X-Git-Url: http://www.chiark.greenend.org.uk/ucgi/~ian/git?p=secnet.git;a=blobdiff_plain;f=NOTES;h=7ead923be70060d708e287b24d63a7dc2093e39a;hp=2443549cb67232ce64fe9b8f127ba20a31e32019;hb=5b5f297f9a9d47ee7e9804d5bdaa552f1953c6b6;hpb=b2a56f7c93d221607864761c590952b9a614dc9f diff --git a/NOTES b/NOTES index 2443549..7ead923 100644 --- a/NOTES +++ b/NOTES @@ -60,6 +60,17 @@ explicit option. NB packets may be routed if the source OR the destination is marked as allowing routing [otherwise packets couldn't get back from eg. chiark to a laptop at greenend]). +[the even newer plan] + +secnet sites are configured to grant access to particular IP address +ranges to the holder of a particular public key. The key can certify +other keys, which will then be permitted to use a subrange of the IP +address range of the certifying key. + +This means that secnet won't know in advance (i.e. at configuration +time) how many tunnels it might be required to support, so we have to +be able to create them (and routes, and so on) on the fly. + ** VPN-level configuration At a high level we just want to be able to indicate which groups of @@ -163,8 +174,9 @@ quite stable so the feature doesn't gain us much. Definitions: -A is the originating gateway machine -B is the destination gateway machine +A is the originating gateway machine name +B is the destination gateway machine name +A+ and B+ are the names with optional additional data, see below PK_A is the public RSA key of A PK_B is the public RSA key of B PK_A^-1 is the private RSA key of A @@ -182,16 +194,45 @@ i? is appropriate index for receiver Note that 'i' may be re-used from one session to the next, whereas 'n' is always fresh. +The optional additional data after the sender's name consists of some +initial subset of the following list of items: + * A 32-bit integer with a set of capability flags, representing the + abilities of the sender. + * More data which is yet to be defined and which must be ignored + by receivers. +The optional additional data after the receiver's name is not +currently used. If any is seen, it must be ignored. + +Capability flag bits must be in one the following two categories: + +1. Early capability flags must be advertised in MSG1 or MSG2, as + applicable. If MSG3 or MSG4 advertise any "early" capability bits, + MSG1 or MSG3 (as applicable) must have advertised them too. Sadly, + advertising an early capability flag will produce MSG1s which are + not understood by versions of secnet which predate the capability + mechanism. + +2. Late capability flags are advertised in MSG2 or MSG3, as + applicable. They may also appear in MSG1, but this is not + guaranteed. MSG4 must advertise the same set as MSG2. + +No capability flags are currently defined. Unknown capability flags +should be treated as late ones. + + Messages: -1) A->B: *,iA,msg1,A,B,protorange-A,nA +1) A->B: *,iA,msg1,A+,B+,nA + +i* must be encoded as 0. (However, it is permitted for a site to use +zero as its "index" for another site.) -2) B->A: iA,iB,msg2,B,A,chosen-protocol,nB,nA +2) B->A: iA,iB,msg2,B+,A+,nB,nA (The order of B and A reverses in alternate messages so that the same code can be used to construct them...) -3) A->B: {iB,iA,msg3,A,B,protorange-A,chosen-protocol,nA,nB,g^x mod m}_PK_A^-1 +3) A->B: {iB,iA,msg3,A+,B+,[chosen-transform],nA,nB,g^x mod m}_PK_A^-1 If message 1 was a replay then A will not generate message 3, because it doesn't recognise nA. @@ -199,18 +240,11 @@ it doesn't recognise nA. If message 2 was from an attacker then B will not generate message 4, because it doesn't recognise nB. -If an attacker is trying to manipulate the chosen protocol, B can spot -this when it sees A's message 3. - -4) B->A: {iA,iB,msg4,B,A,protorange-B,chosen-protocol,nB,nA,g^y mod m}_PK_B^-1 +4) B->A: {iA,iB,msg4,B+,A+,nB,nA,g^y mod m}_PK_B^-1 At this point, A and B share a key, k. B must keep retransmitting message 4 until it receives a packet encrypted using key k. -A can abandon the exchange if the chosen protocol is not the one that -it would have chosen knowing the acceptable protocol ranges of A and -B. - 5) A: iB,iA,msg5,(ping/msg5)_k 6) B: iA,iB,msg6,(pong/msg6)_k @@ -231,28 +265,29 @@ retransmit or confirm reception. It is suggested that this message be sent when a key times out, or the tunnel is forcibly terminated for some reason. -XXX not yet implemented. +**** Protocol sub-goal 3: send a packet + +8) i?,i?,msg0,(send-packet/msg9,packet)_k -8) i?,i?,NAK/msg8 +**** Other messages + +9) i?,i?,NAK (NAK is encoded as zero) If the link-layer can't work out what to do with a packet (session has -gone away, etc.) it can transmit a NAK back to the sender. The sender -can then try to verify whether the session is alive by sending ping -packets, and forget the key if it isn't. Potential denial-of-service -if the attacker can stop the ping/pong packets getting through (the -key will be forgotten and another key setup must take place), but if -they can delete packets then we've lost anyway... +gone away, etc.) it can transmit a NAK back to the sender. -The attacker can of course forge NAKs since they aren't protected. But -if they can only forge packets then they won't be able to stop the -ping/pong working. Trust in NAKs can be rate-limited... +This can alert the sender to the situation where the sender has a key +but the receiver doesn't (eg because it has been restarted). The +sender, on receiving the NAK, will try to initiate a key exchange. -Alternative idea (which is actually implemented): if you receive a -packet you can't decode, because there's no key established, then -initiate key setup... +Forged (or overly delayed) NAKs can cause wasted resources due to +spurious key exchange initiation, but there is a limit on this because +of the key exchange retry timeout. -Keepalives are probably a good idea. +10) i?,i?,msg8,A,B,nA,nB,msg? -**** Protocol sub-goal 3: send a packet +This is an obsolete form of NAK packet which is not sent by any even +vaguely recent version of secnet. (In fact, there is no evidence in +the git history of it ever being sent.) -9) i?,i?,msg0,(send-packet/msg9,packet)_k +This message number is reserved.