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Clarify some comments

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Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
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Manuel Pégourié-Gonnard 2020-07-16 09:48:54 +02:00
parent cadcf4cec6
commit 87a602dd67
1 changed files with 7 additions and 5 deletions
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12
library/rsa.c
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@ -833,25 +833,27 @@ static int rsa_prepare_blinding( mbedtls_rsa_context *ctx,
MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &ctx->Vf, ctx->len - 1, f_rng, p_rng ) ); MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &ctx->Vf, ctx->len - 1, f_rng, p_rng ) );
/* Compute the Vf^1 as R * (R Vf)^-1 to avoid leaks from inv_mod. /* Compute Vf^-1 as R * (R Vf)^-1 to avoid leaks from inv_mod. */
* There's a negligible but non-zero probability that R is not
* invertible mod N, in that case we'd just loop one more time,
* just as if Vf itself wasn't invertible - no need to distinguish. */
MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &R, ctx->len - 1, f_rng, p_rng ) ); MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &R, ctx->len - 1, f_rng, p_rng ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &ctx->Vi, &ctx->Vf, &R ) ); MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &ctx->Vi, &ctx->Vf, &R ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &ctx->Vi, &ctx->Vi, &ctx->N ) ); MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &ctx->Vi, &ctx->Vi, &ctx->N ) );
/* At this point, Vi is invertible mod N if and only if both Vf and R
* are invertible mod N. If one of them isn't, we don't need to know
* which one, we just loop and choose new values for both of them.
* (Each iteration succeeds with overwhelming probability.) */
ret = mbedtls_mpi_inv_mod( &ctx->Vi, &ctx->Vi, &ctx->N ); ret = mbedtls_mpi_inv_mod( &ctx->Vi, &ctx->Vi, &ctx->N );
if( ret == MBEDTLS_ERR_MPI_NOT_ACCEPTABLE ) if( ret == MBEDTLS_ERR_MPI_NOT_ACCEPTABLE )
continue; continue;
if( ret != 0 ) if( ret != 0 )
goto cleanup; goto cleanup;
/* Finish the computation of Vf^-1 = R * (R Vf)^-1 */
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &ctx->Vi, &ctx->Vi, &R ) ); MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &ctx->Vi, &ctx->Vi, &R ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &ctx->Vi, &ctx->Vi, &ctx->N ) ); MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &ctx->Vi, &ctx->Vi, &ctx->N ) );
} while( 0 ); } while( 0 );
/* Blinding value: Vi = Vf^(-e) mod N /* Blinding value: Vi = Vf^(-e) mod N
* (Vi already contains Vf^-1 at this point) */ * (Vi already contains Vf^-1 at this point) */
MBEDTLS_MPI_CHK( mbedtls_mpi_exp_mod( &ctx->Vi, &ctx->Vi, &ctx->E, &ctx->N, &ctx->RN ) ); MBEDTLS_MPI_CHK( mbedtls_mpi_exp_mod( &ctx->Vi, &ctx->Vi, &ctx->E, &ctx->N, &ctx->RN ) );