diff --git a/tests/include/test/bignum_helpers.h b/tests/include/test/bignum_helpers.h
index 22ce7f7..ab3c86a 100644
--- a/tests/include/test/bignum_helpers.h
+++ b/tests/include/test/bignum_helpers.h
@@ -30,6 +30,7 @@
 #if defined(MBEDTLS_BIGNUM_C)
 
 #include <mbedtls/bignum.h>
+#include <bignum_mod.h>
 
 /** Allocate and populate a core MPI from a test case argument.
  *
@@ -54,6 +55,25 @@
 int mbedtls_test_read_mpi_core( mbedtls_mpi_uint **pX, size_t *plimbs,
                                 const char *input );
 
+/** Read a modulus from a hexadecimal string.
+ *
+ * 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] N     A modulus structure. It must be initialized, but
+ *                      not set up.
+ * \param[in] s         The null-terminated hexadecimal string to read from.
+ * \param int_rep       The desired representation of residues.
+ *
+ * \return \c 0 on success, an \c MBEDTLS_ERR_MPI_xxx error code otherwise.
+ */
+int mbedtls_test_read_mpi_modulus( mbedtls_mpi_mod_modulus *N,
+                                   const char *s,
+                                   mbedtls_mpi_mod_rep_selector int_rep );
+
 /** Read an MPI from a hexadecimal string.
  *
  * Like mbedtls_mpi_read_string(), but with tighter guarantees around
diff --git a/tests/src/bignum_helpers.c b/tests/src/bignum_helpers.c
index 575bd03..eb819f5 100644
--- a/tests/src/bignum_helpers.c
+++ b/tests/src/bignum_helpers.c
@@ -85,6 +85,23 @@
     return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
 }
 
+int mbedtls_test_read_mpi_modulus( mbedtls_mpi_mod_modulus *N,
+                                   const char *s,
+                                   mbedtls_mpi_mod_rep_selector int_rep )
+{
+    mbedtls_mpi_uint *p = NULL;
+    size_t limbs = 0;
+    if( N->limbs != 0 )
+        return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
+    int ret = mbedtls_test_read_mpi_core( &p, &limbs, s );
+    if( ret != 0 )
+        return( ret );
+    ret = mbedtls_mpi_mod_modulus_setup( N, p, limbs, int_rep );
+    if( ret != 0 )
+        mbedtls_free( p );
+    return( ret );
+}
+
 int mbedtls_test_read_mpi( mbedtls_mpi *X, const char *s )
 {
     int negative = 0;