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fix(mbedtls): Fix cached Rinv size mismatch under private exponent blinding

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Prevent signature verification failures on targets that do not round hardware words
to 16-word boundaries (e.g. ESP32-S3, ESP32-C6, and ESP32-P4), where exponent blinding
can cause `num_words` to vary between calls, leading to reuse of an incorrectly sized
cached `Rinv`.
This commit is contained in:
Kapil Gupta
2026-06-15 11:39:01 +05:30
parent e9586fa03a
commit 9ce6228b0f
1 changed files with 5 additions and 1 deletions
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6
components/mbedtls/port/bignum/esp_bignum.c
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@@ -208,6 +208,7 @@ static int calculate_rinv(mbedtls_mpi *Rinv, const mbedtls_mpi *M, int num_words
mbedtls_mpi_init(&RR);
MBEDTLS_MPI_CHK(mbedtls_mpi_set_bit(&RR, num_bits * 2, 1));
MBEDTLS_MPI_CHK(mbedtls_mpi_mod_mpi(Rinv, &RR, M));
MBEDTLS_MPI_CHK(mbedtls_mpi_shrink(Rinv, num_words));
cleanup:
mbedtls_mpi_free(&RR);
@@ -415,7 +416,10 @@ static int esp_mpi_exp_mod( mbedtls_mpi *Z, const mbedtls_mpi *X, const mbedtls_
} else {
Rinv = _Rinv;
}
if (Rinv->MBEDTLS_PRIVATE(p) == NULL) {
/* Rinv depends on num_words, which may vary with blinded exponents.
calculate_rinv() stores Rinv with exactly num_words limbs, so the
allocation size is used here as the cache tag. */
if (Rinv->MBEDTLS_PRIVATE(p) == NULL || Rinv->MBEDTLS_PRIVATE(n) != num_words) {
MBEDTLS_MPI_CHK(calculate_rinv(Rinv, M, num_words));
}