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Move some bignum functions out of constant_time module

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Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
This commit is contained in:
Dave Rodgman 2023-05-09 14:01:05 +01:00
parent 19e8cd06fe
commit 7d4f019810
5 changed files with 194 additions and 203 deletions
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136
library/bignum.c
View File

@ -54,6 +54,142 @@
#define MPI_VALIDATE(cond) \
MBEDTLS_INTERNAL_VALIDATE(cond)
/*
* Compare signed values in constant time
*/
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;
MPI_VALIDATE_RET(X != NULL);
MPI_VALIDATE_RET(Y != NULL);
MPI_VALIDATE_RET(ret != NULL);
if (X->n != Y->n) {
return MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
}
/*
* 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;
/*
* 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;
/*
* 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.
*/
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;
/*
* 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;
}
return 0;
}
/*
* Conditionally assign X = Y, without leaking information
* about whether the assignment was made or not.
* (Leaking information about the respective sizes of X and Y is ok however.)
*/
#if defined(_MSC_VER) && defined(_M_ARM64) && (_MSC_FULL_VER < 193131103)
/*
* MSVC miscompiles this function if it's inlined prior to Visual Studio 2022 version 17.1. See:
* https://developercommunity.visualstudio.com/t/c-compiler-miscompiles-part-of-mbedtls-library-on/1646989
*/
__declspec(noinline)
#endif
int mbedtls_mpi_safe_cond_assign(mbedtls_mpi *X,
const mbedtls_mpi *Y,
unsigned char assign)
{
int ret = 0;
MPI_VALIDATE_RET(X != NULL);
MPI_VALIDATE_RET(Y != NULL);
/* all-bits 1 if assign is 1, all-bits 0 if assign is 0 */
mbedtls_mpi_uint limb_mask = mbedtls_ct_mpi_uint_mask(assign);
MBEDTLS_MPI_CHK(mbedtls_mpi_grow(X, Y->n));
X->s = (int) mbedtls_ct_uint_if(assign, Y->s, X->s);
mbedtls_mpi_core_cond_assign(X->p, Y->p, Y->n, assign);
for (size_t i = Y->n; i < X->n; i++) {
X->p[i] &= ~limb_mask;
}
cleanup:
return ret;
}
/*
* Conditionally swap X and Y, without leaking information
* about whether the swap was made or not.
* Here it is not ok to simply swap the pointers, which would lead to
* different memory access patterns when X and Y are used afterwards.
*/
int mbedtls_mpi_safe_cond_swap(mbedtls_mpi *X,
mbedtls_mpi *Y,
unsigned char swap)
{
int ret = 0;
int s;
MPI_VALIDATE_RET(X != NULL);
MPI_VALIDATE_RET(Y != NULL);
if (X == Y) {
return 0;
}
MBEDTLS_MPI_CHK(mbedtls_mpi_grow(X, Y->n));
MBEDTLS_MPI_CHK(mbedtls_mpi_grow(Y, X->n));
s = X->s;
X->s = (int) mbedtls_ct_uint_if(swap, Y->s, X->s);
Y->s = (int) mbedtls_ct_uint_if(swap, s, Y->s);
mbedtls_mpi_core_cond_swap(X->p, Y->p, X->n, swap);
cleanup:
return ret;
}
/* Implementation that should never be optimized out by the compiler */
static void mbedtls_mpi_zeroize(mbedtls_mpi_uint *v, size_t n)
{

40
library/bignum_core.c
View File

@ -168,6 +168,46 @@ unsigned mbedtls_mpi_core_uint_le_mpi(mbedtls_mpi_uint min,
return min_le_lsl | msll_nonzero;
}
unsigned mbedtls_mpi_core_lt_ct(const mbedtls_mpi_uint *A,
const mbedtls_mpi_uint *B,
size_t limbs)
{
unsigned ret, cond, done;
/* The value of any of these variables is either 0 or 1 for the rest of
* their scope. */
ret = cond = done = 0;
for (size_t i = limbs; i > 0; i--) {
/*
* If B[i - 1] < A[i - 1] then A < B is false and the result must
* remain 0.
*
* 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(B[i - 1], A[i - 1]);
done |= cond;
/*
* If A[i - 1] < B[i - 1] then A < B is true.
*
* 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(A[i - 1], B[i - 1]);
ret |= cond & (1 - done);
done |= cond;
}
/*
* If all the limbs were equal, then the numbers are equal, A < B is false
* and leaving the result 0 is correct.
*/
return ret;
}
void mbedtls_mpi_core_cond_assign(mbedtls_mpi_uint *X,
const mbedtls_mpi_uint *A,
size_t limbs,

18
library/bignum_core.h
View File

@ -148,6 +148,24 @@ unsigned mbedtls_mpi_core_uint_le_mpi(mbedtls_mpi_uint min,
const mbedtls_mpi_uint *A,
size_t A_limbs);
/**
* \brief Check if one unsigned MPI is less than another in constant
* time.
*
* \param A The left-hand MPI. This must point to an array of limbs
* with the same allocated length as \p B.
* \param B The right-hand MPI. This must point to an array of limbs
* with the same allocated length as \p A.
* \param limbs The number of limbs in \p A and \p B.
* This must not be 0.
*
* \return The result of the comparison:
* \c 1 if \p A is less than \p B.
* \c 0 if \p A is greater than or equal to \p B.
*/
unsigned mbedtls_mpi_core_lt_ct(const mbedtls_mpi_uint *A,
const mbedtls_mpi_uint *B,
size_t limbs);
/**
* \brief Perform a safe conditional copy of an MPI which doesn't reveal
* whether assignment was done or not.

