wire Payload: [0x00, 0x00, 0x00, 0xE1, 0x31, 0xFF, 0x74, 0x07, 0xD8, 0xA7, 0xA6, 0xB4]
               └────── Header ───────┘  └─ data[2:0] ─┘  Len   └──── data[6:3] ────┘

Channel 0x50 Wire Transmission Example

To transmit the 8-byte signature (D7 FC 94 F5 71 11 FA E1), the coprocessor transmits a 16-byte payload:

Wire Payload: [0x00, 0x00, 0x00, 0xE1, 0x94, 0xFC, 0xD7, 0x08, 0xFA, 0x11, 0x71, 0xF5, 0xE1, 0x00, 0x00, 0x00]
               └────── Header ───────┘  └─ data[2:0] ─┘  Len   └──── data[6:3] ────┘   └────── data[7] ──────┘

Public ID Encoding Details

The 24-character alphanumeric Public ID displayed on the system UI (e.g., 0313VC4Q0XUA6R9RLNA9GCLA) is a custom base32 representation of the Atmel chip's 7-byte unique ID and its 1-byte CRC-8 checksum.

  • Custom Base32 Alphabet: Excludes confusing characters (B, I, O, S): 
    0123456789ACDEFGHJKLMNPQRTUVWXYZ
  • Decoding Mechanics: Decodes as 4 logical words of 6 characters each. Each character represents a 5-bit value, shifted sequentially (27, 22, 17, 12, 7, 2) to reconstruct the Atmel ID and CRC.
  • Checksum Verification: The Atmel ID uses a CRC-8 polynomial with an initial value of 0x0A and a generator polynomial of 0xD8 (ValidateCRC routine, spanning 780 bytes of compiled code).

Complete Coprocessor Verification Handshake Specification (10 Steps)

During boot time, loadmod.ko challenges the coprocessor using a strict 10-step sequence. This table defines the exact commands, response sizes, expected values, and cryptographic outcomes representing the complete verification specification:

Step Command (Bulk OUT) Expected Response (Interrupt IN) Subclass Data Purpose & Outcome
1 [0xB6, 0x00, 0x19, 0x07] 7 bytes: [0x74, 0xFF, 0x31, 0xB4, 0xA6, 0xA7, 0xD8] Read Retrieves coprocessor unique Public ID
2 [0xB8, 0x00, 0x00, 0x00] None Write Triggers dynamic Blowfish state verification
3 [0xB6, 0x00, 0x50, 0x01] 1 byte: 0xD7 Read Queries authentication zone status (must be \(\neq 0xFF\))
4 [0xB4, 0x03, 0x00, 0x00] None Write Directs initial cryptographic challenge data
5 [0xB6, 0x00, 0x50, 0x08] 8 bytes: [0xD7, 0xFC, 0x94, 0xF5, 0x71, 0x11, 0xFA, 0xE1] Read Retrieves signed verification challenge (compares against .data+0x20)
6 [0xB8, ...] (20 bytes) None Write Transmits Host dynamic verification challenge
7 [0xB6, 0x00, 0x50, 0x01] 1 byte: 0xD7 Read Verifies second challenge state (must be \(\neq 0xFF\))
8 [0xB2, 0x00, 0x08, 0x08] 8 bytes: Device-Specific Symmetric Key (Part 1) Read Retrieves symmetric key bytes [0:7] (Blowfish-CFB key)
9 [0xB2, 0x00, 0x08, 0x08] 8 bytes: Device-Specific Symmetric Key (Part 2) Read Retrieves symmetric key bytes [8:15] (Blowfish-CFB key)
10 [0xB2, 0x00, 0x10, 0x08] 8 bytes: Device-Specific Symmetric Key (Part 3) Read Retrieves symmetric IV bytes [16:23] (Blowfish-CFB IV)