Commit | Line | Data |
---|---|---|
974d0468 | 1 | * Design of new, multi-subnet secnet protocol |
2fe58dfd | 2 | |
974d0468 SE |
3 | Like the first (1995/6) version, we're tunnelling IP packets inside |
4 | UDP packets. To defeat various restrictions which may be imposed on us | |
5 | by network providers (like the prohibition of incoming TCP | |
6 | connections) we're sticking with UDP for everything this time, | |
7 | including key setup. | |
2fe58dfd SE |
8 | |
9 | Other new features include being able to deal with subnets hidden | |
10 | behind changing 'real' IP addresses, and the ability to choose | |
11 | algorithms and keys per pair of communicating sites. | |
12 | ||
13 | ** Configuration and structure | |
14 | ||
15 | The network is made up from a number of 'sites'. These are collections | |
16 | of machines with private IP addresses. The new secnet code runs on | |
17 | machines which have interfaces on the private site network and some | |
18 | way of accessing the 'real' internet. | |
19 | ||
20 | Each end of a tunnel is identified by a name. Often it will be | |
21 | convenient for every gateway machine to use the same name for each | |
22 | tunnel endpoint, but this is not vital. Individual tunnels are | |
23 | identified by their two endpoint names. | |
24 | ||
2fe58dfd SE |
25 | ** Protocols |
26 | ||
27 | *** Protocol environment: | |
28 | ||
29 | Each gateway machine serves a particular, well-known set of private IP | |
30 | addresses (i.e. the agreement over which addresses it serves is | |
31 | outside the scope of this discussion). Each gateway machine has an IP | |
32 | address on the interconnecting network (usually the Internet), which | |
33 | may be dynamically allocated and may change at any point. | |
34 | ||
35 | Each gateway knows the RSA public keys of the other gateways with | |
36 | which it wishes to communicate. The mechanism by which this happens is | |
37 | outside the scope of this discussion. There exists a means by which | |
38 | each gateway can look up the probable IP address of any other. | |
39 | ||
40 | *** Protocol goals: | |
41 | ||
42 | The ultimate goal of the protocol is for the originating gateway | |
43 | machine to be able to forward packets from its section of the private | |
44 | network to the appropriate gateway machine for the destination | |
45 | machine, in such a way that it can be sure that the packets are being | |
46 | sent to the correct destination machine, the destination machine can | |
47 | be sure that the source of the packets is the originating gateway | |
48 | machine, and the contents of the packets cannot be understood other | |
49 | than by the two communicating gateways. | |
50 | ||
51 | XXX not sure about the address-change stuff; leave it out of the first | |
52 | version of the protocol. From experience, IP addresses seem to be | |
53 | quite stable so the feature doesn't gain us much. | |
54 | ||
55 | **** Protocol sub-goal 1: establish a shared key | |
56 | ||
57 | Definitions: | |
58 | ||
59 | A is the originating gateway machine | |
60 | B is the destination gateway machine | |
61 | PK_A is the public RSA key of A | |
62 | PK_B is the public RSA key of B | |
63 | PK_A^-1 is the private RSA key of A | |
64 | PK_B^-1 is the private RSA key of B | |
65 | x is the fresh private DH key of A | |
66 | y is the fresh private DH key of B | |
67 | k is g^xy mod m | |
68 | g and m are generator and modulus for Diffie-Hellman | |
69 | nA is a nonce generated by A | |
70 | nB is a nonce generated by B | |
71 | iA is an index generated by A, to be used in packets sent from B to A | |
72 | iB is an index generated by B, to be used in packets sent from A to B | |
73 | i? is appropriate index for receiver | |
74 | ||
75 | Note that 'i' may be re-used from one session to the next, whereas 'n' | |
76 | is always fresh. | |
77 | ||
78 | Messages: | |
79 | ||
baa06aeb | 80 | 1) A->B: *,iA,msg1,A,B,protorange-A,nA |
2fe58dfd | 81 | |
baa06aeb | 82 | 2) B->A: iA,iB,msg2,B,A,chosen-protocol,nB,nA |
2fe58dfd SE |
83 | |
84 | (The order of B and A reverses in alternate messages so that the same | |
85 | code can be used to construct them...) | |
86 | ||
baa06aeb | 87 | 3) A->B: {iB,iA,msg3,A,B,protorange-A,chosen-protocol,nA,nB,g^x mod m}_PK_A^-1 |
2fe58dfd SE |
88 | |
89 | If message 1 was a replay then A will not generate message 3, because | |
90 | it doesn't recognise nA. | |
91 | ||
92 | If message 2 was from an attacker then B will not generate message 4, | |
93 | because it doesn't recognise nB. | |
94 | ||
baa06aeb SE |
95 | If an attacker is trying to manipulate the chosen protocol, B can spot |
96 | this when it sees A's message 3. | |
97 | ||
98 | 4) B->A: {iA,iB,msg4,B,A,protorange-B,chosen-protocol,nB,nA,g^y mod m}_PK_B^-1 | |
2fe58dfd SE |
99 | |
100 | At this point, A and B share a key, k. B must keep retransmitting | |
101 | message 4 until it receives a packet encrypted using key k. | |
102 | ||
baa06aeb SE |
103 | A can abandon the exchange if the chosen protocol is not the one that |
104 | it would have chosen knowing the acceptable protocol ranges of A and | |
105 | B. | |
106 | ||
2fe58dfd SE |
107 | 5) A: iB,iA,msg5,(ping/msg5)_k |
108 | ||
109 | 6) B: iA,iB,msg6,(pong/msg6)_k | |
110 | ||
111 | (Note that these are encrypted using the same transform that's used | |
112 | for normal traffic, so they include sequence number, MAC, etc.) | |
113 | ||
114 | The ping and pong messages can be used by either end of the tunnel at | |
115 | any time, but using msg0 as the unencrypted message type indicator. | |
116 | ||
117 | **** Protocol sub-goal 2: end the use of a shared key | |
118 | ||
119 | 7) i?,i?,msg0,(end-session/msg7,A,B)_k | |
120 | ||
121 | This message can be sent by either party. Once sent, k can be | |
122 | forgotten. Once received and checked, k can be forgotten. No need to | |
123 | retransmit or confirm reception. It is suggested that this message be | |
124 | sent when a key times out, or the tunnel is forcibly terminated for | |
125 | some reason. | |
126 | ||
974d0468 SE |
127 | XXX not yet implemented. |
128 | ||
2fe58dfd SE |
129 | 8) i?,i?,NAK/msg8 |
130 | ||
131 | If the link-layer can't work out what to do with a packet (session has | |
132 | gone away, etc.) it can transmit a NAK back to the sender. The sender | |
133 | can then try to verify whether the session is alive by sending ping | |
134 | packets, and forget the key if it isn't. Potential denial-of-service | |
135 | if the attacker can stop the ping/pong packets getting through (the | |
136 | key will be forgotten and another key setup must take place), but if | |
137 | they can delete packets then we've lost anyway... | |
138 | ||
139 | The attacker can of course forge NAKs since they aren't protected. But | |
140 | if they can only forge packets then they won't be able to stop the | |
141 | ping/pong working. Trust in NAKs can be rate-limited... | |
142 | ||
974d0468 SE |
143 | Alternative idea (which is actually implemented): if you receive a |
144 | packet you can't decode, because there's no key established, then | |
145 | initiate key setup... | |
146 | ||
147 | Keepalives are probably a good idea. | |
2fe58dfd SE |
148 | |
149 | **** Protocol sub-goal 3: send a packet | |
150 | ||
151 | 9) i?,i?,msg0,(send-packet/msg9,packet)_k |