summaryrefslogtreecommitdiffstats
path: root/qemu/roms/ipxe/src/crypto/entropy.c
blob: c7045840e723b8ec91bab116af3d8591c7bce9cd (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
/*
 * Copyright (C) 2012 Michael Brown <mbrown@fensystems.co.uk>.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301, USA.
 */

FILE_LICENCE ( GPL2_OR_LATER );

/** @file
 *
 * Entropy source
 *
 * This algorithm is designed to comply with ANS X9.82 Part 4 (April
 * 2011 Draft) Section 13.3.  This standard is unfortunately not
 * freely available.
 */

#include <stdint.h>
#include <assert.h>
#include <string.h>
#include <errno.h>
#include <ipxe/crypto.h>
#include <ipxe/hash_df.h>
#include <ipxe/entropy.h>

/* Disambiguate the various error causes */
#define EPIPE_REPETITION_COUNT_TEST \
	__einfo_error ( EINFO_EPIPE_REPETITION_COUNT_TEST )
#define EINFO_EPIPE_REPETITION_COUNT_TEST \
	__einfo_uniqify ( EINFO_EPIPE, 0x01, "Repetition count test failed" )
#define EPIPE_ADAPTIVE_PROPORTION_TEST \
	__einfo_error ( EINFO_EPIPE_ADAPTIVE_PROPORTION_TEST )
#define EINFO_EPIPE_ADAPTIVE_PROPORTION_TEST \
	__einfo_uniqify ( EINFO_EPIPE, 0x02, "Adaptive proportion test failed" )

/**
 * Calculate cutoff value for the repetition count test
 *
 * @ret cutoff		Cutoff value
 *
 * This is the cutoff value for the Repetition Count Test defined in
 * ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.2.
 */
static inline __attribute__ (( always_inline )) unsigned int
repetition_count_cutoff ( void ) {
	double max_repetitions;
	unsigned int cutoff;

	/* The cutoff formula for the repetition test is:
	 *
	 *   C = ( 1 + ( -log2(W) / H_min ) )
	 *
	 * where W is set at 2^(-30) (in ANS X9.82 Part 2 (October
	 * 2011 Draft) Section 8.5.2.1.3.1).
	 */
	max_repetitions = ( 1 + ( 30 / min_entropy_per_sample() ) );

	/* Round up to a whole number of repetitions.  We don't have
	 * the ceil() function available, so do the rounding by hand.
	 */
	cutoff = max_repetitions;
	if ( cutoff < max_repetitions )
		cutoff++;
	linker_assert ( ( cutoff >= max_repetitions ), rounding_error );

	/* Floating-point operations are not allowed in iPXE since we
	 * never set up a suitable environment.  Abort the build
	 * unless the calculated number of repetitions is a
	 * compile-time constant.
	 */
	linker_assert ( __builtin_constant_p ( cutoff ),
			repetition_count_cutoff_not_constant );

	return cutoff;
}

/**
 * Perform repetition count test
 *
 * @v sample		Noise sample
 * @ret rc		Return status code
 *
 * This is the Repetition Count Test defined in ANS X9.82 Part 2
 * (October 2011 Draft) Section 8.5.2.1.2.
 */
static int repetition_count_test ( noise_sample_t sample ) {
	static noise_sample_t most_recent_sample;
	static unsigned int repetition_count = 0;

	/* A = the most recently seen sample value
	 * B = the number of times that value A has been seen in a row
	 * C = the cutoff value above which the repetition test should fail
	 */

	/* 1.  For each new sample processed:
	 *
	 * (Note that the test for "repetition_count > 0" ensures that
	 * the initial value of most_recent_sample is treated as being
	 * undefined.)
	 */
	if ( ( sample == most_recent_sample ) && ( repetition_count > 0 ) ) {

		/* a) If the new sample = A, then B is incremented by one. */
		repetition_count++;

		/*    i.  If B >= C, then an error condition is raised
		 *        due to a failure of the test
		 */
		if ( repetition_count >= repetition_count_cutoff() )
			return -EPIPE_REPETITION_COUNT_TEST;

