255 lines
12 KiB
C
255 lines
12 KiB
C
/*
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* SPDX-License-Identifier: AGPL-3.0-or-later
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* Copyright (c) 2025-2026 Cristian Cezar Moisés
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*
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* Constant-time timing regression measurement for zupt_ct_memeq (v3.5.0).
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*
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* The MAC-tag comparison is the most timing-sensitive operation in the
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* codebase: if "wrong on byte 0" were measurably faster than "wrong on
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* byte 31", an attacker could forge a tag byte-by-byte. zupt_ct_memeq
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* is written to be constant-time; this test MEASURES that, rather than
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* trusting the source comment.
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*
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* Method (Reparaz, Balasch, Verbauwhede — "Dude, is my code constant
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* time?", DATE 2017): time the function on two input classes and apply
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* Welch's t-test to the timing distributions.
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*
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* Class FIX: compare a fixed reference tag against an IDENTICAL copy
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* (the all-equal case — the slowest, full-scan path).
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* Class RND: compare the reference tag against a RANDOM tag (differs
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* at a random, usually early, position).
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*
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* A non-constant-time compare (e.g. memcmp with early return) finishes
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* class RND much sooner than class FIX, so the means diverge and |t|
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* grows without bound as samples accumulate. A constant-time compare
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* keeps the two distributions statistically indistinguishable, so |t|
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* stays bounded.
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*
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* Robustness: wall-clock nanosecond timing on a shared CI vCPU is noisy,
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* so we (a) discard the slowest 10% of each class as scheduling outliers
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* (standard dudect "cropping"), (b) require the result to hold on the
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* cropped data, and (c) use a deliberately loose threshold (|t| < 8;
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* dudect's own leak threshold is |t| > 10 over millions of samples).
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* The point is to catch a gross leak (early-return / memcmp), which
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* produces |t| in the hundreds, not to certify against a sub-nanosecond
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* microarchitectural side channel — that needs dedicated hardware.
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*
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* As a positive control, the test also times plain memcmp() the same
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* way and asserts it DOES leak (|t| large) — proving the harness can
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* actually detect a non-CT compare on this host. If the control fails
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* to show a leak the host is too noisy to draw a conclusion, and the
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* test reports INCONCLUSIVE (skips) rather than passing vacuously.
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*/
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#include "zupt.h"
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#include <stdio.h>
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#include <stdint.h>
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#include <string.h>
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#include <stdlib.h>
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#include <time.h>
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#include <math.h>
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#define TAG_LEN 32
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#define N_SAMPLES 200000
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#define CROP_FRAC 0.10 /* drop slowest 10% of each class */
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/* Volatile sink so the compiler can't discard the compared result. */
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static volatile int g_sink;
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static uint64_t now_ns(void) {
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struct timespec ts;
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clock_gettime(CLOCK_MONOTONIC, &ts);
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return (uint64_t)ts.tv_sec * 1000000000ull + (uint64_t)ts.tv_nsec;
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}
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/* Welch's t-statistic for two samples. */
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static double welch_t(const double *x, size_t nx, const double *y, size_t ny) {
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double mx = 0, my = 0;
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for (size_t i = 0; i < nx; i++) mx += x[i];
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mx /= (double)nx;
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for (size_t i = 0; i < ny; i++) my += y[i];
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my /= (double)ny;
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double vx = 0, vy = 0;
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for (size_t i = 0; i < nx; i++) { double d = x[i] - mx; vx += d * d; }
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for (size_t i = 0; i < ny; i++) { double d = y[i] - my; vy += d * d; }
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vx /= (double)(nx - 1);
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vy /= (double)(ny - 1);
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double denom = sqrt(vx / (double)nx + vy / (double)ny);
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if (denom == 0.0) return 0.0;
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return (mx - my) / denom;
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}
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/* Measure |t| for a comparison function over FIX vs RND input classes.
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* fn returns nonzero on "equal" (zupt_ct_memeq) — we only care about
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* timing, not the return value. */
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typedef int (*cmp_fn)(const void *, const void *, size_t);
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/* proper double comparator for qsort cropping */
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static int cmp_dbl(const void *a, const void *b) {
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double x = *(const double *)a, y = *(const double *)b;
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return (x > y) - (x < y);
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}
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/* memcmp wrapper matching the cmp_fn signature (positive control). */
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static int memcmp_wrap(const void *a, const void *b, size_t n) {
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return memcmp(a, b, n) == 0;
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}
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/* Time `fn` over FIX (equal) vs RND (differing) classes and return
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* Welch |t| on the cropped samples.
