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Blind operations on the secret

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I'm not sure this is necessary, because it is only multiplied by xm2 which is
already random and secret, but OTOH, xm2 is related to a public value, so
let's add blinding with a random value that's only use for blinding, just to
be extra sure.
This commit is contained in:
Manuel Pégourié-Gonnard 2015-08-14 15:14:50 +02:00
parent 55f3d84faa
commit d0d8a935b2
1 changed files with 36 additions and 8 deletions
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44
library/ecjpake.c
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@ -111,7 +111,6 @@ int mbedtls_ecjpake_setup( mbedtls_ecjpake_context *ctx,
MBEDTLS_MPI_CHK( mbedtls_ecp_group_load( &ctx->grp, curve ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_read_binary( &ctx->s, secret, len ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &ctx->s, &ctx->s, &ctx->grp.N ) );
cleanup:
if( ret != 0 )
@ -568,6 +567,37 @@ cleanup:
return( ret );
}
/*
* Compute R = +/- X * S mod N, taking care not to leak S
*/
static int ecjpake_mul_secret( mbedtls_mpi *R, int sign,
const mbedtls_mpi *X,
const mbedtls_mpi *S,
const mbedtls_mpi *N,
int (*f_rng)(void *, unsigned char *, size_t),
void *p_rng )
{
int ret;
mbedtls_mpi b; /* Blinding value, then s + N * blinding */
mbedtls_mpi_init( &b );
/* b = s + rnd-128-bit * N */
MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &b, 16, f_rng, p_rng ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &b, &b, N ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_add_mpi( &b, &b, S ) );
/* R = sign * X * b mod N */
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( R, X, &b ) );
R->s *= sign;
MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( R, R, N ) );
cleanup:
mbedtls_mpi_free( &b );
return( ret );
}
/*
* Generate and write the second round message (S: 7.4.2.5, C: 7.4.2.6)
*/
@ -597,8 +627,8 @@ int mbedtls_ecjpake_write_round_two( mbedtls_ecjpake_context *ctx,
*/
MBEDTLS_MPI_CHK( ecjpake_ecp_add3( &ctx->grp, &G,
&ctx->Xp1, &ctx->Xp2, &ctx->Xm1 ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &xm, &ctx->xm2, &ctx->s ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &xm, &xm, &ctx->grp.N ) );
MBEDTLS_MPI_CHK( ecjpake_mul_secret( &xm, 1, &ctx->xm2, &ctx->s,
&ctx->grp.N, f_rng, p_rng ) );
MBEDTLS_MPI_CHK( mbedtls_ecp_mul( &ctx->grp, &Xm, &xm, &G, f_rng, p_rng ) );
/*
@ -671,12 +701,10 @@ int mbedtls_ecjpake_derive_secret( mbedtls_ecjpake_context *ctx,
/*
* Client: K = ( Xs - X4 * x2 * s ) * x2
* Server: K = ( Xc - X2 * x4 * s ) * x4
* Unified: K = ( Xp - Xp2 * xm2 * x ) * xm2
* Unified: K = ( Xp - Xp2 * xm2 * s ) * xm2
*/
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &m_xm2_s, &ctx->xm2, &ctx->s ) );
m_xm2_s.s *= -1;
MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &m_xm2_s, &m_xm2_s, &ctx->grp.N ) );
MBEDTLS_MPI_CHK( ecjpake_mul_secret( &m_xm2_s, -1, &ctx->xm2, &ctx->s,
&ctx->grp.N, f_rng, p_rng ) );
MBEDTLS_MPI_CHK( mbedtls_ecp_muladd( &ctx->grp, &K,
&one, &ctx->Xp,
&m_xm2_s, &ctx->Xp2 ) );