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Update mbedtls_mpi_lt_mpi_ct to new interface

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Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
This commit is contained in:
Dave Rodgman 2023-05-17 13:47:56 +01:00
parent 589ccb8aaa
commit 1a7a5626ec
1 changed files with 18 additions and 33 deletions
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51
library/bignum.c
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@ -61,9 +61,7 @@ int mbedtls_mpi_lt_mpi_ct(const mbedtls_mpi *X,
const mbedtls_mpi *Y,
unsigned *ret)
{
size_t i;
/* The value of any of these variables is either 0 or 1 at all times. */
unsigned cond, done, X_is_negative, Y_is_negative;
mbedtls_ct_condition_t cond, X_is_negative, Y_is_negative, result;
MPI_VALIDATE_RET(X != NULL);
MPI_VALIDATE_RET(Y != NULL);
@ -77,46 +75,33 @@ int mbedtls_mpi_lt_mpi_ct(const mbedtls_mpi *X,
* Set sign_N to 1 if N >= 0, 0 if N < 0.
* We know that N->s == 1 if N >= 0 and N->s == -1 if N < 0.
*/
X_is_negative = (X->s & 2) >> 1;
Y_is_negative = (Y->s & 2) >> 1;
X_is_negative = mbedtls_ct_bool((X->s & 2) >> 1);
Y_is_negative = mbedtls_ct_bool((Y->s & 2) >> 1);
/*
* If the signs are different, then the positive operand is the bigger.
* That is if X is negative (X_is_negative == 1), then X < Y is true and it
* is false if X is positive (X_is_negative == 0).
*/
cond = (X_is_negative ^ Y_is_negative);
*ret = cond & X_is_negative;
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);
/*
* This is a constant-time function. We might have the result, but we still
* need to go through the loop. Record if we have the result already.
/* 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
* sign.
*
* Store in ret iff the signs are the same (i.e., iff cond == 0). If
* the signs differ, done has already been set.
*/
done = cond;
for (i = X->n; i > 0; i--) {
/*
* If Y->p[i - 1] < X->p[i - 1] then X < Y is true if and only if both
* X and Y are negative.
*
* Again even if we can make a decision, we just mark the result and
* the fact that we are done and continue looping.
*/
cond = mbedtls_ct_mpi_uint_lt(Y->p[i - 1], X->p[i - 1]);
*ret |= cond & (1 - done) & X_is_negative;
done |= cond;
/* This is used to conditionally swap the pointers in const time */
void * const p[2] = { X->p, Y->p };
mbedtls_ct_condition_t lt = mbedtls_mpi_core_lt_ct(
p[X_is_negative & 1], p[(X_is_negative & 1) ^ 1], X->n);
/*
* If X->p[i - 1] < Y->p[i - 1] then X < Y is true if and only if both
* X and Y are positive.
*
* Again even if we can make a decision, we just mark the result and
* the fact that we are done and continue looping.
*/
cond = mbedtls_ct_mpi_uint_lt(X->p[i - 1], Y->p[i - 1]);
*ret |= cond & (1 - done) & (1 - X_is_negative);
done |= cond;
}
result = mbedtls_ct_bool_or(result, mbedtls_ct_bool_and(mbedtls_ct_bool_not(cond), lt));
*ret = mbedtls_ct_uint_if0(result, 1);
return 0;
}