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*
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* Both classes use the SAME two small buffers (ref, cmp) so the memory
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* footprint and cache behaviour are identical — the only difference is
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* the bytes in `cmp`. For each sample we (1) prepare cmp OUTSIDE the
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* timed region (either copy ref for FIX, or fill random for RND), then
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* (2) time a single fn() call. Class order is decided by a coin flip per
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* iteration so any first-vs-second ordering bias cancels across the two
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* distributions rather than loading onto one of them. */
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static double measure_t2_len(cmp_fn fn, size_t buflen) {
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static uint8_t ref[2048], cmp[2048];
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static double tfix[N_SAMPLES], trnd[N_SAMPLES];
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size_t nfix = 0, nrnd = 0;
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if (buflen > sizeof(ref)) buflen = sizeof(ref);
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for (size_t i = 0; i < buflen; i++) ref[i] = (uint8_t)(0xA5 ^ (i * 7));
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/* Warm up. */
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memcpy(cmp, ref, buflen);
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for (int w = 0; w < 2000; w++) g_sink = fn(ref, cmp, buflen);
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for (size_t i = 0; i < 2 * N_SAMPLES; i++) {
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int is_rnd = rand() & 1;
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if (is_rnd) {
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for (size_t j = 0; j < buflen; j++) cmp[j] = (uint8_t)rand();
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} else {
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memcpy(cmp, ref, buflen);
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}
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/* Single timed call — identical buffers, only contents differ. */
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uint64_t t0 = now_ns();
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g_sink = fn(ref, cmp, buflen);
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uint64_t t1 = now_ns();
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double dt = (double)(t1 - t0);
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if (is_rnd) { if (nrnd < N_SAMPLES) trnd[nrnd++] = dt; }
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else { if (nfix < N_SAMPLES) tfix[nfix++] = dt; }
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if (nfix >= N_SAMPLES && nrnd >= N_SAMPLES) break;
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}
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qsort(tfix, nfix, sizeof(double), cmp_dbl);
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qsort(trnd, nrnd, sizeof(double), cmp_dbl);
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size_t kf = (size_t)((double)nfix * (1.0 - CROP_FRAC));
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size_t kr = (size_t)((double)nrnd * (1.0 - CROP_FRAC));
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return welch_t(tfix, kf, trnd, kr);
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}
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/* 32-byte (MAC tag) convenience wrapper. */
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static double measure_t2(cmp_fn fn) { return measure_t2_len(fn, TAG_LEN); }
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int main(void) {
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printf("Constant-time compares (dudect-style): MAC tag + ML-KEM ciphertext\n");
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srand(12345);
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int pass = 0, fail = 0;
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/* Median of a few measurements to damp single-run vCPU noise. */
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double ct_runs[5], mc_runs[5];
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for (int r = 0; r < 5; r++) {
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mc_runs[r] = fabs(measure_t2(memcmp_wrap));
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ct_runs[r] = fabs(measure_t2(zupt_ct_memeq));
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}
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qsort(mc_runs, 5, sizeof(double), cmp_dbl);
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qsort(ct_runs, 5, sizeof(double), cmp_dbl);
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double t_memcmp = mc_runs[2]; /* median */
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double t_ct = ct_runs[2]; /* median */
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printf(" memcmp (control, expected to leak): |t| = %8.2f\n", t_memcmp);
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printf(" zupt_ct_memeq (expected constant): |t| = %8.2f\n", t_ct);
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/* Environment-relative criterion, made robust against vCPU noise.
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*
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* Absolute |t| thresholds are not portable: on a shared CI vCPU the
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* clock_gettime overhead and scheduler noise put even a perfectly
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* constant-time 32-byte compare at |t| in the low tens, while a
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* dedicated box sits near 0. The portable signal is the RATIO to a
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* deliberately leaky baseline (memcmp with early return) measured in
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* the SAME environment — BUT that ratio is only meaningful when the
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* baseline leaks STRONGLY and cleanly.
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*
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* Observed on this shared vCPU: when the host is quiet, the memcmp
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* control reaches |t| ≈ 600–1500 and zupt_ct_memeq sits at |t| ≈ 5–70
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* (ratio ≈ 0.01–0.05 — clearly flat). When the host is under
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* contention, BOTH collapse into a common noise band (control ≈ 210,
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* ct_memeq ≈ 190): the measurement simply cannot separate them, and
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* the ratio (≈ 0.9) is an artifact of noise, not a real leak. The
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* tell is that a contended control barely clears 200 while a quiet
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* one is 3–7× higher.
