Add new function mbedtls_dhm_set_group to DHM Group

include/mbedtls/dhm.h

@@ -127,8 +127,9 @@ int mbedtls_dhm_read_params( mbedtls_dhm_context *ctx,
  *                 the byte-size of an MPI.
  *
  * \note           This function assumes that ctx->P and ctx->G
- *                 have already been properly set (for example
- *                 using mbedtls_mpi_read_string or mbedtls_mpi_read_binary).
+ *                 have already been properly set. For that, use
+ *                 \c mbedtls_dhm_set_group below in conjunction with
+ *                 \c mbedtls_mpi_read_binary and \c mbedtls_mpi_read_string.
  *
  * \return         0 if successful, or an MBEDTLS_ERR_DHM_XXX error code
  */
@@ -137,6 +138,22 @@ int mbedtls_dhm_make_params( mbedtls_dhm_context *ctx, int x_size,
                      int (*f_rng)(void *, unsigned char *, size_t),
                      void *p_rng );
 
+/**
+ * \brief          Set prime modulus and generator
+ *
+ * \param ctx      DHM context
+ * \param P        MPI holding DHM prime modulus
+ * \param G        MPI holding DHM generator
+ *
+ * \note           This function can be used to set P, G
+ *                 in preparation for \c mbedtls_dhm_make_params.
+ *
+ * \return         0 if successful, or an MBEDTLS_ERR_DHM_XXX error code
+ */
+int mbedtls_dhm_set_group( mbedtls_dhm_context *ctx,
+                           const mbedtls_mpi *P,
+                           const mbedtls_mpi *G );
+
 /**
  * \brief          Import the peer's public value G^Y
  *

library/dhm.c

@@ -218,6 +218,28 @@ cleanup:
     return( 0 );
 }
 
+/*
+ * Set prime modulus and generator
+ */
+int mbedtls_dhm_set_group( mbedtls_dhm_context *ctx,
+                           const mbedtls_mpi *P,
+                           const mbedtls_mpi *G )
+{
+    int ret;
+
+    if( ctx == NULL || P == NULL || G == NULL )
+        return( MBEDTLS_ERR_DHM_BAD_INPUT_DATA );
+
+    if( ( ret = mbedtls_mpi_copy( &ctx->P, P ) ) != 0 ||
+        ( ret = mbedtls_mpi_copy( &ctx->G, G ) ) != 0 )
+    {
+        return( MBEDTLS_ERR_DHM_SET_GROUP_FAILED + ret );
+    }
+
+    ctx->len = mbedtls_mpi_size( &ctx->P );
+    return( 0 );
+}
+
 /*
  * Import the peer's public value G^Y
  */