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icao_filter.[ch] from the experimental branch.

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This commit is contained in:
Oliver Jowett 2015-01-20 23:53:26 +00:00
parent 270a22e06a
commit 0433ed3f5d
7 changed files with 197 additions and 65 deletions

54
mode_s.c
View file

@ -67,46 +67,6 @@ int modesMessageLenByType(int type) {
return (type & 0x10) ? MODES_LONG_MSG_BITS : MODES_SHORT_MSG_BITS ;
}
//=========================================================================
//
// Hash the ICAO address to index our cache of MODES_ICAO_CACHE_LEN
// elements, that is assumed to be a power of two
//
uint32_t ICAOCacheHashAddress(uint32_t a) {
// The following three rounds wil make sure that every bit affects
// every output bit with ~ 50% of probability.
a = ((a >> 16) ^ a) * 0x45d9f3b;
a = ((a >> 16) ^ a) * 0x45d9f3b;
a = ((a >> 16) ^ a);
return a & (MODES_ICAO_CACHE_LEN-1);
}
//
//=========================================================================
//
// Add the specified entry to the cache of recently seen ICAO addresses.
// Note that we also add a timestamp so that we can make sure that the
// entry is only valid for MODES_ICAO_CACHE_TTL seconds.
//
void addRecentlySeenICAOAddr(uint32_t addr) {
uint32_t h = ICAOCacheHashAddress(addr);
Modes.icao_cache[h*2] = addr;
Modes.icao_cache[h*2+1] = (uint32_t) time(NULL);
}
//
//=========================================================================
//
// Returns 1 if the specified ICAO address was seen in a DF format with
// proper checksum (not xored with address) no more than * MODES_ICAO_CACHE_TTL
// seconds ago. Otherwise returns 0.
//
int ICAOAddressWasRecentlySeen(uint32_t addr) {
uint32_t h = ICAOCacheHashAddress(addr);
uint32_t a = Modes.icao_cache[h*2];
uint32_t t = Modes.icao_cache[h*2+1];
uint64_t tn = time(NULL);
return ( (a) && (a == addr) && ( (tn - t) <= MODES_ICAO_CACHE_TTL) );
}
//
//=========================================================================
//
@ -342,7 +302,7 @@ void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) {
// If we correct, validate ICAO addr to help filter birthday paradox solutions.
if (mm->correctedbits) {
uint32_t ulAddr = (msg[1] << 16) | (msg[2] << 8) | (msg[3]);
if (!ICAOAddressWasRecentlySeen(ulAddr))
if (!icaoFilterTest(ulAddr))
mm->correctedbits = 0;
}
}
@ -357,11 +317,11 @@ void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) {
if ((mm->crcok = (0 == mm->crc))) {
// DF 11 : if crc == 0 try to populate our ICAO addresses whitelist.
addRecentlySeenICAOAddr(mm->addr);
icaoFilterAdd(mm->addr);
} else if (mm->crc < 80) {
mm->crcok = ICAOAddressWasRecentlySeen(mm->addr);
mm->crcok = icaoFilterTest(mm->addr);
if (mm->crcok) {
addRecentlySeenICAOAddr(mm->addr);
icaoFilterAdd(mm->addr);
}
}
@ -371,7 +331,7 @@ void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) {
if ((mm->crcok = (0 == mm->crc))) {
// DF 17 : if crc == 0 try to populate our ICAO addresses whitelist.
addRecentlySeenICAOAddr(mm->addr);
icaoFilterAdd(mm->addr);
}
} else if (mm->msgtype == 18) { // DF 18
@ -380,13 +340,13 @@ void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) {
if ((mm->crcok = (0 == mm->crc))) {
// DF 18 : if crc == 0 try to populate our ICAO addresses whitelist.
addRecentlySeenICAOAddr(mm->addr);
icaoFilterAdd(mm->addr);
}
} else { // All other DF's
// Compare the checksum with the whitelist of recently seen ICAO
// addresses. If it matches one, then declare the message as valid
mm->crcok = ICAOAddressWasRecentlySeen(mm->addr = mm->crc);
mm->crcok = icaoFilterTest(mm->addr = mm->crc);
}
// If we're checking CRC and the CRC is invalid, then we can't trust any