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stegdetect.c
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stegdetect.c
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/*
* Copyright 2001 Niels Provos <[email protected]>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. All advertising materials mentioning features or use of this software
* must display the following acknowledgement:
* This product includes software developed by Niels Provos.
* 4. The name of the author may not be used to endorse or promote products
* derived from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
* THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#include <sys/types.h>
#include "config.h"
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <err.h>
#include <string.h>
#include <math.h>
#include <ctype.h>
#include <jpeglib.h>
#include <file.h>
#include "common.h"
#include "extraction.h"
#include "discrimination.h"
#define DBG_PRINTHIST 0x0001
#define DBG_CHIDIFF 0x0002
#define DBG_CHICALC 0x0004
#define DBG_CHIEND 0x0008
#define DBG_PRINTONES 0x0010
#define DBG_CHI 0x0020
#define DBG_ENDVAL 0x0040
#define DBG_BINSRCH 0x0080
#define DBG_PRINTZERO 0x0100
#define FLAG_DOOUTGUESS 0x0001
#define FLAG_DOJPHIDE 0x0002
#define FLAG_DOJSTEG 0x0004
#define FLAG_DOINVIS 0x0008
#define FLAG_DOF5 0x0010
#define FLAG_DOF5_SLOW 0x0020
#define FLAG_DOAPPEND 0x0040
#define FLAG_DOTRANSF 0x0080
#define FLAG_DOCLASSDIS 0x0100
#define FLAG_CHECKHDRS 0x1000
#define FLAG_JPHIDESTAT 0x2000
float chi2cdf(float chi, int dgf);
double detect_f5(char *);
char *progname;
float DCThist[257];
float scale = 1; /* Sensitivity scaling */
static int sd_debug = 0;
static int quiet = 0;
static int ispositive = 0; /* Current images contain stego */
static char *transformname; /* Current transform name */
static short *olddata;
static int oldx, oldy;
static transform_t transform;
void
buildDCTreset(void)
{
olddata = NULL;
oldx = oldy = 0;
}
void
buildDCThist(short *data, int x, int y)
{
int i, min, max;
int off, count, sum;
if (olddata != data || x < oldx || y < oldy ||
x - oldx + y - oldy >= y - x) {
olddata = data;
oldx = x;
oldy = y;
memset(DCThist, 0, sizeof(DCThist));
} else {
for (i = oldx; i < x; i++) {
off = data[i];
/* Don't know what to do about DC! */
if (off < -128)
continue;
else if (off > 127)
continue;
DCThist[off + 128]--;
}
olddata = data;
oldx = x;
x = oldy;
oldy = y;
}
min = 2048;
max = -2048;
/* Calculate coefficent frequencies */
sum = count = 0;
for (i = x; i < y; i++) {
if ((i & ~63) == i) {
if (sd_debug & DBG_PRINTONES)
fprintf(stdout, "%d] %d\n", i, count);
sum += count;
count = 0;
}
off = data[i];
if (off == 1)
count++;
if (off < min)
min = off;
if (off > max)
max = off;
/* Don't know what to do about DC! */
if (off < -128)
continue;
else if (off > 127)
continue;
DCThist[off + 128]++;
}
if (sd_debug & DBG_PRINTHIST) {
for (i = 0; i < 256; i++) {
fprintf(stdout, "%4d: %8.1f\n", i - 128, DCThist[i]);
}
fprintf(stdout, "Min: %d, Max: %d, Sum-1: %d\n",
min, max, sum);
}
}
/*
* Self calibration on bad test example.