185
library/constant_time.c
View File

@ -405,188 +405,3 @@ void mbedtls_ct_memcpy_offset(unsigned char *dest,
#endif /* MBEDTLS_SSL_SOME_SUITES_USE_MAC */
#if defined(MBEDTLS_BIGNUM_C)
#define MPI_VALIDATE_RET(cond) \
MBEDTLS_INTERNAL_VALIDATE_RET(cond, MBEDTLS_ERR_MPI_BAD_INPUT_DATA)
/*
* Conditionally assign X = Y, without leaking information
* about whether the assignment was made or not.
* (Leaking information about the respective sizes of X and Y is ok however.)
*/
#if defined(_MSC_VER) && defined(_M_ARM64) && (_MSC_FULL_VER < 193131103)
/*
* MSVC miscompiles this function if it's inlined prior to Visual Studio 2022 version 17.1. See:
* https://developercommunity.visualstudio.com/t/c-compiler-miscompiles-part-of-mbedtls-library-on/1646989
*/
__declspec(noinline)
#endif
int mbedtls_mpi_safe_cond_assign(mbedtls_mpi *X,
const mbedtls_mpi *Y,
unsigned char assign)
{
int ret = 0;
MPI_VALIDATE_RET(X != NULL);
MPI_VALIDATE_RET(Y != NULL);
/* all-bits 1 if assign is 1, all-bits 0 if assign is 0 */
mbedtls_mpi_uint limb_mask = mbedtls_ct_mpi_uint_mask(assign);
MBEDTLS_MPI_CHK(mbedtls_mpi_grow(X, Y->n));
X->s = (int) mbedtls_ct_uint_if(assign, Y->s, X->s);
mbedtls_mpi_core_cond_assign(X->p, Y->p, Y->n, assign);
for (size_t i = Y->n; i < X->n; i++) {
X->p[i] &= ~limb_mask;
}
cleanup:
return ret;
}
/*
* Conditionally swap X and Y, without leaking information
* about whether the swap was made or not.
* Here it is not ok to simply swap the pointers, which would lead to
* different memory access patterns when X and Y are used afterwards.
*/
int mbedtls_mpi_safe_cond_swap(mbedtls_mpi *X,
mbedtls_mpi *Y,
unsigned char swap)
{
int ret = 0;
int s;
MPI_VALIDATE_RET(X != NULL);
MPI_VALIDATE_RET(Y != NULL);
if (X == Y) {
return 0;
}
MBEDTLS_MPI_CHK(mbedtls_mpi_grow(X, Y->n));
MBEDTLS_MPI_CHK(mbedtls_mpi_grow(Y, X->n));
s = X->s;
X->s = (int) mbedtls_ct_uint_if(swap, Y->s, X->s);
Y->s = (int) mbedtls_ct_uint_if(swap, s, Y->s);
mbedtls_mpi_core_cond_swap(X->p, Y->p, X->n, swap);
cleanup:
return ret;
}
/*
* Compare unsigned values in constant time
*/
unsigned mbedtls_mpi_core_lt_ct(const mbedtls_mpi_uint *A,
const mbedtls_mpi_uint *B,
size_t limbs)
{
unsigned ret, cond, done;
/* The value of any of these variables is either 0 or 1 for the rest of
* their scope. */
ret = cond = done = 0;
for (size_t i = limbs; i > 0; i--) {
/*
* If B[i - 1] < A[i - 1] then A < B is false and the result must
* remain 0.
*
* 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(B[i - 1], A[i - 1]);
done |= cond;
/*
* If A[i - 1] < B[i - 1] then A < B is true.
*
* 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(A[i - 1], B[i - 1]);
ret |= cond & (1 - done);
done |= cond;
}
/*
* If all the limbs were equal, then the numbers are equal, A < B is false
* and leaving the result 0 is correct.
*/
return ret;
}
/*
* Compare signed values in constant time
*/
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;
MPI_VALIDATE_RET(X != NULL);
MPI_VALIDATE_RET(Y != NULL);
MPI_VALIDATE_RET(ret != NULL);
if (X->n != Y->n) {
return MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
}
/*
* 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;
/*
* 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;
/*
* 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.
*/
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;
/*
* 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;
}
return 0;
}
#endif /* MBEDTLS_BIGNUM_C */

18
library/constant_time_internal.h
View File

@ -129,24 +129,6 @@ unsigned mbedtls_ct_size_bool_eq(size_t x,
unsigned mbedtls_ct_mpi_uint_lt(const mbedtls_mpi_uint x,
const mbedtls_mpi_uint y);
/**
* \brief Check if one unsigned MPI is less than another in constant
* time.
*
* \param A The left-hand MPI. This must point to an array of limbs
* with the same allocated length as \p B.
* \param B The right-hand MPI. This must point to an array of limbs
* with the same allocated length as \p A.
* \param limbs The number of limbs in \p A and \p B.
* This must not be 0.
*
* \return The result of the comparison:
* \c 1 if \p A is less than \p B.
* \c 0 if \p A is greater than or equal to \p B.
*/
unsigned mbedtls_mpi_core_lt_ct(const mbedtls_mpi_uint *A,
const mbedtls_mpi_uint *B,
size_t limbs);
#endif /* MBEDTLS_BIGNUM_C */
/** Choose between two integer values without branches.