	} else {

		/* b) Else:
		 *    i.  A = new sample
		 */
		most_recent_sample = sample;

		/*    ii. B = 1 */
		repetition_count = 1;
	}

	return 0;
}

/**
 * Window size for the adaptive proportion test
 *
 * ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.3.1.1 allows
 * five possible window sizes: 16, 64, 256, 4096 and 65536.
 *
 * We expect to generate relatively few (<256) entropy samples during
 * a typical iPXE run; the use of a large window size would mean that
 * the test would never complete a single cycle.  We use a window size
 * of 64, which is the smallest window size that permits values of
 * H_min down to one bit per sample.
 */
#define ADAPTIVE_PROPORTION_WINDOW_SIZE 64

/**
 * Combine adaptive proportion test window size and min-entropy
 *
 * @v n			N (window size)
 * @v h			H (min-entropy)
 * @ret n_h		(N,H) combined value
 */
#define APC_N_H( n, h ) ( ( (n) << 8 ) | (h) )

/**
 * Define a row of the adaptive proportion cutoff table
 *
 * @v h			H (min-entropy)
 * @v c16		Cutoff for N=16
 * @v c64		Cutoff for N=64
 * @v c256		Cutoff for N=256
 * @v c4096		Cutoff for N=4096
 * @v c65536		Cutoff for N=65536
 */
#define APC_TABLE_ROW( h, c16, c64, c256, c4096, c65536)	   \
	case APC_N_H ( 16, h ) :	return c16;		   \
	case APC_N_H ( 64, h ) :	return c64;   		   \
	case APC_N_H ( 256, h ) :	return c256;		   \
	case APC_N_H ( 4096, h ) :	return c4096;		   \
	case APC_N_H ( 65536, h ) :	return c65536;

/** Value used to represent "N/A" in adaptive proportion cutoff table */
#define APC_NA 0

/**
 * Look up value in adaptive proportion test cutoff table
 *
 * @v n			N (window size)
 * @v h			H (min-entropy)
 * @ret cutoff		Cutoff
 *
 * This is the table of cutoff values defined in ANS X9.82 Part 2
 * (October 2011 Draft) Section 8.5.2.1.3.1.2.
 */
static inline __attribute__ (( always_inline )) unsigned int
adaptive_proportion_cutoff_lookup ( unsigned int n, unsigned int h ) {
	switch ( APC_N_H ( n, h ) ) {
		APC_TABLE_ROW (  1, APC_NA,     51,    168,   2240,  33537 );
		APC_TABLE_ROW (  2, APC_NA,     35,    100,   1193,  17053 );
		APC_TABLE_ROW (  3,     10,     24,     61,    643,   8705 );
		APC_TABLE_ROW (  4,      8,     16,     38,    354,   4473 );
		APC_TABLE_ROW (  5,      6,     12,     25,    200,   2321 );
		APC_TABLE_ROW (  6,      5,      9,     17,    117,   1220 );
		APC_TABLE_ROW (  7,      4,      7,     15,     71,    653 );
		APC_TABLE_ROW (  8,      4,      5,      9,     45,    358 );
		APC_TABLE_ROW (  9,      3,      4,      7,     30,    202 );
		APC_TABLE_ROW ( 10,      3,      4,      5,     21,    118 );
		APC_TABLE_ROW ( 11,      2,      3,      4,     15,     71 );
		APC_TABLE_ROW ( 12,      2,      3,      4,     11,     45 );
		APC_TABLE_ROW ( 13,      2,      2,      3,      9,     30 );
		APC_TABLE_ROW ( 14,      2,      2,      3,      7,     21 );
		APC_TABLE_ROW ( 15,      1,      2,      2,      6,     15 );
		APC_TABLE_ROW ( 16,      1,      2,      2,      5,     11 );
		APC_TABLE_ROW ( 17,      1,      1,      2,      4,      9 );
		APC_TABLE_ROW ( 18,      1,      1,      2,      4,      7 );
		APC_TABLE_ROW ( 19,      1,      1,      1,      3,      6 );
		APC_TABLE_ROW ( 20,      1,      1,      1,      3,      5 );
	default:
		return APC_NA;
	}
}

