## This file is part of Scapy ## See http://www.secdev.org/projects/scapy for more informations ## Copyright (C) Philippe Biondi ## This program is published under a GPLv2 license ## Copyright (C) 2005 Guillaume Valadon ## Arnaud Ebalard """ Utility functions for IPv6. """ from config import conf from data import * from utils import * def construct_source_candidate_set(addr, plen, laddr, loname): """ Given all addresses assigned to a specific interface ('laddr' parameter), this function returns the "candidate set" associated with 'addr/plen'. Basically, the function filters all interface addresses to keep only those that have the same scope as provided prefix. This is on this list of addresses that the source selection mechanism will then be performed to select the best source address associated with some specific destination that uses this prefix. """ def cset_sort(x,y): x_global = 0 if in6_isgladdr(x): x_global = 1 y_global = 0 if in6_isgladdr(y): y_global = 1 res = y_global - x_global if res != 0 or y_global != 1: return res # two global addresses: if one is native, it wins. if not in6_isaddr6to4(x): return -1; return -res cset = [] if in6_isgladdr(addr) or in6_isuladdr(addr): cset = filter(lambda x: x[1] == IPV6_ADDR_GLOBAL, laddr) elif in6_islladdr(addr): cset = filter(lambda x: x[1] == IPV6_ADDR_LINKLOCAL, laddr) elif in6_issladdr(addr): cset = filter(lambda x: x[1] == IPV6_ADDR_SITELOCAL, laddr) elif in6_ismaddr(addr): if in6_ismnladdr(addr): cset = [('::1', 16, loname)] elif in6_ismgladdr(addr): cset = filter(lambda x: x[1] == IPV6_ADDR_GLOBAL, laddr) elif in6_ismlladdr(addr): cset = filter(lambda x: x[1] == IPV6_ADDR_LINKLOCAL, laddr) elif in6_ismsladdr(addr): cset = filter(lambda x: x[1] == IPV6_ADDR_SITELOCAL, laddr) elif addr == '::' and plen == 0: cset = filter(lambda x: x[1] == IPV6_ADDR_GLOBAL, laddr) cset = map(lambda x: x[0], cset) cset.sort(cmp=cset_sort) # Sort with global addresses first return cset def get_source_addr_from_candidate_set(dst, candidate_set): """ This function implement a limited version of source address selection algorithm defined in section 5 of RFC 3484. The format is very different from that described in the document because it operates on a set of candidate source address for some specific route. """ def scope_cmp(a, b): """ Given two addresses, returns -1, 0 or 1 based on comparison of their scope """ scope_mapper = {IPV6_ADDR_GLOBAL: 4, IPV6_ADDR_SITELOCAL: 3, IPV6_ADDR_LINKLOCAL: 2, IPV6_ADDR_LOOPBACK: 1} sa = in6_getscope(a) if sa == -1: sa = IPV6_ADDR_LOOPBACK sb = in6_getscope(b) if sb == -1: sb = IPV6_ADDR_LOOPBACK sa = scope_mapper[sa] sb = scope_mapper[sb] if sa == sb: return 0 if sa > sb: return 1 return -1 def rfc3484_cmp(source_a, source_b): """ The function implements a limited version of the rules from Source Address selection algorithm defined section of RFC 3484. """ # Rule 1: Prefer same address if source_a == dst: return 1 if source_b == dst: return 1 # Rule 2: Prefer appropriate scope tmp = scope_cmp(source_a, source_b) if tmp == -1: if scope_cmp(source_a, dst) == -1: return 1 else: return -1 elif tmp == 1: if scope_cmp(source_b, dst) == -1: return 1 else: return -1 # Rule 3: cannot be easily implemented # Rule 4: cannot be easily implemented # Rule 5: does not make sense here # Rule 6: cannot be implemented # Rule 7: cannot be implemented # Rule 8: Longest prefix match tmp1 = in6_get_common_plen(source_a, dst) tmp2 = in6_get_common_plen(source_b, dst) if tmp1 > tmp2: return 1 elif tmp2 > tmp1: return -1 return 0 if not candidate_set: # Should not happen return None candidate_set.sort(cmp=rfc3484_cmp, reverse=True) return candidate_set[0] def find_ifaddr2(addr, plen, laddr): dstAddrType = in6_getAddrType(addr) if dstAddrType == IPV6_ADDR_UNSPECIFIED: # Shouldn't happen as dst addr return None if dstAddrType == IPV6_ADDR_LOOPBACK: return None tmp = [[]] + map(lambda (x,y,z): (in6_getAddrType(x), x, y, z), laddr) def filterSameScope(l, t): if (t[0] & dstAddrType & IPV6_ADDR_SCOPE_MASK) == 0: l.append(t) return l sameScope = reduce(filterSameScope, tmp) l = len(sameScope) if l == 1: # Only one address for our scope return sameScope[0][1] elif l > 1: # Muliple addresses for our scope stfAddr = filter(lambda x: x[0] & IPV6_ADDR_6TO4, sameScope) nativeAddr = filter(lambda x: not (x[0] & IPV6_ADDR_6TO4), sameScope) if not (dstAddrType & IPV6_ADDR_6TO4): # destination is not 6to4 if len(nativeAddr) != 0: return nativeAddr[0][1] return stfAddr[0][1] else: # Destination is 6to4, try to use source 6to4 addr if any if len(stfAddr) != 0: return stfAddr[0][1] return nativeAddr[0][1] else: return None # Think before modify it : for instance, FE::1 does exist and is unicast # there are many others like that. # TODO : integrate Unique Local Addresses def in6_getAddrType(addr): naddr = inet_pton(socket.AF_INET6, addr) paddr = inet_ntop(socket.AF_INET6, naddr) # normalize addrType = 0 # _Assignable_ Global Unicast Address space # is defined in RFC 3513 as those in 2000::/3 if ((struct.unpack("B", naddr[0])[0] & 0xE0) == 0x20): addrType = (IPV6_ADDR_UNICAST | IPV6_ADDR_GLOBAL) if naddr[:2] == ' \x02': # Mark 6to4 @ addrType |= IPV6_ADDR_6TO4 elif naddr[0] == '\xff': # multicast addrScope = paddr[3] if addrScope == '2': addrType = (IPV6_ADDR_LINKLOCAL | IPV6_ADDR_MULTICAST) elif addrScope == 'e': addrType = (IPV6_ADDR_GLOBAL | IPV6_ADDR_MULTICAST) else: addrType = (IPV6_ADDR_GLOBAL | IPV6_ADDR_MULTICAST) elif ((naddr[0] == '\xfe') and ((int(paddr[2], 16) & 0xC) == 0x8)): addrType = (IPV6_ADDR_UNICAST | IPV6_ADDR_LINKLOCAL) elif paddr == "::1": addrType = IPV6_ADDR_LOOPBACK elif paddr == "::": addrType = IPV6_ADDR_UNSPECIFIED else: # Everything else is global unicast (RFC 3513) # Even old deprecated (RFC3879) Site-Local addresses addrType = (IPV6_ADDR_GLOBAL | IPV6_ADDR_UNICAST) return addrType def in6_mactoifaceid(mac, ulbit=None): """ Compute the interface ID in modified EUI-64 format associated to the Ethernet address provided as input. value taken by U/L bit in the interface identifier is basically the reversed value of that in given MAC address it can be forced to a specific value by using optional 'ulbit' parameter. """ if len(mac) != 17: return None m = "".join(mac.split(':')) if len(m) != 12: return None first = int(m[0:2], 16) if ulbit is None or not (ulbit == 0 or ulbit == 1): ulbit = [1,'-',0][first & 0x02] ulbit *= 2 first = "%.02x" % ((first & 0xFD) | ulbit) eui64 = first + m[2:4] + ":" + m[4:6] + "FF:FE" + m[6:8] + ":" + m[8:12] return eui64.upper() def in6_ifaceidtomac(ifaceid): # TODO: finish commenting function behavior """ Extract the mac address from provided iface ID. Iface ID is provided in printable format ("XXXX:XXFF:FEXX:XXXX", eventually compressed). None is returned on error. """ try: ifaceid = inet_pton(socket.AF_INET6, "::"+ifaceid)[8:16] except: return None if ifaceid[3:5] != '\xff\xfe': return None first = struct.unpack("B", ifaceid[:1])[0] ulbit = 2*[1,'-',0][first & 0x02] first = struct.pack("B", ((first & 0xFD) | ulbit)) oui = first + ifaceid[1:3] end = ifaceid[5:] l = map(lambda x: "%.02x" % struct.unpack("B", x)[0], list(oui+end)) return ":".join(l) def in6_addrtomac(addr): """ Extract the mac address from provided address. None is returned on error. """ mask = inet_pton(socket.AF_INET6, "::ffff:ffff:ffff:ffff") x = in6_and(mask, inet_pton(socket.AF_INET6, addr)) ifaceid = inet_ntop(socket.AF_INET6, x)[2:] return in6_ifaceidtomac(ifaceid) def in6_addrtovendor(addr): """ Extract the MAC address from a modified EUI-64 constructed IPv6 address provided and use the IANA oui.txt file to get the vendor. The database used for the conversion is the one loaded by Scapy, based on Wireshark (/usr/share/wireshark/wireshark/manuf) None is returned on error, "UNKNOWN" if the vendor is unknown. """ mac = in6_addrtomac(addr) if mac is None: return None res = conf.manufdb._get_manuf(mac) if len(res) == 17 and res.count(':') != 5: # Mac address, i.e. unknown res = "UNKNOWN" return res def in6_getLinkScopedMcastAddr(addr, grpid=None, scope=2): """ Generate a Link-Scoped Multicast Address as described in RFC 4489. Returned value is in printable notation. 'addr' parameter specifies the link-local address to use for generating Link-scoped multicast address IID. By default, the function returns a ::/96 prefix (aka last 32 bits of returned address are null). If a group id is provided through 'grpid' parameter, last 32 bits of the address are set to that value (accepted formats : '\x12\x34\x56\x78' or '12345678' or 0x12345678 or 305419896). By default, generated address scope is Link-Local (2). That value can be modified by passing a specific 'scope' value as an argument of the function. RFC 4489 only authorizes scope values <= 2. Enforcement is performed by the function (None will be returned). If no link-local address can be used to generate the Link-Scoped IPv6 Multicast address, or if another error occurs, None is returned. """ if not scope in [0, 1, 2]: return None try: if not in6_islladdr(addr): return None addr = inet_pton(socket.AF_INET6, addr) except: warning("in6_getLinkScopedMcastPrefix(): Invalid address provided") return None iid = addr[8:] if grpid is None: grpid = '\x00\x00\x00\x00' else: if type(grpid) is str: if len(grpid) == 8: try: grpid = int(grpid, 16) & 0xffffffff except: warning("in6_getLinkScopedMcastPrefix(): Invalid group id provided") return None elif len(grpid) == 4: try: grpid = struct.unpack("!I", grpid)[0] except: warning("in6_getLinkScopedMcastPrefix(): Invalid group id provided") return None grpid = struct.pack("!I", grpid) flgscope = struct.pack("B", 0xff & ((0x3 << 4) | scope)) plen = '\xff' res = '\x00' a = '\xff' + flgscope + res + plen + iid + grpid return inet_ntop(socket.AF_INET6, a) def in6_get6to4Prefix(addr): """ Returns the /48 6to4 prefix associated with provided IPv4 address On error, None is returned. No check is performed on public/private status of the address """ try: addr = inet_pton(socket.AF_INET, addr) addr = inet_ntop(socket.AF_INET6, '\x20\x02'+addr+'\x00'*10) except: return None return addr def in6_6to4ExtractAddr(addr): """ Extract IPv4 address embbeded in 6to4 address. Passed address must be a 6to4 addrees. None is returned on error. """ try: addr = inet_pton(socket.AF_INET6, addr) except: return None if addr[:2] != " \x02": return None return inet_ntop(socket.AF_INET, addr[2:6]) def in6_getLocalUniquePrefix(): """ Returns a pseudo-randomly generated Local Unique prefix. Function follows recommandation of Section 3.2.2 of RFC 4193 for prefix generation. """ # Extracted from RFC 1305 (NTP) : # NTP timestamps are represented as a 64-bit unsigned fixed-point number, # in seconds relative to 0h on 1 January 1900. The integer part is in the # first 32 bits and the fraction part in the last 32 bits. # epoch = (1900, 1, 1, 0, 0, 0, 5, 1, 0) # x = time.time() # from time import gmtime, strftime, gmtime, mktime # delta = mktime(gmtime(0)) - mktime(self.epoch) # x = x-delta tod = time.time() # time of day. Will bother with epoch later i = int(tod) j = int((tod - i)*(2**32)) tod = struct.pack("!II", i,j) # TODO: Add some check regarding system address gathering rawmac = get_if_raw_hwaddr(conf.iface6)[1] mac = ":".join(map(lambda x: "%.02x" % ord(x), list(rawmac))) # construct modified EUI-64 ID eui64 = inet_pton(socket.AF_INET6, '::' + in6_mactoifaceid(mac))[8:] import sha globalid = sha.new(tod+eui64).digest()[:5] return inet_ntop(socket.AF_INET6, '\xfd' + globalid + '\x00'*10) def in6_getRandomizedIfaceId(ifaceid, previous=None): """ Implements the interface ID generation algorithm described in RFC 3041. The function takes the Modified EUI-64 interface identifier generated as described in RFC 4291 and an optional previous history value (the first element of the output of this function). If no previous interface identifier is provided, a random one is generated. The function returns a tuple containing the randomized interface identifier and the history value (for possible future use). Input and output values are provided in a "printable" format as depicted below. ex: >>> in6_getRandomizedIfaceId('20b:93ff:feeb:2d3') ('4c61:76ff:f46a:a5f3', 'd006:d540:db11:b092') >>> in6_getRandomizedIfaceId('20b:93ff:feeb:2d3', previous='d006:d540:db11:b092') ('fe97:46fe:9871:bd38', 'eeed:d79c:2e3f:62e') """ s = "" if previous is None: d = "".join(map(chr, range(256))) for i in range(8): s += random.choice(d) previous = s s = inet_pton(socket.AF_INET6, "::"+ifaceid)[8:] + previous import md5 s = md5.new(s).digest() s1,s2 = s[:8],s[8:] s1 = chr(ord(s1[0]) | 0x04) + s1[1:] s1 = inet_ntop(socket.AF_INET6, "\xff"*8 + s1)[20:] s2 = inet_ntop(socket.AF_INET6, "\xff"*8 + s2)[20:] return (s1, s2) _rfc1924map = [ '0','1','2','3','4','5','6','7','8','9','A','B','C','D','E', 'F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T', 'U','V','W','X','Y','Z','a','b','c','d','e','f','g','h','i', 'j','k','l','m','n','o','p','q','r','s','t','u','v','w','x', 'y','z','!','#','$','%','&','(',')','*','+','-',';','<','=', '>','?','@','^','_','`','{','|','}','~' ] def in6_ctop(addr): """ Convert an IPv6 address in Compact Representation Notation (RFC 1924) to printable representation ;-) Returns None on error. """ if len(addr) != 20 or not reduce(lambda x,y: x and y, map(lambda x: x in _rfc1924map, addr)): return None i = 0 for c in addr: j = _rfc1924map.index(c) i = 85*i + j res = [] for j in range(4): res.append(struct.pack("!I", i%2**32)) i = i/(2**32) res.reverse() return inet_ntop(socket.AF_INET6, "".join(res)) def in6_ptoc(addr): """ Converts an IPv6 address in printable representation to RFC 1924 Compact Representation ;-) Returns None on error. """ try: d=struct.unpack("!IIII", inet_pton(socket.AF_INET6, addr)) except: return None res = 0 m = [2**96, 2**64, 2**32, 1] for i in range(4): res += d[i]*m[i] rem = res res = [] while rem: res.append(_rfc1924map[rem%85]) rem = rem/85 res.reverse() return "".join(res) def in6_isaddr6to4(x): """ Return True if provided address (in printable format) is a 6to4 address (being in 2002::/16). """ x = inet_pton(socket.AF_INET6, x) return x[:2] == ' \x02' conf.teredoPrefix = "2001::" # old one was 3ffe:831f (it is a /32) conf.teredoServerPort = 3544 def in6_isaddrTeredo(x): """ Return True if provided address is a Teredo, meaning it is under the /32 conf.teredoPrefix prefix value (by default, 2001::). Otherwise, False is returned. Address must be passed in printable format. """ our = inet_pton(socket.AF_INET6, x)[0:4] teredoPrefix = inet_pton(socket.AF_INET6, conf.teredoPrefix)[0:4] return teredoPrefix == our def teredoAddrExtractInfo(x): """ Extract information from a Teredo address. Return value is a 4-tuple made of IPv4 address of Teredo server, flag value (int), mapped address (non obfuscated) and mapped port (non obfuscated). No specific checks are performed on passed address. """ addr = inet_pton(socket.AF_INET6, x) server = inet_ntop(socket.AF_INET, addr[4:8]) flag = struct.unpack("!H",addr[8:10])[0] mappedport = struct.unpack("!H",strxor(addr[10:12],'\xff'*2))[0] mappedaddr = inet_ntop(socket.AF_INET, strxor(addr[12:16],'\xff'*4)) return server, flag, mappedaddr, mappedport def in6_iseui64(x): """ Return True if provided address has an interface identifier part created in modified EUI-64 format (meaning it matches *::*:*ff:fe*:*). Otherwise, False is returned. Address must be passed in printable format. """ eui64 = inet_pton(socket.AF_INET6, '::ff:fe00:0') x = in6_and(inet_pton(socket.AF_INET6, x), eui64) return x == eui64 def in6_isanycast(x): # RFC 2526 if in6_iseui64(x): s = '::fdff:ffff:ffff:ff80' packed_x = inet_pton(socket.AF_INET6, x) packed_s = inet_pton(socket.AF_INET6, s) x_and_s = in6_and(packed_x, packed_s) return x_and_s == packed_s else: # not EUI-64 #| n bits | 121-n bits | 7 bits | #+---------------------------------+------------------+------------+ #| subnet prefix | 1111111...111111 | anycast ID | #+---------------------------------+------------------+------------+ # | interface identifier field | warning('in6_isanycast(): TODO not EUI-64') return 0 def _in6_bitops(a1, a2, operator=0): a1 = struct.unpack('4I', a1) a2 = struct.unpack('4I', a2) fop = [ lambda x,y: x | y, lambda x,y: x & y, lambda x,y: x ^ y ] ret = map(fop[operator%len(fop)], a1, a2) t = ''.join(map(lambda x: struct.pack('I', x), ret)) return t def in6_or(a1, a2): """ Provides a bit to bit OR of provided addresses. They must be passed in network format. Return value is also an IPv6 address in network format. """ return _in6_bitops(a1, a2, 0) def in6_and(a1, a2): """ Provides a bit to bit AND of provided addresses. They must be passed in network format. Return value is also an IPv6 address in network format. """ return _in6_bitops(a1, a2, 1) def in6_xor(a1, a2): """ Provides a bit to bit XOR of provided addresses. They must be passed in network format. Return value is also an IPv6 address in network format. """ return _in6_bitops(a1, a2, 2) def in6_cidr2mask(m): """ Return the mask (bitstring) associated with provided length value. For instance if function is called on 48, return value is '\xff\xff\xff\xff\xff\xff\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'. """ if m > 128 or m < 0: raise Scapy_Exception("value provided to in6_cidr2mask outside [0, 128] domain (%d)" % m) t = [] for i in xrange(0, 4): t.append(max(0, 2**32 - 2**(32-min(32, m)))) m -= 32 return ''.join(map(lambda x: struct.pack('!I', x), t)) def in6_getnsma(a): """ Return link-local solicited-node multicast address for given address. Passed address must be provided in network format. Returned value is also in network format. """ r = in6_and(a, inet_pton(socket.AF_INET6, '::ff:ffff')) r = in6_or(inet_pton(socket.AF_INET6, 'ff02::1:ff00:0'), r) return r def in6_getnsmac(a): # return multicast Ethernet address associated with multicast v6 destination """ Return the multicast mac address associated with provided IPv6 address. Passed address must be in network format. """ a = struct.unpack('16B', a)[-4:] mac = '33:33:' mac += ':'.join(map(lambda x: '%.2x' %x, a)) return mac def in6_getha(prefix): """ Return the anycast address associated with all home agents on a given subnet. """ r = in6_and(inet_pton(socket.AF_INET6, prefix), in6_cidr2mask(64)) r = in6_or(r, inet_pton(socket.AF_INET6, '::fdff:ffff:ffff:fffe')) return inet_ntop(socket.AF_INET6, r) def in6_ptop(str): """ Normalizes IPv6 addresses provided in printable format, returning the same address in printable format. (2001:0db8:0:0::1 -> 2001:db8::1) """ return inet_ntop(socket.AF_INET6, inet_pton(socket.AF_INET6, str)) def in6_isincluded(addr, prefix, plen): """ Returns True when 'addr' belongs to prefix/plen. False otherwise. """ temp = inet_pton(socket.AF_INET6, addr) pref = in6_cidr2mask(plen) zero = inet_pton(socket.AF_INET6, prefix) return zero == in6_and(temp, pref) def in6_isdocaddr(str): """ Returns True if provided address in printable format belongs to 2001:db8::/32 address space reserved for documentation (as defined in RFC 3849). """ return in6_isincluded(str, '2001:db8::', 32) def in6_islladdr(str): """ Returns True if provided address in printable format belongs to _allocated_ link-local unicast address space (fe80::/10) """ return in6_isincluded(str, 'fe80::', 10) def in6_issladdr(str): """ Returns True if provided address in printable format belongs to _allocated_ site-local address space (fec0::/10). This prefix has been deprecated, address being now reserved by IANA. Function will remain for historic reasons. """ return in6_isincluded(str, 'fec0::', 10) def in6_isuladdr(str): """ Returns True if provided address in printable format belongs to Unique local address space (fc00::/7). """ return in6_isincluded(str, 'fc00::', 7) # TODO : we should see the status of Unique Local addresses against # global address space. # Up-to-date information is available through RFC 3587. # We should review function behavior based on its content. def in6_isgladdr(str): """ Returns True if provided address in printable format belongs to _allocated_ global address space (2000::/3). Please note that, Unique Local addresses (FC00::/7) are not part of global address space, and won't match. """ return in6_isincluded(str, '2000::', 3) def in6_ismaddr(str): """ Returns True if provided address in printable format belongs to allocated Multicast address space (ff00::/8). """ return in6_isincluded(str, 'ff00::', 8) def in6_ismnladdr(str): """ Returns True if address belongs to node-local multicast address space (ff01::/16) as defined in RFC """ return in6_isincluded(str, 'ff01::', 16) def in6_ismgladdr(str): """ Returns True if address belongs to global multicast address space (ff0e::/16). """ return in6_isincluded(str, 'ff0e::', 16) def in6_ismlladdr(str): """ Returns True if address balongs to link-local multicast address space (ff02::/16) """ return in6_isincluded(str, 'ff02::', 16) def in6_ismsladdr(str): """ Returns True if address belongs to site-local multicast address space (ff05::/16). Site local address space has been deprecated. Function remains for historic reasons. """ return in6_isincluded(str, 'ff05::', 16) def in6_isaddrllallnodes(str): """ Returns True if address is the link-local all-nodes multicast address (ff02::1). """ return (inet_pton(socket.AF_INET6, "ff02::1") == inet_pton(socket.AF_INET6, str)) def in6_isaddrllallservers(str): """ Returns True if address is the link-local all-servers multicast address (ff02::2). """ return (inet_pton(socket.AF_INET6, "ff02::2") == inet_pton(socket.AF_INET6, str)) def in6_getscope(addr): """ Returns the scope of the address. """ if in6_isgladdr(addr) or in6_isuladdr(addr): scope = IPV6_ADDR_GLOBAL elif in6_islladdr(addr): scope = IPV6_ADDR_LINKLOCAL elif in6_issladdr(addr): scope = IPV6_ADDR_SITELOCAL elif in6_ismaddr(addr): if in6_ismgladdr(addr): scope = IPV6_ADDR_GLOBAL elif in6_ismlladdr(addr): scope = IPV6_ADDR_LINKLOCAL elif in6_ismsladdr(addr): scope = IPV6_ADDR_SITELOCAL elif in6_ismnladdr(addr): scope = IPV6_ADDR_LOOPBACK else: scope = -1 elif addr == '::1': scope = IPV6_ADDR_LOOPBACK else: scope = -1 return scope def in6_get_common_plen(a, b): """ Return common prefix length of IPv6 addresses a and b. """ def matching_bits(byte1, byte2): for i in range(8): cur_mask = 0x80 >> i if (byte1 & cur_mask) != (byte2 & cur_mask): return i return 8 tmpA = inet_pton(socket.AF_INET6, a) tmpB = inet_pton(socket.AF_INET6, b) for i in range(16): mbits = matching_bits(ord(tmpA[i]), ord(tmpB[i])) if mbits != 8: return 8*i + mbits return 128