diff --git a/tests/include/test/helpers.h b/tests/include/test/helpers.h
index 823635c00..fe3b787fd 100644
--- a/tests/include/test/helpers.h
+++ b/tests/include/test/helpers.h
@@ -283,6 +283,28 @@ void mbedtls_test_err_add_check( int high, int low,
 #endif
 
 #if defined(MBEDTLS_BIGNUM_C)
+/** Allocate and populate a core MPI from a test case argument.
+ *
+ * This function allocates exactly as many limbs as necessary to fit
+ * the length of the input. In other words, it preserves leading zeros.
+ *
+ * The limb array is allocated with mbedtls_calloc() and must later be
+ * freed with mbedtls_free().
+ *
+ * \param[in,out] pX    The address where a pointer to the allocated limb
+ *                      array will be stored.
+ *                      \c *pX must be null on entry.
+ *                      On exit, \c *pX is null on error or if the number
+ *                      of limbs is 0.
+ * \param[out] plimbs   The address where the number of limbs will be stored.
+ * \param[in] input     The test argument to read.
+ *                      It is interpreted as a big-endian integer in base 256.
+ *
+ * \return \c 0 on success, an \c MBEDTLS_ERR_MPI_xxx error code otherwise.
+ */
+int mbedtls_test_read_mpi_core( mbedtls_mpi_uint **pX, size_t *plimbs,
+                                const data_t *input );
+
 /** Read an MPI from a hexadecimal string.
  *
  * Like mbedtls_mpi_read_string(), but size the resulting bignum based
diff --git a/tests/src/helpers.c b/tests/src/helpers.c
index 673a8412b..da2c047b4 100644
--- a/tests/src/helpers.c
+++ b/tests/src/helpers.c
@@ -345,6 +345,24 @@ void mbedtls_test_err_add_check( int high, int low,
 #endif /* MBEDTLS_TEST_HOOKS */
 
 #if defined(MBEDTLS_BIGNUM_C)
+#include "bignum_core.h"
+
+int mbedtls_test_read_mpi_core( mbedtls_mpi_uint **pX, size_t *plimbs,
+                                const data_t *input )
+{
+    /* Sanity check */
+    if( *pX != NULL )
+        return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
+
+    *plimbs = ( input->len + sizeof( **pX ) - 1 ) / sizeof( **pX );
+    if( *plimbs == 0 )
+        return( 0 );
+    *pX = mbedtls_calloc( *plimbs, sizeof( **pX ) );
+    if( *pX == NULL )
+        return( MBEDTLS_ERR_MPI_ALLOC_FAILED );
+    return( mbedtls_mpi_core_read_be( *pX, *plimbs, input->x, input->len ) );
+}
+
 int mbedtls_test_read_mpi( mbedtls_mpi *X, const char *s )
 {
     /* mbedtls_mpi_read_string() currently retains leading zeros.