Parse RSA parameters DP, DQ and QP from PKCS1 private keys
Otherwise these values are recomputed in mbedtls_rsa_deduce_crt, which
currently suffers from side channel issues in the computation of QP
(see https://eprint.iacr.org/2020/055). By loading the pre-computed
values not only is the side channel avoided, but runtime overhead of
loading RSA keys is reduced.
Discussion in https://github.com/ARMmbed/mbed-crypto/issues/347
Backport of https://github.com/ARMmbed/mbed-crypto/pull/352
diff --git a/library/rsa.c b/library/rsa.c
index af1a878..09fd379 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -249,6 +249,9 @@
{
int ret = 0;
int have_N, have_P, have_Q, have_D, have_E;
+#if !defined(MBEDTLS_RSA_NO_CRT)
+ int have_DP, have_DQ, have_QP;
+#endif
int n_missing, pq_missing, d_missing, is_pub, is_priv;
RSA_VALIDATE_RET( ctx != NULL );
@@ -259,6 +262,12 @@
have_D = ( mbedtls_mpi_cmp_int( &ctx->D, 0 ) != 0 );
have_E = ( mbedtls_mpi_cmp_int( &ctx->E, 0 ) != 0 );
+#if !defined(MBEDTLS_RSA_NO_CRT)
+ have_DP = ( mbedtls_mpi_cmp_int( &ctx->DP, 0 ) != 0 );
+ have_DQ = ( mbedtls_mpi_cmp_int( &ctx->DQ, 0 ) != 0 );
+ have_QP = ( mbedtls_mpi_cmp_int( &ctx->QP, 0 ) != 0 );
+#endif
+
/*
* Check whether provided parameters are enough
* to deduce all others. The following incomplete
@@ -324,7 +333,7 @@
*/
#if !defined(MBEDTLS_RSA_NO_CRT)
- if( is_priv )
+ if( is_priv && ! ( have_DP && have_DQ && have_QP ) )
{
ret = mbedtls_rsa_deduce_crt( &ctx->P, &ctx->Q, &ctx->D,
&ctx->DP, &ctx->DQ, &ctx->QP );