Doc ID024209 Rev 2

11/30

AN4240

Secure storage for cryptographic keys

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Secure storage for cryptographic keys

The CSE provides secure, and non-volatile storage for cryptographic keys as described in

the SHE Functional Specification. The keys are stored in fifteen memory slots, with one

ROM slot, thirteen non-volatile slots, and one RAM slot as shown in Table 2. The first four

slots have a dedicated usage, the other slots are available for application specific keys. The

BOOT_MAC slot is loaded with a MAC value used by the secure boot process. All other

slots are used for encryption or message authentication keys. The SECRET_KEY slot is

programmed with a random value during device fabrication same as the Unique Identifier

Number (UID). It is unique for every part and is programmed into the secure flash when it is

tested in wafer form. UID is 120 bits long. UID is used during inter ECU communications to

confirm that external controllers is not substituted. SECRET KEY may only be used to

import/export keys.

All CSE encryption and message authentication commands specify a key, by its Key ID.

Table 3. describes that each memory slot holds a 128-bit value, a 28-bit counter and five

security flags.

Table 2. Memory Slots

Slot Name

Key ID

Type

SECRET_KEY

0x0

ROM

MASTER_ECU_KEY

0x1

non-volatile

BOOT_MAC_KEY

0x2

non-volatile

BOOT_MAC

0x3

non-volatile

KEY_1

0x4

non-volatile

KEY_2

0x5

non-volatile

KEY_3

0x6

non-volatile

KEY_4

0x7

non-volatile

KEY_5

0x8

non-volatile

KEY_6

0x9

non-volatile

KEY_7

0xA

non-volatile

KEY_8

0xB

non-volatile

KEY_9

0xC

non-volatile

KEY_10

0xD

non-volatile

RAM_KEY

0xE

RAM

Table 3. Key attributes

Flag Name

Description

WRITE_PROT

If set, memory slot cannot be updated

BOOT_PROT

If set, memory slot is disabled if Secure Boot is not enabled

DEBUG_PROT

If set. memory slot is disabled if a debugger is connected