From: marshmellow42 <marshmellowrf@gmail.com>
Date: Mon, 6 Jul 2015 03:35:00 +0000 (-0400)
Subject: more verification on FDX-B tag demod - reduce...
X-Git-Tag: v2.3.0~33^2~4
X-Git-Url: https://git.zerfleddert.de/cgi-bin/gitweb.cgi/proxmark3-svn/commitdiff_plain/29b6cacc6ffece36f48bb8634b590cd82d96bf8b

more verification on FDX-B tag demod - reduce...

... false positives
---

diff --git a/client/cmddata.c b/client/cmddata.c
index bec1b5aa..bf10a6ec 100644
--- a/client/cmddata.c
+++ b/client/cmddata.c
@@ -1491,9 +1491,9 @@ int CmdFDXBdemodBI(const char *Cmd){
 
 	setDemodBuf(BitStream, 128, preambleIndex);
 
-	// remove but don't verify parity. (pType = 2)
+	// remove marker bits (1's every 9th digit after preamble) (pType = 2)
 	size = removeParity(BitStream, preambleIndex + 11, 9, 2, 117);
-	if ( size <= 103 ) {
+	if ( size != 104 ) {
 		if (g_debugMode) PrintAndLog("Error removeParity:: %d", size);
 		return 0;
 	}
diff --git a/common/lfdemod.c b/common/lfdemod.c
index f13a567c..a3a7a500 100644
--- a/common/lfdemod.c
+++ b/common/lfdemod.c
@@ -580,7 +580,7 @@ int IOdemodFSK(uint8_t *dest, size_t size)
 
 // by marshmellow
 // takes a array of binary values, start position, length of bits per parity (includes parity bit),
-//   Parity Type (1 for odd; 0 for even; 2 for just drop it), and binary Length (length to run) 
+//   Parity Type (1 for odd; 0 for even; 2 Always 1's), and binary Length (length to run) 
 size_t removeParity(uint8_t *BitStream, size_t startIdx, uint8_t pLen, uint8_t pType, size_t bLen)
 {
 	uint32_t parityWd = 0;
@@ -590,10 +590,12 @@ size_t removeParity(uint8_t *BitStream, size_t startIdx, uint8_t pLen, uint8_t p
 			parityWd = (parityWd << 1) | BitStream[startIdx+word+bit];
 			BitStream[j++] = (BitStream[startIdx+word+bit]);
 		}
-		j--;
+		j--; // overwrite parity with next data
 		// if parity fails then return 0
-		if (pType != 2) {
-			if (parityTest(parityWd, pLen, pType) == 0) return -1;
+		if (pType == 2) { // then marker bit which should be a 1
+			if (!BitStream[j]) return 0;
+		} else {
+			if (parityTest(parityWd, pLen, pType) == 0) return 0;			
 		}
 		bitCnt+=(pLen-1);
 		parityWd = 0;