Implement and test mbedtls_mpi_mod_raw_random

In the basic/XXX=core test cases, use odd upper bounds, because the mod
version of random() only supports odd upper bounds (the upper bound is a
modulus and the mod modules only support odd moduli).

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

library/bignum_mod_raw.c

@@ -176,6 +176,18 @@ void mbedtls_mpi_mod_raw_add( mbedtls_mpi_uint *X,
 
 /* BEGIN MERGE SLOT 6 */
 
+int mbedtls_mpi_mod_raw_random( mbedtls_mpi_uint *X,
+                                mbedtls_mpi_uint min,
+                                const mbedtls_mpi_mod_modulus *N,
+                                int (*f_rng)(void *, unsigned char *, size_t),
+                                void *p_rng )
+{
+    int ret = mbedtls_mpi_core_random( X, min, N->p, N->limbs, f_rng, p_rng );
+    if( ret != 0 )
+        return( ret );
+    return( mbedtls_mpi_mod_raw_to_mont_rep( X, N ) );
+}
+
 /* END MERGE SLOT 6 */
 
 /* BEGIN MERGE SLOT 7 */

library/bignum_mod_raw.h

@@ -297,6 +297,40 @@ void mbedtls_mpi_mod_raw_add( mbedtls_mpi_uint *X,
 
 /* BEGIN MERGE SLOT 6 */
 
+/** Generate a random number uniformly in a range.
+ *
+ * This function generates a random number between \p min inclusive and
+ * \p N exclusive.
+ *
+ * The procedure complies with RFC 6979 §3.3 (deterministic ECDSA)
+ * when the RNG is a suitably parametrized instance of HMAC_DRBG
+ * and \p min is \c 1.
+ *
+ * \note           There are `N - min` possible outputs. The lower bound
+ *                 \p min can be reached, but the upper bound \p N cannot.
+ *
+ * \param X        The destination MPI, in canonical representation modulo \p N.
+ *                 It must not be aliased with \p N or otherwise overlap it.
+ * \param min      The minimum value to return. It must be strictly smaller
+ *                 than \b N.
+ * \param N        The modulus.
+ *                 This is the upper bound of the output range, exclusive.
+ * \param f_rng    The RNG function to use. This must not be \c NULL.
+ * \param p_rng    The RNG parameter to be passed to \p f_rng.
+ *
+ * \return         \c 0 if successful.
+ * \return         #MBEDTLS_ERR_MPI_NOT_ACCEPTABLE if the implementation was
+ *                 unable to find a suitable value within a limited number
+ *                 of attempts. This has a negligible probability if \p N
+ *                 is significantly larger than \p min, which is the case
+ *                 for all usual cryptographic applications.
+ */
+int mbedtls_mpi_mod_raw_random( mbedtls_mpi_uint *X,
+                                mbedtls_mpi_uint min,
+                                const mbedtls_mpi_mod_modulus *N,
+                                int (*f_rng)(void *, unsigned char *, size_t),
+                                void *p_rng );
+
 /* END MERGE SLOT 6 */
 
 /* BEGIN MERGE SLOT 7 */

tests/suites/test_suite_bignum_random.data

@@ -17,31 +17,43 @@ MPI core random basic: 2^30..2^129
 mpi_core_random_basic:0x40000000:"0200000000000000000000000000000000":0
 
 # Use the same data values for mpi_core_random_basic->NOT_ACCEPTABLE
-# and for mpi_legacy_random_values where we want to return NOT_ACCEPTABLE
+# and for mpi_XXX_random_values where we want to return NOT_ACCEPTABLE
 # but this isn't checked at runtime.
-MPI core random basic: 2^28-1..2^28 (NOT_ACCEPTABLE)
+MPI core random basic: 2^28-1..2^28+1 (NOT_ACCEPTABLE)
-mpi_core_random_basic:0x0fffffff:"10000000":MBEDTLS_ERR_MPI_NOT_ACCEPTABLE
+mpi_core_random_basic:0x0fffffff:"10000001":MBEDTLS_ERR_MPI_NOT_ACCEPTABLE
 
