ESB aes-ccm MIC verify

RE: ESB aes-ccm MIC verify

ovrebekk — Tue, 08 Mar 2022 19:41:32 GMT

What is "AAD" a reference to?

I can't find this acronym in our CCM documentation.

The problem with using the CCM when S0 is set to 0 is that the CCM module always assume that the S0 field is there, and if you are receiving data over the air without S0 included then the CCM module will process the data incorrectly.

Best regards
Torbjørn

RE: ESB aes-ccm MIC verify

imo — Tue, 08 Mar 2022 01:59:39 GMT

clear_buffer[CCM_IN_HEADER_INDEX] = 0;
clear_buffer[CCM_IN_LENGTH_INDEX] = (uint8_t)packet_length;
clear_buffer[CCM_IN_RFU_INDEX] = 0;
memcpy(&clear_buffer[CCM_IN_PAYLOAD_INDEX], packet, packet_length);
NRF_CCM->INPTR = (uint32_t)clear_buffer;

I understood  the above code , when the S0 field is set to 1, it is used as follows.

 AAD                  plain text

| S0 | length | S1(RFU) | payload |



How to use 'AAD' when S0 field is set to 0?

AAD?                   plain text

| X | length | S1(RFU) | payload |

RE: ESB aes-ccm MIC verify

ovrebekk — Mon, 07 Mar 2022 12:32:47 GMT

The team confirmed that S0 has to be used in order for the CCM to work properly.

Best regards
Torbjørn

RE: ESB aes-ccm MIC verify

imo — Fri, 25 Feb 2022 08:32:55 GMT

OK. Thanks.

RE: ESB aes-ccm MIC verify

ovrebekk — Thu, 24 Feb 2022 15:16:49 GMT

The CCM module is designed with Bluetooth encryption in mind, and Bluetooth uses a 2-byte packet header that requires the S0 field to be set to 1. I will have to double check this with the hardware team, but I believe the MIC check will only work when the radio is configured in a Bluetooth compliant mode, which would explain why S0 has to be 1 byte long.

I will get this confirmed and get back to you.

Best regards
Torbjørn