*/
int
unify_false_jsteg(float *hist, float *theo, float *obs, float *discard)
{
int i, size = 0;
/* Build theoretical histogram */
for (i = 0; i < 128; i++) {
if (i == 64 || i == 65 || i == 0)
continue;
theo[size] = (float)(hist[2*i - 1] + hist[2*i])/2;
obs[size++] = hist[2*i];
}
return (size);
}
int
unify_false_outguess(float *hist, float *theo, float *obs, float *discard)
{
int i, size = 0;
int one, two;
/* Build theoretical histogram */
for (i = 0; i < 128; i++) {
if (i == 64 || i == 65 || i == 0)
continue;
one = hist[2*i - 1];
two = hist[2*i];
theo[size] = (float)(one + two)/2;
obs[size++] = two;
}
return (size);
}
int
unify_false_jphide(float *hist, float *theo, float *obs, float *discard)
{
int i, size = 0;
/* Build theoretical histogram */
for (i = 0; i < 128; i++) {
if (i == 64)
continue;
if (i < 64) {
theo[size] = (float)(hist[2*i] + hist[2*i + 1])/2;
obs[size++] = hist[2*i + 1];
} else {
theo[size] = (float)(hist[2*i - 1] + hist[2*i])/2;
obs[size++] = hist[2*i];
}
}
return (size);
}
int
unify_normal(float *hist, float *theo, float *obs, float *discard)
{
int i, size = 0;
/* Build theoretical histogram */
for (i = 0; i < 128; i++) {
if (i == 64)
continue;
theo[size] = (float)(hist[2*i] + hist[2*i + 1])/2;
obs[size++] = hist[2*i + 1];
}
return (size);
}
int
unify_outguess(float *hist, float *theo, float *obs, float *pdiscard)
{
int i, size = 0;
int one, two, sum, discard;
float f, fbar;
sum = 0;
for (i = 0; i < 256; i++) {
if (i == 64 || i == 65)
continue;
sum += hist[i];
}
discard = 0;
/* Build theoretical histogram */
for (i = 0; i < 128; i++) {
if (i == 64)
continue;
one = hist[2*i];
two = hist[2*i + 1];
if (one > two) {
f = one;
fbar= two;
} else {
f = two;
fbar = one;
}
/* Try to check if outguess could have been used here
* If the smaller coefficient is less than a quarter
* of the larger one, then outguess has probably been
* not used, and we included the coefficient.
* Otherwise check if outguess modifications could
* have reduced the difference significantly.
*/
if ((fbar > f/4) &&
((f - f/3) - (fbar + f/3) > 0)) {
if ((sd_debug & DBG_CHIDIFF) && (one || two))
fprintf(stdout,
"%4d: %8.3f - %8.3f skipped (%f)\n",
i*2 - 128,
(float)two,
(float)(one + two)/2,
(float)(one + two)/sum);
discard += one + two;
continue;
}
theo[size] = (float)(one + two)/2;
obs[size++] = two;
}
*pdiscard = (float)discard/sum;
return (size);
}
int
unify_jphide(float *hist, float *theo, float *obs, float *discard)
{
int i, size;
size = 0;
/* First pass */
for (i = 0; i < 256; i++) {
/* Exclude special cases */
if (i >= (-1 + 128) && i <= (1 + 128))
continue;
/* Lower bit = 0 */
if (i < 128 && !(i & 1))
continue;
else if ((i >= 128) && (i & 1))
continue;
theo[size] = (hist[i] + hist[i + 1])/2;
obs[size++] = hist[i];
}
/* Special case for 1 and -1 */
/*
theo[size] = (hist[-1 + 128] + hist[1 + 128] + hist[0 + 128])/2;
obs[size++] = hist[-1 + 128] + hist[1 + 128];
*/
return (size);
}
float
chi2(float *DCTtheo, float *DCTobs, int size, float discard)
{
int i, dgf;
float chi, sumchi, ymt, ytt, f;
ymt = ytt = 0;
sumchi = 0;
dgf = 0;
for (i = 0; i < size; i++) {
ymt += DCTobs[i];
ytt += DCTtheo[i];
if (sd_debug & DBG_CHIDIFF) {
if (DCTobs[i] || DCTtheo[i])
fprintf(stdout, "%4d: %8.3f - %8.3f\n", i,
DCTobs[i],
DCTtheo[i]);
}
if (ytt >= 5) {
/* Calculate chi^2 */
chi = ymt - ytt;
if (sd_debug & DBG_CHICALC) {
