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add a brief description of SIT

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Mark Andrews 2014-02-25 09:50:17 +11:00
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Source Identity Token
Source Identity Token (SIT) is based in Donald Eastlake 3rd's DNS Cookies[1].
The main differences are that the error code has been dropped and
that the server cookie doesn't have a fixed length and may be
missing.
The error code has been dropped because it served no useful purpose
for us. If it was to be restored it should be the first element
of the option.
We extended the server cookie to transmit server time and to include
a server generated nonce. The purpose of these is to provide a
short window of time (1 hour with a 5 minutes of clock skew for
cluster time) where a previous cookie can be used for and to not
require the server secret to be updated when it is shared by a
cluster of servers. In particular the time of generation needed
to be passed between servers via the client so that old cookie can
be rejected.
The option structure is:
client cookie (64 bits)
server cookie (128 bits) broken up into:
- nonce (32 bits)
- time (32 bits)
- hash (64 bits)
The initial requests just sends the client cookie. If the response
contains a matching client cookie the entire response is saved and
sent on the next transaction. A new server cookie is generated for
every response.
We are currently using EDNS Experimental code point 65001. This is
subject to change.
We have three supported hash method. AES, HMAC SHA 1 and HMAC SHA 256.
A cluster of servers needs to choose one of them.
AES
memset(input, 0, sizeof(input));
cp = isc_buffer_used(buf);
isc_buffer_putmem(buf, client->cookie, 8);
isc_buffer_putuint32(buf, nonce);
isc_buffer_putuint32(buf, when);
memmove(input, cp, 16);
isc_aes128_crypt(ns_g_server->secret, input, digest);
for (i = 0; i < 8; i++)
input[i] = digest[i] ^ digest[i + 8];
isc_netaddr_fromsockaddr(&netaddr, &client->peeraddr);
switch (netaddr.family) {
case AF_INET:
memmove(input + 8, (unsigned char *)&netaddr.type.in, 4);
memset(input + 12, 0, 4);
isc_aes128_crypt(ns_g_server->secret, input, digest);
break;
case AF_INET6:
memmove(input + 8, (unsigned char *)&netaddr.type.in6, 16);
isc_aes128_crypt(ns_g_server->secret, input, digest);
for (i = 0; i < 8; i++)
input[i + 8] = digest[i] ^ digest[i + 8];
isc_aes128_crypt(ns_g_server->secret, input + 8, digest);
break;
}
for (i = 0; i < 8; i++)
digest[i] ^= digest[i + 8];
isc_buffer_putmem(buf, digest, 8);
HMAC SHA1
hash = trunc(hmacsha1(client|nonce|when|address, secret), 8);
HMAC SHA256
hash = trunc(hmacsha256(client|nonce|when|address, secret), 8);
[1]
INTERNET-DRAFT Donald Eastlake
Intended Status: Proposed Standard Huawei
Expires: July 21, 2014 January 22, 2014
Domain Name System (DNS) Cookies
<draft-eastlake-dnsext-cookies-04.txt>