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[rng] Add dummy entropy source
Cryptographic random number generation requires an entropy source, which is used as the input to a Deterministic Random Bit Generator (DRBG). iPXE does not currently have a suitable entropy source. Provide a dummy source to allow the DRBG code to be implemented. Signed-off-by: Michael Brown <mcb30@ipxe.org>
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src/crypto/entropy.c
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src/crypto/entropy.c
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/*
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* Copyright (C) 2012 Michael Brown <mbrown@fensystems.co.uk>.
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License as
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* published by the Free Software Foundation; either version 2 of the
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* License, or any later version.
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*
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* This program is distributed in the hope that it will be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*/
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FILE_LICENCE ( GPL2_OR_LATER );
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/** @file
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*
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* Entropy source
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*
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*/
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#include <string.h>
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#include <ipxe/entropy.h>
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/**
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* Obtain entropy input
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*
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* @v entropy_bits Minimum amount of entropy, in bits
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* @v data Data buffer
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* @v min_len Minimum length of entropy input, in bytes
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* @v max_len Maximum length of entropy input, in bytes
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* @ret len Length of entropy input, in bytes
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*/
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int get_entropy_input ( unsigned int entropy_bits, void *data, size_t min_len,
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size_t max_len ) {
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/* Placeholder to allow remainder of RBG code to be tested */
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( void ) entropy_bits;
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( void ) min_len;
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memset ( data, 0x01, max_len );
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return max_len;
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}
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90
src/include/ipxe/entropy.h
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src/include/ipxe/entropy.h
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#ifndef _IPXE_ENTROPY_H
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#define _IPXE_ENTROPY_H
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/** @file
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*
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* Entropy source
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*
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*/
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FILE_LICENCE ( GPL2_OR_LATER );
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#include <stdint.h>
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#include <assert.h>
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/** min-entropy per entropy sample
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*
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* min-entropy is defined in ANS X9.82 Part 1-2006 Section 8.3 and in
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* NIST SP 800-90 Appendix C.3 as
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*
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* H_min = -log2 ( p_max )
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*
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* where p_max is the probability of the most likely sample value.
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*/
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#define MIN_ENTROPY_PER_SAMPLE 0.16
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/** Length of each entropy sample (in bits) */
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#define ENTROPY_SAMPLE_LEN_BITS 12
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/**
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* Calculate entropy buffer size
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*
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* @v entropy_bits Amount of entropy required, in bits
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* @v min_len Minimum buffer size, in bytes
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* @v max_len Maximum buffer size, in bytes
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* @ret len Buffer size, in bytes
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*/
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static inline __attribute__ (( const, always_inline )) size_t
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entropy_bufsize ( unsigned int entropy_bits, size_t min_len, size_t max_len ) {
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unsigned int min_len_bits;
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double min_samples;
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double samples;
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unsigned int samples_int;
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unsigned int len_bits;
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size_t len;
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/* Sanity check */
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linker_assert ( MIN_ENTROPY_PER_SAMPLE <= ENTROPY_SAMPLE_LEN_BITS,
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min_entropy_per_sample_is_impossibly_high );
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/* Calculate number of samples required to contain sufficient entropy */
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samples = ( ( entropy_bits * 1.0 ) / MIN_ENTROPY_PER_SAMPLE );
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/* Increase to minimum length if necessary */
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min_len_bits = ( min_len * 8 );
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min_samples = ( ( min_len_bits * 1.0 ) / ENTROPY_SAMPLE_LEN_BITS );
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if ( samples < min_samples )
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samples = min_samples;
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/* Round up to a whole number of samples. We don't have the
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* ceil() function available, so do the rounding by hand.
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*/
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samples_int = samples;
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if ( samples_int < samples )
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samples_int++;
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assert ( samples_int >= samples );
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/* Calculate buffer length in bits */
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len_bits = ( samples_int * ENTROPY_SAMPLE_LEN_BITS );
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/* Calculate buffer length in bytes (rounding up) */
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len = ( ( len_bits + 7 ) / 8 );
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/* Check that buffer is within allowed lengths */
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linker_assert ( len >= min_len, entropy_bufsize_too_short );
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linker_assert ( len <= max_len, entropy_bufsize_too_long );
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/* Floating-point operations are not allowed in iPXE since we
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* never set up a suitable environment. Abort the build
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* unless the calculated length is a compile-time constant.
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*/
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linker_assert ( __builtin_constant_p ( len ),
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entropy_bufsize_not_constant );
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return len;
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}
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extern int get_entropy_input ( unsigned int entropy_bits, void *data,
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size_t min_len, size_t max_len );
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#endif /* _IPXE_ENTROPY_H */
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