EnOcean EASYFIT EMDCB, User Manual

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USER MANUAL
EMDCB – BLUETOOTH LOW ENERGY MOTION AND ILLUMINATION SENSOR
© 2019 EnOcean | www.enocean.com EMDCB User Manual | v1.3 | August 2019 | Page 62/67
B.2 Constant algorithm parameters
The RFC3610 implementation requires two constant algorithm parameters:
◼
Length field size (in byte)
This is the size of the field used to encode the length of the input data (which is the
payload to be authenticated).
The maximum size of sensor payload to be authenticated is 31 byte; therefore one
byte would be easily sufficient to encode the payload size. The minimum value per-
mitted by the standard is however 2 bytes which is therefore chosen.
◼
Signature size (in byte)
The desired signature size is 4 byte for sensor data telegrams
Table 27 below summarizes these algorithm parameters.
Parameter Comment / Description Example
Length
Field Size Size (in bytes) of the field used to
encode the input length
2
(always, minimum permissible size)
Signature
Size
Desired size (in byte) of the signa-
ture generated by the algorithm
4
(always)
Table 27 – Constant algorithm parameters
The RFC3610 implementation derives two algorithm parameters – M’ and L’ – based on the
constant algorithm parameters and uses them to construct A0_Flag and B_0_Flag which –
together with the iteration counter i – are required for subsequent processing.
The value of these internal parameters - listed in Table 28 below - is the same for all
EnOcean BLE telegrams.
Parameter Comment / Description Example
M’
Binary encoded output length
M’ = (Output length / 2) - 1
0b001
(always)
L’
Binary encoded length field size
L’ = length field size - 1
0b001
(always)
A0_Flag
L’ 0x01
(always)
B0_Flag
(0b01<<6) + ( M’<<3) + L’
0x49
(always)
I Iteration counter
0x0000 (always)
Table 28 – Constant internal parameters