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upstream: randomise keystroke obfuscation intervals and average 

interval rate. ok dtucker@

OpenBSD-Commit-ID: 05f61d051ab418fcfc4857ff306e420037502382
This commit is contained in:
djm@openbsd.org 2023-09-10 03:25:53 +00:00 committed by Damien Miller
parent bd1b9e52f5
commit 90ccc5918e
No known key found for this signature in database
1 changed files with 46 additions and 6 deletions
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52
clientloop.c
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@ -1,4 +1,4 @@
/* $OpenBSD: clientloop.c,v 1.396 2023/09/04 00:08:14 djm Exp $ */
/* $OpenBSD: clientloop.c,v 1.397 2023/09/10 03:25:53 djm Exp $ */
/*
* Author: Tatu Ylonen <ylo@cs.hut.fi>
* Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@ -118,6 +118,9 @@
/* Permitted RSA signature algorithms for UpdateHostkeys proofs */
#define HOSTKEY_PROOF_RSA_ALGS "rsa-sha2-512,rsa-sha2-256"
/* Uncertainty (in percent) of keystroke timing intervals */
#define SSH_KEYSTROKE_TIMING_FUZZ 10
/* import options */
extern Options options;
@ -528,6 +531,43 @@ send_chaff(struct ssh *ssh)
return 1;
}
/* Sets the next interval to send a keystroke or chaff packet */
static void
set_next_interval(const struct timespec *now, struct timespec *next_interval,
u_int interval_ms, int starting)
{
struct timespec tmp;
long long interval_ns, fuzz_ns;
static long long rate_fuzz;
interval_ns = interval_ms * (1000LL * 1000);
fuzz_ns = (interval_ns * SSH_KEYSTROKE_TIMING_FUZZ) / 100;
/* Center fuzz around requested interval */
if (fuzz_ns > INT_MAX)
fuzz_ns = INT_MAX;
if (fuzz_ns > interval_ns) {
/* Shouldn't happen */
fatal_f("internal error: fuzz %u%% %lldns > interval %lldns",
SSH_KEYSTROKE_TIMING_FUZZ, fuzz_ns, interval_ns);
}
/*
* Randomise the keystroke/chaff intervals in two ways:
* 1. Each interval has some random jitter applied to make the
* interval-to-interval time unpredictable.
* 2. The overall interval rate is also randomly perturbed for each
* each chaffing session to make the average rate unpredictable.
*/
if (starting)
rate_fuzz = arc4random_uniform(fuzz_ns);
interval_ns -= fuzz_ns;
interval_ns += arc4random_uniform(fuzz_ns) + rate_fuzz;
tmp.tv_sec = interval_ns / (1000 * 1000 * 1000);
tmp.tv_nsec = interval_ns % (1000 * 1000 * 1000);
timespecadd(now, &tmp, next_interval);
}
/*
* Performs keystroke timing obfuscation. Returns non-zero if the
* output fd should be polled.
@ -591,12 +631,12 @@ obfuscate_keystroke_timing(struct ssh *ssh, struct timespec *timeout,
*/
if (!active && ssh_packet_interactive_data_to_write(ssh) &&
channel_did_enqueue && ssh_packet_have_data_to_write(ssh)) {
debug3_f("starting: interval %d",
debug3_f("starting: interval ~%dms",
options.obscure_keystroke_timing_interval);
just_started = had_keystroke = active = 1;
nchaff = 0;
ms_to_timespec(&tmp, options.obscure_keystroke_timing_interval);
timespecadd(&now, &tmp, &next_interval);
set_next_interval(&now, &next_interval,
options.obscure_keystroke_timing_interval, 1);
}
/* Don't hold off if obfuscation inactive */
@ -629,8 +669,8 @@ obfuscate_keystroke_timing(struct ssh *ssh, struct timespec *timeout,
n = (n < 0) ? 1 : n + 1;
/* Advance to the next interval */
ms_to_timespec(&tmp, options.obscure_keystroke_timing_interval * n);
timespecadd(&now, &tmp, &next_interval);
set_next_interval(&now, &next_interval,
options.obscure_keystroke_timing_interval * n, 0);
return 1;
}