X-Git-Url: http://www.chiark.greenend.org.uk/ucgi/~ian/git?a=blobdiff_plain;f=NOTES;h=a681d75f231cb21c0e6b316ee12e77444907bdd9;hb=4bc0381500403be5f3e40bc39f65a2d8fb75cf19;hp=33c010e47d18100f2edc5b53bd8d63afbfdd4f13;hpb=ddc1d9e0ad064de11d8bd735f7c86ab56555330e;p=secnet.git diff --git a/NOTES b/NOTES index 33c010e..a681d75 100644 --- a/NOTES +++ b/NOTES @@ -140,6 +140,75 @@ before going to background. 'normal' and 'failed' runs output to stdout/stderr before backgrounding, then thereafter output only to log destinations. +** Site long-term keys + +We use authenticated DH. Sites identify themselves to each other +using long-term signing keys. + +These signing keys may be for a variety of algorithms. (An algorithm +specifies completely how to do a signature and verification.) + +Each site may have several keys. This helps support key rollover and +algorithm agility. Several keys of different algorithms can form a +key group. Usually a key group consists of keys generated at the same +time. A key is identified by a 4-byte group id (invented by its +publisher and opaque) plus a 1-byte algorithm id (defined by the +protocol spec for each algorithm). + +Keys are published in key sets. A key set is a collection of key +groups (including older keys as well as newer ones) published at a +particular time. Key sets have their own 4-byte ids; these are +invented by the publisher but are ordered using sequence number +arithmetic. This allows reliers to favour new sets over old ones. + +Within each key set, some groups may be marked as `fallback'. This +means a group that should be tolerated by a relier only if the relier +doesn't support any non-fallback keys. + +Keys within groups, and groups within sets, are ordered (by the +publisher of the set), from most to least preferred. + +When deciding which public keys to accept, a relier should: + Process each group within the key set. + Discard unknown algorithms. + Choose a preferred algorithm: + Earliest in the group + (or local config could have algorithm prefererence). + Discard empty groups. + Discard unneeded fallback groups: + If any (non-empty) non-fallback groups found, discard all + fallback groups. Otherwise there are only fallback groups; + discard all but first group in the set. + Discard any keys exceeding limit on number of keys honoured: + Limit is at least 4 + Discard keys later in the set + In wire protocol, offer the resulting subset of keyids to + the peer and a allow the signer to select which key to use + from that subset. + +In configuration and key management, long-term private and public keys +are octet strings. Private keys are generally stored in disk files, +one key per file. The octet string for a private key should identify +the algorithm so that passing the private key to the code for the +wrong algorithm does not produce results which would leak or weaken +the key. The octet string for a public key need not identify the +algorithm; when it's loaded the algorithm will be known from context. + +The group id 00000000 is special. It should contain only one key, +algorithm 00. Key 0000000000 refers to the rsa1 key promulgated +before the key rollover/advertisement protocols, or the key which +should be used by sites running old software. + +The key set id 00000000 is special and is considered older than all +othere key sets (ie this is an exception to the sequence number +arithmetic). It is the implied key set id of the rsa1 key +promulgated before the key rollover/advertisement protocols. + +The algorithm 00 is special and refers to the old rsa1 signature +protocol but unusually does not identify the hash function. The hash +function is conventional and must be specified out of band. In known +existing installations it is SHA-1. + ** Protocols *** Protocol environment: @@ -174,8 +243,9 @@ 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 @@ -193,18 +263,104 @@ 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 protocol version selection stuff is not yet implemented. +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. + * In MSG3/MSG4: a 16-bit integer being the sender's MTU, or zero. + (In other messages: nothing.) See below. + * In MSG2/MSG3: a list of the peer's public keys that the sender will + accept: (i) a 1-byte integer count (ii) that many 5-byte key ids. + If not present, implicitly only the special key id 0000000000. + * In MSG3/MSG4: an 8-bit integer being an index into the + receiver's public key acceptance list, with which the message + is signed. If not present, implicitly the key id 00000000000. + * 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. + +Currently, the low 16 bits are allocated for negotiating bulk-crypto +transforms. Bits 