Decrypt & encrypt BMW CAS3+/CAS4+ ISN (3-byte XOR cipher)
The ISN stored in CAS EEPROM (encrypted)
The plain ISN read from ECU (e.g. DME, DDE, EGS)
The ISN stored in CAS EEPROM (encrypted)
The 3-byte repeating XOR key
The plain ISN to encrypt for CAS EEPROM storage
The 3-byte repeating XOR key
BMW CAS3+ and CAS4+ modules store the ISN (Individual Serial Number) in EEPROM using a 3-byte XOR cipher repeated across the 16-byte (128-bit) ISN.
Verification: After decrypting, the tool checks that the decrypted ISN matches the known ECU ISN — confirming that CAS and ECU are paired.
Use cases: ECU replacement (match ISN to new DME), key programming (verify CAS-ECU ISN match), used CAS installation, and EWS/CAS synchronization.
This tool allows you to decrypt or encrypt the ISN (Individual Serial Number) stored in BMW CAS3+ and CAS4+ modules. The CAS stores the ISN in encrypted form in its EEPROM using a simple 3-byte XOR repeating cipher applied across the 16-byte (128-bit) ISN value.
The ISN is a shared secret between the CAS (Car Access System) module and the engine ECU (DME/DDE). Both modules must have a matching ISN for the vehicle to start. When replacing an ECU, the ISN must be synchronized between the new ECU and the existing CAS — this calculator helps verify and prepare the correct ISN values.
BMW CAS3+ and CAS4+ modules store the ISN encrypted in EEPROM using a 3-byte repeating cipher. This tool handles the encryption and decryption automatically — just provide the encrypted value and a known ECU ISN (or the key directly) and the plain ISN is recovered instantly.
This calculator works with CAS3+ (E-series, 2007+) and CAS4/CAS4+ (F-series, 2010+) modules. Earlier CAS1/CAS2 modules use different encryption methods not covered here.
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