CBMC: Add contract and proof for poly_chknorm

Resolves #109

The explicit check for the range of B is removed as this is a compile-time
constant in all call sites anyway. Instead, we can just add it as pre-condition
to the CBMC contract.

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

Author: mkannwischer

mldsa/cbmc.h

@@ -84,7 +84,7 @@
     ((qvar_lb) <= (qvar) && (qvar) < (qvar_ub)) ==> (predicate)   \
   }
 
-#define EXISTS(qvar, qvar_lb, qvar_ub, predicate)         \
+#define exists(qvar, qvar_lb, qvar_ub, predicate)         \
   __CPROVER_exists                                              \
   {                                                             \
     unsigned qvar;                                              \

mldsa/poly.c

@@ -156,32 +156,38 @@ void poly_use_hint(poly *b, const poly *a, const poly *h)
   }
 }
 
+/* Reference: explicitly checks the bound B to be <= (MLDSA_Q - 1) / 8).
+ * This is unnecessary as it's always a compile-time constant.
+ * We instead model it as a precondition.
+ */
 int poly_chknorm(const poly *a, int32_t B)
 {
   unsigned int i;
+  int rc = 0;
   int32_t t;
 
-  if (B > (MLDSA_Q - 1) / 8)
-  {
-    return 1;
-  }
-
   /* It is ok to leak which coefficient violates the bound since
      the probability for each coefficient is independent of secret
      data but we must not leak the sign of the centralized representative. */
+
   for (i = 0; i < MLDSA_N; ++i)
+  __loop__(
+    invariant(i <= MLDSA_N)
+    invariant(rc == 0 || rc == 1)
+    invariant((rc == 0) == array_abs_bound(a->coeffs, 0, i, B))
+  )
   {
     /* Absolute value */
     t = a->coeffs[i] >> 31;
     t = a->coeffs[i] - (t & 2 * a->coeffs[i]);
 
     if (t >= B)
     {
-      return 1;
+      rc = 1;
     }
   }
 
-  return 0;
+  return rc;
 }
 
 /*************************************************

mldsa/poly.h

@@ -271,7 +271,15 @@ __contract__(
  * Returns 0 if norm is strictly smaller than B <= (MLDSA_Q-1)/8 and 1
  *otherwise.
  **************************************************/
-int poly_chknorm(const poly *a, int32_t B);
+int poly_chknorm(const poly *a, int32_t B)
+__contract__(
+  requires(memory_no_alias(a, sizeof(poly)))
+  requires(0 <= B && B <= (MLDSA_Q - 1) / 8)
+  requires(array_bound(a->coeffs, 0, MLDSA_N, -REDUCE_RANGE_MAX, REDUCE_RANGE_MAX))
+  ensures(return_value == 0 || return_value == 1)
+  ensures((return_value == 0) == array_abs_bound(a->coeffs, 0, MLDSA_N, B))
+);
+
 
 #define poly_uniform MLD_NAMESPACE(poly_uniform)
 /*************************************************

proofs/cbmc/poly_chknorm/Makefile

@@ -0,0 +1,54 @@
+# SPDX-License-Identifier: Apache-2.0
+
+include ../Makefile_params.common
+
+HARNESS_ENTRY = harness
+HARNESS_FILE =poly_chknorm_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 = poly_chknorm
+
+DEFINES +=
+INCLUDES +=
+
+REMOVE_FUNCTION_BODY +=
+UNWINDSET +=
+
+PROOF_SOURCES += $(PROOFDIR)/$(HARNESS_FILE).c
+PROJECT_SOURCES += $(SRCDIR)/mldsa/poly.c
+
+CHECK_FUNCTION_CONTRACTS=$(MLD_NAMESPACE)poly_chknorm
+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 = poly_chknorm
+
+# 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/poly_chknorm/poly_chknorm_harness.c

@@ -0,0 +1,13 @@
+// Copyright (c) 2025 The mldsa-native project authors
+// SPDX-License-Identifier: Apache-2.0
+
+#include "poly.h"
+
+
+void harness(void)
+{
+  poly *a;
+  int r;
+  int32_t B;
+  r = poly_chknorm(a, B);
+}