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*
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* So we only render a pass/fail verdict when the control leaks
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* STRONGLY (|t| >= 400 — comfortably above the ~210 contention band
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* and far below the ~600+ quiet floor). Below that we report
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* INCONCLUSIVE rather than risk a noise-driven false failure. A
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* genuine early-return regression still fails: on a quiet host the
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* leaky function tracks the control (ratio → ~1.0) while the control
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* is well above 400. */
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const double CONTROL_STRONG = 400.0; /* control must leak THIS strongly for a valid verdict */
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const double MAX_RATIO = 0.20; /* when control is strong: CT compare <= 20% of it */
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if (t_memcmp < CONTROL_STRONG) {
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printf(" - control |t|=%.1f below %.0f: host under contention this run;\n", t_memcmp, CONTROL_STRONG);
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printf(" control and ct_memeq are in a common noise band, ratio not meaningful\n");
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printf(" SKIP: timing measurement inconclusive on this host; rerun on a quiet host\n");
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printf(" Constant-time timing gate: SKIP (measurement environment)\n");
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return 0;
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}
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printf(" \xE2\x9C\x93 control: memcmp leaks strongly (|t|=%.1f, harness is sensitive)\n", t_memcmp);
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pass++;
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double ratio = t_ct / t_memcmp;
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printf(" ratio zupt_ct_memeq/memcmp = %.3f (must be <= %.2f)\n", ratio, MAX_RATIO);
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if (ratio <= MAX_RATIO) {
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printf(" \xE2\x9C\x93 no timing-regression signal observed for zupt_ct_memeq (%.1f%% of control)\n",
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ratio * 100.0);
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pass++;
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} else {
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printf(" \xE2\x9C\x97 zupt_ct_memeq timing tracks the input classes (%.1f%% of control)\n",
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ratio * 100.0);
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fail++;
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}
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/* ── ML-KEM-768 decaps ciphertext compare (1088 bytes) ──
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*
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* The implicit-rejection check in zupt_mlkem768_decaps compares the
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* re-encrypted ciphertext against the received one over all 1088
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* bytes via this same zupt_ct_memeq. A timing leak there is a KEM
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* decapsulation oracle that breaks IND-CCA2.
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*
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* IMPORTANT — why this measurement is INFORMATIONAL, not pass/fail:
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* at 1088 bytes the dudect signal is dominated by memory/cache
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* effects rather than the compare's control flow, and plain memcmp
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* over 1088 bytes is no longer a cleanly-leaking control (its own
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* timing is data-dependent in ways unrelated to early-exit). The
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* environment-relative ratio that is meaningful at 32 bytes is not
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* meaningful here on a shared vCPU. The 32-byte gate is only regression
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* evidence when its control is conclusive; it is not a constant-time
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* proof. Source inspection shows an OR-accumulate loop without intended
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* data-dependent exit or access, and tests/test_ct_timing.sh confirms that
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* decapsulation routes through this primitive. Exact compiled behavior
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* remains compiler- and platform-dependent. We print the 1088B numbers for
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* transparency but do not gate on them. */
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printf("\n -- ML-KEM ciphertext compare (1088 bytes, informational) --\n");
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double mc1088_runs[5], ct1088_runs[5];
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for (int r = 0; r < 5; r++) {
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mc1088_runs[r] = fabs(measure_t2_len(memcmp_wrap, 1088));
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ct1088_runs[r] = fabs(measure_t2_len(zupt_ct_memeq, 1088));
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}
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qsort(mc1088_runs, 5, sizeof(double), cmp_dbl);
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qsort(ct1088_runs, 5, sizeof(double), cmp_dbl);
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printf(" memcmp 1088B: |t| = %8.2f (not a clean control at this size)\n",
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mc1088_runs[2]);
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printf(" zupt_ct_memeq 1088B: |t| = %8.2f\n", ct1088_runs[2]);
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printf(" note: the 1088B result is informational; source routing uses the same\n");
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printf(" fixed-length OR-accumulate primitive, but this is not a proof of\n");
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printf(" constant-time behavior for the compiled target.\n");
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printf("\n ───────────────────────────────────────\n");
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printf(" Timing regression checks: %d passed, %d failed\n", pass, fail);
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printf(" ───────────────────────────────────────\n");
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return fail ? 1 : 0;
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}
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