fprintf(stdout,
" (%8.3f - %8.3f)^2 = %8.3f / %8.3f = %8.3f | %8.3f\n",
ymt, ytt,
chi*chi, ytt, chi*chi/ytt, sumchi);
}
chi = chi*chi;
chi /= ytt;
sumchi += chi;
dgf++;
ymt = ytt = 0;
}
}
f = 1 - chi2cdf(sumchi, dgf - 1);
if (sd_debug & DBG_CHIEND) {
fprintf(stdout,
"Categories: %d, Chi: %f, Q: %f, dis: %f -> %f\n",
dgf, sumchi, f, discard, f * (1 - discard));
}
return (f * (1 - discard));
}
float
chi2test(short *data, int bits,
int (*unify)(float *, float *, float *, float *),
int a, int b)
{
float DCTtheo[128], DCTobs[128], discard;
int size;
if (a < 0)
a = 0;
if (b > bits)
b = bits;
if (a >= b)
return (-1);
buildDCThist(data, a, b);
discard = 0;
size = (*unify)(DCThist, DCTtheo, DCTobs, &discard);
return (chi2(DCTtheo, DCTobs, size, discard));
}
#define BINSEARCHVAR \
float _max, _min, _good; \
int _iteration
#define BINSEARCH(imin, imax, imaxiter) \
percent = (imax); \
_good = (imax) + 1; \
_iteration = 0; \
_min = (imin); \
_max = (imax); \
buildDCTreset(); \
while (_iteration < (imaxiter))
#define BINSEARCH_NEXT(thresh) \
if (sd_debug & DBG_BINSRCH) \
fprintf(stdout, "sum: %f, percent: %f, good: %f\n", \
sum, percent, _good); \
if (_iteration == 0) { \
if (sum >= (thresh)) \
break; \
_good = percent; \
percent = _min; \
} else \
if (sum < (thresh)) { \
_good = percent; \
if (_good == _min) /* XXX */\
break; /* XXX */\
_max = percent; \
percent = (_max - _min)/2 + _min; \
} else { \
_min = percent; \
percent = (_max - _min)/2 + _min; \
} \
_iteration++
#define BINSEARCH_IFABORT(thresh) \
percent = _good; \
if (_good > (thresh))
int
histogram_chi_jsteg(short *data, int bits)
{
int length, minlen, maxlen, end;
float f, sum, percent, i, count, where;
float max, aftercount, scale, fs;
BINSEARCHVAR;
if (bits == 0)
goto abort;
end = bits/100;
if (end < 4000)
end = 4000;
BINSEARCH(200, end, 6) {
sum = 0;
for (i = percent; i <= bits; i += percent) {
f = chi2test(data, bits, unify_false_jsteg, 0, i);
if (f == 0)
break;
if (f > 0.4)
sum += f * percent;
if ((sd_debug & DBG_CHI) && f != 0)
fprintf(stdout, "%04f[:] %8.5f%% %f\n",
i, f * 100, sum);
}
BINSEARCH_NEXT(400);
}
BINSEARCH_IFABORT(end) {
abort:
if (sd_debug & DBG_ENDVAL)
fprintf(stdout,
"Accumulation: no detection possible\n");
return (-1);
}
where = count = 0;
aftercount = max = 0;
scale = 0.95;
sum = 0;
for (i = percent; i <= bits; i += percent) {
f = chi2test(data, bits, unify_normal, 0, i);
if (f == 0)
break;
if (f > 0.4) {
sum += f * percent;
count++;
}
if (f >= (max * scale)) {
if (f > max) {
max = f;
/* More latitude for high values */
fs = (max - 0.4) / 0.6;
if (fs > 0)
scale = 1 - (0.15*fs + (1 - fs)*0.05);
}
aftercount = -3;
where = i;
} else if (f > 0.05 * max) {
if (aftercount >= 0)
aftercount += f * percent;
else {
aftercount++;
where = i;
}
}
if ((sd_debug & DBG_CHI) &&
((sd_debug & DBG_PRINTZERO) || f != 0))
fprintf(stdout, "%04f: %8.5f%%\n",
i, f * 100);
}
length = jsteg_size(data, bits, NULL);
minlen = where/8;
maxlen = (where + percent)/8;
if (sd_debug & DBG_ENDVAL) {
fprintf(stdout,
"Accumulation (%d): %f%% - %f (%f) (%d:%d - %d)\n",
(int)percent,
sum/percent, aftercount, count,
length, minlen, maxlen);
}
if (aftercount > 0)
sum -= aftercount;
/* Require a positive sum and at least two working samples */
if (sum < 0 || count < 3)
sum = 0;
if (length < minlen/2 || length > maxlen*2)
sum = 0;
return (scale * sum / (2 * percent));
}
