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Add optionally unsafe variant of exp_mod for perf

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Attempt to partially solve the performance regression in 3.6.0 without
adding too much code size.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
This commit is contained in:
Manuel Pégourié-Gonnard 2024-06-18 12:52:45 +02:00
parent 8fb5df859c
commit 75ed58723e
4 changed files with 148 additions and 17 deletions
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49
include/mbedtls/bignum.h
View File

@ -44,6 +44,22 @@
goto cleanup; \
} while (0)
/* Constants to identify whether a value is public or secret.
*
* Parameters should be named X_public where X is the name of the
* corresponding input parameter.
*
* Implementation should always check using
* if (X_public == MBEDTLS_MPI_IS_PUBLIC) {
* // unsafe path
* } else {
* // safe path
* }
* not the other way round, in order to prevent misuse. (This is, if a value
* other than the two below is passed, default to the safe path.) */
#define MBEDTLS_MPI_IS_PUBLIC 0x2a2a
#define MBEDTLS_MPI_IS_SECRET 0
/*
* Maximum size MPIs are allowed to grow to in number of limbs.
*/
@ -880,7 +896,38 @@ int mbedtls_mpi_mod_int(mbedtls_mpi_uint *r, const mbedtls_mpi *A,
mbedtls_mpi_sint b);
/**
* \brief Perform a sliding-window exponentiation: X = A^E mod N
* \brief Perform a modular exponentiation: X = A^E mod N
*
* \param X The destination MPI. This must point to an initialized MPI.
* This must not alias E or N.
* \param A The base of the exponentiation.
* This must point to an initialized MPI.
* \param E The exponent MPI. This must point to an initialized MPI.
* \param N The base for the modular reduction. This must point to an
* initialized MPI.
* \param prec_RR A helper MPI depending solely on \p N which can be used to
* speed-up multiple modular exponentiations for the same value
* of \p N. This may be \c NULL. If it is not \c NULL, it must
* point to an initialized MPI. If it hasn't been used after
* the call to mbedtls_mpi_init(), this function will compute
* the helper value and store it in \p prec_RR for reuse on
* subsequent calls to this function. Otherwise, the function
* will assume that \p prec_RR holds the helper value set by a
* previous call to mbedtls_mpi_exp_mod(), and reuse it.
*
* \return \c 0 if successful.
* \return #MBEDTLS_ERR_MPI_ALLOC_FAILED if a memory allocation failed.
* \return #MBEDTLS_ERR_MPI_BAD_INPUT_DATA if \c N is negative or
* even, or if \c E is negative.
* \return Another negative error code on different kinds of failures.
*
*/
int mbedtls_mpi_exp_mod_optionally_safe(mbedtls_mpi *X, const mbedtls_mpi *A,
const mbedtls_mpi *E, const mbedtls_mpi *N,
mbedtls_mpi *prec_RR, int E_public);
/**
* \brief Perform a modular exponentiation: X = A^E mod N
*
* \param X The destination MPI. This must point to an initialized MPI.
* This must not alias E or N.

23
library/bignum.c
View File

@ -1610,9 +1610,9 @@ int mbedtls_mpi_mod_int(mbedtls_mpi_uint *r, const mbedtls_mpi *A, mbedtls_mpi_s
return 0;
}
int mbedtls_mpi_exp_mod(mbedtls_mpi *X, const mbedtls_mpi *A,
const mbedtls_mpi *E, const mbedtls_mpi *N,
mbedtls_mpi *prec_RR)
int mbedtls_mpi_exp_mod_optionally_safe(mbedtls_mpi *X, const mbedtls_mpi *A,
const mbedtls_mpi *E, const mbedtls_mpi *N,
mbedtls_mpi *prec_RR, int E_public)
{
int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
@ -1695,7 +1695,15 @@ int mbedtls_mpi_exp_mod(mbedtls_mpi *X, const mbedtls_mpi *A,
{
mbedtls_mpi_uint mm = mbedtls_mpi_core_montmul_init(N->p);
mbedtls_mpi_core_to_mont_rep(X->p, X->p, N->p, N->n, mm, RR.p, T);
mbedtls_mpi_core_exp_mod(X->p, X->p, N->p, N->n, E->p, E->n, RR.p, T);
mbedtls_mpi_core_exp_mod_optionally_safe(X->p,
X->p,
N->p,
N->n,
E->p,
E->n,
RR.p,
T,
E_public);
mbedtls_mpi_core_from_mont_rep(X->p, X->p, N->p, N->n, mm, T);
}
@ -1720,6 +1728,13 @@ cleanup:
return ret;
}
int mbedtls_mpi_exp_mod(mbedtls_mpi *X, const mbedtls_mpi *A,
const mbedtls_mpi *E, const mbedtls_mpi *N,
mbedtls_mpi *prec_RR)
{
return mbedtls_mpi_exp_mod_optionally_safe(X, A, E, N, prec_RR, MBEDTLS_MPI_IS_SECRET);
}
/*
* Greatest common divisor: G = gcd(A, B) (HAC 14.54)
*/

