Difference between revisions of "Messages Format"

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  PPRZ-message: ABCxxxxxxxDE
  PPRZ-message: ABCxxxxxxxDE
byte:    |  value:
     A PPRZ_STX (0x99)
     0        PPRZ_STX (0x99)
     B LENGTH (PPRZ_STX->PPRZ_CHECKSUM_B)
     1        LENGTH (PPRZ_STX->PPRZ_CHECKSUM_B)
     C PPRZ_DATA
     2       SENDER_ID
       0 SENDER_ID
    3       MSG_ID
       1 MSG_ID
    4..(N-2) MSG_PAYLOAD (messages.xml)
      2 MSG_PAYLOAD
     N-1      PPRZ_CHECKSUM_A (sum[B->C])
      . DATA (messages.xml)
     N        PPRZ_CHECKSUM_B (sum[ck_a])
     D PPRZ_CHECKSUM_A (sum[B->C])
     E PPRZ_CHECKSUM_B (sum[ck_a])


===Pprzlink v2.0===
===Pprzlink v2.0===
Line 35: Line 36:
       4 MSG_PAYLOAD
       4 MSG_PAYLOAD
       . DATA (messages.xml)
       . DATA (messages.xml)
    D PPRZ_CHECKSUM_A (sum[B->C])
    E PPRZ_CHECKSUM_B (sum[ck_a])
==Secure Paparazzi (S-PPRZ) for serial and transparent modem communication==
Secure Paparazzi link ([https://github.com/paparazzi/pprzlink/tree/hacl-c pprzlink]) uses [https://en.wikipedia.org/wiki/Symmetric-key_algorithm symmetric key encryption] with [https://en.wikipedia.org/wiki/Salsa20#ChaCha_variant Chacha20] cipher.
'''s-pprzlink''' uses a formally verified cryptographic library [https://github.com/mitls/hacl-star HACL*] developed by the [http://prosecco.inria.fr/ Prosecco] team at [https://www.inria.fr/en/centre/paris INRIA Paris] in collaboration with Microsoft Research, as part of [https://github.com/project-everest Project Everest].
Because certain data have to be send in plaintext (such as the message counter), '''s-pprzlink''' uses ''Authenticated Encryption with Associated Data'' (AEAD) algorithm to authenticate such data before use. Authentication means that any unauthorized change in data is detected upon decryption. Authentication is important because if we decide to send SENDER_ID or DESTINATION_ID in plaintext - so the receiver can decide whether to decrypt the message - we don't want it to be tampered with. '''s-pprzlink''' uses a modified [https://github.com/GaloisInc/gec Galois Embedded Crypto] algorithm, refer there for more details about the algorithm itself.
The cryptographic overhead during regular communication is 20 bytes (4 bytes of the counter, 16 bytes of the authentication tag), plus the necessary key exchange at the beginning of communication.
===S-Pprzlink v1.0===
PPRZ-message: ABCxxxxxxxDE
byte:      |    value:
    0            PPRZ_STX (0x99)
    1            LENGTH (PPRZ_STX->PPRZ_CHECKSUM_B)
    2-5          COUNTER (4 bytes)
    6            encrypted SENDER_ID
    7            encrypted MSG_ID
    8..(N-16)    encrypted MSG_PAYLOAD (messages.xml)
    (N-15)..(N-2) TAG (16 bytes)
    N-1          PPRZ_CHECKSUM_A (sum[B->C])
    N            PPRZ_CHECKSUM_B (sum[ck_a])
{|border="1"  cellspacing="0" style="text-align:center" cellpadding="2%" width="70%"
|+'''S-Pprzlink v1.0'''
!width="7%"|''byte''!!width="10%"|''Value''!!width="10%"|''Type''
|-
|0||PPRZ_STX (0x99)||style="background:orange; color:black"|Header
|-
|1||length N (PPRZ_STX->checksum B)||style="background:orange; color:black"|Header
|-
|2|| sender ID ||style="background:green; color:white"|Authenticated data
|-
|3||counter LSB 1 ||style="background:green; color:white"|Authenticated data
|-
|4||counter LSB 2 ||style="background:green; color:white"|Authenticated data
|-
|5||counter LSB 3 ||style="background:green; color:white"|Authenticated data
|-
|6||counter LSB 4 ||style="background:green; color:white"|Authenticated data
|-
|7|| message ID ||style="background:yellow; color:black"|Encrypted payload
|-
|8..(N-16)|| (optional) message payload (0-234 bytes) ||style="background:yellow; color:black"|Encrypted payload
|-
|(N-15)..(N-2)|| tag (16 bytes) ||style="background:blue; color:white"|Authentication tag
|-
|N-1||checksum A||style="background:orange; color:black"|Header
|-
|N||checksum B||style="background:orange; color:black"|Header
|}
===S-Pprzlink v2.0===
In this case we decrypt only packets with matching DESTINATION (or broadcast), to avoid decrypting unwanted data.
PPRZ-message: ABCxxxxxxxDE
    A PPRZ_STX (0x99)
    B LENGTH (A->E)
    C PPRZ_DATA
      0 SOURCE (~sender_ID)
      1 DESTINATION (can be a broadcast ID)
      2-5 COUNTER (4 bytes)
      // ciphertext start
      6 CLASS/COMPONENT
        bits 0-3: 16 class ID available
        bits 4-7: 16 component ID available
      7 MSG_ID
      8 MSG_PAYLOAD
      . DATA (messages.xml)
      // ciphertext end
      8-19 TAG (12 bytes)
     D PPRZ_CHECKSUM_A (sum[B->C])
     D PPRZ_CHECKSUM_A (sum[B->C])
     E PPRZ_CHECKSUM_B (sum[ck_a])
     E PPRZ_CHECKSUM_B (sum[ck_a])

