CBMC: Add contract and proof for polyvecl_reduce

Resolves #116

Signed-off-by: Matthias J. Kannwischer <[email protected]>

Author: mkannwischer

mldsa/poly.h

@@ -29,7 +29,7 @@ typedef struct
 void poly_reduce(poly *a)
 __contract__(
   requires(memory_no_alias(a, sizeof(poly)))
-  requires(forall(k0, 0, MLDSA_N, a->coeffs[k0] <= REDUCE_DOMAIN_MAX))
+  requires(array_bound(a->coeffs, 0, MLDSA_N, INT32_MIN, REDUCE_DOMAIN_MAX))
   assigns(memory_slice(a, sizeof(poly)))
   ensures(array_bound(a->coeffs, 0, MLDSA_N, -REDUCE_RANGE_MAX, REDUCE_RANGE_MAX))
 );

mldsa/polyvec.c

@@ -65,8 +65,18 @@ void polyvecl_reduce(polyvecl *v)
   unsigned int i;
 
   for (i = 0; i < MLDSA_L; ++i)
+  __loop__(
+    invariant(i <= MLDSA_L)
+    invariant(forall(k0, i, MLDSA_L, forall(k1, 0, MLDSA_N, v->vec[k0].coeffs[k1] == loop_entry(*v).vec[k0].coeffs[k1])))
+    invariant(forall(k2, 0, i,
+      array_bound(v->vec[k2].coeffs, 0, MLDSA_N, -REDUCE_RANGE_MAX, REDUCE_RANGE_MAX))))
   {
-    poly_reduce(&v->vec[i]);
+    poly t = v->vec[i];
+    poly_reduce(&t);
+    /* Full struct assignment from local variables to simplify proof */
+    /* TODO: eliminate once CBMC resolves
+     * https://github.com/diffblue/cbmc/issues/8617 */
+    v->vec[i] = t;
   }
 }
 

mldsa/polyvec.h

@@ -25,7 +25,24 @@ void polyvecl_uniform_gamma1(polyvecl *v, const uint8_t seed[MLDSA_CRHBYTES],
                              uint16_t nonce);
 
 #define polyvecl_reduce MLD_NAMESPACE(polyvecl_reduce)
-void polyvecl_reduce(polyvecl *v);
+/*************************************************
+ * Name:        polyvecl_reduce
+ *
+ * Description: Inplace reduction of all coefficients of all polynomial in a
+ *              vector of length MLDSA_L to
+ *              representative in [-6283008,6283008].
+ *
+ * Arguments:   - poly *v: pointer to input/output vector
+ **************************************************/
+void polyvecl_reduce(polyvecl *v)
+__contract__(
+  requires(memory_no_alias(v, sizeof(polyvecl)))
+  requires(forall(k0, 0, MLDSA_L,
+    array_bound(v->vec[k0].coeffs, 0, MLDSA_N, INT32_MIN, REDUCE_DOMAIN_MAX)))
+  assigns(memory_slice(v, sizeof(polyvecl)))
+  ensures(forall(k1, 0, MLDSA_L,
+    array_bound(v->vec[k1].coeffs, 0, MLDSA_N, -REDUCE_RANGE_MAX, REDUCE_RANGE_MAX)))
+);
 
 #define polyvecl_add MLD_NAMESPACE(polyvecl_add)
 /*************************************************

proofs/cbmc/polyvecl_reduce/Makefile

@@ -0,0 +1,54 @@
+# SPDX-License-Identifier: Apache-2.0
+
+include ../Makefile_params.common
+
+HARNESS_ENTRY = harness
+HARNESS_FILE = polyvecl_reduce_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 = polyvecl_reduce
+
+DEFINES +=
+INCLUDES +=
+
+REMOVE_FUNCTION_BODY +=
+UNWINDSET +=
+
+PROOF_SOURCES += $(PROOFDIR)/$(HARNESS_FILE).c
+PROJECT_SOURCES += $(SRCDIR)/mldsa/polyvec.c
+
+CHECK_FUNCTION_CONTRACTS=$(MLD_NAMESPACE)polyvecl_reduce
+USE_FUNCTION_CONTRACTS=$(MLD_NAMESPACE)poly_reduce
+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 = polyvecl_reduce
+
+# 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/polyvecl_reduce/polyvecl_reduce_harness.c

@@ -0,0 +1,10 @@
+// Copyright (c) 2025 The mldsa-native project authors
+// SPDX-License-Identifier: Apache-2.0
+
+#include "polyvec.h"
+
+void harness(void)
+{
+  polyvecl *a;
+  polyvecl_reduce(a);
+}