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no edit summary
= IR communications =
= IR communications =
Packets are sent using IrDA-SIR (using [[ir:USER]]), with a 8N1 encoding (eight data bits, one stop bit, without parity). Each one is formed by a 2-byte header, a varint with the payload size, an obfuscated payload, and trailing error detection byte.
== Packet header ==
The packet header is fixed and consists in a synchronization byte (0xA5), followed by a unused (possibly RFU) zero byte. After these two hardcoded bytes, there's a varint representing the payload size, which may use one byte or two, depending on the how big the payload is.
* For payloads with less than 64 bytes, the third byte represents the payload size.
* For packets with up to 16383 bytes, the size is split in two bytes, with the third byte being the upper 6 bits of the payload size, OR'd with 0x40, and the fourth being the lower eight bits of the payload size
For packets with less than 64 bytes:
{| class="wikitable" border="1"
! Sync
! RFU
! Size
|-
| 0xA5
| 0x00
| size
|}
For packets with up to 16383 bytes:
{| class="wikitable" border="1"
! Sync
! RFU
! Size (1)
! Size (2)
|-
| 0xA5
| 0x00
| (size >> 8) <nowiki>|</nowiki> 0x40
| size & 0xFF
|}
In C:
<nowiki>uint8_t * setPacketHeader(uint8_t * buffer, size_t payloadSize) {
assert(payloadSize < 16384);
buffer[0] = 0xA5;
buffer[1] = 0x00;
if (payloadSize < 64) {
buffer[2] = payloadSize;
buffer += 3;
} else {
buffer[2] = 0x40 | (payloadSize >> 8);
buffer[3] = payloadSize;
buffer += 4;
}
return buffer;
}</nowiki>
== Payload ==
The payload is obfuscated using a XOR-based encryption. In C:
<nowiki>void payloadObfuscate(const void * voidplain, void * voidcipher, size_t size) {
uint16_t * plain = (uint16_t *) voidplain;
uint16_t * cipher = (uint16_t *) voidcipher;
size_t halfCount = size / sizeof(uint16_t);
uint16_t xorval = htobe16(0xE963);
size_t i;
for (i = 0; i < halfCount; i++) {
xorval ^= plain[i];
cipher[i] = xorval;
}
}
void payloadDeobfuscate(const void * voidcipher, void * voidplain, size_t size) {
uint16_t * cipher = (uint16_t *) voidcipher;
uint16_t * plain = (uint16_t *) voidplain;
size_t halfCount = size / sizeof(uint16_t);
if (halfCount) {
size_t i;
for (i = halfCount - 1; i > 0; i--) {
plain[i] = cipher[i] ^ cipher[i - 1];
}
plain[0] = cipher[0] ^ htobe16(0xE963);
}
}</nowiki>
== Error detection ==
The trailing error detection byte is calculated using [[CRC-8-CCITT]] <b>over the whole packet</b> (both the header and the payload)
== Samples ==
== Samples ==