/**
 * Calculate cutoff value for the adaptive proportion test
 *
 * @ret cutoff		Cutoff value
 *
 * This is the cutoff value for the Adaptive Proportion Test defined
 * in ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.3.1.2.
 */
static inline __attribute__ (( always_inline )) unsigned int
adaptive_proportion_cutoff ( void ) {
	unsigned int h;
	unsigned int n;
	unsigned int cutoff;

	/* Look up cutoff value in cutoff table */
	n = ADAPTIVE_PROPORTION_WINDOW_SIZE;
	h = min_entropy_per_sample();
	cutoff = adaptive_proportion_cutoff_lookup ( n, h );

	/* Fail unless cutoff value is a build-time constant */
	linker_assert ( __builtin_constant_p ( cutoff ),
			adaptive_proportion_cutoff_not_constant );

	/* Fail if cutoff value is N/A */
	linker_assert ( ( cutoff != APC_NA ),
			adaptive_proportion_cutoff_not_applicable );

	return cutoff;
}

/**
 * Perform adaptive proportion test
 *
 * @v sample		Noise sample
 * @ret rc		Return status code
 *
 * This is the Adaptive Proportion Test for the Most Common Value
 * defined in ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.3.
 */
static int adaptive_proportion_test ( noise_sample_t sample ) {
	static noise_sample_t current_counted_sample;
	static unsigned int sample_count = ADAPTIVE_PROPORTION_WINDOW_SIZE;
	static unsigned int repetition_count;

	/* A = the sample value currently being counted
	 * B = the number of samples examined in this run of the test so far
	 * N = the total number of samples that must be observed in
	 *     one run of the test, also known as the "window size" of
	 *     the test
	 * B = the current number of times that S (sic) has been seen
	 *     in the W (sic) samples examined so far
	 * C = the cutoff value above which the repetition test should fail
	 * W = the probability of a false positive: 2^-30
	 */

	/* 1.  The entropy source draws the current sample from the
	 *     noise source.
	 *
	 * (Nothing to do; we already have the current sample.)
	 */

	/* 2.  If S = N, then a new run of the test begins: */
	if ( sample_count == ADAPTIVE_PROPORTION_WINDOW_SIZE ) {

		/* a.  A = the current sample */
		current_counted_sample = sample;

		/* b.  S = 0 */
		sample_count = 0;

		/* c. B = 0 */
		repetition_count = 0;

	} else {

		/* Else: (the test is already running)
		 * a.  S = S + 1
		 */
		sample_count++;

		/* b.  If A = the current sample, then: */
		if ( sample == current_counted_sample ) {

			/* i.   B = B + 1 */
			repetition_count++;

			/* ii.  If S (sic) > C then raise an error
			 *      condition, because the test has
			 *      detected a failure
			 */
			if ( repetition_count > adaptive_proportion_cutoff() )
				return -EPIPE_ADAPTIVE_PROPORTION_TEST;

		}
	}

	return 0;
}

/**
 * Get entropy sample
 *
 * @ret entropy		Entropy sample
 * @ret rc		Return status code
 *
 * This is the GetEntropy function defined in ANS X9.82 Part 2
 * (October 2011 Draft) Section 6.5.1.
 */
static int get_entropy ( entropy_sample_t *entropy ) {
	static int rc = 0;
	noise_sample_t noise;

	/* Any failure is permanent */
	if ( rc != 0 )
		return rc;

	/* Get noise sample */
	if ( ( rc = get_noise ( &noise ) ) != 0 )
		return rc;

	/* Perform Repetition Count Test and Adaptive Proportion Test
	 * as mandated by ANS X9.82 Part 2 (October 2011 Draft)
	 * Section 8.5.2.1.1.
	 */
	if ( ( rc = repetition_count_test ( noise ) ) != 0 )
		return rc;
	if ( ( rc = adaptive_proportion_test ( noise ) ) != 0 )
		return rc;