8 to 15 are used by Secnet as default capability +numbers for the various kinds of transform closures: bit 8 is for the +original CBCMAC-based transform, and bit 9 for the new EAX transform; +bits 10 to 15 are reserved for future expansion. The the low eight bits +are reserved for local use, e.g., to allow migration from one set of +parameters for a particular transform to a different, incompatible set +of parameters for the same transform. Bit 31, if advertised by both +ends, indicates that a mobile end gets priority in case of crossed MSG1. +The remaining bits have not yet been assigned a purpose. + +Whether a capability number is early depends on its meaning, rather than +being a static property of its number. That said, the mobile-end-gets +priority bit (31) is always sent as an `early' capability bit. + + +MTU handling + +In older versions of secnet, secnet was not capable of fragmentation +or sending ICMP Frag Needed. Administrators were expected to configure +consistent MTUs across the network. + +It is still the case in the current version that the MTUs need to be +configured reasonably coherently across the network: the allocated +buffer sizes must be sufficient to cope with packets from all other +peers. + +However, provided the buffers are sufficient, all packets will be +processed properly: a secnet receiving a packet larger than the +applicable MTU for its delivery will either fragment it, or reject it +with ICMP Frag Needed. + +The MTU additional data field allows secnet to advertise an MTU to the +peer. This allows the sending end to handle overlarge packets, before +they are transmitted across the underlying public network. This can +therefore be used to work around underlying network braindamage +affecting large packets. + +If the MTU additional data field is zero or not present, then the peer +should use locally-configured MTU information (normally, its local +netlink MTU) instead. + +If it is nonzero, the peer may send packets up to the advertised size +(and if that size is bigger than the peer's administratively +configured size, the advertiser promises that its buffers can handle +such a large packet). + +A secnet instance should not assume that just because it has +advertised an mtu which is lower than usual for the vpn, the peer will +honour it, unless the administrator knows that the peers are +sufficiently modern to understand the mtu advertisement option. So +secnet will still accept packets which exceed the link MTU (whether +negotiated or assumed). + Messages: -1) A->B: *,iA,msg1,A,B,nA +1) A->B: i*,iA,msg1,A+,B+,nA -2) B->A: iA,iB,msg2,B,A,nB,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+,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,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. @@ -212,7 +368,7 @@ 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. -4) B->A: {iA,iB,msg4,B,A,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. @@ -263,3 +419,37 @@ vaguely recent version of secnet. (In fact, there is no evidence in the git history of it ever being sent.) This message number is reserved. + +11) *,*,PROD,A,B + +Sent in response to a NAK from B to A. Requests that B initiates a +key exchange with A, if B is willing and lacks a transport key for A. +(If B doesn't have A's address configured, implicitly supplies A's +public address.) + +This is necessary because if one end of a link (B) is restarted while +a key exchange is in progress, the following bad state can persist: +the non-restarted end (A) thinks that the key is still valid and keeps +sending packets, but B either doesn't realise that a key exchange with +A is necessary or (if A is a mobile site) doesn't know A's public IP +address. + +Normally in these circumstances B would send NAKs to A, causing A to +initiate a key exchange. However if A and B were already in the +middle of a key exchange then A will not want to try another one until +the first one has timed out ("setup-time" x "setup-retries") and then +the key exchange retry timeout ("wait-time") has elapsed. + +However if B's setup has timed out, B would be willing to participate +in a key exchange initiated by A, if A could be induced to do so. +This is the purpose of the PROD packet. + +We send no more PRODs than we would want to send data packets, to +avoid a traffic amplification attack. We also send them only in state +WAIT, as in other states we wouldn't respond favourably. And we only +honour them if we don't already have a key. + +With PROD, the period of broken communication due to a key exchange +interrupted by a restart is limited to the key exchange total +retransmission timeout, rather than also including the key exchange +retry timeout.