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Exp mod: simplify 0 exponent handling

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Removing E_core and returning early achieves the same and is simpler
(easier to read and maintain).

Signed-off-by: Janos Follath <janos.follath@arm.com>
This commit is contained in:
Janos Follath 2024-02-19 11:16:44 +00:00
parent 576087d836
commit 583f047c9f
1 changed files with 11 additions and 18 deletions
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29
library/bignum.c
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@ -1616,12 +1616,18 @@ int mbedtls_mpi_exp_mod(mbedtls_mpi *X, const mbedtls_mpi *A,
return MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
}
/*
* Ensure that the exponent that we are passing to the core is not NULL.
*/
if (E->n == 0) {
ret = mbedtls_mpi_lset(X, 1);
return ret;
}
mbedtls_mpi RR;
mbedtls_mpi_init(&RR);
mbedtls_mpi T;
mbedtls_mpi_init(&T);
mbedtls_mpi E_core;
mbedtls_mpi_init(&E_core);
/*
* If 1st call, pre-compute R^2 mod N
@ -1636,15 +1642,6 @@ int mbedtls_mpi_exp_mod(mbedtls_mpi *X, const mbedtls_mpi *A,
RR = *prec_RR;
}
/*
* Ensure that the exponent that we are passing to the core is not NULL.
*/
if (E->n == 0) {
mbedtls_mpi_lset(&E_core, 0);
} else {
E_core = *E;
}
/*
* To preserve constness we need to make a copy of A. Using X for this to
* save memory.
@ -1668,21 +1665,21 @@ int mbedtls_mpi_exp_mod(mbedtls_mpi *X, const mbedtls_mpi *A,
* Allocate working memory for mbedtls_mpi_core_exp_mod()
*/
MBEDTLS_MPI_CHK(mbedtls_mpi_grow(&T,
mbedtls_mpi_core_exp_mod_working_limbs(N->n, E_core.n)));
mbedtls_mpi_core_exp_mod_working_limbs(N->n, E->n)));
/*
* Convert to and from Montgomery around mbedtls_mpi_core_exp_mod().
*/
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.p);
mbedtls_mpi_core_exp_mod(X->p, X->p, N->p, N->n, E_core.p, E_core.n, RR.p,
mbedtls_mpi_core_exp_mod(X->p, X->p, N->p, N->n, E->p, E->n, RR.p,
T.p);
mbedtls_mpi_core_from_mont_rep(X->p, X->p, N->p, N->n, mm, T.p);
/*
* Correct for negative A.
*/
if (A->s == -1 && (E_core.p[0] & 1) != 0) {
if (A->s == -1 && (E->p[0] & 1) != 0) {
X->s = -1;
MBEDTLS_MPI_CHK(mbedtls_mpi_add_mpi(X, N, X));
}
@ -1695,10 +1692,6 @@ cleanup:
mbedtls_mpi_free(&RR);
}
if (E->n == 0) {
mbedtls_mpi_free(&E_core);
}
return ret;
}