-MPI random legacy=core: 2^28-1..2^28 (NOT_ACCEPTABLE)
+MPI random legacy=core: 2^28-1..2^28+1 (NOT_ACCEPTABLE)
-mpi_legacy_random_values:0x0fffffff:"10000000"
+mpi_legacy_random_values:0x0fffffff:"10000001"
 
-MPI core random basic: 2^29-1..2^29 (NOT_ACCEPTABLE)
+MPI random mod=core: 2^28-1..2^28+1 (NOT_ACCEPTABLE)
-mpi_core_random_basic:0x1fffffff:"20000000":MBEDTLS_ERR_MPI_NOT_ACCEPTABLE
+mpi_mod_random_values:0x0fffffff:"10000001"
 
-MPI random legacy=core: 2^29-1..2^29 (NOT_ACCEPTABLE)
+MPI core random basic: 2^29-1..2^29+1 (NOT_ACCEPTABLE)
-mpi_legacy_random_values:0x1fffffff:"20000000"
+mpi_core_random_basic:0x1fffffff:"20000001":MBEDTLS_ERR_MPI_NOT_ACCEPTABLE
 
-MPI core random basic: 2^30-1..2^30 (NOT_ACCEPTABLE)
+MPI random legacy=core: 2^29-1..2^29+1 (NOT_ACCEPTABLE)
-mpi_core_random_basic:0x3fffffff:"40000000":MBEDTLS_ERR_MPI_NOT_ACCEPTABLE
+mpi_legacy_random_values:0x1fffffff:"20000001"
 
-MPI random legacy=core: 2^30-1..2^30 (NOT_ACCEPTABLE)
+MPI random mod=core: 2^29-1..2^29+1 (NOT_ACCEPTABLE)
-mpi_legacy_random_values:0x3fffffff:"40000000"
+mpi_mod_random_values:0x1fffffff:"20000001"
 
-MPI core random basic: 2^31-1..2^31 (NOT_ACCEPTABLE)
+MPI core random basic: 2^30-1..2^30+1 (NOT_ACCEPTABLE)
-mpi_core_random_basic:0x7fffffff:"80000000":MBEDTLS_ERR_MPI_NOT_ACCEPTABLE
+mpi_core_random_basic:0x3fffffff:"40000001":MBEDTLS_ERR_MPI_NOT_ACCEPTABLE
 
-MPI random legacy=core: 2^31-1..2^31 (NOT_ACCEPTABLE)
+MPI random legacy=core: 2^30-1..2^30+1 (NOT_ACCEPTABLE)
-mpi_legacy_random_values:0x7fffffff:"80000000"
+mpi_legacy_random_values:0x3fffffff:"40000001"
+
+MPI random mod=core: 2^30-1..2^30+1 (NOT_ACCEPTABLE)
+mpi_mod_random_values:0x3fffffff:"40000001"
+
+MPI core random basic: 2^31-1..2^31+1 (NOT_ACCEPTABLE)
+mpi_core_random_basic:0x7fffffff:"80000001":MBEDTLS_ERR_MPI_NOT_ACCEPTABLE
+
+MPI random legacy=core: 2^31-1..2^31+1 (NOT_ACCEPTABLE)
+mpi_legacy_random_values:0x7fffffff:"80000001"
+
+MPI random mod=core: 2^31-1..2^31+1 (NOT_ACCEPTABLE)
+mpi_mod_random_values:0x7fffffff:"80000001"
 
 MPI random in range: 1..2
 mpi_random_many:1:"02":1000
@@ -233,3 +245,21 @@ mpi_legacy_random_values:0:"0100000000000000000000000000000000000000000000000000
 
 MPI random legacy=core: 0..2^256+1
 mpi_legacy_random_values:0:"010000000000000000000000000000000000000000000000000000000000000001"
+
+MPI random mod=core: 0..1
+mpi_mod_random_values:0:"01"
+
+MPI random mod=core: 0..3
+mpi_mod_random_values:0:"03"
+
+MPI random mod=core: 1..3
+mpi_mod_random_values:1:"03"
+
+MPI random mod=core: 2^30..2^31-1
+mpi_mod_random_values:0x40000000:"7fffffff"
+
+MPI random mod=core: 2^31-1..2^32-1
+mpi_mod_random_values:0x7fffffff:"ffffffff"
+
+MPI random mod=core: 0..2^256+1
+mpi_mod_random_values:0:"010000000000000000000000000000000000000000000000000000000000000001"

tests/suites/test_suite_bignum_random.function

@@ -8,6 +8,7 @@
 #include "mbedtls/bignum.h"
 #include "mbedtls/entropy.h"
 #include "bignum_core.h"
+#include "bignum_mod_raw.h"
 #include "constant_time_internal.h"
 
 /* This test suite only manipulates non-negative bignums. */
@@ -158,6 +159,61 @@ exit:
 }
 /* END_CASE */
 
+/* BEGIN_CASE */
+void mpi_mod_random_values( int min, char *max_hex )
+{
+    /* Same RNG as in mpi_core_random_basic */
+    mbedtls_test_rnd_pseudo_info rnd_core = {
+        {'T', 'h', 'i', 's', ' ', 'i', ',', 'a',
+         's', 'e', 'e', 'd', '!', 0},
+        0, 0};
+    mbedtls_test_rnd_pseudo_info rnd_mod_raw;
+    memcpy( &rnd_mod_raw, &rnd_core, sizeof( rnd_core ) );
+    mbedtls_mpi_uint *R_core = NULL;
+    mbedtls_mpi_uint *R_mod_raw = NULL;
+    mbedtls_mpi_mod_modulus N;
+    mbedtls_mpi_mod_modulus_init( &N );
+
+    TEST_EQUAL( mbedtls_test_read_mpi_modulus( &N, max_hex,
+                                               MBEDTLS_MPI_MOD_REP_MONTGOMERY ),
+                0 );
+    ASSERT_ALLOC( R_core, N.limbs );
+    ASSERT_ALLOC( R_mod_raw, N.limbs );
+
+    /* Call the core and mod random() functions with the same random stream. */
+    int core_ret = mbedtls_mpi_core_random( R_core,
+                                            min, N.p, N.limbs,
+                                            mbedtls_test_rnd_pseudo_rand,
+                                            &rnd_core );
+    int mod_raw_ret = mbedtls_mpi_mod_raw_random( R_mod_raw,
+                                                  min, &N,
+                                                  mbedtls_test_rnd_pseudo_rand,
+                                                  &rnd_mod_raw );
+
+    /* They must return the same status, and, on success, output the
+     * same number, with the same limb count. */
+    TEST_EQUAL( core_ret, mod_raw_ret );
+    if( core_ret == 0 )
+    {
+        TEST_EQUAL( mbedtls_mpi_mod_raw_from_mont_rep( R_mod_raw, &N ), 0 );
+        ASSERT_COMPARE( R_core, N.limbs * ciL,
+                        R_mod_raw, N.limbs * ciL );
+    }
+
+    /* Also check that they have consumed the RNG in the same way. */
+    /* This may theoretically fail on rare platforms with padding in
+     * the structure! If this is a problem in practice, change to a
+     * field-by-field comparison. */
+    ASSERT_COMPARE( &rnd_core, sizeof( rnd_core ),
+                    &rnd_mod_raw, sizeof( rnd_mod_raw ) );
+
+exit:
+    mbedtls_mpi_mod_modulus_free( &N );
+    mbedtls_free( R_core );
+    mbedtls_free( R_mod_raw );
+}
+/* END_CASE */
+
 /* BEGIN_CASE */
 void mpi_random_many( int min, char *bound_hex, int iterations )
 {