UM0932 datasheet(10/43 Pages) STMICROELECTRONICS

10 / 43 page

Functional overview

UM0932

10/43

DocID17385 Rev 3

Table 3 lists the fields of DL frame.

Security services (SS) frames, providing authentication to payload using cryptographic

algorithms based on AES with 128-bit keys. Authentication is provided appending to

user data an AES-CMAC digest. A dedicated key (stored in the MIB object SS key 02h,

Section 4.2.3) is used for both transmitting and receiving frames. Figure 4 shows the

SS frame structure.

Figure 4. Security services frame structure

Table 4 lists the fields of SS frame.

Table 3. DL frame description

Field name

Length

Description and values

DL length

Total length of payload and CRC fields.

Payload

DL length – length of CRC

Data bytes

CRC

1, 2 or 4

CRC calculated in accordance with

algorithm chosen in dedicated MIB object

(modem configuration 00h, Section 4.2.1).

Table 4. SS frame description

Field name

Length

Description and values

Header

Part of user data transmitted in clear.

Encrypted payload

LEN-29-HL

Part of user data transmitted ciphered.

Initialization vector of AES-CTR algorithm.

Header field length.

Digest

AES-CMAC digest computed on header, encrypted

payload, IV and HL fields.

UM0932 Datasheet(PDF) 10 Page - STMicroelectronics