Merge pull request #89 from pq-code-package/cbmc-power2round

CBMC: Add contract and proof for power2round

Author: mkannwischer

mldsa/poly.c

@@ -212,7 +212,7 @@ void poly_power2round(poly *a1, poly *a0, const poly *a)
 
   for (i = 0; i < MLDSA_N; ++i)
   {
-    a1->coeffs[i] = power2round(&a0->coeffs[i], a->coeffs[i]);
+    power2round(&a0->coeffs[i], &a1->coeffs[i], a->coeffs[i]);
   }
 
   DBENCH_STOP(*tround);

mldsa/rounding.c

@@ -6,25 +6,11 @@
 #include <stdint.h>
 #include "params.h"
 
-/*************************************************
- * Name:        power2round
- *
- * Description: For finite field element a, compute a0, a1 such that
- *              a mod^+ MLDSA_Q = a1*2^MLDSA_D + a0 with -2^{MLDSA_D-1} < a0 <=
- *2^{MLDSA_D-1}. Assumes a to be standard representative.
- *
- * Arguments:   - int32_t a: input element
- *              - int32_t *a0: pointer to output element a0
- *
- * Returns a1.
- **************************************************/
-int32_t power2round(int32_t *a0, int32_t a)
-{
-  int32_t a1;
 
-  a1 = (a + (1 << (MLDSA_D - 1)) - 1) >> MLDSA_D;
-  *a0 = a - (a1 << MLDSA_D);
-  return a1;
+void power2round(int32_t *a0, int32_t *a1, const int32_t a)
+{
+  *a1 = (a + (1 << (MLDSA_D - 1)) - 1) >> MLDSA_D;
+  *a0 = a - (*a1 << MLDSA_D);
 }
 
 /*************************************************

mldsa/rounding.h

@@ -9,8 +9,34 @@
 #include "cbmc.h"
 #include "params.h"
 
+#define MLD_2_POW_D (1 << MLDSA_D)
+
 #define power2round MLD_NAMESPACE(power2round)
-int32_t power2round(int32_t *a0, int32_t a);
+/*************************************************
+ * Name:        power2round
+ *
+ * Description: For finite field element a, compute a0, a1 such that
+ *              a mod^+ MLDSA_Q = a1*2^MLDSA_D + a0 with -2^{MLDSA_D-1} < a0 <=
+ *              2^{MLDSA_D-1}. Assumes a to be standard representative.
+ *
+ * Arguments:   - int32_t a: input element
+ *              - int32_t *a0: pointer to output element a0
+ *              - int32_t *a1: pointer to output element a1
+ *
+ * Reference: a1 is passed as a return value instead
+ **************************************************/
+void power2round(int32_t *a0, int32_t *a1, int32_t a)
+__contract__(
+  requires(memory_no_alias(a0, sizeof(int32_t)))
+  requires(memory_no_alias(a1, sizeof(int32_t)))
+  requires(a >= 0 && a < MLDSA_Q)
+  assigns(memory_slice(a0, sizeof(int32_t)))
+  assigns(memory_slice(a1, sizeof(int32_t)))
+  ensures(*a0 > -(MLD_2_POW_D/2) && *a0 <= (MLD_2_POW_D/2))
+  ensures(*a1 >= 0 && *a1 <= (MLDSA_Q - 1) / MLD_2_POW_D)
+  ensures((*a1 * MLD_2_POW_D + *a0 - a) % MLDSA_Q == 0)
+);
+
 
 #define decompose MLD_NAMESPACE(decompose)
 int32_t decompose(int32_t *a0, int32_t a);

proofs/cbmc/power2round/Makefile

@@ -0,0 +1,54 @@
+# SPDX-License-Identifier: Apache-2.0
+
+include ../Makefile_params.common
+
+HARNESS_ENTRY = harness
+HARNESS_FILE = power2round_harness
+
+# This should be a unique identifier for this proof, and will appear on the
+# Litani dashboard. It can be human-readable and contain spaces if you wish.
+PROOF_UID = power2round
+
+DEFINES +=
+INCLUDES +=
+
+REMOVE_FUNCTION_BODY +=
+UNWINDSET +=
+
+PROOF_SOURCES += $(PROOFDIR)/$(HARNESS_FILE).c
+PROJECT_SOURCES += $(SRCDIR)/mldsa/rounding.c
+
+CHECK_FUNCTION_CONTRACTS=$(MLD_NAMESPACE)power2round
+USE_FUNCTION_CONTRACTS=
+APPLY_LOOP_CONTRACTS=on
+USE_DYNAMIC_FRAMES=1
+
+# Disable any setting of EXTERNAL_SAT_SOLVER, and choose SMT backend instead
+EXTERNAL_SAT_SOLVER=
+CBMCFLAGS=--smt2
+
+FUNCTION_NAME = power2round
+
+# If this proof is found to consume huge amounts of RAM, you can set the
+# EXPENSIVE variable. With new enough versions of the proof tools, this will
+# restrict the number of EXPENSIVE CBMC jobs running at once. See the
+# documentation in Makefile.common under the "Job Pools" heading for details.
+# EXPENSIVE = true
+
+# This function is large enough to need...
+CBMC_OBJECT_BITS = 8
+
+# If you require access to a file-local ("static") function or object to conduct
+# your proof, set the following (and do not include the original source file
+# ("mldsa/poly.c") in PROJECT_SOURCES).
+# REWRITTEN_SOURCES = $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i
+# include ../Makefile.common
+# $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i_SOURCE = $(SRCDIR)/mldsa/poly.c
+# $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i_FUNCTIONS = foo bar
+# $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i_OBJECTS = baz
+# Care is required with variables on the left-hand side: REWRITTEN_SOURCES must
+# be set before including Makefile.common, but any use of variables on the
+# left-hand side requires those variables to be defined. Hence, _SOURCE,
+# _FUNCTIONS, _OBJECTS is set after including Makefile.common.
+
+include ../Makefile.common

proofs/cbmc/power2round/power2round_harness.c

@@ -0,0 +1,12 @@
+// Copyright (c) 2025 The mldsa-native project authors
+// SPDX-License-Identifier: Apache-2.0
+
+#include "rounding.h"
+
+
+void harness(void)
+{
+  int32_t *a0, *a1;
+  int32_t a;
+  power2round(a0, a1, a);
+}