Secure storage for cryptographic keys

AN4240

12/30

Doc ID024209 Rev 2

In general, knowledge of a specific key is needed in order to update that specific key.

MASTER_ECU_KEY is a key with special meaning. It is used to authorize updating other

keys (BOOT_MAC_KEY, BOOT_MAC, BOOT_MAC_KEY and all KEY_1 to KEY_10)

without knowledge of those keys. See Table 4: Memory Update Policy of the SHE

specification, reported here for convenience:

Table 4 shows that the knowledge of the MASTER ECU KEY allows updating of all the other

keys. This implies that it is the most important key as it allows to reset the device to its

flag set.

Setting this flag is an irreversible step and for this reason, the user has to be very carefull

during the creation of the key attributes of the cryptographic keys.

KEY_USAGE

If set, memory slot holds a MAC key, otherwise it holds an encryption key

WILDCARD

If set, memory slot cannot be updated with the wildcard UID

Table 4. Memory Update Policy

Slot to update

MASTER ECU KEY

BOOT MAC KEY BOOT MAC KEY<n> RAM KEY

MASTER ECU KEY

X

BOOT MAC KEY

X

X

BOOT MAC

X

X

KEY<n>

X

X

RAM KEY

X

Table 3. Key attributes (continued)

Flag Name

Description