Latest revision as of 18:25, 17 March 2018

Introduction

Messages Format and how to use them

For the message definitions see http://docs.paparazziuav.org/latest/paparazzi_messages.html

Types

Paparazzi "PPRZ" standard message for direct serial connection

Pprzlink v1.0

PPRZ-message: ABCxxxxxxxDE
    A PPRZ_STX (0x99)
    B LENGTH (PPRZ_STX->PPRZ_CHECKSUM_B)
    C PPRZ_DATA
      0 SENDER_ID
      1 MSG_ID
      2 MSG_PAYLOAD
      . DATA (messages.xml)
    D PPRZ_CHECKSUM_A (sum[B->C])
    E PPRZ_CHECKSUM_B (sum[ck_a])

Pprzlink v2.0

Note that there is currently no explicit way to recognize whether the protocol is v1.0 or 2.0 so it has to be known before the communication. 

PPRZ-message: ABCxxxxxxxDE
    A PPRZ_STX (0x99)
    B LENGTH (A->E)
    C PPRZ_DATA
      0 SOURCE (~sender_ID)
      1 DESTINATION (can be a broadcast ID)
      2 CLASS/COMPONENT
        bits 0-3: 16 class ID available
        bits 4-7: 16 component ID available
      3 MSG_ID
      4 MSG_PAYLOAD
      . DATA (messages.xml)
    D PPRZ_CHECKSUM_A (sum[B->C])
    E PPRZ_CHECKSUM_B (sum[ck_a])

XBee API format

XBee-message: ABCDxxxxxxxE
    A XBEE_START (0x7E)
    B LENGTH_MSB (D->D)
    C LENGTH_LSB
    D XBEE_PAYLOAD
      0 XBEE_TX16 (0x01) / XBEE_RX16 (0x81)
      1 FRAME_ID (0)     / SRC_ID_MSB
      2 DEST_ID_MSB      / SRC_ID_LSB
      3 DEST_ID_LSB      / XBEE_RSSI
      4 TX16_OPTIONS (0) / RX16_OPTIONS
      5 PPRZ_DATA
        0 SENDER_ID
        1 MSG_ID
        2 MSG_PAYLOAD
        . DATA (messages.xml)
    E XBEE_CHECKSUM (sum[D->D])

   ID is AC_ID for aircraft, 0x100 for ground station

Telemetry storage format for data logger

TLM-message: ABCDEFGHxxxxxxxI
    A PPRZ_STX (0x99)
    B LENGTH (H->H)
    C SOURCE (0=uart0, 1=uart1, 2=i2c0, ...)
    D TIMESTAMP_LSB (100 microsecond raster)
    E TIMESTAMP
    F TIMESTAMP
    G TIMESTAMP_MSB
    H PPRZ_DATA
      0 SENDER_ID
      1 MSG_ID
      2 MSG_PAYLOAD
      . DATA (messages.xml)
    I CHECKSUM (sum[B->H])
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