CBMC: Add contract and proof for polyvecl_ntt

Resolves #118

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

Author: mkannwischer

mldsa/polyvec.h

@@ -72,11 +72,18 @@ __contract__(
  * Name:        polyvecl_ntt
  *
  * Description: Forward NTT of all polynomials in vector of length MLDSA_L.
- *Output coefficients can be up to 16*MLDSA_Q larger than input coefficients.
+ *              Coefficients can grow by 8*MLDSA_Q in absolute value.
  *
  * Arguments:   - polyvecl *v: pointer to input/output vector
  **************************************************/
-void polyvecl_ntt(polyvecl *v);
+void polyvecl_ntt(polyvecl *v)
+__contract__(
+  requires(memory_no_alias(v, sizeof(polyvecl)))
+  requires(forall(k0, 0, MLDSA_L, array_abs_bound(v->vec[k0].coeffs, 0, MLDSA_N, MLDSA_Q)))
+  assigns(memory_slice(v, sizeof(polyvecl)))
+  ensures(forall(k1, 0, MLDSA_L, array_abs_bound(v->vec[k1].coeffs, 0, MLDSA_N, MLD_NTT_BOUND)))
+);
+
 #define polyvecl_invntt_tomont MLD_NAMESPACE(polyvecl_invntt_tomont)
 void polyvecl_invntt_tomont(polyvecl *v);
 #define polyvecl_pointwise_poly_montgomery \

proofs/cbmc/polyvecl_ntt/Makefile

@@ -0,0 +1,54 @@
+# SPDX-License-Identifier: Apache-2.0
+
+include ../Makefile_params.common
+
+HARNESS_ENTRY = harness
+HARNESS_FILE = polyvecl_ntt_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_ntt
+
+DEFINES +=
+INCLUDES +=
+
+REMOVE_FUNCTION_BODY +=
+UNWINDSET += $(MLD_NAMESPACE)polyvecl_ntt.0:7 # Largest value of MLDSA_L
+
+PROOF_SOURCES += $(PROOFDIR)/$(HARNESS_FILE).c
+PROJECT_SOURCES += $(SRCDIR)/mldsa/polyvec.c
+
+CHECK_FUNCTION_CONTRACTS=$(MLD_NAMESPACE)polyvecl_ntt
+USE_FUNCTION_CONTRACTS=$(MLD_NAMESPACE)poly_ntt
+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_ntt
+
+# 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_ntt/polyvecl_ntt_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_ntt(a);
+}