fields: decode BitLenField integers with int.from_bytes

scapy/fields.py

@@ -2433,12 +2433,9 @@ def getfield(self,  # type: ignore
         nb_bytes = (self.size + bn - 1) // 8 + 1
         w = s[:nb_bytes]
 
-        # split the substring byte by byte
-        _bytes = struct.unpack('!%dB' % nb_bytes, w)
-
-        b = 0
-        for c in range(nb_bytes):
-            b |= int(_bytes[c]) << (nb_bytes - c - 1) * 8
+        if len(w) < nb_bytes:
+            raise struct.error("unpack requires a buffer of %d bytes" % nb_bytes)
+        b = int.from_bytes(w, byteorder="big")
 
         # get rid of high order bits
         b &= (1 << (nb_bytes * 8 - bn)) - 1

test/scapy/layers/ssh.uts

@@ -23,6 +23,31 @@ assert pkts[8].pay.first_kex_packet_follows == 0
 
 assert pkts[9].pay.e.value == 2350579254774455149352841074576538343990628078324267734527140871419932900538846241321452574779013468175018290651674793284015144587365340014962083771650859331476013596977123734998706481058518220105378857548427645559226229797476788395456646389818256564838400739135010680681163456095677232493468587230912056633743356721223955966756143014970734820639779746710511156996619195651512803235508645669051962031830043263352566212925544898655158819407252176433755590900240990111833619058714386338971655960765233885975850331922799954445954999296511309262036243757363804224821843032668273263064448847356873248470173458896243397517402866118125112555466428030166305568609671333602038983517505792245686243281766138834921907336198416449172686346486278292406454736650900489703602941311383274571253473117352192402069818356338637658276508681778175858698292872544113899589241205559671386224999719303843179598966006636814844667908804397048115462945951578163865384872376314062218622823399683168509281546378022234848416282276373248755506541244682438394426446625521247772730497030387798046748675856950435919346613694108323269457293633349708282520556429147475379754811181108827452704284587405668562299209768965780662855380191554093502874303015220051947466960323628390298028334555285261078171376959928596505395834904631983924930632066431620277301098016669514461539415396461120090857273167687140578309080011600491384868409678444601854368584606594031430672989514629489628693896623746874263590779323124144977231406480674784862894820443935735009785417326990153059454525335480905124180809168192695170554860881795940302149730125034860576014797577021907464402342887433222178995989216549376816454492040151004613910636289921361266911700572137074963622410473946132501034758965925448585513804452940921661377181371668124810916661795313840472724039889785883499146147917756012320556865741641210760458632895093416475238323450214341924898457846364890041182141641464569458439289152005646151462927271290045368143992045845833755058875621892664952349241777000175525150234301417133611325716295866942917806328460205857145603834255471170372323643072574234093090258963511137747272416302757220165305443240348220594316744411327905569373474701071080394539231361841208268626239088329642014216286141161796678306068065801822739617419769840590119143287370990841250896367782388086153939896000077437989471526045058990545907990089943059323343511158016141104461822684535344848072465778114834380180144470998703244485996404968078774187171096252472206846575112317045051585989901412734220984229769099373462781991394933642599850052765470790711255335450011529534223200229563089616493679704133500670071803056311472370457584460617950784473886654145020569892882621834458180061425761806601776138949785217977296683442504030198235793054259542236826904098257847794792743244091577002001218915723232763883537852453240602434246755006330557239933
 
+= Decode an mpint with one bulk numeric conversion
+from builtins import int as builtin_int
+import scapy.fields as fields_module
+from scapy.layers.ssh import Mpint
+
+class CountingInt:
+    calls = 0
+    def __call__(self, value):
+        self.calls += 1
+        return value
+    def from_bytes(self, value, byteorder):
+        self.calls += 1
+        return builtin_int.from_bytes(value, byteorder)
+
+field = Mpint(length=8).get_field("value")
+counter = CountingInt()
+fields_module.int = counter
+try:
+    _, value = field.getfield(Mpint(length=8), b"\x01" * 8)
+finally:
+    del fields_module.int
+
+assert value == builtin_int.from_bytes(b"\x01" * 8, "big")
+assert counter.calls == 1
+
 = Check for SSH Kex DH Reply
 
 assert isinstance(pkts[10].pay, SSHKexDHReply)