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= 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 ==