def stripBCBHead(imgxor):
	return imgxor[imgxor.index(b"\x1A\x01") + 2:]

def freqtable(data, klen):
	freqtable = {}
	for x in range (0, klen):
		curfreq = {}
		for y in range (0, 256):
			curfreq[y] = 0
		freqtable[x] = curfreq

	for nr in range(len(data)):
		freqtable[nr % klen][data[nr]] += 1
		
	return freqtable

def findXORkeyfreq(bcbdata, byte, confidence, maxlen):
	data = stripBCBHead(bcbdata)
	keyFound = False
	key = bytearray()

	for curlen in reversed(range(4, maxlen+1)):
		fqtable = freqtable(data, curlen)
		key = bytearray(curlen)
		
		avgconf1 = 0
		for fqpos in sorted(fqtable):
			sortedlist = sorted(fqtable[fqpos].items(), key=lambda x: x[1], reverse=True)
			avgconf1 += 100 - sortedlist[1][1]/sortedlist[0][1]*100
			key[fqpos] = sortedlist[0][0] ^ byte
		
		avgconf1 = avgconf1/curlen
		
		if (avgconf1 >= confidence):
			key = deleterepeat(key)
			return key
	return b""