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Improve comments in mbedtls_mpi_lt_mpi_ct

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Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
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Dave Rodgman 2023-07-31 12:28:05 +01:00
parent 07f853713d
commit 32d726033b
1 changed files with 7 additions and 5 deletions
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12
library/bignum.c
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@ -86,19 +86,21 @@ int mbedtls_mpi_lt_mpi_ct(const mbedtls_mpi *X,
cond = mbedtls_ct_bool_xor(X_is_negative, Y_is_negative); // non-zero if different sign cond = mbedtls_ct_bool_xor(X_is_negative, Y_is_negative); // non-zero if different sign
result = mbedtls_ct_bool_and(cond, X_is_negative); result = mbedtls_ct_bool_and(cond, X_is_negative);
/* Assuming signs are the same, compare X and Y. We switch the comparison /*
* Assuming signs are the same, compare X and Y. We switch the comparison
* order if they are negative so that we get the right result, regardles of * order if they are negative so that we get the right result, regardles of
* sign. * sign.
*
* Store in ret iff the signs are the same (i.e., iff cond == 0). If
* the signs differ, done has already been set.
*/ */
/* This is used to conditionally swap the pointers in const time */ /* This array is used to conditionally swap the pointers in const time */
void * const p[2] = { X->p, Y->p }; void * const p[2] = { X->p, Y->p };
size_t i = mbedtls_ct_size_if0(X_is_negative, 1); size_t i = mbedtls_ct_size_if0(X_is_negative, 1);
mbedtls_ct_condition_t lt = mbedtls_mpi_core_lt_ct(p[i], p[i ^ 1], X->n); mbedtls_ct_condition_t lt = mbedtls_mpi_core_lt_ct(p[i], p[i ^ 1], X->n);
/*
* Store in result iff the signs are the same (i.e., iff cond == false). If
* the signs differ, result has already been set, so we don't change it.
*/
result = mbedtls_ct_bool_or(result, mbedtls_ct_bool_and(mbedtls_ct_bool_not(cond), lt)); result = mbedtls_ct_bool_or(result, mbedtls_ct_bool_and(mbedtls_ct_bool_not(cond), lt));
*ret = mbedtls_ct_uint_if0(result, 1); *ret = mbedtls_ct_uint_if0(result, 1);