USER MANUAL
STM 550 / EMSI
–
ENOCEAN MULTISENSOR FOR IOT APPLICATIONS
© 2020 EnOcean | www.enocean.com F-710-017, V1.0
STM 550 / EMSI User Manual | v1.3 | September 2020 | Page 53/97
9.5.5
USER_KEY
Each STM 550 module is pre-programmed at the factory with a randomly generated 128 bit
security key (FACTORY_KEY). This key will by default be used to encrypt and authenticate
STM 550 radio telegrams when operating in high security mode.
In certain applications it might be desirable to assign a different (user-defined) security key
(USER_KEY) during commissioning to STM 550. This can be done by writing the user-de-
fined security key to the USER_KEY register and setting KEY SELECTION field of the SECU-
RITY_KEY_MODE to 0b01 as described below.
Note that the USER_SECURITY_KEY register is a write-only register meaning that it is not
possible to read back a user-defined security key.
9.5.6
SECURITY_KEY_MODE
The register SECURITY_KEY_MODE allows selecting if FACTORY_KEY or USER_KEY should
be used to encrypt and authenticate STM 550 radio telegrams in high security mode. In ad-
dition, it allows disabling the transmission of Secure Teach-in telegrams in order to protect-
ing the security key.
Note that if the transmission of a secure teach-in telegram has been disabled and is subse-
quently re-enabled then USER_KEY will be reset to FACTORY_KEY.
Figure 33 below shows the structure of the SECURITY_KEY_MODE register.
SECURITY_KEY_MODE (Default: 0x00)
Bit 7 Bit 6 Bit 5 Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
RFU
SECURE LRN TELEGRAM
KEY SELECTION
Figure 33
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SECURITY_KEY_MODE register
The encoding for the KEY SELECTION bit field is shown in Table 10 below.
KEY SELECTION Security key used
0b00 (Default)
FACTORY_KEY is used
0b01
USER_KEY is used
0b10, 0b11
Reserved, do not use
Table 10
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KEY SELECTION bit field encoding