float norm_outguess[21] = {
0.5,
0.7071067811865475244,
1,
1.2247448713915890491,
1.4142135623730950488,
1.581138830084189666,
1.73205080756887729353,
1.87082869338697069279,
2,
2.1213203435596425732,
2.23606797749978969641,
2.34520787991171477728,
2.4494897427831780982,
2.54950975679639241501,
2.6457513110645905905,
2.73861278752583056728,
2.8284271247461900976,
2.91547594742265023544,
3,
3.08220700148448822513,
3.162277660168379332
};
int
histogram_chi_outguess(short *data, int bits)
{
int i, off, range;
float percent, count;
float f, sum, norm;
BINSEARCHVAR;
BINSEARCH(0.1, 10, 9) {
range = percent*bits/100;
sum = 0;
for (i = 0; i <= 100; i ++) {
off = i*bits/100;
f = chi2test(data, bits, unify_false_outguess,
off - range, off + range);
sum += f;
if ((sd_debug & DBG_CHI) && f != 0)
fprintf(stdout, "%04d[:] %8.5f%%\n",
i, f * 100);
}
BINSEARCH_NEXT(0.6);
}
/* XXX */
BINSEARCH_IFABORT(10)
return (0);
range = percent*bits/100;
count = 0;
sum = 0;
for (i = 0; i <= 100; i ++) {
off = i*bits/100;
f = chi2test(data, bits, unify_outguess,
off - range, off + range);
if (f > 0.25)
sum += f;
if (f > 0.001)
count++;
if ((sd_debug & DBG_CHI) &&
((sd_debug & DBG_PRINTZERO) || f != 0))
fprintf(stdout, "%04d: %8.5f%%\n", i, f * 100);
}
count /= percent;
off = percent * 2;
if (off >= sizeof(norm_outguess)/sizeof(float))
off = sizeof(norm_outguess)/sizeof(float) - 1;
norm = sum / norm_outguess[off];
if (sd_debug & DBG_ENDVAL)
fprintf(stdout,
"Accumulation (%4.1f%%): %8.3f%% (%8.3f%%) (%4.1f)\n",
percent,
sum * 100,
norm * 100,
count);
/* XXX - some wild adjustment */
if (count < 15) {
sum -= (15 - count) * 0.5;
if (sum < 0)
sum = 0;
}
return (scale * sum / 0.5);
}
int
jphide_runlength(short *data, int bits)
{
int i, max = -1;
short coeff, rundct[128], runmdct[128];
int runlen[128], runmlen[128], off;
memset(rundct, 0, sizeof(rundct));
memset(runlen, 0, sizeof(runlen));
memset(runmdct, 0, sizeof(runmdct));
memset(runmlen, 0, sizeof(runmlen));
for (i = 0; i < bits; i++) {
coeff = data[i];
if (coeff < -127 || coeff > 127)
continue;
if (coeff >= -1 && coeff <= 1)
continue;
if (coeff < 0)
off = -coeff/2;
else
off = coeff/2 + 64;
if (rundct[off] != coeff) {
if (runlen[off] > 1 && runlen[off] > runmlen[off]) {
runmlen[off] = runlen[off];
runmdct[off] = rundct[off];
}
rundct[off] = coeff;
runlen[off] = 1;
} else {
runlen[off]++;
if (runlen[off] > max)
max = runlen[off];
}
}
return (max);
}
int
jphide_zero_one(void)
{
int one, zero, res, sum;
int negative = 0;
/* Zero and One have a 1/4 chance to be modified, back project */
one = DCThist[-1 + 128] + DCThist[1 + 128];
zero = DCThist[0 + 128];
sum = one + zero;
if (sum > 10) {
if (one > zero)
res = (3*zero - one);
else
res = (3*one - zero);
if (res < -1)
negative = 1;
else if (sum >= 15 && res <= -1)
negative = 1;
if (sd_debug & DBG_ENDVAL)
printf("Zero/One: %d : %d -> %5.1f%s\n",
one, zero, (float)res/2, negative ? " **" : "");
}
return (negative);
}
int
jphide_empty_pair(void)
{
int i, res;
res = 0;
for (i = 0; i < 256; i++) {
if (i >= (-1 + 128) && i <= (1 + 128))
continue;
if (i < 128 && !(i & 1))
continue;
else if (i >= 128 && (i & 1))
continue;
if ((DCThist[i] + DCThist[i+1]) >= 5 &&
(!DCThist[i] || !DCThist[i+1]))
res++;
}
if (sd_debug & DBG_ENDVAL)
printf("Empty pairs: %d\n", res);
if (res > 3)
return (1);
return (0);
}
/*
* Calculate liklihood of JPHide embedding.
* Pos is the last bit position where we are guaranteed to have
* a 0.5 modification chance.
*/
int stat_runlength = 0;
int stat_zero_one = 0;
int stat_empty_pair = 0;
int
histogram_chi_jphide(short *data, int bits)
{
int i, range, highpeak, negative;
extern int jphpos[];
float f, f2, sum, false;
/* Image is too small */
if (jphpos[0] < 500)
return (0);
buildDCTreset();
f = chi2test(data, bits, unify_jphide, 0, jphpos[0]);
if (sd_debug & DBG_ENDVAL)
fprintf(stdout, "Pos[0]: %04d: %8.5f%%\n", jphpos[0], f*100);
/* If JPhide was used, we should get a high value at this position */
if (f < 0.9)
return (0);
if (jphide_runlength(data, jphpos[0]) > 16) {
stat_runlength++;
return (0);
}
if (jphide_zero_one()) {
stat_zero_one++;
return (0);
}
if (jphide_empty_pair()) {
stat_empty_pair++;
return (0);
}
false = 0;
f2 = chi2test(data, bits, unify_false_jphide, 0, jphpos[0]);
if (sd_debug & DBG_ENDVAL)
fprintf(stdout, "Pos[0]: %04d[:] %8.5f%%: %8.5f%%\n",
jphpos[0], f2*100, (f2 - f)*100);
/* JPHide embedding reduces f2 and increases f */
if (f2 * 0.95 > f)
return (0);
f = chi2test(data, bits, unify_jphide, jphpos[0]/2, jphpos[0]);
if (sd_debug & DBG_ENDVAL)
fprintf(stdout, "Pos[0]/2: %04d: %8.5f%%\n", jphpos[0], f*100);
if (f < 0.9)
return (0);
f2 = chi2test(data, bits, unify_false_jphide, jphpos[0]/2, jphpos[0]);
if (sd_debug & DBG_ENDVAL)
fprintf(stdout, "Pos[0]/2: %04d[:] %8.5f%%: %8.5f%%\n",
jphpos[0], f2*100, (f2 - f)*100);
if (f2 * 0.95 > f)
return (0);
f = chi2test(data, bits, unify_jphide, 0, jphpos[0]/2);
f2 = chi2test(data, bits, unify_false_jphide, 0, jphpos[0]/2);
if (sd_debug & DBG_ENDVAL)
fprintf(stdout, "0->1/2: %04d[:] %8.5f%% %8.5f%%\n",
jphpos[0], f*100, f2*100);
if (f2 * 0.95 > f)
return (0);
range = jphpos[0]/12;
for (i = 11; i >= 1 && range < 250; i--)
range = jphpos[0]/i;
if (range < 250)
range = 250;
negative = highpeak = 0;
false = sum = 0;
for (i = range; i <= bits && (!negative || i < 4*jphpos[0]);
i += range) {
f = chi2test(data, bits, unify_jphide, 0, i);
f2 = chi2test(data, bits, unify_false_jphide, 0, i);
if (i <= jphpos[0] && jphide_zero_one()) {
stat_zero_one++;
negative++;
}
if (i <= jphpos[0] && jphide_empty_pair()) {
stat_empty_pair++;
negative++;
}
if (i <= jphpos[1] && f2 >= 0.95) {
false += f2 * range;
if (false * 1.10 >= jphpos[1])
negative++;
}
/* Special tests */
if (f >= 0.95)
highpeak = 1;
if (i > jphpos[0] && !highpeak)
negative++;
