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Improve const-timeness of mbedtls_ct_bool_lt

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Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
This commit is contained in:
Dave Rodgman
2023-05-17 12:21:32 +01:00
parent 40a41d0461
commit 74e18ebf77
1 changed files with 3 additions and 2 deletions
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5
library/constant_time_impl.h
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@@ -123,12 +123,13 @@ static inline mbedtls_ct_condition_t mbedtls_ct_bool_lt(mbedtls_ct_uint_t x, mbe
/* Ensure that the compiler cannot optimise the following operations over x and y,
* even if it knows the value of x and y.
*/
const mbedtls_ct_uint_t xo = mbedtls_ct_compiler_opaque(x);
const mbedtls_ct_uint_t yo = mbedtls_ct_compiler_opaque(y);
/*
* Check if the most significant bits (MSB) of the operands are different.
* cond is true iff the MSBs differ.
*/
mbedtls_ct_condition_t cond = mbedtls_ct_bool((x ^ yo) >> (MBEDTLS_CT_SIZE - 1));
mbedtls_ct_condition_t cond = mbedtls_ct_bool((xo ^ yo) >> (MBEDTLS_CT_SIZE - 1));
/*
* If the MSB are the same then the difference x-y will be negative (and
@@ -140,7 +141,7 @@ static inline mbedtls_ct_condition_t mbedtls_ct_bool_lt(mbedtls_ct_uint_t x, mbe
*/
// Select either y, or x - y
mbedtls_ct_uint_t ret = mbedtls_ct_if(cond, yo, (mbedtls_ct_uint_t) (x - yo));
mbedtls_ct_uint_t ret = mbedtls_ct_if(cond, yo, (mbedtls_ct_uint_t) (xo - yo));
// Extract only the MSB of ret
ret = ret >> (MBEDTLS_CT_SIZE - 1);