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Parse RSA parameters DP, DQ and QP from PKCS1 private keys

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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
This commit is contained in:
Jack Lloyd 2020-01-29 13:13:04 -05:00
parent a67508e066
commit 100e147c71
2 changed files with 37 additions and 5 deletions
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32
library/pkparse.c
View file

@ -754,15 +754,41 @@ static int pk_parse_key_pkcs1_der( mbedtls_rsa_context *rsa,
goto cleanup;
p += len;
/* Complete the RSA private key */
if( ( ret = mbedtls_rsa_complete( rsa ) ) != 0 )
#if !defined(MBEDTLS_RSA_NO_CRT)
/*
* The RSA CRT parameters DP, DQ and QP are nominally redundant, in
* that they can be easily recomputed from D, P and Q. However by
* parsing them from the PKCS1 structure it is possible to avoid
* recalculating them which both reduces the overhead of loading
* RSA private keys into memory and also avoids side channels which
* can arise when computing those values, since all of D, P, and Q
* are secret. See https://eprint.iacr.org/2020/055 for a
* description of one such attack.
*/
/* Import DP */
if( ( ret = mbedtls_asn1_get_mpi( &p, end, &rsa->DP ) ) != 0)
goto cleanup;
/* Check optional parameters */
/* Import DQ */
if( ( ret = mbedtls_asn1_get_mpi( &p, end, &rsa->DQ ) ) != 0)
goto cleanup;
/* Import QP */
if( ( ret = mbedtls_asn1_get_mpi( &p, end, &rsa->QP ) ) != 0)
goto cleanup;
#else
/* Verify existance of the CRT params */
if( ( ret = mbedtls_asn1_get_mpi( &p, end, &T ) ) != 0 ||
( ret = mbedtls_asn1_get_mpi( &p, end, &T ) ) != 0 ||
( ret = mbedtls_asn1_get_mpi( &p, end, &T ) ) != 0 )
goto cleanup;
#endif
/* Complete the RSA private key */
if( ( ret = mbedtls_rsa_complete( rsa ) ) != 0 )
goto cleanup;
if( p != end )
{

8
library/rsa.c
View file

@ -251,6 +251,12 @@ int mbedtls_rsa_complete( mbedtls_rsa_context *ctx )
const int have_D = ( mbedtls_mpi_cmp_int( &ctx->D, 0 ) != 0 );
const int have_E = ( mbedtls_mpi_cmp_int( &ctx->E, 0 ) != 0 );
#if !defined(MBEDTLS_RSA_NO_CRT)
const int have_DP = ( mbedtls_mpi_cmp_int( &ctx->DP, 0 ) != 0 );
const int have_DQ = ( mbedtls_mpi_cmp_int( &ctx->DQ, 0 ) != 0 );
const int have_QP = ( mbedtls_mpi_cmp_int( &ctx->QP, 0 ) != 0 );
#endif
/*
* Check whether provided parameters are enough
* to deduce all others. The following incomplete
@ -316,7 +322,7 @@ int mbedtls_rsa_complete( mbedtls_rsa_context *ctx )
*/
#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 );