if (highpeak && f < 0.90 && sum < jphpos[0])
negative++;
if (i <= jphpos[1] && f2*0.99 > f)
negative++;
if (f >= 0.9)
sum += f * range;
else if (f < 0.2)
break;
if ((sd_debug & DBG_CHI) &&
((sd_debug & DBG_PRINTZERO) || f != 0))
fprintf(stdout, "%04d: %8.5f%% %8.5f%% %.2f %.2f %s\n",
i, f * 100, f2*100, sum, false,
(i <= jphpos[0] && f2*0.99 > f) ||
(i <= jphpos[1] && false * 1.10 >= jphpos[1])
? "**" : "");
}
sum /= 1000;
if (sd_debug & DBG_ENDVAL)
fprintf(stdout, "Accumulation (neg = %d, %d): %f%% [%d]\n",
negative, range, sum * 100, jphpos[1]);
if (negative)
return (0);
sum *= (float)1100/jphpos[0];
return (scale * sum );
}
int
histogram_chi_jphide_old(short *data, int bits)
{
int i, highpeak, range;
extern int jphpos[];
float f, sum, percent;
int start, end;
BINSEARCHVAR;
end = bits/10;
start = jphpos[0]/2;
if (start > end)
return (0);
BINSEARCH(start, end, 7) {
range = percent;
sum = 0;
for (i = 0; i <= bits; i += range) {
f = chi2test(data, bits, unify_false_jphide,
0, i + range);
if (f > 0.3)
sum += f;
else if (f < 0.2)
break;
if ((sd_debug & DBG_CHI) && f != 0)
fprintf(stdout, "%04d[:] %8.5f%%\n",
i, f * 100);
}
BINSEARCH_NEXT(3);
}
BINSEARCH_IFABORT(end)
return (0);
range = percent;
highpeak = sum = 0;
for (i = 0; i <= bits; i += range) {
f = chi2test(data, bits, unify_jphide,
0, i + range);
if (!highpeak && f > 0.9)
highpeak = 1;
if (highpeak && f < 0.75)
break;
if (f > 0.3)
sum += f;
else if (f < 0.2)
break;
if ((sd_debug & DBG_CHI) &&
((sd_debug & DBG_PRINTZERO) || f != 0))
fprintf(stdout, "%04d: %8.5f%%\n", i, f * 100);
}
if (sd_debug & DBG_ENDVAL)
fprintf(stdout, "Accumulation (%4.0f): %f%%\n",
percent,
sum * 100);
return (scale * sum / 7);
}
float
histogram_f5(float *hist, int size)
{
int i, n;
float DCTtheo[8], DCTobs[8];
float mean;
if (size < 63)
return (0);
n = 4;
for (i = 0; i < n; i++) {
DCTobs[i] = hist[i*8];
mean = hist[i*8 + 1];
if (i != 0) {
float tmp;
tmp = hist[i*8 - 1];
mean = (mean + tmp)/2;
}
DCTtheo[i] = mean;
}
return (chi2(DCTtheo, DCTobs, n, 0));
}
char detect_buffer[4096];
size_t detect_buflen;
/* Copy data into buffer */
#define DETECT_MINAPPEND 128
void
detect_append(void *arg)
{
j_decompress_ptr dinfo = arg;
u_char *buf = (u_char *)dinfo->src->next_input_byte;
size_t buflen = dinfo->src->bytes_in_buffer;
if (buflen > sizeof(detect_buffer))
buflen = sizeof(detect_buffer);
memcpy(detect_buffer, buf, buflen);
detect_buflen = buflen;