55
library/bignum_core.c
View File

@ -758,14 +758,15 @@ static void exp_mod_precompute_window(const mbedtls_mpi_uint *A,
* (The difference is that the body in our loop processes a single bit instead
* of a full window.)
*/
void mbedtls_mpi_core_exp_mod(mbedtls_mpi_uint *X,
const mbedtls_mpi_uint *A,
const mbedtls_mpi_uint *N,
size_t AN_limbs,
const mbedtls_mpi_uint *E,
size_t E_limbs,
const mbedtls_mpi_uint *RR,
mbedtls_mpi_uint *T)
void mbedtls_mpi_core_exp_mod_optionally_safe(mbedtls_mpi_uint *X,
const mbedtls_mpi_uint *A,
const mbedtls_mpi_uint *N,
size_t AN_limbs,
const mbedtls_mpi_uint *E,
size_t E_limbs,
const mbedtls_mpi_uint *RR,
mbedtls_mpi_uint *T,
int E_public)
{
const size_t wsize = exp_mod_get_window_size(E_limbs * biL);
const size_t welem = ((size_t) 1) << wsize;
@ -803,6 +804,14 @@ void mbedtls_mpi_core_exp_mod(mbedtls_mpi_uint *X,
* (limb_index=0, E_bit_index=0). */
size_t E_limb_index = E_limbs;
size_t E_bit_index = 0;
if (E_public == MBEDTLS_MPI_IS_PUBLIC) {
size_t E_bits = mbedtls_mpi_core_bitlen(E, E_limbs);
if (E_bits != 0) {
E_limb_index = E_bits / biL;
E_bit_index = E_bits % biL;
}
}
/* At any given time, window contains window_bits bits from E.
* window_bits can go up to wsize. */
size_t window_bits = 0;
@ -828,10 +837,14 @@ void mbedtls_mpi_core_exp_mod(mbedtls_mpi_uint *X,
* when we've finished processing the exponent. */
if (window_bits == wsize ||
(E_bit_index == 0 && E_limb_index == 0)) {
/* Select Wtable[window] without leaking window through
* memory access patterns. */
mbedtls_mpi_core_ct_uint_table_lookup(Wselect, Wtable,
AN_limbs, welem, window);
if (E_public == MBEDTLS_MPI_IS_PUBLIC) {
memcpy(Wselect, Wtable + window * AN_limbs, AN_limbs * ciL);
} else {
/* Select Wtable[window] without leaking window through
* memory access patterns. */
mbedtls_mpi_core_ct_uint_table_lookup(Wselect, Wtable,
AN_limbs, welem, window);
}
/* Multiply X by the selected element. */
mbedtls_mpi_core_montmul(X, X, Wselect, AN_limbs, N, AN_limbs, mm,
temp);
@ -841,6 +854,24 @@ void mbedtls_mpi_core_exp_mod(mbedtls_mpi_uint *X,
} while (!(E_bit_index == 0 && E_limb_index == 0));
}
void mbedtls_mpi_core_exp_mod(mbedtls_mpi_uint *X,
const mbedtls_mpi_uint *A,
const mbedtls_mpi_uint *N, size_t AN_limbs,
const mbedtls_mpi_uint *E, size_t E_limbs,
const mbedtls_mpi_uint *RR,
mbedtls_mpi_uint *T)
{
mbedtls_mpi_core_exp_mod_optionally_safe(X,
A,
N,
AN_limbs,
E,
E_limbs,
RR,
T,
MBEDTLS_MPI_IS_SECRET);
}
mbedtls_mpi_uint mbedtls_mpi_core_sub_int(mbedtls_mpi_uint *X,
const mbedtls_mpi_uint *A,
mbedtls_mpi_uint c, /* doubles as carry */

38
library/bignum_core.h
View File

@ -604,6 +604,44 @@ int mbedtls_mpi_core_random(mbedtls_mpi_uint *X,
*/
size_t mbedtls_mpi_core_exp_mod_working_limbs(size_t AN_limbs, size_t E_limbs);
/**
* \brief Perform a modular exponentiation with public or secret exponent:
* X = A^E mod N, where \p A is already in Montgomery form.
*
* \p X may be aliased to \p A, but not to \p RR or \p E, even if \p E_limbs ==
* \p AN_limbs.
*
* \param[out] X The destination MPI, as a little endian array of length
* \p AN_limbs.
* \param[in] A The base MPI, as a little endian array of length \p AN_limbs.
* Must be in Montgomery form.
* \param[in] N The modulus, as a little endian array of length \p AN_limbs.
* \param AN_limbs The number of limbs in \p X, \p A, \p N, \p RR.
* \param[in] E The exponent, as a little endian array of length \p E_limbs.
* \param E_limbs The number of limbs in \p E.
* \param[in] RR The precomputed residue of 2^{2*biL} modulo N, as a little
* endian array of length \p AN_limbs.
* \param[in,out] T Temporary storage of at least the number of limbs returned
* by `mbedtls_mpi_core_exp_mod_working_limbs()`.
* Its initial content is unused and its final content is
* indeterminate.
* It must not alias or otherwise overlap any of the other
* parameters.
* It is up to the caller to zeroize \p T when it is no
* longer needed, and before freeing it if it was dynamically
* allocated.
* \param[in] E_public Set to MBEDTLS_MPI_IS_PUBLIC to gain some performance
* when the value of E is public.
* Set to MBEDTLS_MPI_IS_SECRET when the value of E is secret.
*/
void mbedtls_mpi_core_exp_mod_optionally_safe(mbedtls_mpi_uint *X,
const mbedtls_mpi_uint *A,
const mbedtls_mpi_uint *N, size_t AN_limbs,
const mbedtls_mpi_uint *E, size_t E_limbs,
const mbedtls_mpi_uint *RR,
mbedtls_mpi_uint *T,
int E_public);
/**
* \brief Perform a modular exponentiation with secret exponent:
* X = A^E mod N, where \p A is already in Montgomery form.