	/* We do not use any optional conditioning component */
	*entropy = noise;

	return 0;
}

/**
 * Calculate number of samples required for startup tests
 *
 * @ret num_samples	Number of samples required
 *
 * ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.5 requires
 * that at least one full cycle of the continuous tests must be
 * performed at start-up.
 */
static inline __attribute__ (( always_inline )) unsigned int
startup_test_count ( void ) {
	unsigned int num_samples;

	/* At least max(N,C) samples shall be generated by the noise
	 * source for start-up testing.
	 */
	num_samples = repetition_count_cutoff();
	if ( num_samples < adaptive_proportion_cutoff() )
		num_samples = adaptive_proportion_cutoff();
	linker_assert ( __builtin_constant_p ( num_samples ),
			startup_test_count_not_constant );

	return num_samples;
}

/**
 * Create next nonce value
 *
 * @ret nonce		Nonce
 *
 * This is the MakeNextNonce function defined in ANS X9.82 Part 4
 * (April 2011 Draft) Section 13.3.4.2.
 */
static uint32_t make_next_nonce ( void ) {
	static uint32_t nonce;

	/* The simplest implementation of a nonce uses a large counter */
	nonce++;

	return nonce;
}

/**
 * Obtain entropy input temporary buffer
 *
 * @v num_samples	Number of entropy samples
 * @v tmp		Temporary buffer
 * @v tmp_len		Length of temporary buffer
 * @ret rc		Return status code
 *
 * This is (part of) the implementation of the Get_entropy_input
 * function (using an entropy source as the source of entropy input
 * and condensing each entropy source output after each GetEntropy
 * call) as defined in ANS X9.82 Part 4 (April 2011 Draft) Section
 * 13.3.4.2.
 *
 * To minimise code size, the number of samples required is calculated
 * at compilation time.
 */
int get_entropy_input_tmp ( unsigned int num_samples, uint8_t *tmp,
			    size_t tmp_len ) {
	static unsigned int startup_tested = 0;
	struct {
		uint32_t nonce;
		entropy_sample_t sample;
	} __attribute__ (( packed )) data;;
	uint8_t df_buf[tmp_len];
	unsigned int i;
	int rc;

	/* Enable entropy gathering */
	if ( ( rc = entropy_enable() ) != 0 )
		return rc;

	/* Perform mandatory startup tests, if not yet performed */
	for ( ; startup_tested < startup_test_count() ; startup_tested++ ) {
		if ( ( rc = get_entropy ( &data.sample ) ) != 0 )
			goto err_get_entropy;
	}

	/* 3.  entropy_total = 0
	 *
	 * (Nothing to do; the number of entropy samples required has
	 * already been precalculated.)
	 */

	/* 4.  tmp = a fixed n-bit value, such as 0^n */
	memset ( tmp, 0, tmp_len );

	/* 5.  While ( entropy_total < min_entropy ) */
	while ( num_samples-- ) {
		/* 5.1.  ( status, entropy_bitstring, assessed_entropy )
		 *       = GetEntropy()
		 * 5.2.  If status indicates an error, return ( status, Null )
		 */
		if ( ( rc = get_entropy ( &data.sample ) ) != 0 )
			goto err_get_entropy;

		/* 5.3.  nonce = MakeNextNonce() */
		data.nonce = make_next_nonce();

		/* 5.4.  tmp = tmp XOR
		 *             df ( ( nonce || entropy_bitstring ), n )
		 */
		hash_df ( &entropy_hash_df_algorithm, &data, sizeof ( data ),
			  df_buf, sizeof ( df_buf ) );
		for ( i = 0 ; i < tmp_len ; i++ )
			tmp[i] ^= df_buf[i];

		/* 5.5.  entropy_total = entropy_total + assessed_entropy
		 *
		 * (Nothing to do; the number of entropy samples
		 * required has already been precalculated.)
		 */
	}

	/* Disable entropy gathering */
	entropy_disable();

	return 0;

 err_get_entropy:
	entropy_disable();
	return rc;
}