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[crypto] Extract bigint_reduce_supremum() from bigint_mod_exp() 

Calculating the Montgomery constant (R^2 mod N) is done in our
implementation by zeroing the double-width representation of N,
subtracting N once to give (R^2 - N) in order to obtain a positive
value, then reducing this value modulo N.

Extract this logic from bigint_mod_exp() to a separate function
bigint_reduce_supremum(), to allow for reuse by other code.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
pull/1386/head
Michael Brown 2025-01-10 13:44:13 +00:00
parent 83ba34076a
commit d88eb0a193
2 changed files with 44 additions and 7 deletions
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30
src/crypto/bigint.c
View File

@ -277,6 +277,30 @@ void bigint_reduce_raw ( bigint_element_t *modulus0, bigint_element_t *value0,
profile_stop ( &bigint_mod_profiler );
}
/**
* Reduce supremum of big integer representation
*
* @v modulus0 Element 0 of big integer modulus
* @v result0 Element 0 of big integer to hold result
* @v size Number of elements in modulus and value
*
* Reduce the value 2^k (where k is the bit width of the big integer
* representation) modulo the specified modulus.
*/
void bigint_reduce_supremum_raw ( bigint_element_t *modulus0,
bigint_element_t *result0,
unsigned int size ) {
bigint_t ( size ) __attribute__ (( may_alias ))
*modulus = ( ( void * ) modulus0 );
bigint_t ( size ) __attribute__ (( may_alias ))
*result = ( ( void * ) result0 );
/* Calculate (2^k) mod N via direct reduction of (2^k - N) mod N */
memset ( result, 0, sizeof ( *result ) );
bigint_subtract ( modulus, result );
bigint_reduce ( modulus, result );
}
/**
* Compute inverse of odd big integer modulo any power of two
*
@ -629,10 +653,8 @@ void bigint_mod_exp_raw ( const bigint_element_t *base0,
if ( ! submask )
submask = ~submask;
/* Calculate (R^2 mod N) via direct reduction of (R^2 - N) */
memset ( &temp->product.full, 0, sizeof ( temp->product.full ) );
bigint_subtract ( &temp->padded_modulus, &temp->product.full );
bigint_reduce ( &temp->padded_modulus, &temp->product.full );
/* Calculate (R^2 mod N) */
bigint_reduce_supremum ( &temp->padded_modulus, &temp->product.full );
bigint_copy ( &temp->product.low, &temp->stash );
/* Initialise result = Montgomery(1, R^2 mod N) */

21
src/include/ipxe/bigint.h
View File

@ -236,9 +236,21 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
* @v value Big integer to be reduced
*/
#define bigint_reduce( modulus, value ) do { \
unsigned int size = bigint_size (modulus); \
bigint_reduce_raw ( (modulus)->element, \
(value)->element, size ); \
unsigned int size = bigint_size (modulus); \
bigint_reduce_raw ( (modulus)->element, (value)->element, \
size ); \
} while ( 0 )
/**
* Reduce supremum of big integer representation
*
* @v modulus0 Big integer modulus
* @v result0 Big integer to hold result
*/
#define bigint_reduce_supremum( modulus, result ) do { \
unsigned int size = bigint_size (modulus); \
bigint_reduce_supremum_raw ( (modulus)->element, \
(result)->element, size ); \
} while ( 0 )
/**
@ -385,6 +397,9 @@ void bigint_multiply_raw ( const bigint_element_t *multiplicand0,
bigint_element_t *result0 );
void bigint_reduce_raw ( bigint_element_t *modulus0, bigint_element_t *value0,
unsigned int size );
void bigint_reduce_supremum_raw ( bigint_element_t *modulus0,
bigint_element_t *value0,
unsigned int size );
void bigint_mod_invert_raw ( const bigint_element_t *invertend0,
bigint_element_t *inverse0, unsigned int size );
int bigint_montgomery_relaxed_raw ( const bigint_element_t *modulus0,