/* * Copyright (C) 2012 Michael Brown . * * 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. * * You can also choose to distribute this program under the terms of * the Unmodified Binary Distribution Licence (as given in the file * COPYING.UBDL), provided that you have satisfied its requirements. */ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL ); /** @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 #include #include #include #include #include #include /* 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" ) /** Current entropy source */ static struct entropy_source *entropy_source; /** * Enable entropy gathering * * @ret rc Return status code */ int entropy_enable ( void ) { int rc; /* Enable selected source, if applicable */ if ( entropy_source ) { /* Enable entropy source */ if ( ( rc = entropy_source->enable() ) != 0 ) { DBGC ( &entropy_source, "ENTROPY could not enable " "source \"%s\": %s\n", entropy_source->name, strerror ( rc ) ); return rc; } /* Sanity checks */ assert ( entropy_source->min_entropy_per_sample > 0 ); assert ( entropy_source->repetition_count_cutoff > 0 ); assert ( entropy_source->adaptive_proportion_cutoff > 0 ); assert ( entropy_source->startup_test_count > 0 ); return 0; } /* Find the first working source */ rc = -ENOENT; for_each_table_entry ( entropy_source, ENTROPY_SOURCES ) { if ( ( rc = entropy_enable() ) == 0 ) { DBGC ( &entropy_source, "ENTROPY using source \"%s\"\n", entropy_source->name ); break; } } return rc; } /** * Disable entropy gathering * */ void entropy_disable ( void ) { /* Sanity check */ assert ( entropy_source != NULL ); /* Disable entropy gathering, if applicable */ if ( entropy_source->disable ) entropy_source->disable(); } /** * 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; unsigned int repetition_count_cutoff = entropy_source->repetition_count_cutoff; /* 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; } /** * 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; unsigned int adaptive_proportion_cutoff = entropy_source->adaptive_proportion_cutoff; /* 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 noise sample * * @ret noise Noise sample * @ret rc Return status code * * This is the GetNoise function defined in ANS X9.82 Part 2 * (October 2011 Draft) Section 6.5.2. */ int get_noise ( noise_sample_t *noise ) { /* Sanity check */ assert ( entropy_source != NULL ); return entropy_source->get_noise ( noise ); } /** * 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; /* Sanity check */ assert ( entropy_source != NULL ); /* 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; } /** * 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 min_entropy Min-entropy required * @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. */ int get_entropy_input_tmp ( min_entropy_t min_entropy, 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]; min_entropy_t entropy_total; unsigned int num_samples; 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 < entropy_source->startup_test_count ; startup_tested++ ) { if ( ( rc = get_entropy ( &data.sample ) ) != 0 ) goto err_get_entropy; } /* 3. entropy_total = 0 */ entropy_total = MIN_ENTROPY ( 0 ); /* 4. tmp = a fixed n-bit value, such as 0^n */ memset ( tmp, 0, tmp_len ); /* 5. While ( entropy_total < min_entropy ) */ for ( num_samples = 0 ; entropy_total < min_entropy ; 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 */ entropy_total += entropy_source->min_entropy_per_sample; } /* Disable entropy gathering */ entropy_disable(); DBGC ( &entropy_source, "ENTROPY gathered %d bits in %d samples\n", ( min_entropy / MIN_ENTROPY_SCALE ), num_samples ); return 0; err_get_entropy: entropy_disable(); return rc; } /* Drag in objects via entropy_enable */ REQUIRING_SYMBOL ( entropy_enable ); /* Drag in entropy configuration */ REQUIRE_OBJECT ( config_entropy );