From 1a7a5626ec3bd8cb86d6e8c51725c72987a98a08 Mon Sep 17 00:00:00 2001 From: Dave Rodgman Date: Wed, 17 May 2023 13:47:56 +0100 Subject: [PATCH] Update mbedtls_mpi_lt_mpi_ct to new interface Signed-off-by: Dave Rodgman --- library/bignum.c | 51 +++++++++++++++++------------------------------- 1 file changed, 18 insertions(+), 33 deletions(-) diff --git a/library/bignum.c b/library/bignum.c index dd77bfc9f..9e215d8e5 100644 --- a/library/bignum.c +++ b/library/bignum.c @@ -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; }