commit 75525aec527bc9d3bba2cd3214c3c8fc2d9961af
author Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com> Tue Jun 15 11:29:26 2021 +0200
committer Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com> Thu Jun 17 09:38:38 2021 +0200
tree 73733eb903edab26b0137adf99a8bd6d2957065b
parent aa3ed6f9872e3def71a557a940ab6f81364f274b

Fix mbedtls_ecp_muladd()

It was indirectly calling ecp_mul() without an RNG. That's actually the
rare case where this should be allowed, as ecp_muladd() is typically
used on non-secret data (to verify signatures or ZKPs) and documented as
not being constant-time.

Refactor a bit in order to keep the ability to call ecp_mul() without a
RNG, but not exposed publicly (except though muladd).

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

library/ecp.c

diff --git a/library/ecp.c b/library/ecp.c
index 873b4c8..bd560b5 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -2669,8 +2669,11 @@
 
 /*
  * Restartable multiplication R = m * P
+ *
+ * This internal function can be called without an RNG in case where we know
+ * the inputs are not sensitive.
  */
-int mbedtls_ecp_mul_restartable( mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
+static int ecp_mul_restartable_internal( mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
              const mbedtls_mpi *m, const mbedtls_ecp_point *P,
              int (*f_rng)(void *, unsigned char *, size_t), void *p_rng,
              mbedtls_ecp_restart_ctx *rs_ctx )
@@ -2679,13 +2682,6 @@
 #if defined(MBEDTLS_ECP_INTERNAL_ALT)
     char is_grp_capable = 0;
 #endif
-    ECP_VALIDATE_RET( grp != NULL );
-    ECP_VALIDATE_RET( R   != NULL );
-    ECP_VALIDATE_RET( m   != NULL );
-    ECP_VALIDATE_RET( P   != NULL );
-
-    if( f_rng == NULL )
-        return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );
 
 #if defined(MBEDTLS_ECP_RESTARTABLE)
     /* reset ops count for this call if top-level */
@@ -2739,6 +2735,25 @@
 }
 
 /*
+ * Restartable multiplication R = m * P
+ */
+int mbedtls_ecp_mul_restartable( mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
+             const mbedtls_mpi *m, const mbedtls_ecp_point *P,
+             int (*f_rng)(void *, unsigned char *, size_t), void *p_rng,
+             mbedtls_ecp_restart_ctx *rs_ctx )
+{
+    ECP_VALIDATE_RET( grp != NULL );
+    ECP_VALIDATE_RET( R   != NULL );
+    ECP_VALIDATE_RET( m   != NULL );
+    ECP_VALIDATE_RET( P   != NULL );
+
+    if( f_rng == NULL )
+        return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );
+
+    return( ecp_mul_restartable_internal( grp, R, m, P, f_rng, p_rng, rs_ctx ) );
+}
+
+/*
  * Multiplication R = m * P
  */
 int mbedtls_ecp_mul( mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
@@ -2831,8 +2846,8 @@
     }
     else
     {
-        MBEDTLS_MPI_CHK( mbedtls_ecp_mul_restartable( grp, R, m, P,
-                                                      NULL, NULL, rs_ctx ) );
+        MBEDTLS_MPI_CHK( ecp_mul_restartable_internal( grp, R, m, P,
+                                                       NULL, NULL, rs_ctx ) );
     }
 
 cleanup: